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The gametophyte-sporophyte junction: unequivocal hints for two evolutionary lines of archegoniate land plants
Flora (2001) 196,431-445 http://www.urbanfischer.de/jouma]s/flora
© by Urban & Fischer VerLag
The gametophyte-sporophyte junction: unequivocal hints for two evolutionary lines of archegoniate land plants WOLFGANG FREY*, MARIA HOFMANN
& HARTMUT H. HILGER
Institut fUr Biologie, Systematische Botanik und Pflanzengeographie, Freie Universitat Berlin, AltensteinstraBe 6, D-14195 Berlin, Germany *e-mail correspondingauthor:[email protected] Accepted: May 4, 2001
Summary New results have added to our knowledge about the gametophyte-sporophyte junction in bryophytes, hornworts and pteridophytes and its significance for archegoniate land plant evolution. Bryophyta (liverworts and mosses) show a uniform structure: the sporophyte foot is separated from the surrounding gametophyte tissue by a placental space. At maturity the space is in most cases filled with residues of collapsed cells of gametophyte origin. The taxa with a placental space represent the bryophyte lineage of land plant evolution, indicating a common ancestor of the bryophyte groups Hepaticophytina and Bryophytina. To emphasize the separate position of Fossombronia within liverworts, the new class Fossombroniopsida is proposed. In Anthocerotophyta and Psilotopsida sporophyte haustorial cells intermingle resp. interdigitate with gametophyte transfer cells, and electron-dense fibrillar material fills the interface. In Lycopodiopsida, Equisetopsida and Pteridopsida, a direct contact exists between gametophyte and sporophyte placental layers. In some cases, sporophyte placental cells tend to interdigitate between gametophyte placental cells. No residues of collapsed gametophyte cells are found in the interface between the two generations. Key words: Gametophyte-sporophyte junction, evolution, Anthocerotophyta, Bryophyta, Fossombroniopsida class. nov., Pteridophyta, placental space, transfer cells.
1. Introduction Besides the archegonium the gametophyte-sporophyte junction is one of the most characteristic features common to all archegoniate plants [Bryophyta (mosses and liverworts), Anthocerotophyta (hornworts) and Pteridophyta (ferns)]. Since the last reviews (e.g. LIGRONE et al. 1993; FREY et al. 1994 a) the number of investigated species has increased to a total of 63 mosses, 43 liverworts, 15 hornworts and 9 ferns, including our own results on 38 mosses and liverworts, one hornwort and 2 ferns. The comparison of the structure of the gametophytesporophyte junction in combination with the arrangement of transfer cells in the placenta raises the question of the systematic value of these features and their relevance as important evolutionary characters. They furthermore may give an answer on the divergence of archegoniate plant groups. The shape of the sporophyte foot, the distribution of transfer cells in both parts of the placenta, the degree of development of the wall in0367-2530101/196/06-431
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growths, and the nature of the contact between the two generations (intermingling, interdigitating or not) are central in the study here presented.
2. Materials and methods Plant specimens investigated (species printed in bold in Table 1) were collected during field trips in Europe, New Zealand and Patagonia and are deposited in the Herbarium W. Frey or BSB. The specimens of Haplomitrium hookeri sent by D. G. Long, Edinburgh, and microtome slides of Haplomitrium gibbsiae by E. O. Campbell, North Palmerston, are gratefully acknowledged. The study of the junction of Tmesipteris elongata, published by FREY et al. (1994 a, b), was based on material found in New Zealand 1992 and 1994 on tree fern trunks. Fern specimens from the Botanic Garden Marburg, Division of Fern Cultivation, are preserved in ethanol and deposited at the Institut fur Biologie - Systematische Botanik und Pflanzengeographie. Specimens were either fixed in AFE for SEM, in glutaraldehyde/Os0 4 for TEM, or herbaFLORA (2001) 196
431
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"'" N 0
~
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N
Subclass Order
+ + + + +
Aneura pinguis (L.) Dum. Riccardia multifida (L.) S. Gray Cryptothallus mirabilis Malmb. Metzgeria conjugata Lindb.
Aneuraceae
Metzgeriaceae
+
Pallavicinia innovans Steph.5 , P. indica Hook., P. lyellii Carruth, Symphyogyna subsimplex Mitt.6 , S. hymenophyllum (Hook.) Mont. & Nees6
Pallaviciniaceae
Hymenophytaceae Hymenophyton leptopodum (Hook. f. & Tayl.) Steph. 6
+
Pellia epiphylla (L.) Corda, P. endiviifolia (Dicks.) Dum.
Pelliaceae
Metzgeriidae Metzgeriales
Jungermanniopsida
+
+
Haplomitrium blumii (Nees) Schust., H. hookeri (Smith) Nees, H. gibbsiae (Steph.) Schnst.
Fossombroniaceae Fossombronia echinata Macv. Haplomitriaceae
Calobryidae
+
Apotreubia hortonae Schust. & Konstantinova2
Fossombroniopsida
Treubiaceae
Treubiopsida
+
+
+
+
Placental space present (+) or not (-)
Blasia pusilla L.
Riccia sorocarpa Bisch.
Haplomitriopsida
Blasiaceae
Ricciaceae
Carrpaceae: Carrpos monocarpos Prosk., Targioniaceae: Targionia hypophylla L., Conocephalaceae: Conocephalum conicum (L.) Underw., Aytoniaceae: Reboulia hemisphaerica (L.) Raddi, Mannia androgyna (L.) Evans, Plagiochasma rupestre (R. & G. Forst.) Steph., Exormothecaceae: Exormotheca pustulosa Mitt., Marchantiaceae: Preissia quadrata (Scop.) Nees, Dumortiera hirsuta (Sw.) Nees
Sphaerocarpos donnellii Aust., S. texanus Aust.
Sphaerocarpaceae
Sphaerocarpidae
Marchantiidae
Monoclea gottschei Lindb.
Monocleaceae
Species (resp. families/species) (bold = own results, others after Ligrone et a1. 1993 1, resp. cited2 - 16
Monocleidae
Family
Blasiopsida
Marchantiopsida
Hepaticophytina (Hepaticae)
Bryophyta
Class
Systematics of species investigated
? + (2)
+ (2-3)
+ (2-3)
+ (2-3)
+ (2-4)
+ (2-3)
+ (2-3)
Gametophyte
+ (1)
+ (1-2)
+ (1)
+ (1-2)
+ (1)
+ (1) + (1-2)
+ (1)
+ (1)
Sporophyte
Distribution of transfer cells (layers)
conical
conical conical conical
conical
conical
conical
bulbous
bulbous
bulbous
conical
spheroidal, conical, bulbous, cylindrical, cup-shaped
spheroidal
conical
Foot shape
clustering of transfer cells
see 3 and 4
Remarks
Table 1. Overview on analysed gametophyte-sporophyte junctions in Bryophyta-Anthocerotophyta-Pteridophyta. Bold-printed species were investigated by the authors . Full references see References.
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~
0
Bryopsida
Andreaeopsida
Sphagnopsida
Bryophytina (Musci)
Dawsoniaceae Polytrichaceae
Polytrichidae Dawsoniales
Polytrichales
Tetraphididae
Tetraphidaceae
Diphysciaceae
Buxbaumiaceae
Andreaeaceae
Andreaeidae Andreaeales
Buxbaumiidae Buxbaurniales
Takakiaceae
Sphagnaceae
Radulaceae
Takakiidae Takakiales
Jungermanniidae Jungermanniales
+ (2-3)
Tetraphis pellucida Hedw.
Diphysciumfoliosum (Hedw.) Mohr
+
+
+ (1)
-?
+ (1)
+ (2)
+ (1)
elongatetapering
conicalbulbous
elongatetapering
elongatetapering
+ (2-3) +
Polytrichumformosum Hedw., P. commune Hedw., P. piliferum Hedw., Pogonatum aloides (Hedw.) P. Beauv., P. neesii (C. Miill.) Dozy, Oligotrichum hercynicum (Hedw.) Lam. & DC., Atrichum undulatum (Hedw.) P. Beauv., Dendroligotrichum dendroides Hedw.
+
elongatetapering
+ (2-3) +
Dawsonia superba Grev.
Buxbaumia piperi Best.
conical
+ (1) +
Andreaea rupestris Hedw.,A. rothii Web. & Mohr, Andreaeobryum macrosporum Steeve & B. Murr.
bulbous
bulbous
conical bulbous spheroidal
elongatetapering
+
Takakia ceratophylla (Mitt.) Grolle l •7
+ (1) + (1-2)
+ (1)
cavernous
partly
+
Sphagnum fimbria tum Wils., S. fallax (Klinggr.) Klinggr., S. subnitens Russ. & Warnst., S. cuspidatum Hoffm.
Radula complanata (L.) Dum.
Herbertaceae: Herberta spp., Lepidoziaceae: + Kurzia trichoclados (K. Miill.) Grolle, Zoopsis linkiensis Horik., Cephaloziaceae: Cephalozia bicuspidata (L.) Dum., C. lunulifolia (Dum.) Dum., Jungermanniaceae: Jungermannia gracillima Sm., Gyrnnomitriaceae: Marsupella funckii (Web. & Mohr) Dum., Scapaniaceae: Scapania gracilis Lindb., Diplophyllum albicans (L.) Dum., Geocalycaceae: Lophocolea heterophylla (Schrad.) Dum., Chiloscyphus pallescens (Erh. ex Hoftin.) Dum., Leptoscyphus cf. horizontalis (Hook.) Herz.
NO INTERMINGLING (as reported in I)
NO INTERMINGLING (as reported in I)
obliterating gametophyte cells
NO INTERMINGLING (as reported in I)
~
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~
~
tl
Bryopsida
Class
Bryidae
Subclass Order
Systematics of species investigated
Table 1. (continued)
Family
+
Orthotrichaceae: Orthotrichum cupulatum Brid., Neckeraceae: Neckera crispa Hedw., N. cephalonica + Jur. & Ungerll, N. intermedia Brid. lI , Leptodon longisetus Mont ll , Leucodontaceae: Leucodon canariensis (Brid.) Schwaegr.lI, Hypopterygiaceae: Hypopterygium arbuscula Brid., Thuidiaceae: Thuidium tamariscinum (Hedw.) B.S.G., Amblystegiaceae: Amblystegium serpens (Hedw.) B.S.G., Drepanocladus uncinatus (Hedw.) Warnst.5, Brachytheciaceae: Brachythecium rivulare B.S.G., B. velutinum (Hedw.) B.S.G.5, Isothecium myosuroides Brid. 1I , Plagiotheciaceae: Isopterygiopsis pulchella (Hedw.) Iwats.
Dicranum majus Sm.
Fissidentaceae: Fissidens dubius P. Beauv., Archidiaceae: Archidium tenerrimum Mitt., Ditrichaceae: + Ceratodon purpureus (Hedw.) Brid., Seligeriaceae: Blindia acuta (Hedw.) B.S.G., Dicranaceae: Dicranum scoparium Hedw., Leucobryum glaucum (Hedw.) Angstr., Cladophascum gymnomitrioides (Dix.) Sim., Pottiaceae: Acaulon muticum (Hedw.) C. Miill.8, Tortula muratis Hedw., Timmiella barbuloides (Brid.) Monk., Phascum cuspidatum Hedw., Grimmiaceae: Grimmia pulvinata (Hedw.) Sm., Schistidium apocarpum (Hedw.) B.S.G., Bartramiaceae: Bartramia stricta Brid., Aulacomniaceae : Aulacomnium palustre (Hedw.) Schwaegr., Gigaspermaceae: Lorentziella imbricata (Mitt.) Broth.9 , Ephemeraceae: Ephemerum cohaerens (Hedw.) HampelO, Funariaceae: Funaria hygrometrica Hedw., Physcomitrium coorgense Broth., P. cyathicarpum Mitt., Bryaceae: Bryum elegans Nees ex Brid., B. capillare Hedw., Pohlia nutans (Hedw.) Lindb., Mniaceae: Mnium hornum Hedw., Plagiomnium undulatum (Hedw.) T. Kop., P. cuspidatum (Hedw.) T. Kop., Hypnodendraceae: Hypnodendron menziesii (Hook.) Par.
Species (resp. families/species) (bold =own results, others after Ligrone et al. 1993 1 , resp. cited2 - 16
+ (1)
+ (1)
Gametophyte
elongatetapering elongatetapering
+ (1) + (1) + (1-2) (3-4)
elongatetapering
NO INTERMINGLING in Dicranum scoparium, Bryum elegans and Pohlia nutans (as reported in 1,5)
Foot shape Remarks
+ (1) + (1-2) + (3-4)
Sporophyte
Distribution of Placental space transfer cells (layers) present (+) or not (-)
V\
w
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=--
~
-
~
~
fl
Lycopodiaceae
Equisetaceae Adiantaceae
Polypodiaceae
Lycopodiales
Equisetales
Pteridales
Polypodiales
Lycopodiopsida
Equisetopsida
Pteridopsida
Polypodium vulgare L. Polypodium spec.
Pteridium aquilinum (L.) Kuhn l6
+ (1) + (1) +
+ (1) +
+
+ (1)
+
+ (1)
+ (1)
+
+ (1)
Pelkleafalcata (R.Br.) Fee
+
-** -* -* -* -*
Adiantum capillus-veneris L. 15
No intra- + (1) placental space, no intermingling
Lycopodium appressum (Chapm.) Lloyd & UnderwP, L. cernuum L.14
Equisetum spec.
Sporophyte + (1-3) haustorial cells intermingling with gametophyte transfer cells
Tmesipteris elongata P. A. Dangeard l2
Sporophyte + (1-3) haustorial cells intermingling with gametophyte transfer cells
bulbous
bulbous
bulbous
bulbous or ovoidal
** Sporophyte placental cells tend to interdigitate ("intermingling"), no residues of collapsed gametophyte cells
* Without clear-cut demarcation zone. Interface between both generations. No collapsed gametophyte cells.
I LIGRONE et al. (1993),2 FREY & HILGER (2001), 3 STECH & FREY (2001),4 PASS & RENZAGLIA (1995),5 HOFMANN et al. (1996),6 FREY et al. (1996),7 RENZAGLIA et al. (1997),8 RusHING & ANDERSON (1996),9 RUSHING (1999).10 YIP & RUSHING (1999), II ALFAYATE et al. (2000), 12 FREY et al. (1994a), 13 PETERSON & WHITIER (1991), 14 DUCKETT & LIGRONE (1992),15 GUNNING & PATE (1969), 16 KHATOON (1986) .
Psilotaceae
Psilotales
--------
Anthoceros punctatus L., A. formosae Haseg., A. granulosa Haseg., A. caucasicus Steph. in Woronow, Phaeoceros laevis (L.) Prosk., P. carolinianus (Michx.) Prosk., Foliocerosfuciformis Bharadw., F. appendiculatus (Steph.) Hasegawa, Nothothylas orbicularis Sull., N. temperata Haseg., Dendroceros cavernosa Haseg., D. javanicus Nees, D. tubercularis Hatt.,D. validus Steph. (E. o. Campbell, pers. comm.), Megaceros flagellaris Steph.
--- - -
Psilotopsida
Pteridophyta
Anthocerotopsida
Anthocerotophyta
.._-----_. --.---
rium specimens were rehydrated with Aerosol aT (HOFMANN et al. 1996). For SEM-analysis, junctions were razor-sectioned, dehydrated, critical-point-dried (C0 2), sputtered with gold, and analyzed in a Stereoscan S 100 (Cambridge). Specimens for TEM were embedded in Spurr's resin and dissected with a Reichert Ultracut microtome, contrasted with lead citrate and viewed with a Zeiss EM 109 TEM. Results from literature are included in this survey (non bold-printed species in Table 1).
placental space is filled with residues of collapsed cells of gametophyte origin. Intermingling resp. interdigitating of sporophyte and gametophyte placental cells as in hornworts and pteridophytes (see below) does not occur. The main groups can be characterized as follows:
Hepaticophytina (liverworts) Marchantiopsida (Figs. 1-2)
3. Results and discussion Terminology/characters The gametophyte-sporophyte junction of archegoniate plants consists of the sporophyte foot and the surrounding parental gametophyte tissue. The outermost sporophyte and gametophyte cell layers represent the placenta and a space between gametophyte and sporophyte tissue is thus called placental space. The junctions of the different archegoniate groups can be characterized on the basis of a) the existence of a placental space, b) the number and distribution of transfer cell layers in both generations, c) peculiar features concerning the degree of development of wall ingrowths in transfer cells and d) the shape of the sporophyte foot, (Table 1, Figs. 1-19):
Bryophyta (liverworts and mosses) To the present day, a total of 106 species comprising all major groups of liverworts and mosses have been investigated. The young sporophyte (embryo) penetrates with its foot into the enlarged tissue of the archegonial stalk, resp. into the stem or thallus tissue. In some moss taxa an appendage (haustorium) is developed which originates from the hypobasal cell of the dividing zygote. In liverworts it generally becomes indistinct. A clear-cut demarcation zone between the gametophyte and the sporophyte is apparent. In many taxa the separating
The sporophyte foot is embedded in the parental gametophyte tissue and is variously shaped. It ranges from spheroidal to cup-shaped. Prominent wall ingrowths exist on both the gametophyte (2-4 cell layers) and the sporophyte (1-2 cell layers) side of the placenta. The sporophyte placental transfer cells have highly branched and anastomosing wall ingrowths, the gametophyte placental transfer cells extensive or short and coarse wall ingrowths ("Marchantiopsidalean type"). In some taxa (e.g. Dumortiera hirsuta) the placental space is filled with collapsed gametophyte cells. In the Ricciales only the capsule is developed, seta and sporophyte foot are not developed. Transfer cells are lacking. The Marchantiopsida are clearly separated from the Jungermanniopsidalean liverworts on the basis of the number and distribution of transfer cell layers, the wall ingrowth types, and the shape of the sporophyte foot, lacking mostly a collar (see below and Table 1). Placental organisation confIrms the Marchantiopsida as a taxonomically well circumscribed group.
Blasiopsida The structure of the gametophyte-sporophyte junction, the number of sporophyte (one) and gametophyte placental layers (two to three) with Marchantialean type wall ingrowths points to a relationship with Marchantiopsidalean, not with Jungermanniopsidalean liverworts. In addition, the spermatozoid ultrastructure (PASS & RENZAGLIA 1995) and recent molecular results (STECH &
Figs. 1- 6. Longitudinal sections of the gametophyte-sporophyte junction in liverworts. 1- 2. Marchantiopsida-Marchantiidae. 1- 2. Mannia androgyna. 1. SEM micrograph of a razor-sectioned "hanging" sporogon. The bulbous sporophyte foot is embedded in gametophyte tissue, the capsule has ruptured the calyptra. 2. TEM micrograph of the placental region. Sporophyte and gametophyte placental cells with wall ingrowths. Placental space present. 3-4. Jungermanniopsida-Metzgeriidae. 3. Aneura pinguis. SEM micrograph of a razor-sectioned junction. The conical sporophyte foot bears a collar at the transition to the seta. 4. Metzgeria conjugata. TEM micrograph of the placental region. The placental space is filled with collapsed cells of gametophyte origin (arrows). The gametophyte placental cells have no wall ingrowths but nacreous wall thickenings, the sporophyte placental cells show only thickened tangential walls with radial wall deposits. 5-6. Jungermanniopsida-Jungermanniidae. 5-6. Lophocolea heterophylla. 5. Light micrograph of the junction. Spheroidal-shaped foot with a prominent collar at the transition to the seta. 6. TEM micrograph of the placental region. Placental space filled with collapsed gametophyte cells; gametophyte placental cells with tangential wall thickenings, sporophyte placental cells show prominent thin and branched wall ingrowths. c collar, ca capsule, f foot, g gametophyte, I lipid droplet, p plastid, ps placental space, s sporophyte, se seta. Scale bars: 1, 3, 5 =200 !tm; 2,4,6 =5 !tm.
436
FLORA (2001) 196
f
ffii ii'
IliI lill d •
FLORA (2001) 196
437
FREY 2001) indicate an isolated position of this taxon (incl. Cavicularia), best recognized at the rank of a separate class as established by STECH & FREY (2001).
Treubiopsida The character-complex of a bulbous foot with an indistinct collar, correlated with the lack of gametophyte placental transfer cells, but with the occurrence of several layers of collapsed gametophyte placental cells and 1-2 layers of sporophyte placental transfer cells indicates a separate position within Hepaticophytina, but with Jungermanniopsidalean relationships. Compared with all gametophyte-sporophyte junctions investigated in Bryophyta, Anthocerotophyta, and Pteridophyta until now, the distribution pattern of the 1-2 layered transfer cells in the sporophyte placenta is unique (FREY & HILGER 2001). Clusters of sporophyte placenta cells with coarse wall ingrowths (transfer cells) alternate with clusters of sporophyte placental cells lacking wall ingrowths. Such a distribution pattern of transfer cells is not known from any other archegoniate land plant group.
Jungermanniopsida - Metzgeriidae (Figs. 3-4) The sporophyte foot of the thalloid and semifoliose Metzgerialean liverworts is of conical shape, and often a prominent collar delimits the transition to the seta. Collapsed cells of gametophyte origin are always present in the placental space. The sporophyte side of the placental region is variable concerning the presence or absence of a single layer of transfer cells. If wall ingrowths are present as e.g. in the Pallaviciniaceae they are short and coarse. Lack of transfer cells in the gametophyte generation is interpreted as a reduction (LIGRONE et al. 1993). Some of the species are characterized by nacreous wall thickenings in the gametophyte placental cell layer if wall ingrowths are lacking, and, in Metzgeria conjugata and Aneura pinguis, nacreous walls also occur in the sporophyte placental cell layer.
Jungermanniopsida - Jungermanniidae (Figs. 5 -6) Leafy Jungermannialean liverworts show a very uniform gametophyte-sporophyte structure. Usually the conical or bulbous foot bears a collar. Collapsed gametophyte cells mark the placental space. Prominent wall labyrinths with thin, highly branched wall ingrowths are restricted to usually one or rarely two layers of the sporophyte side of the placenta, the gametophyte placental cell walls are smooth. According to LIGRONE et al. (1993) irregular, radially elongated peripheral sporophyte placental cells in Radula seem to interdigitate into adjacent gametophyte placental cells like in bryopsid mosses, e.g., Diphyscium, Polytrichum and Bryum. Neither the knowledge of 438
FLORA (2001) 196
the situation in the three mentioned genera (see Bryophytina, mosses) nor the figures in LIGRONE et al. (1993) give hints that true interdigitation of sporophyte placental cells into gametophyte placental cells exists. A distinct placental space is present.
Haplomitrium and Fossombronia An outstanding placental organisation within liverworts of Jungermanniopsidalean affinity is found in Haplomitrium spp. and Fossombronia spp. Two to three layers of placental transfer cells are present on the gametophyte side, and only one on the sporophyte side of the placenta. These features separate these taxa resp. their gametophyte-sporophyte junction from all other Jungermanniopsidalean liverworts. In Haplomitrium spp. the wall ingrowths within the 2-3 layers of gametophyte placental transfer cells are generally longer and more highly branched than in the 1-2 layers of sporophyte placental transfer cells. Haplomitrium shares a similar placenta rather to the Marchantiopsida (LIGRONE et al. 1993, Figs. 74, 75), than to Jungermanniidae, where the taxon is currently placed. This character, the bulbous foot and also the absence of a collar indicate a separate position from Jungermanniopsida (LIGRONE et al. 1993), and the placement as class Haplomitriopsida R. Stotl. & B. Stotl. (STOTLER & CRANDALL-STOTLER 1977) seems to be justified. On the other hand, molecular systematic studies show that this taxon is related with the Metzgerialean line of Jungermanniopsidalean liverworts (STECH & FREY 2001).
Fossombroniopsida W. Frey & H. H. Hilger, classis nova Differt ab aliis Hepaticophytinis gametangiis masculinis et femininis singulariis ordine acropeto supra axis insertis, capsulis sine valvis, irregulariter dilabentibus, sporophyto pseudoperianthio unistratosa vestito. Type family: Fossombroniaceae Hazsl., Magyar Birodalom Moh-Fl6nija: 20,36. Mai - Dec 1885 (nom. fam. cons.). In Fossombronia echinata the gametophyte placenta transfer cells have strongly branched wall ingrowths that form a highly complex three-dimensional network (LIGRONE & GAMBARDELLA 1988). Similar elaborate wall ingrowths are common in the placentae of Blasia and Marchantiopsidalean liverworts, but are not found in Jungermanniopsida s.str. (LIGRONE et al. 1993). This and the following characters, viz. the bulbous foot and the lacking collar urge to exclude Fossombronia from the Jungermanniopsida. The establishment of the new class "Fossombroniopsida" instead of Fossombroniales Schljakow indicates, as in Haplomitriopsida, the independent position of a plant group that is "immensely old,
isolated and relict" (SCHUSTER 1984: 943). But as in Haplomitriopsida, molecular systematic results indicate Fossombronia as a member of the Metzgeriidae (STECH & FREY 2001).
Bryophytina (mosses) The Sphagnopsida are an extraordinary, highly specialized moss group. They are phylogenetic ally very old and may date back to Late Permian time. It is the only moss taxon in which wall ingrowths (transfer cells) are lacking in the placental cell layers of both generations. A distinct mucilage-containing cavernous tissue which develops from obliterating gametophyte placental cells between the bulbous sporophyte foot and the gametophyte tissue separates the two generations (cf. LIGRONE et al. 1993, p. 241 ,Fig. 8 ; also confirmed by own observations). The following moss groups are characterized by at least one cell layer of transfer cells on the sporophyte side of the placenta. A distinct placental space is present. With few exceptions, the foot is elongate-tapering. In Andreaeopsida (incl. Takakiales), the granite mosses, the outer sporophyte placental cell layer consists of transfer cells. Gametophyte placental cells lack wall ingrowths. In Andreaeaceae the foot is conical.
Bryopsida (Figs. 7 -12) The Polytrichidae represent a subclass of acrocarpous mosses which are plesiomorphic with respect to their conducting system and peristome type. The elongatetapering foot is appendaged (Fig. 7). Wall ingrowths are restricted to the sporophyte side of the placenta (2-3 layers). A placental space is always present, especially in the foot tip region (Figs. 7 -8). In contrast to the observations of BLAIKLEY (1933) and LIGRONE et al. (1993), no intermingling of sporophyte and gametophyte placental cells could be observed in Polytrichum. The reports about the junction in the Buxbaumiidae should be reinvestigated. For Buxbaumia piperi, LIGRONE et al. (1982) stated 2-3 layers of gametophyte placental transfer cells and one layer of sporophyte placental transfer cells. Diphyscium foliosum was mentioned by LIGRONE et al. (1993) with a description of the placental ultrastructure that contradicts our results. They figured wall ingrowths in the sporophyte projecting placental cells and in the outer gametophyte placental cell layer. Our re-investigations on Diphyscium foliosum indicate, apart from the conical-bulbous foot, no intermingling of gametophyte and sporophyte placental cells. Furthermore, there are two layers of cells with coarse wall ingrowths in the sporophyte projecting cells of the placenta, but probably no wall ingrowths on the gametophyte side of the placenta. The placental space is filled with collapsed cells of gametophyte origin.
The Tetraphididae possess the bryalean gametophyte-sporophyte junction with one layer of transfer cells on each placenta side (LIGRONE et al. 1993). Concerning the junction, together with the Tetraphididae the Bryidae form a homogeneous unit. Transfer cells are present on both generations of the placenta, in most cases one layer on each side, in apparently more advanced acrocarpous taxa two layers on the sporophyte side. The elongate-tapering foot is always separated by a placental space from the surrounding gametophyte tissue often containing residues of collapsed cells of gametophyte origin without wall ingrowths. The outermost sporophyte cells in Dicranum, Bryum and Pohlia are horizontally elongate (Figs. 9-10), but no intermingling of the two generations could be observed (cf. also HOFMANN et al. 1996,Fig.l,Bryum elegans) as reported by LIGRONE et al. (1993). Regarding the distribution of transfer cells, most pleurocarpous species (Figs. 11-12) show a uniform structure of the junction: gametophyte transfer cells (1 layer) and sporophyte transfer cells (1 or 2 layers ) are universally present. In Brachythecium spp. and Drepanocladus spp. the appendage consists of living cells. A placental space is apparent throughout this group and the gametophyte origin of collapsed cells filling the interface of the two generations could be proven for Drepanocladus (Fig. 12). Exceptions from this general appearance were recently reported by ALFAYATE et al. (2000), who described 3-4 layers of sporophyte transfer cells for Isothecium myosuroides and 2-3 layers for Leptodon longisetus.
Anthocerotophyta (hornworts) (Figs. 13-14) So far, 15 taxa have been investigated in hornworts. The basal part of the sporophyte forms a typical bulbous or ovoidal foot. The placental region is characterized by elongated haustorium-like sporophyte cells intermingling with gametophyte transfer cells, most distinctly in the genus Folioceros (VAUGHN & HASEGAWA 1993). A typical placental space is absent, both cell types are separated by intercellular spaces forming lacunae, filled with fibrillar material. Gametophyte cells (1- 3 layers) adjacent to the sporophyte haustorial cells are smaller than ordinary parenchyma cells of the thallus. They show prominent wall labyrinths, whereas the haustorium-like sporophyte placental cells have rather thin walls without wall ingrowths.
Pteridophyta (ferns and fern allies) The number of taxa investigated (9 taxa) is relatively low due to the problems of finding gametophyte-sporophyte junctions. There are at least two types of junctions within this group. FLORA (2001) 196
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FLORA (2001) 196
Psilotopsida (Figs. 15 -16)
4. Conclusions
The two genera Psilatum and Tmesipteris are either regarded as "some of the most primitive plants surviving on earth today" (label in the Botanical Garden Sydney) lacking roots, but with unicellular rhizoids and eusporangiate sporangia or they are believed to be highly evolved members of the Gleicheniaceae of the Pteridopsida (BIERHORST 1971). Beside this controversy, the gametophyte-sporophyte junction with intermingling gametophyte and sporophyte placental cells ("parasitic phase") gives the impression of a plesiomorphic situation in the Anthocerotophyta-Psilotopsida-LycopodiopsidaJEquisetopsidaJPteridopsida clade of land plant evolution. The sporophyte foot is bulbous (Fig. 15). Sporophyte placental cells intermingle resp. interdigitate with gametophyte placental cells (transfer cells). No demarcation zone and also no lacunae occur between the sporophyte and the gametophyte generation. Wall ingrowths (transfer cells) in 1-3 layers are restricted to the gametophyte side of the placenta (Fig .16).
By now there is a good overview of the structure of the gametophyte-sporophyte junction especially in bryophytes, but much less and still un sufficient in pteridophytes (Table 1). Nevertheless, the findings permit well supported statements on systematic and phylogenetic aspects and especially on the divergence of archegoniate land plant groups. The structure of the gametophyte-sporophyte junction, especially
Lycopodiopsida, Equisetopsida, Pteridopsida (Figs. 17-18) The investigated species of Lycopodiopsida, Equisetopsida and Pteridopsida show a bulbous sporophyte foot. The sporophyte and gametophyte placental cells are in direct contact: No clear-cut demarcation zone between the two generations can be proven, no placental space is obvious and no collapsed cells fill the interface of the two generations. In Adiantum capillus-veneris sporophyte and gametophyte placental cells tend to interdigitate (GUNNING & PATE 1969). Wall labyrinths with very thin wall ingrowths, extending a considerable distance into the cell interior, are present in both generations on the outermost cell layers.
- the distribution and number of gametophyte and sporophyte transfer cell layers of the placenta, - the degree of development of wall labyrinth ingrowths (coarse/fine, highly branched), - the nature of the contact (intermingling, interdigitating, or not), resp. the presence of a placental space with collapsed cells of gametophyte origin between the two generations indicate that there are two different lineages of land plant evolution apparent (Fig. 19, Table 1). In the bryophyte lineage of land plant evolution the sporophyte foot is separated by a distinct placental space from the surrounding gametophyte tissue (placental taxa sensu LIGRONE et al. 1993). The placental space is in most taxa filled with residues of collapsed cells of gametophyte origin. This basic structure of the gametophyte-sporophyte junction within Hepaticophytina and Bryophytina points out that the Bryophyta are an evolutionary line of their own (monophyletic group). This common type of junction in bryophytes must have been separated from a junction precursor without intermingling of sporophyte and gametophyte placental cells, i.e. prior to the evolution of pteridophytalean land plants. This assumption is supported by fossil data, where plants with hepaticophytic characters are believed to be the earliest land plants (EDWARDS et al. 1995; KENRICK & CRANE 1997a, b).
Figs. 7 -12. Bryopsida. Longitudinal sections (except Fig. 12) of the sporophyte-gametophyte junction in mosses. 7 - 8. Polytrichum piliferum. 7. SEM micrograph of a razor-sectioned junction. Foot tip with appendage (arrow), the placental space is obvious. 8. TEM micrograph of the placental region. The outermost sporophyte placental cells with prominent wall ingrowths on the outer tangential walls, the second cell layer with smaller wall ingrowths. The gametophyte placental cells lack wall ingrowths. The placental space is filled with cell-remnants of gametophytic origin. There is no intermingling of gametophyte and sporophyte placental cells. 9-10. Pohlia nutans. 9. SEM micrograph of razor-sectioned junction. Foot tip with appendage (arrow). The placental space is obvious. 10. Detail of 9. with the lateral part of the placental region. The outermost cells of the sporophyte foot, with wall ingrowths, are radially elongated, but no intermingling with gametophyte placental cells occurs. 11-12. Drepanocladus uncinatus. 11. SEM micrograph of a razor-sectioned junction. The foot tip consists of living cells, the placental space is obvious. Wall ingrowths are prominent in the outermost sporophyte placental layer. 14. Light micrograph of a cross section of the placental region. The outermost cell layer of the sporophyte placental cells possesses thickened outer tangential walls. Adjacent two cell layers with gametophyte placental cells with thin cell walls, not yet collapsed, the third cell layer with thickened outer tangential walls. g gametophyte, ps placental space, s sporophyte, w wall ingrowths. Scale bars: 7,9 = 100 flm; 11 = 50 flm; 10,12 = 25 flm; 8 = 10 flm. FLORA (2001) 196
441
442
FLORA (2001) 196
Iii
liiiiA
a
II
M_
Gametophyte-sporophyte junction No intermingling Placental space between the sporophyte and gametophyte generation, in most cases filled with collapsed cells of gametophyte origin spo
Intermingling
No intermingling
of sporophyte and gametophyte placental cells
Direct contact or tend to interdigitate
spo
spo foot
Hepaticophytina
Bryophytina
- Bryophyte lineage of land plant evolution -
Anthocerotophyta Psilotopsida
Pteridophyta (Lycopodiopsida, Equisetopsida, Pteridopsida)
- Pteridophyte lineage of land plant evolution -
Fig. 19. Gametophyte-sporophyte junction types within bryophyte resp. pteridophyte lineages of land plant evolution. - collapsed gametophyte cells, ap appendage, co collar, gam gametophyte, ps placental space, spo sporophyte.
Within this bryophytalean type of gametophytesporophyte junction three subtypes are obvious (Table 1). Marchantiopsida exhibit 2-3 layers of gametophyte placental transfer cells and one to two layers of sporophyte placental transfer cells. The foot is of different shape and lacks a collar. The junction of Blasiopsida, Fossombroniopsida, and Haplomitriopsida is related to the Marchantiopsidalean junction type and may represent a separate subtype.
The junctions in Treubiopsida and Jungermanniopsidalean taxa lack in most cases gametophyte placental transfer cells, and have 0-1, rarely 2 layers of sporophyte placental transfer cells. A conical foot with a collar is obvious in most Jungermanniopsidalean taxa. The gametophyte-sporophyte junction in Bryophytina (excl. Sphagnopsida and Buxbaumiidae) is very uniform and constant. In most cases the foot is elongate-
Figs. 13 -18. Longitudinal sections of the sporophyte-gametophyte junction in the homworts (Anthoceros caucasicus). 13. Light micrograph of the junction. The bulbous foot is surrounded by the parental gametophyte. Notice the dark staining outer cell layers of the gametophyte. The inner part of the sporophyte foot consists of horizontally elongated haustorial cells. 14. TEM micrograph of the placental region. Sporophyte placental cells with rather thin cell walls are intermingled with gametophyte placental cells with prominent wall labyrinths. The typical placental space is lacking, the two cellular types are separated by intercellular spaces forming lacunae. 15 -18. Longitudinal sections of the sporophyte-gametophyte junction in Pteridophyta. 15 -16. Psilotopsida. Tmesipteris elongata. 15. SEM micrograph of a razor-sectioned sporophyte part of the junction. The sporophyte foot is surrounded by the vaginula. Conducting strand (arrow). 16. TEM micrograph of the placental region. Sporophyte placental cells (with thin cell walls) intermingle with gametophyte transfer cells, no placental space present. 17 -18. Pteridopsida. Pellaea Jalcata. 17. SEM micrograph of a razor-sectioned junction. The bulbous sporophyte foot is in direct contact with the gametophyte, no placental space is apparent. 18. TEM micrograph of the placental region. The outermost gametophyte placental cells with a prominent wall labyrinth on the outer tangential walls, wall ingrowths on the sporophyte placental cells are very small. Both generations are in direct contact. f foot, g gametophyte, s sporophyte, v vaginula, w wall ingrowths. Scale bars: 15 = 500 ftm; 13, 17 = 100 ftm; 14, 18 = 10 ftm; 16=5ftm. FLORA (200 1) 196
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±!±£!S2
M
tapering and lacks a collar. Reinvestigation is necessary in Buxbaumia. For the pteridophyte lineage of land plant evolution we assume a basic structure of the gametophytesporophyte junction with intermingling resp. interdigitating sporophyte placental cells and gametophyte placental cells, present only in extant Anthocerotophyta and Psilotopsida. It is assumed that in both groups ancestral conditions of the gametophyte-sporophyte structure have been retained throughout their separate evolutionary history. This view of the pteridophyte (and spermatophyte) lineage of land plant evolution including the Anthocerotophyta clade corresponds with the "old" palaeobotanical idea of closer relationships between certain fern groups and the Anthocerotophyta (CAMPBELL 1895; BLAIKLEY 1933; SMITH 1938 ; STEWART & ROTHWELL 1993; FREY et al. 1994a, b). The opinion that Anthocerotophyta (hornworts) are not closely related with Bryophyta is now supported also by different molecular data sets, e.g. that one of HEDDERSON et al. (1996), classifying the Anthocerotophyta as a sister group to all other Embryophyta, and more clearly by nucleotid sequence data of the chloroplast gene rbeL, which implicates the Anthocerotophyta as the lineage of bryophytalean plants closest to vascular plants (LEWIS et al. 1997), and by nucleotid sequences of the plastid coded genes and the 18S rRNA gene (NISHIYAMA & KATO 1999,Fig. 2). QIU & LEE (2000: 81) recently stated referring to LEWIS et al. (1997) that "Hornworts, when not occupying the basal position in land plants, are sister to vascular plants.". This position is also in contrast to the traditional view of horn worts occupying an intermediate position between liverworts and mosses. The gametophyte-sporophyte junction is very similar in Lycopodiopsida and Pteridopsida. This does not agree with the recently presented view (KENRICK & CRANE 1997 a) of two independent lineages: microphyllous Lycophytina including extant Lycopodiales, Selaginellales and Isoetales, and the megaphyllous Euphyllophytina including Psilotopsida, Equisetopsida, and Pteridopsida and seed plants. It rather argues for the relationships between Lycopodiopsida and Pteridophyta. Gametophyte-sporophyte junction research should concentrate onto these plant groups in future.
5. Acknowledgement We thank the Departments of Conservation in Tongariro, Gisborne, Nelson and Hokitika in New Zealand for collection permits (Tmesipteris), the Botanical Garden Marburg (Germany) for fern specimens, D.G. Long, Edinburgh for specimens of Haplomitrium hookeri, and Dame Dr E.O. Campbell, Palmerston North, for slides of the junction of Haplomitrium gibbsiae. We are also grateful to Ms C. Macmillan and Ms 444
FLORA (2001) 196
C. Gruber for skilful technical assistance (TEM, SEM), Mr H. Ltinser for drawing Fig. 19, Dr M. Stech for help with the Latin diagnosis, and Dr M. Weigend for critically reading the English manuscript.
6. References ALFAYATE, C.; ESTEBANEZ, B. & RON, E. (2000): The sporophyte-gametophyte junction in five species of pleurocarpous mosses. - Bryologist 103: 467 -474. BIERHORST, D.W. (1971): Morphology of vascular plants. Macmillan, New York. BLAIKLEY, N. M. (1933): The structure of the foot in certain mosses and in Anthoceros. - Trans. Roy. Soc. Edinburgh 57: 691-709. CAMPBELL, D.H. (1895): The structure and development of mosses and ferns. 1. ed. Macmillan, London. DUCKETT, J.G. & LIGRONE, R. (1992): A light and electron microscope study of the fungal endophytes in the sporophyte and gametophyte of Lycopodium cernuum L. with observations on the gametophyte-sporophyte junction. Can. J. Bot. 70: 58-72. EDWARDS, D.; DUCKETT, J.G. & RICHARDSON, J. B. (1995): Hepatic characters in the earliest land plants. - Nature 374: 635-636. FREY, W. & HILGER, H.H. (2001): The gametophyte-sporophyte junction in Apotreubia hortonae (Treubiaceae, Hepaticophytina). Structure and systematic implications. - Nova Hedwigia 72,339-345. FREY, W.; CAMPBELL, E.O. & HILGER, H.H. (1994a): The sporophyte-gametophyte junction in Tmesipteris (Psilotaceae, Psilotopsida). - Beitr. BioI. Pflanzen 68:
105-111. FREY, W.; CAMPBELL, E.O. & HILGER, H.H. (1994b): Structure of the sporophyte-gametophyte junction in Tmesipteris elongata P.A. DANGEARD (Psilotaceae, Psilotopsida) and its phylogenetic implications. - A SEM analysis. Nova Hedwigia 59: 21-32. FREY, W.; HOFMANN,M. & HILGER,H.H. (1996): The sporophyte-gametophyte junction in Hymenophyton and Symphyogyna (Metzgeriidae, Hepaticae): structure and phylogenetic implications. - Flora 191: 245-252. GUNNING, B.E.S. & PATE, J.S. (1969): Cells with wall ingrowths (transfer cells) in the placenta of ferns. - Planta 87: 271-274. HEDDERSON, T.A.; CHAPMAN, R.L. & RooTES, W.L. (1996): Phylogenetic relationships of bryophytes inferred from nuclear rRNA gene sequences. - PI. Syst. Evol. 200: 213-224. HOFMANN, M.; HILGER, H.H. & FREY, W. (1996): Preparation of bryophyte herbarium specimens for the SEM using Aerosol aT solution in combination with FDA rapid dehydration. - Bryologist 99: 385-389. KENRICK, P. & CRANE, P.R. (1997a): The origin and early evolution of plants on land. - Nature 389: 33-39. KENRICK, P. & CRANE, P.R. (l997b): The origin and early diversification of land plants. Smithsonian Inst. Press, Washington and London.
44
4a
KHATOON, K. (1986): Occurrence of transfer cells in the sporophyte of Pteridium aquilinum L. - Pakistan J. Bot. 18: 9-13. LEWIS, L.A.; MISHLER, B.D. & VILGALYS, R. (1997): Phylogenetic relationships of liverworts (Hepaticae), a basal embryophyte lineage, inferred from nucleotide sequence data of the chloroplast gene rbcL. - Mol. Phylog. Evol. 7: 377-393. LIGRONE, R. & GAMBARDELLA, R. (1988): The sporophytegametophyte junction in bryophytes. - Adv. Bryol. 3: 225-274. LIGRONE, R.; GAMBARDELLA, R.; CASTALDO, R.; GIORDANO, S. & LUCIA SPOSITO, M.L. (1982): Gametophyte and sporophyte ultrastructure in Buxbaumia piperi BEST (Buxbaumiales, Musci). - J. Hattori Bot. Lab. 52: 465-499. LIGRONE, R.; DUCKETT, J.G., & RENZAGLIA, K.S. (1993): The gametophyte-sporophyte junction in land plants. Adv. Bot. Res. 19: 231-317. NISHIYAMA, T. & KATO, M. (1999): Molecular-phylogenetic analysis among bryophytes and tracheophytes based on combined data of plastid coded genes and the 18S rRNA gene. - Mol. BioI. EvoI. 16: 1027 -1036. PASS, J.M. & RENZAGLIA, K.S. (1995): Comparative microanatomy of the locomotory apparatus of Conocephalum conicum (Hepaticae, Conocephalaceae). - Fragm. Flor. Geobot. 40: 365-377. PETERSON, R.L. & WHITTIER, D.P. (1991): Transfer cells in the sporophyte-gametophyte junction of Lycopodium appressum. - Can. J. Bot. 69: 222- 226. QIU, Y.-L. & LEE, J. (2000): Transition to a land flora: a molecular phylogenetic perspective. - J. Phycol. 36: 799-802.
RENZAGLIA, K.S.; McFARLAND, K.D. & SMITH, D.K. (1997): Anatomy and ultrastructure of the sporophyte of Takakia ceratophylla (Bryophyta). - Amer. J. Bot. 84: 1337-1350. RUSHING, A. E. (1999): The mature sporophyte-gametophyte junction of Lorentzella imbricata. - Bryologist 102: 92-98. RUSHING, A.E. & ANDERSON, W.B. (1996): The sporophytegametophyte junction in the moss Acaulon muticum (Pottiaceae): early stages of development. - Amer. J. Bot. 83: 1274-1281. SCHUSTER, R. M. (1984): Evolution, phylogeny and classification of the Hepaticae. In: SCHUSTER, R. M.: New manual of Bryology 2: 892-1070. Hattori Bot. Lab., Nichinan. SMITH, G.M. (1938): Cryptogamic Botany. 2: Bryophyta and Pteridophyta. 1. ed. McGraw-Hill, New York. STECH, M. & FREY, W. (2001): cpDNA-relationship and classification of the liverworts (Hepaticophytina, Bryophyta). - Nova Hedwigia 72 (in press). STEWART, W.N. & ROTHWELL, G.W. (1993): Palaeobotany and the evolution of plants. 2. ed. Cambridge University Press, Cambridge. STOTLER, R. & CRANDALL-STOTLER, B. (1977): A checklist of the liverworts and hornworts of North America. - Bryologist 80: 405-428. VAUGHN, K.C. & HASEGAWA, I. (1993): Ultrastructural characteristics of the placental region of Folioceros and their taxonomic significance. - Bryologist 96: 112-121. YIP, K.L. & RUSHING, A.E. (1999): An ultrastructural study of the sporophyte-gametophyte junction in Ephemerum cohaerens. - Bryologist 102: 179-195.
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The gametophyte-sporophyte junction: unequivocal hints for two evolutionary lines of archegoniate land plants WOLFGANG FREY*, MARIA HOFMANN
& HARTMUT H. HILGER
Institut fUr Biologie, Systematische Botanik und Pflanzengeographie, Freie Universitat Berlin, AltensteinstraBe 6, D-14195 Berlin, Germany *e-mail correspondingauthor:[email protected] Accepted: May 4, 2001
Summary New results have added to our knowledge about the gametophyte-sporophyte junction in bryophytes, hornworts and pteridophytes and its significance for archegoniate land plant evolution. Bryophyta (liverworts and mosses) show a uniform structure: the sporophyte foot is separated from the surrounding gametophyte tissue by a placental space. At maturity the space is in most cases filled with residues of collapsed cells of gametophyte origin. The taxa with a placental space represent the bryophyte lineage of land plant evolution, indicating a common ancestor of the bryophyte groups Hepaticophytina and Bryophytina. To emphasize the separate position of Fossombronia within liverworts, the new class Fossombroniopsida is proposed. In Anthocerotophyta and Psilotopsida sporophyte haustorial cells intermingle resp. interdigitate with gametophyte transfer cells, and electron-dense fibrillar material fills the interface. In Lycopodiopsida, Equisetopsida and Pteridopsida, a direct contact exists between gametophyte and sporophyte placental layers. In some cases, sporophyte placental cells tend to interdigitate between gametophyte placental cells. No residues of collapsed gametophyte cells are found in the interface between the two generations. Key words: Gametophyte-sporophyte junction, evolution, Anthocerotophyta, Bryophyta, Fossombroniopsida class. nov., Pteridophyta, placental space, transfer cells.
1. Introduction Besides the archegonium the gametophyte-sporophyte junction is one of the most characteristic features common to all archegoniate plants [Bryophyta (mosses and liverworts), Anthocerotophyta (hornworts) and Pteridophyta (ferns)]. Since the last reviews (e.g. LIGRONE et al. 1993; FREY et al. 1994 a) the number of investigated species has increased to a total of 63 mosses, 43 liverworts, 15 hornworts and 9 ferns, including our own results on 38 mosses and liverworts, one hornwort and 2 ferns. The comparison of the structure of the gametophytesporophyte junction in combination with the arrangement of transfer cells in the placenta raises the question of the systematic value of these features and their relevance as important evolutionary characters. They furthermore may give an answer on the divergence of archegoniate plant groups. The shape of the sporophyte foot, the distribution of transfer cells in both parts of the placenta, the degree of development of the wall in0367-2530101/196/06-431
$ 15.00/0
growths, and the nature of the contact between the two generations (intermingling, interdigitating or not) are central in the study here presented.
2. Materials and methods Plant specimens investigated (species printed in bold in Table 1) were collected during field trips in Europe, New Zealand and Patagonia and are deposited in the Herbarium W. Frey or BSB. The specimens of Haplomitrium hookeri sent by D. G. Long, Edinburgh, and microtome slides of Haplomitrium gibbsiae by E. O. Campbell, North Palmerston, are gratefully acknowledged. The study of the junction of Tmesipteris elongata, published by FREY et al. (1994 a, b), was based on material found in New Zealand 1992 and 1994 on tree fern trunks. Fern specimens from the Botanic Garden Marburg, Division of Fern Cultivation, are preserved in ethanol and deposited at the Institut fur Biologie - Systematische Botanik und Pflanzengeographie. Specimens were either fixed in AFE for SEM, in glutaraldehyde/Os0 4 for TEM, or herbaFLORA (2001) 196
431
.j>.
w
c:I\
~
'-' I-'
0 ......
"'" N 0
~
fl 0
N
Subclass Order
+ + + + +
Aneura pinguis (L.) Dum. Riccardia multifida (L.) S. Gray Cryptothallus mirabilis Malmb. Metzgeria conjugata Lindb.
Aneuraceae
Metzgeriaceae
+
Pallavicinia innovans Steph.5 , P. indica Hook., P. lyellii Carruth, Symphyogyna subsimplex Mitt.6 , S. hymenophyllum (Hook.) Mont. & Nees6
Pallaviciniaceae
Hymenophytaceae Hymenophyton leptopodum (Hook. f. & Tayl.) Steph. 6
+
Pellia epiphylla (L.) Corda, P. endiviifolia (Dicks.) Dum.
Pelliaceae
Metzgeriidae Metzgeriales
Jungermanniopsida
+
+
Haplomitrium blumii (Nees) Schust., H. hookeri (Smith) Nees, H. gibbsiae (Steph.) Schnst.
Fossombroniaceae Fossombronia echinata Macv. Haplomitriaceae
Calobryidae
+
Apotreubia hortonae Schust. & Konstantinova2
Fossombroniopsida
Treubiaceae
Treubiopsida
+
+
+
+
Placental space present (+) or not (-)
Blasia pusilla L.
Riccia sorocarpa Bisch.
Haplomitriopsida
Blasiaceae
Ricciaceae
Carrpaceae: Carrpos monocarpos Prosk., Targioniaceae: Targionia hypophylla L., Conocephalaceae: Conocephalum conicum (L.) Underw., Aytoniaceae: Reboulia hemisphaerica (L.) Raddi, Mannia androgyna (L.) Evans, Plagiochasma rupestre (R. & G. Forst.) Steph., Exormothecaceae: Exormotheca pustulosa Mitt., Marchantiaceae: Preissia quadrata (Scop.) Nees, Dumortiera hirsuta (Sw.) Nees
Sphaerocarpos donnellii Aust., S. texanus Aust.
Sphaerocarpaceae
Sphaerocarpidae
Marchantiidae
Monoclea gottschei Lindb.
Monocleaceae
Species (resp. families/species) (bold = own results, others after Ligrone et a1. 1993 1, resp. cited2 - 16
Monocleidae
Family
Blasiopsida
Marchantiopsida
Hepaticophytina (Hepaticae)
Bryophyta
Class
Systematics of species investigated
? + (2)
+ (2-3)
+ (2-3)
+ (2-3)
+ (2-4)
+ (2-3)
+ (2-3)
Gametophyte
+ (1)
+ (1-2)
+ (1)
+ (1-2)
+ (1)
+ (1) + (1-2)
+ (1)
+ (1)
Sporophyte
Distribution of transfer cells (layers)
conical
conical conical conical
conical
conical
conical
bulbous
bulbous
bulbous
conical
spheroidal, conical, bulbous, cylindrical, cup-shaped
spheroidal
conical
Foot shape
clustering of transfer cells
see 3 and 4
Remarks
Table 1. Overview on analysed gametophyte-sporophyte junctions in Bryophyta-Anthocerotophyta-Pteridophyta. Bold-printed species were investigated by the authors . Full references see References.
fl
w w
.j>.
="
\C
'-' I-'
0 ......
""' N 0
~
0
Bryopsida
Andreaeopsida
Sphagnopsida
Bryophytina (Musci)
Dawsoniaceae Polytrichaceae
Polytrichidae Dawsoniales
Polytrichales
Tetraphididae
Tetraphidaceae
Diphysciaceae
Buxbaumiaceae
Andreaeaceae
Andreaeidae Andreaeales
Buxbaumiidae Buxbaurniales
Takakiaceae
Sphagnaceae
Radulaceae
Takakiidae Takakiales
Jungermanniidae Jungermanniales
+ (2-3)
Tetraphis pellucida Hedw.
Diphysciumfoliosum (Hedw.) Mohr
+
+
+ (1)
-?
+ (1)
+ (2)
+ (1)
elongatetapering
conicalbulbous
elongatetapering
elongatetapering
+ (2-3) +
Polytrichumformosum Hedw., P. commune Hedw., P. piliferum Hedw., Pogonatum aloides (Hedw.) P. Beauv., P. neesii (C. Miill.) Dozy, Oligotrichum hercynicum (Hedw.) Lam. & DC., Atrichum undulatum (Hedw.) P. Beauv., Dendroligotrichum dendroides Hedw.
+
elongatetapering
+ (2-3) +
Dawsonia superba Grev.
Buxbaumia piperi Best.
conical
+ (1) +
Andreaea rupestris Hedw.,A. rothii Web. & Mohr, Andreaeobryum macrosporum Steeve & B. Murr.
bulbous
bulbous
conical bulbous spheroidal
elongatetapering
+
Takakia ceratophylla (Mitt.) Grolle l •7
+ (1) + (1-2)
+ (1)
cavernous
partly
+
Sphagnum fimbria tum Wils., S. fallax (Klinggr.) Klinggr., S. subnitens Russ. & Warnst., S. cuspidatum Hoffm.
Radula complanata (L.) Dum.
Herbertaceae: Herberta spp., Lepidoziaceae: + Kurzia trichoclados (K. Miill.) Grolle, Zoopsis linkiensis Horik., Cephaloziaceae: Cephalozia bicuspidata (L.) Dum., C. lunulifolia (Dum.) Dum., Jungermanniaceae: Jungermannia gracillima Sm., Gyrnnomitriaceae: Marsupella funckii (Web. & Mohr) Dum., Scapaniaceae: Scapania gracilis Lindb., Diplophyllum albicans (L.) Dum., Geocalycaceae: Lophocolea heterophylla (Schrad.) Dum., Chiloscyphus pallescens (Erh. ex Hoftin.) Dum., Leptoscyphus cf. horizontalis (Hook.) Herz.
NO INTERMINGLING (as reported in I)
NO INTERMINGLING (as reported in I)
obliterating gametophyte cells
NO INTERMINGLING (as reported in I)
~
'-' I-'
~......
~
~
tl
Bryopsida
Class
Bryidae
Subclass Order
Systematics of species investigated
Table 1. (continued)
Family
+
Orthotrichaceae: Orthotrichum cupulatum Brid., Neckeraceae: Neckera crispa Hedw., N. cephalonica + Jur. & Ungerll, N. intermedia Brid. lI , Leptodon longisetus Mont ll , Leucodontaceae: Leucodon canariensis (Brid.) Schwaegr.lI, Hypopterygiaceae: Hypopterygium arbuscula Brid., Thuidiaceae: Thuidium tamariscinum (Hedw.) B.S.G., Amblystegiaceae: Amblystegium serpens (Hedw.) B.S.G., Drepanocladus uncinatus (Hedw.) Warnst.5, Brachytheciaceae: Brachythecium rivulare B.S.G., B. velutinum (Hedw.) B.S.G.5, Isothecium myosuroides Brid. 1I , Plagiotheciaceae: Isopterygiopsis pulchella (Hedw.) Iwats.
Dicranum majus Sm.
Fissidentaceae: Fissidens dubius P. Beauv., Archidiaceae: Archidium tenerrimum Mitt., Ditrichaceae: + Ceratodon purpureus (Hedw.) Brid., Seligeriaceae: Blindia acuta (Hedw.) B.S.G., Dicranaceae: Dicranum scoparium Hedw., Leucobryum glaucum (Hedw.) Angstr., Cladophascum gymnomitrioides (Dix.) Sim., Pottiaceae: Acaulon muticum (Hedw.) C. Miill.8, Tortula muratis Hedw., Timmiella barbuloides (Brid.) Monk., Phascum cuspidatum Hedw., Grimmiaceae: Grimmia pulvinata (Hedw.) Sm., Schistidium apocarpum (Hedw.) B.S.G., Bartramiaceae: Bartramia stricta Brid., Aulacomniaceae : Aulacomnium palustre (Hedw.) Schwaegr., Gigaspermaceae: Lorentziella imbricata (Mitt.) Broth.9 , Ephemeraceae: Ephemerum cohaerens (Hedw.) HampelO, Funariaceae: Funaria hygrometrica Hedw., Physcomitrium coorgense Broth., P. cyathicarpum Mitt., Bryaceae: Bryum elegans Nees ex Brid., B. capillare Hedw., Pohlia nutans (Hedw.) Lindb., Mniaceae: Mnium hornum Hedw., Plagiomnium undulatum (Hedw.) T. Kop., P. cuspidatum (Hedw.) T. Kop., Hypnodendraceae: Hypnodendron menziesii (Hook.) Par.
Species (resp. families/species) (bold =own results, others after Ligrone et al. 1993 1 , resp. cited2 - 16
+ (1)
+ (1)
Gametophyte
elongatetapering elongatetapering
+ (1) + (1) + (1-2) (3-4)
elongatetapering
NO INTERMINGLING in Dicranum scoparium, Bryum elegans and Pohlia nutans (as reported in 1,5)
Foot shape Remarks
+ (1) + (1-2) + (3-4)
Sporophyte
Distribution of Placental space transfer cells (layers) present (+) or not (-)
V\
w
.j:.
=--
~
-
~
~
fl
Lycopodiaceae
Equisetaceae Adiantaceae
Polypodiaceae
Lycopodiales
Equisetales
Pteridales
Polypodiales
Lycopodiopsida
Equisetopsida
Pteridopsida
Polypodium vulgare L. Polypodium spec.
Pteridium aquilinum (L.) Kuhn l6
+ (1) + (1) +
+ (1) +
+
+ (1)
+
+ (1)
+ (1)
+
+ (1)
Pelkleafalcata (R.Br.) Fee
+
-** -* -* -* -*
Adiantum capillus-veneris L. 15
No intra- + (1) placental space, no intermingling
Lycopodium appressum (Chapm.) Lloyd & UnderwP, L. cernuum L.14
Equisetum spec.
Sporophyte + (1-3) haustorial cells intermingling with gametophyte transfer cells
Tmesipteris elongata P. A. Dangeard l2
Sporophyte + (1-3) haustorial cells intermingling with gametophyte transfer cells
bulbous
bulbous
bulbous
bulbous or ovoidal
** Sporophyte placental cells tend to interdigitate ("intermingling"), no residues of collapsed gametophyte cells
* Without clear-cut demarcation zone. Interface between both generations. No collapsed gametophyte cells.
I LIGRONE et al. (1993),2 FREY & HILGER (2001), 3 STECH & FREY (2001),4 PASS & RENZAGLIA (1995),5 HOFMANN et al. (1996),6 FREY et al. (1996),7 RENZAGLIA et al. (1997),8 RusHING & ANDERSON (1996),9 RUSHING (1999).10 YIP & RUSHING (1999), II ALFAYATE et al. (2000), 12 FREY et al. (1994a), 13 PETERSON & WHITIER (1991), 14 DUCKETT & LIGRONE (1992),15 GUNNING & PATE (1969), 16 KHATOON (1986) .
Psilotaceae
Psilotales
--------
Anthoceros punctatus L., A. formosae Haseg., A. granulosa Haseg., A. caucasicus Steph. in Woronow, Phaeoceros laevis (L.) Prosk., P. carolinianus (Michx.) Prosk., Foliocerosfuciformis Bharadw., F. appendiculatus (Steph.) Hasegawa, Nothothylas orbicularis Sull., N. temperata Haseg., Dendroceros cavernosa Haseg., D. javanicus Nees, D. tubercularis Hatt.,D. validus Steph. (E. o. Campbell, pers. comm.), Megaceros flagellaris Steph.
--- - -
Psilotopsida
Pteridophyta
Anthocerotopsida
Anthocerotophyta
.._-----_. --.---
rium specimens were rehydrated with Aerosol aT (HOFMANN et al. 1996). For SEM-analysis, junctions were razor-sectioned, dehydrated, critical-point-dried (C0 2), sputtered with gold, and analyzed in a Stereoscan S 100 (Cambridge). Specimens for TEM were embedded in Spurr's resin and dissected with a Reichert Ultracut microtome, contrasted with lead citrate and viewed with a Zeiss EM 109 TEM. Results from literature are included in this survey (non bold-printed species in Table 1).
placental space is filled with residues of collapsed cells of gametophyte origin. Intermingling resp. interdigitating of sporophyte and gametophyte placental cells as in hornworts and pteridophytes (see below) does not occur. The main groups can be characterized as follows:
Hepaticophytina (liverworts) Marchantiopsida (Figs. 1-2)
3. Results and discussion Terminology/characters The gametophyte-sporophyte junction of archegoniate plants consists of the sporophyte foot and the surrounding parental gametophyte tissue. The outermost sporophyte and gametophyte cell layers represent the placenta and a space between gametophyte and sporophyte tissue is thus called placental space. The junctions of the different archegoniate groups can be characterized on the basis of a) the existence of a placental space, b) the number and distribution of transfer cell layers in both generations, c) peculiar features concerning the degree of development of wall ingrowths in transfer cells and d) the shape of the sporophyte foot, (Table 1, Figs. 1-19):
Bryophyta (liverworts and mosses) To the present day, a total of 106 species comprising all major groups of liverworts and mosses have been investigated. The young sporophyte (embryo) penetrates with its foot into the enlarged tissue of the archegonial stalk, resp. into the stem or thallus tissue. In some moss taxa an appendage (haustorium) is developed which originates from the hypobasal cell of the dividing zygote. In liverworts it generally becomes indistinct. A clear-cut demarcation zone between the gametophyte and the sporophyte is apparent. In many taxa the separating
The sporophyte foot is embedded in the parental gametophyte tissue and is variously shaped. It ranges from spheroidal to cup-shaped. Prominent wall ingrowths exist on both the gametophyte (2-4 cell layers) and the sporophyte (1-2 cell layers) side of the placenta. The sporophyte placental transfer cells have highly branched and anastomosing wall ingrowths, the gametophyte placental transfer cells extensive or short and coarse wall ingrowths ("Marchantiopsidalean type"). In some taxa (e.g. Dumortiera hirsuta) the placental space is filled with collapsed gametophyte cells. In the Ricciales only the capsule is developed, seta and sporophyte foot are not developed. Transfer cells are lacking. The Marchantiopsida are clearly separated from the Jungermanniopsidalean liverworts on the basis of the number and distribution of transfer cell layers, the wall ingrowth types, and the shape of the sporophyte foot, lacking mostly a collar (see below and Table 1). Placental organisation confIrms the Marchantiopsida as a taxonomically well circumscribed group.
Blasiopsida The structure of the gametophyte-sporophyte junction, the number of sporophyte (one) and gametophyte placental layers (two to three) with Marchantialean type wall ingrowths points to a relationship with Marchantiopsidalean, not with Jungermanniopsidalean liverworts. In addition, the spermatozoid ultrastructure (PASS & RENZAGLIA 1995) and recent molecular results (STECH &
Figs. 1- 6. Longitudinal sections of the gametophyte-sporophyte junction in liverworts. 1- 2. Marchantiopsida-Marchantiidae. 1- 2. Mannia androgyna. 1. SEM micrograph of a razor-sectioned "hanging" sporogon. The bulbous sporophyte foot is embedded in gametophyte tissue, the capsule has ruptured the calyptra. 2. TEM micrograph of the placental region. Sporophyte and gametophyte placental cells with wall ingrowths. Placental space present. 3-4. Jungermanniopsida-Metzgeriidae. 3. Aneura pinguis. SEM micrograph of a razor-sectioned junction. The conical sporophyte foot bears a collar at the transition to the seta. 4. Metzgeria conjugata. TEM micrograph of the placental region. The placental space is filled with collapsed cells of gametophyte origin (arrows). The gametophyte placental cells have no wall ingrowths but nacreous wall thickenings, the sporophyte placental cells show only thickened tangential walls with radial wall deposits. 5-6. Jungermanniopsida-Jungermanniidae. 5-6. Lophocolea heterophylla. 5. Light micrograph of the junction. Spheroidal-shaped foot with a prominent collar at the transition to the seta. 6. TEM micrograph of the placental region. Placental space filled with collapsed gametophyte cells; gametophyte placental cells with tangential wall thickenings, sporophyte placental cells show prominent thin and branched wall ingrowths. c collar, ca capsule, f foot, g gametophyte, I lipid droplet, p plastid, ps placental space, s sporophyte, se seta. Scale bars: 1, 3, 5 =200 !tm; 2,4,6 =5 !tm.
436
FLORA (2001) 196
f
ffii ii'
IliI lill d •
FLORA (2001) 196
437
FREY 2001) indicate an isolated position of this taxon (incl. Cavicularia), best recognized at the rank of a separate class as established by STECH & FREY (2001).
Treubiopsida The character-complex of a bulbous foot with an indistinct collar, correlated with the lack of gametophyte placental transfer cells, but with the occurrence of several layers of collapsed gametophyte placental cells and 1-2 layers of sporophyte placental transfer cells indicates a separate position within Hepaticophytina, but with Jungermanniopsidalean relationships. Compared with all gametophyte-sporophyte junctions investigated in Bryophyta, Anthocerotophyta, and Pteridophyta until now, the distribution pattern of the 1-2 layered transfer cells in the sporophyte placenta is unique (FREY & HILGER 2001). Clusters of sporophyte placenta cells with coarse wall ingrowths (transfer cells) alternate with clusters of sporophyte placental cells lacking wall ingrowths. Such a distribution pattern of transfer cells is not known from any other archegoniate land plant group.
Jungermanniopsida - Metzgeriidae (Figs. 3-4) The sporophyte foot of the thalloid and semifoliose Metzgerialean liverworts is of conical shape, and often a prominent collar delimits the transition to the seta. Collapsed cells of gametophyte origin are always present in the placental space. The sporophyte side of the placental region is variable concerning the presence or absence of a single layer of transfer cells. If wall ingrowths are present as e.g. in the Pallaviciniaceae they are short and coarse. Lack of transfer cells in the gametophyte generation is interpreted as a reduction (LIGRONE et al. 1993). Some of the species are characterized by nacreous wall thickenings in the gametophyte placental cell layer if wall ingrowths are lacking, and, in Metzgeria conjugata and Aneura pinguis, nacreous walls also occur in the sporophyte placental cell layer.
Jungermanniopsida - Jungermanniidae (Figs. 5 -6) Leafy Jungermannialean liverworts show a very uniform gametophyte-sporophyte structure. Usually the conical or bulbous foot bears a collar. Collapsed gametophyte cells mark the placental space. Prominent wall labyrinths with thin, highly branched wall ingrowths are restricted to usually one or rarely two layers of the sporophyte side of the placenta, the gametophyte placental cell walls are smooth. According to LIGRONE et al. (1993) irregular, radially elongated peripheral sporophyte placental cells in Radula seem to interdigitate into adjacent gametophyte placental cells like in bryopsid mosses, e.g., Diphyscium, Polytrichum and Bryum. Neither the knowledge of 438
FLORA (2001) 196
the situation in the three mentioned genera (see Bryophytina, mosses) nor the figures in LIGRONE et al. (1993) give hints that true interdigitation of sporophyte placental cells into gametophyte placental cells exists. A distinct placental space is present.
Haplomitrium and Fossombronia An outstanding placental organisation within liverworts of Jungermanniopsidalean affinity is found in Haplomitrium spp. and Fossombronia spp. Two to three layers of placental transfer cells are present on the gametophyte side, and only one on the sporophyte side of the placenta. These features separate these taxa resp. their gametophyte-sporophyte junction from all other Jungermanniopsidalean liverworts. In Haplomitrium spp. the wall ingrowths within the 2-3 layers of gametophyte placental transfer cells are generally longer and more highly branched than in the 1-2 layers of sporophyte placental transfer cells. Haplomitrium shares a similar placenta rather to the Marchantiopsida (LIGRONE et al. 1993, Figs. 74, 75), than to Jungermanniidae, where the taxon is currently placed. This character, the bulbous foot and also the absence of a collar indicate a separate position from Jungermanniopsida (LIGRONE et al. 1993), and the placement as class Haplomitriopsida R. Stotl. & B. Stotl. (STOTLER & CRANDALL-STOTLER 1977) seems to be justified. On the other hand, molecular systematic studies show that this taxon is related with the Metzgerialean line of Jungermanniopsidalean liverworts (STECH & FREY 2001).
Fossombroniopsida W. Frey & H. H. Hilger, classis nova Differt ab aliis Hepaticophytinis gametangiis masculinis et femininis singulariis ordine acropeto supra axis insertis, capsulis sine valvis, irregulariter dilabentibus, sporophyto pseudoperianthio unistratosa vestito. Type family: Fossombroniaceae Hazsl., Magyar Birodalom Moh-Fl6nija: 20,36. Mai - Dec 1885 (nom. fam. cons.). In Fossombronia echinata the gametophyte placenta transfer cells have strongly branched wall ingrowths that form a highly complex three-dimensional network (LIGRONE & GAMBARDELLA 1988). Similar elaborate wall ingrowths are common in the placentae of Blasia and Marchantiopsidalean liverworts, but are not found in Jungermanniopsida s.str. (LIGRONE et al. 1993). This and the following characters, viz. the bulbous foot and the lacking collar urge to exclude Fossombronia from the Jungermanniopsida. The establishment of the new class "Fossombroniopsida" instead of Fossombroniales Schljakow indicates, as in Haplomitriopsida, the independent position of a plant group that is "immensely old,
isolated and relict" (SCHUSTER 1984: 943). But as in Haplomitriopsida, molecular systematic results indicate Fossombronia as a member of the Metzgeriidae (STECH & FREY 2001).
Bryophytina (mosses) The Sphagnopsida are an extraordinary, highly specialized moss group. They are phylogenetic ally very old and may date back to Late Permian time. It is the only moss taxon in which wall ingrowths (transfer cells) are lacking in the placental cell layers of both generations. A distinct mucilage-containing cavernous tissue which develops from obliterating gametophyte placental cells between the bulbous sporophyte foot and the gametophyte tissue separates the two generations (cf. LIGRONE et al. 1993, p. 241 ,Fig. 8 ; also confirmed by own observations). The following moss groups are characterized by at least one cell layer of transfer cells on the sporophyte side of the placenta. A distinct placental space is present. With few exceptions, the foot is elongate-tapering. In Andreaeopsida (incl. Takakiales), the granite mosses, the outer sporophyte placental cell layer consists of transfer cells. Gametophyte placental cells lack wall ingrowths. In Andreaeaceae the foot is conical.
Bryopsida (Figs. 7 -12) The Polytrichidae represent a subclass of acrocarpous mosses which are plesiomorphic with respect to their conducting system and peristome type. The elongatetapering foot is appendaged (Fig. 7). Wall ingrowths are restricted to the sporophyte side of the placenta (2-3 layers). A placental space is always present, especially in the foot tip region (Figs. 7 -8). In contrast to the observations of BLAIKLEY (1933) and LIGRONE et al. (1993), no intermingling of sporophyte and gametophyte placental cells could be observed in Polytrichum. The reports about the junction in the Buxbaumiidae should be reinvestigated. For Buxbaumia piperi, LIGRONE et al. (1982) stated 2-3 layers of gametophyte placental transfer cells and one layer of sporophyte placental transfer cells. Diphyscium foliosum was mentioned by LIGRONE et al. (1993) with a description of the placental ultrastructure that contradicts our results. They figured wall ingrowths in the sporophyte projecting placental cells and in the outer gametophyte placental cell layer. Our re-investigations on Diphyscium foliosum indicate, apart from the conical-bulbous foot, no intermingling of gametophyte and sporophyte placental cells. Furthermore, there are two layers of cells with coarse wall ingrowths in the sporophyte projecting cells of the placenta, but probably no wall ingrowths on the gametophyte side of the placenta. The placental space is filled with collapsed cells of gametophyte origin.
The Tetraphididae possess the bryalean gametophyte-sporophyte junction with one layer of transfer cells on each placenta side (LIGRONE et al. 1993). Concerning the junction, together with the Tetraphididae the Bryidae form a homogeneous unit. Transfer cells are present on both generations of the placenta, in most cases one layer on each side, in apparently more advanced acrocarpous taxa two layers on the sporophyte side. The elongate-tapering foot is always separated by a placental space from the surrounding gametophyte tissue often containing residues of collapsed cells of gametophyte origin without wall ingrowths. The outermost sporophyte cells in Dicranum, Bryum and Pohlia are horizontally elongate (Figs. 9-10), but no intermingling of the two generations could be observed (cf. also HOFMANN et al. 1996,Fig.l,Bryum elegans) as reported by LIGRONE et al. (1993). Regarding the distribution of transfer cells, most pleurocarpous species (Figs. 11-12) show a uniform structure of the junction: gametophyte transfer cells (1 layer) and sporophyte transfer cells (1 or 2 layers ) are universally present. In Brachythecium spp. and Drepanocladus spp. the appendage consists of living cells. A placental space is apparent throughout this group and the gametophyte origin of collapsed cells filling the interface of the two generations could be proven for Drepanocladus (Fig. 12). Exceptions from this general appearance were recently reported by ALFAYATE et al. (2000), who described 3-4 layers of sporophyte transfer cells for Isothecium myosuroides and 2-3 layers for Leptodon longisetus.
Anthocerotophyta (hornworts) (Figs. 13-14) So far, 15 taxa have been investigated in hornworts. The basal part of the sporophyte forms a typical bulbous or ovoidal foot. The placental region is characterized by elongated haustorium-like sporophyte cells intermingling with gametophyte transfer cells, most distinctly in the genus Folioceros (VAUGHN & HASEGAWA 1993). A typical placental space is absent, both cell types are separated by intercellular spaces forming lacunae, filled with fibrillar material. Gametophyte cells (1- 3 layers) adjacent to the sporophyte haustorial cells are smaller than ordinary parenchyma cells of the thallus. They show prominent wall labyrinths, whereas the haustorium-like sporophyte placental cells have rather thin walls without wall ingrowths.
Pteridophyta (ferns and fern allies) The number of taxa investigated (9 taxa) is relatively low due to the problems of finding gametophyte-sporophyte junctions. There are at least two types of junctions within this group. FLORA (2001) 196
439
±!TI±M
440
FLORA (2001) 196
Psilotopsida (Figs. 15 -16)
4. Conclusions
The two genera Psilatum and Tmesipteris are either regarded as "some of the most primitive plants surviving on earth today" (label in the Botanical Garden Sydney) lacking roots, but with unicellular rhizoids and eusporangiate sporangia or they are believed to be highly evolved members of the Gleicheniaceae of the Pteridopsida (BIERHORST 1971). Beside this controversy, the gametophyte-sporophyte junction with intermingling gametophyte and sporophyte placental cells ("parasitic phase") gives the impression of a plesiomorphic situation in the Anthocerotophyta-Psilotopsida-LycopodiopsidaJEquisetopsidaJPteridopsida clade of land plant evolution. The sporophyte foot is bulbous (Fig. 15). Sporophyte placental cells intermingle resp. interdigitate with gametophyte placental cells (transfer cells). No demarcation zone and also no lacunae occur between the sporophyte and the gametophyte generation. Wall ingrowths (transfer cells) in 1-3 layers are restricted to the gametophyte side of the placenta (Fig .16).
By now there is a good overview of the structure of the gametophyte-sporophyte junction especially in bryophytes, but much less and still un sufficient in pteridophytes (Table 1). Nevertheless, the findings permit well supported statements on systematic and phylogenetic aspects and especially on the divergence of archegoniate land plant groups. The structure of the gametophyte-sporophyte junction, especially
Lycopodiopsida, Equisetopsida, Pteridopsida (Figs. 17-18) The investigated species of Lycopodiopsida, Equisetopsida and Pteridopsida show a bulbous sporophyte foot. The sporophyte and gametophyte placental cells are in direct contact: No clear-cut demarcation zone between the two generations can be proven, no placental space is obvious and no collapsed cells fill the interface of the two generations. In Adiantum capillus-veneris sporophyte and gametophyte placental cells tend to interdigitate (GUNNING & PATE 1969). Wall labyrinths with very thin wall ingrowths, extending a considerable distance into the cell interior, are present in both generations on the outermost cell layers.
- the distribution and number of gametophyte and sporophyte transfer cell layers of the placenta, - the degree of development of wall labyrinth ingrowths (coarse/fine, highly branched), - the nature of the contact (intermingling, interdigitating, or not), resp. the presence of a placental space with collapsed cells of gametophyte origin between the two generations indicate that there are two different lineages of land plant evolution apparent (Fig. 19, Table 1). In the bryophyte lineage of land plant evolution the sporophyte foot is separated by a distinct placental space from the surrounding gametophyte tissue (placental taxa sensu LIGRONE et al. 1993). The placental space is in most taxa filled with residues of collapsed cells of gametophyte origin. This basic structure of the gametophyte-sporophyte junction within Hepaticophytina and Bryophytina points out that the Bryophyta are an evolutionary line of their own (monophyletic group). This common type of junction in bryophytes must have been separated from a junction precursor without intermingling of sporophyte and gametophyte placental cells, i.e. prior to the evolution of pteridophytalean land plants. This assumption is supported by fossil data, where plants with hepaticophytic characters are believed to be the earliest land plants (EDWARDS et al. 1995; KENRICK & CRANE 1997a, b).
Figs. 7 -12. Bryopsida. Longitudinal sections (except Fig. 12) of the sporophyte-gametophyte junction in mosses. 7 - 8. Polytrichum piliferum. 7. SEM micrograph of a razor-sectioned junction. Foot tip with appendage (arrow), the placental space is obvious. 8. TEM micrograph of the placental region. The outermost sporophyte placental cells with prominent wall ingrowths on the outer tangential walls, the second cell layer with smaller wall ingrowths. The gametophyte placental cells lack wall ingrowths. The placental space is filled with cell-remnants of gametophytic origin. There is no intermingling of gametophyte and sporophyte placental cells. 9-10. Pohlia nutans. 9. SEM micrograph of razor-sectioned junction. Foot tip with appendage (arrow). The placental space is obvious. 10. Detail of 9. with the lateral part of the placental region. The outermost cells of the sporophyte foot, with wall ingrowths, are radially elongated, but no intermingling with gametophyte placental cells occurs. 11-12. Drepanocladus uncinatus. 11. SEM micrograph of a razor-sectioned junction. The foot tip consists of living cells, the placental space is obvious. Wall ingrowths are prominent in the outermost sporophyte placental layer. 14. Light micrograph of a cross section of the placental region. The outermost cell layer of the sporophyte placental cells possesses thickened outer tangential walls. Adjacent two cell layers with gametophyte placental cells with thin cell walls, not yet collapsed, the third cell layer with thickened outer tangential walls. g gametophyte, ps placental space, s sporophyte, w wall ingrowths. Scale bars: 7,9 = 100 flm; 11 = 50 flm; 10,12 = 25 flm; 8 = 10 flm. FLORA (2001) 196
441
442
FLORA (2001) 196
Iii
liiiiA
a
II
M_
Gametophyte-sporophyte junction No intermingling Placental space between the sporophyte and gametophyte generation, in most cases filled with collapsed cells of gametophyte origin spo
Intermingling
No intermingling
of sporophyte and gametophyte placental cells
Direct contact or tend to interdigitate
spo
spo foot
Hepaticophytina
Bryophytina
- Bryophyte lineage of land plant evolution -
Anthocerotophyta Psilotopsida
Pteridophyta (Lycopodiopsida, Equisetopsida, Pteridopsida)
- Pteridophyte lineage of land plant evolution -
Fig. 19. Gametophyte-sporophyte junction types within bryophyte resp. pteridophyte lineages of land plant evolution. - collapsed gametophyte cells, ap appendage, co collar, gam gametophyte, ps placental space, spo sporophyte.
Within this bryophytalean type of gametophytesporophyte junction three subtypes are obvious (Table 1). Marchantiopsida exhibit 2-3 layers of gametophyte placental transfer cells and one to two layers of sporophyte placental transfer cells. The foot is of different shape and lacks a collar. The junction of Blasiopsida, Fossombroniopsida, and Haplomitriopsida is related to the Marchantiopsidalean junction type and may represent a separate subtype.
The junctions in Treubiopsida and Jungermanniopsidalean taxa lack in most cases gametophyte placental transfer cells, and have 0-1, rarely 2 layers of sporophyte placental transfer cells. A conical foot with a collar is obvious in most Jungermanniopsidalean taxa. The gametophyte-sporophyte junction in Bryophytina (excl. Sphagnopsida and Buxbaumiidae) is very uniform and constant. In most cases the foot is elongate-
Figs. 13 -18. Longitudinal sections of the sporophyte-gametophyte junction in the homworts (Anthoceros caucasicus). 13. Light micrograph of the junction. The bulbous foot is surrounded by the parental gametophyte. Notice the dark staining outer cell layers of the gametophyte. The inner part of the sporophyte foot consists of horizontally elongated haustorial cells. 14. TEM micrograph of the placental region. Sporophyte placental cells with rather thin cell walls are intermingled with gametophyte placental cells with prominent wall labyrinths. The typical placental space is lacking, the two cellular types are separated by intercellular spaces forming lacunae. 15 -18. Longitudinal sections of the sporophyte-gametophyte junction in Pteridophyta. 15 -16. Psilotopsida. Tmesipteris elongata. 15. SEM micrograph of a razor-sectioned sporophyte part of the junction. The sporophyte foot is surrounded by the vaginula. Conducting strand (arrow). 16. TEM micrograph of the placental region. Sporophyte placental cells (with thin cell walls) intermingle with gametophyte transfer cells, no placental space present. 17 -18. Pteridopsida. Pellaea Jalcata. 17. SEM micrograph of a razor-sectioned junction. The bulbous sporophyte foot is in direct contact with the gametophyte, no placental space is apparent. 18. TEM micrograph of the placental region. The outermost gametophyte placental cells with a prominent wall labyrinth on the outer tangential walls, wall ingrowths on the sporophyte placental cells are very small. Both generations are in direct contact. f foot, g gametophyte, s sporophyte, v vaginula, w wall ingrowths. Scale bars: 15 = 500 ftm; 13, 17 = 100 ftm; 14, 18 = 10 ftm; 16=5ftm. FLORA (200 1) 196
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tapering and lacks a collar. Reinvestigation is necessary in Buxbaumia. For the pteridophyte lineage of land plant evolution we assume a basic structure of the gametophytesporophyte junction with intermingling resp. interdigitating sporophyte placental cells and gametophyte placental cells, present only in extant Anthocerotophyta and Psilotopsida. It is assumed that in both groups ancestral conditions of the gametophyte-sporophyte structure have been retained throughout their separate evolutionary history. This view of the pteridophyte (and spermatophyte) lineage of land plant evolution including the Anthocerotophyta clade corresponds with the "old" palaeobotanical idea of closer relationships between certain fern groups and the Anthocerotophyta (CAMPBELL 1895; BLAIKLEY 1933; SMITH 1938 ; STEWART & ROTHWELL 1993; FREY et al. 1994a, b). The opinion that Anthocerotophyta (hornworts) are not closely related with Bryophyta is now supported also by different molecular data sets, e.g. that one of HEDDERSON et al. (1996), classifying the Anthocerotophyta as a sister group to all other Embryophyta, and more clearly by nucleotid sequence data of the chloroplast gene rbeL, which implicates the Anthocerotophyta as the lineage of bryophytalean plants closest to vascular plants (LEWIS et al. 1997), and by nucleotid sequences of the plastid coded genes and the 18S rRNA gene (NISHIYAMA & KATO 1999,Fig. 2). QIU & LEE (2000: 81) recently stated referring to LEWIS et al. (1997) that "Hornworts, when not occupying the basal position in land plants, are sister to vascular plants.". This position is also in contrast to the traditional view of horn worts occupying an intermediate position between liverworts and mosses. The gametophyte-sporophyte junction is very similar in Lycopodiopsida and Pteridopsida. This does not agree with the recently presented view (KENRICK & CRANE 1997 a) of two independent lineages: microphyllous Lycophytina including extant Lycopodiales, Selaginellales and Isoetales, and the megaphyllous Euphyllophytina including Psilotopsida, Equisetopsida, and Pteridopsida and seed plants. It rather argues for the relationships between Lycopodiopsida and Pteridophyta. Gametophyte-sporophyte junction research should concentrate onto these plant groups in future.
5. Acknowledgement We thank the Departments of Conservation in Tongariro, Gisborne, Nelson and Hokitika in New Zealand for collection permits (Tmesipteris), the Botanical Garden Marburg (Germany) for fern specimens, D.G. Long, Edinburgh for specimens of Haplomitrium hookeri, and Dame Dr E.O. Campbell, Palmerston North, for slides of the junction of Haplomitrium gibbsiae. We are also grateful to Ms C. Macmillan and Ms 444
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C. Gruber for skilful technical assistance (TEM, SEM), Mr H. Ltinser for drawing Fig. 19, Dr M. Stech for help with the Latin diagnosis, and Dr M. Weigend for critically reading the English manuscript.
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