Cortical morphogenesis and conjugation process in Caenomorpha medusula (Ciliophora, Heterotrichida)

European Journal of

Europ.j.Proristol, 23,111-121 (1987)

PROTISTOLOGY

Cortical Morphogenesis and Conjugation Process in Caenomorpha medusula (Ciliophora, Heterotrichida) Ana Martin-Gonzalez, Susana Serrano and Dimas FernandezGaliano Departamento de Microbiologia, Facultad de Biologia, Universidad Complutense. Madrid, Espana

SUMMARY The new oral infraciliature comes from the posterior longitudinal proliferation of numerous somatic kineties of the parental perizonal zone during the bipartition of Caenomorpha medusula. The new perizonal zone of the opisthe originates from proliferation of the anterior extremes of all the somatic kineties that make up the parental perizonal zone. The proter retains both, the oral infraciliature and the perizonal zone of the parental cell. Conjugation in C. medusula presents three maturation divisions and three postzygotic divisions. During conjugation the oral infraciliature of each conjugant degenerates to be replaced by a new one.

Introduction Caenomorphidae,

which

includes

the

genera

Caenomorpha, Cirranter and Ludic, is a single family in the suborder Armophorina (order Heterotrichida) [7,21] . The genus Caenomorpba has several fresh water species [19] with polysaprobic habitats and Caenomorpha medusula is one of the most frequent to be found in zones with a high concentration of H 2S. An ultrastructural work [30] and several morphological studies [16, 19, 40] have been carried out on this species. But as far as we know, no study has yet been made of stomatogenesis and conjugation in members of the suborder Armophorina. In this present paper the morphogenetical events and nuclear phenomena occurring the asexual and sexual phases of the life cyclein C. medusula are described for the first time. Material and Methods Samples with C. medusula were collected from the Aulencia river (Madrid, Spain) and cultures were made using a soil extract medium previously inoculated with Enterobacter aerogenes and maintained at a constant temperature of 20 ± 1 "C. Subcultures were made periodically to obtain a high population growth © 1988 by GustavFischerVerlag, Stuttgart

necessary to studybinary fission. Some cultures were transferred to test tubes with Dryl's solution [9] to induce conjugation. Aliquotsof these cultures were stained every Y2 hour to threee hours once the greater part of the pairs had separated. The staining proceding used was the pyridinated silver carbonate method [13]. The orcein acetochlorhidric stain[20] wasalso used to record thedifferent nuclear stages during the asexual and sexual phases of the life cycle. Results

General Morphology Caenomorpba medusula is a bell-like shaped heterotrich with a posterior spine. The nuclear apparatus is constituted by two or three macronuclear nodes (although up to five nodes were exceptionally observed), and an oval micronucleus placed near the macronuclear nodes (Figs. 1, 2). The somatic infraciliature consists only of the following structures: one perizonal zone, two kineties of the bell, and two very short kinetics situated at the base of the spine. The perizonal zone (PZ) begins in the right ventral edge of the bell, continues along the dorsal edge and then curves inwards on the left side towards the left ventral part were it ends near the spine. This zone is composed of more than 100 oblique kineties, each averaging 20-24 pairs of kinetosomes, except for those on the left ventral zone 0932-4739/88/0023-0111 $0.00 /0

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6 Fig. 1. Caenomorpha medusula. Ventral view of a vegetative cell showing the infraciliature of the two kineties of the bell (BK),of the perizonal zone (PZ) and also the two kineties of the spine (SK) (x 720). - Fig. 2. Specimen of C. medusula with three spine kineties (SK). AZM-adoral zone of membranelles, PM-paroral membrane. Note that there are two macronuclear nodes (MA) and one micronucleus (MI) (x 720). - Fig. 3. Detail of the left ventral zone. In this part of the cell each adoral membranelle has four rows of kinetosomes, one of which is very short having only two or three kinetosomes (~). The paroral membrane (PM) and the two kineties of the spine can also be observed (SK) (x 1400). - Fig. 4. Morphogenetic stage of division in C. medusula. The cytoplasmic prolongation of the spine disappears although the kineties of this structure (SK) do not degenerate. Arrow points the antiapical proliferation of the most of the kineties of the perizonal zone (x 550). - Fig. 5. Detail of the left ventral zone of the same specimen of Fig. 4. Note the fibrillar connections between the submembranellar and subparoral fibres (~) (x 1320). - Fig. 6. Photomicrograph showing the oral primordia in a dividing cell. The submembranellar and subparoral fibres have degenerated and there is a partial desdifferentiation of the parental buccal infraciliature (X 1550).

Cortical Morphogenesis and Conjugation in Caenomorpha medusula . 113

7

10

11

12

Fig. 7. Morphogenesis in C. medusula. Formation of the paroral membrane primordium of the opisthe (x 1550). - Fig. 8. The new paroral membrane is almost organ ized (--+) ; the alineation process in each membranellar primordium and the apical proliferation of the perizonal zone kineties have stated (X 1550). - Figs. 9-12. Last stages of the division process in C. medusula (for explanation see text). BK-kineties of the bell, PZP-perizonal zone of the proter, PZO-perizonal zone of the opisthe, AZMP-adoral zone of membranelles of the proter, AZMO-adoral zone of membranelles of the opisthe (x 620).

114 . A. Martin-Gonzales, S. Serrano and D. Fernandez-Galiano

where the number of kinetosomes per kinety decreases progressively (Figs. 1,2). The two kineties of the bell are different in length; the shortest one is in the middle ventral part and has a longitudinal trajectory whilst the second one extends anteriorly towards the dorsal side (Fig. 1). Each of these kineties has two rows of kinetosomes. The kineties of the spine are made up of two rows of kinetosomes and sometimes three kineties can be observed in this zone (Figs. 1, 2).

The oral infraciliature consists of one adoral zone of membranelles (AZM) and one paroral membrane. The AZM is parallel to the perizonal zone and has 37-45 membranelles, each with three or four rows of kinetosomes. Not all the membranelles are of the same length, the most posterior ones being longer than the others. The paroral membrane is located between the adoral zone of membranelles and the perizonal zone and it is a diplostichomonade which extends from the base of the spine on

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Cortical Morphogenesis and Conjugation in Caenomorpha medusula . 115

Stage 4. - The anterior extremes of these kinetal segments or membranellar primordia break off and then a lengthened and anarchic field of pairs of kinetosomes is formed between the membranellar primordia and the parental perizonal zone (Figs. 13d, 7). Stage 5. - The somatic kineties of the perizonal zone increase in length due to an apical proliferation (Fig. Be). Stage 6. - The kinetosomal field placed between the perizonal zone and the membranellar primordia is arranged giving rise to a row of pairs of kinetosomes which represents the new paroral membrane of the opisthe. A rearrangement of the kinetosomes in the old paroral membrane was also observed (Fig. 13g). Stage 7. - When the new paroral membrane is completely formed a delineation of one or two rows of kinetosomes takes place, so that each membranelle exhibits the same number of rows as those in the vegetative cell (Figs. 13g,8). At the same time, the kineties of the spine and those of the bell, which had lost their fibrillar derivates, increase in length. Stage 8. - When all the infraciliar structures are formed in the proter and in the opisthe, a complex succession of morphogenetic movements takes place in the cell. The perizonal zone breaks into two parts and later an unciliated area appears between them, the anterior part of which constitutes the perizonal zone of the opisthe and the posterior part the perizonal zone of the proter. Both parts are ciliated but their kinetosomes do not bear fibrillar derivates (Figs. 9, 10). Initially, the cell elongates transversally so that its width is greater than its length. As a result, the two kineties of the spine break into two fragments, one of which is in the right posterior half of the body. Later, the posterior peri-

the ventral side to the dorsal part where it ends before reaching it. Located beneath the AZM and the paroral membrane are two fibres: the submembranellar fibre and the subparoral fibre with some fibrillar connections between them (Figs. 2, 3, 5).

Binary Fission (Fig. 13) During the asexual division of the vegetative cell the body adopts an ellipsoidal shape and the cytoplasmic prolongation of the spine dissappears although the infraciliature of this zone remains intact. The cellular movement decreases greatly during morphogenesis even though the ciliate never loses its cilia with the result that these cells sink down to the bottom of the culture container. We have divided morphogenesis into eight stages: Stage 1. - The kinetosomes of the somatic kineties in the perizonal zone lose their fibrillar derivates. Then these somatic kineties undergo a longitudinal and antiapical proliferation following a left to right gradient except for the most right kineties where no sign of proliferation can be observed (Figs. 13a, 4, 5). Stage 2. - The kineties in the perizonal zone continue proliferating and the fibrillar derivates between the AZM and the paroral membrane disappear. Simultaneously a partial desdifferentiation of the parental paroral membrane occurs (Fig. 13b and 6). Stage 3. - The posterior extremes of the kineties of the perizonal zone that have been proliferating gradually break and imigrate to an intermediate zone closer to the oral structures. Each of these kinetal segments is made up of two rows of kinetosomes which decrease in length as they follow a right (Figs. Be, 6).

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116 . A. Martin-Gonzales, S. Serrano and D. Fernandez-Galiano zonal zone and the oral structures of the proter advance to the left dorsal side and the anterior perizonal zone and the new oral structures of the opisthe retreat to the right dorsal side. Then a partial spiralization of the infraciliary structures occurs and the fibrillar systems associated to the somatic and oral infraciliatures are formed (Figs. 13h, 11,

12). . . . The separation of the somanc kineties of the bell can be observed in the apical zone.

Conjugation The union of the conjugants occurs in theventral middle zone of each cell. No infraciliary structure participates in that union, and the infraciliatures of the conjugants adopt an enantiomorphic position (Figs. 15, 16). a) Nuclear Phenomena (Fig. 14). . In the early meiotic prophase I the micronucleus mcreases its size greatly and the chracteristic disposition of the "parachute stage" corresponding to the zygotene can be

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Cortical Morphogenesis and Conjugation in Caenomorpha medusula . 117

observed . In general, this first maturation division occurs simultaneously in both conjugant s, but sometimes there is a slight out of phase in the micronuclear stage of the conjugants (Fig. 17). The second pregamic division originates four haploid nuclei (Fig. 18), three of which degenerate whilst the fourth divides yet again to produce the sta tionary and the migratory pronuclei in each cell. These pronuclei appear slightly poorly stained. When the exchange and fusion of pronuclei tak e place, a single and voluminous synkaryon is originated. This synkaryon undergoe s two consecutive mitoses, giving rise to four nuclei. One of these four derivates degenerates and the other three divide again to produce six nuclei. After the third postzygotic division, the conjugants separate from each other, one of the nuclei

becomes a micronucleus, and the other nuclear derivates follow diverse development. Frequently , two or thr ee derivates become macronuclear anlagen and the other s degenerate; but sometimes, ther e are excon jugants with four or five macronuclear anlagen (Fig. 24). Simultaneously to the micronuclear changes during conjugation, the old macronuclear nodes of each specimen degenerate by pycnosis without fragmentation. b) Changes in the oral and somati c infraciliatures In C. medusula, several modifications in the somatic infraciliature and an oral reorganization occur dur ing conjugation. In the third maturation division the somati c kinetosomes of the perizonal zone lose their associated fibrillar systems, but the kinetosomes bear cilia. As in the biparti-

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24 Figs. 15, 16. Union of the conjugants in C. medusula. Somatic and oral infraciliatures are not involved in this process. Note the different number of macronuclear nodes of each partner of Fig. 15 (x 480). - Fig. 17. First maturation division. Note the different stage of development of the micronucleus in each conjugant (X 480). - Fig. 18. Second maturation division (x 560). - Fig. 19. First stage of the oral reorganization during the conjugation in C. medusula. Arrow points the proliferation of the kineties of the perizonal zone (x 560). - Figs. 20, 21. Photomicrograph of an exconjugant of C. medusula at different planes. The formation of the new oral primordia can be observed (X 560). - Fig. 22. Arrangement of the new paroral membrane in an exconjugant (x 560). Fig. 23. Detail of Fig.22 showing the degeneration of the old oral structures (..... ). The new adoral membranelles have reached their mature configuration (X 1600). - Fig.24. Exconjugantin the last stage of the oral reorganization process. There has not been apical proliferation of the kineties of the perizonal zone (x 560).

118 . A. Martin-Gonzales, S. Serrano and D. Fernandez-Galiano

tion process, the disappearance of the fibrillar systems associated to the kinetosomes produces a decrease in the movement of the conjugants that sink to the bottom. At this moment the fibrillar structures associated to the adoral zone of membranelles and the paroral membrane disappear. We have not observed changes in the kineties of the spine and the bell. The somatic infraciliature does not undergo any other changes during conjugation. After the formation of the synkaryon, a slow and gradual degeneration of the paroral membrane and the adoral zone of membranelles begins in each conjugant. An antiapical proliferation of the kineties in the perizonal zone appears during the three synkaryon divisions and this proliferation gives rise to the membranellar primordia (Fig. 19). When the conjugant pairs separate, the oral reorganization continues according to the same pattern as that in the stomatogenesis of this ciliate, that is, the pararal membrane and the new adoral zone of membranelles are formed and the parental oral structures degenerate (Figs. 20-24).

Discussion Our observations on the general morphology in Caenomorpha medusula differ on several points with previous descriptions by Villeneuve-Brachon [40] and Jankowski [19] although we basically agree with the results obtained by Fernandez-Galiano and Fernandez-Leborans [16]. The somatic infraciliature of this ciliate is very reduced [19] and it is not uniformly distributed as oblique rows on the dorsal side and as horizontal rows on the ventral surface [40]. However, as pointed out by Fernandez-Galiano and Fernandez-Leborans [16], Jankowski's results present two important erroneous interpretations; in the first place, each kinety of the bell is not formed by eight to ten cirri but has in fact two rows of kinetosomes as Rodrigues de Santa Rosa (30] stated in his brief ultrastructural study. In the second place, the perizonal zone is not a polykinety made up of oblique rows with five kinetosomes: on the contrary, this zone is composed of numerous oblique rows, each with 20 to 24 pairs of kinetosomes. With regard to the oral infraciliature, Rodrigues de Santa Rosa [30] indicated that each membranelle has four rows of kinetosomes whereas Fernandez-Galiano and Fernandez-Leborans [16] only observed three rows of kinetosomes per oral membranelle, each one with ten kinetosomes. Both affirmations are certainly true because they are the consequence of a partial observation of the adoral zone of membranelles. In C. medusula this structure consists of 37 to 45 membranelles of different sizes; the most posterior or left ventral ones present four rows of kinetosomes, three of which have nine kinetosomes whilst the fourth is very short with only two or three kinetosomeso The other oral membranelles are smaller and each has only three rows with five kinetosomes. The paroral membrane presents two close rows of kinetosomes. Its ultrastructure (30J corresponds to a diplostichomonade [28].

Finally, we have to emphasize that in our strain a rather high percentage of cells appear with three kineties in the spine and not two as described by all the previously mentioned authors. Furthermore, there are two to four macronuclear nodes although in a few cases we have found five nodes although Jankowski [19] pointed out that C. medusula had three or four macronuclear nodes instead of two nodes as C. lata. As mentioned above, no study on the morphogenetical process has been made up to now in any of the species included in the suborder Armophorina. The only reference in the matter denotes that this stomatogenesis "is very likely of an apokinetal nature" [7]. As can be observed clearly in our photomicrographs, at least in C. medusula, all the new oral structures of the opisthe proceed from the proliferation of the greater part of the somatic kineties of the parental perizonal zone and therefore they derive from the parental somatic infraciliature. In other studied species included in the different suborders of the order Heterotrichida, stomatogenesis is apokinetal or parakinetaI. In the suborder Heterotrichina, the most detailed studies are centred on the genera Blepbarisma [26, 32], Condylostoma [3J, Climacostomum [10] and Stentor [26]. In all of these genera the oral primordium originates from the left lateral proliferation of some postoral somatic kineties which constitute an outstanding stomatogenic territory. This territory coincides with or is placed to the right of the discontinuity zone or area "V". As a result of this proliferation a kinetosomal field is formed which later breaks into two unequal parts: a left one which always gives rise to the new adoral zone of membranelles and a right one which originates the paroral membrane in all the species and other structures of paroral complex, if the cell has them, but not the frontal field of Stentor which has a somatic origin [26J. In the suborder Coliphorina stomatogenesis is parakinetal whereas the members in the suborder Licnophorina present an apokinetal stomatogenesis [7, 21J. In Nyctotherus ovalis (31], included in the suborder Clevelandellina, stomatogenesis begins at the zone of discontinuity originating an orderly suboral kinetosomal field which represents the primordium of the infundibular adoral zone of membranelles. The new peristomial adoral zone of membranelles results from the proliferation of the 8-10 most ventral kineties of the proliferation zone. The paroral membrane is formed by proliferation of the somatic kineties which are placed over the proliferation furrow. If we compare our results in C. medusula with stomatogenic patterns of other members of the order Heterotrichida, many differences are made evident. Firstly, the somatic infraciliature is very scant in C. medusula and the vegetative cell has neither a suboral outstanding territory nor is it originated during stomatogenesis although many somatic kineties of the perizonal zone (included the dorsal ones) take part in this process of stomatogenesis. On the other hand, the formation gradient of the adoral membranelles is postero-anterior and extends from the left ventral zone towards the right ventral zone. In contrast, in other heterotrichous ciliates, for instance in Blepharisma (26, 32J and Stentor [26J, the arrangement gradient of the

Cortical Morphogenesis and Conjugation in Caenomorpha medusula . 119 membranellar primordia is antero-posterior and in Nyctotherus ovalis [31] this gradient is dorso-ventral. Stomatogenesis in Nyctotherus ovalis [31] presents two similarities with C. medusula. (1) The oral primordium derives from a longitudinal proliferation of the somatic kineties exclusively and (2) this oral primordium always appears ordered. As can be verified, at least in C. medusula, stomatogenesis-is not apokinetal as Corliss [7] presupposed; rather, the oral structures of the opisthe derive directly from the somatic kineties of the parental perizonal zone. However, the stomatogenic pattern of this species can not be included in any of the two types of stomatogenesis with somatic origin defined by Corliss [6, 7] and Corliss and Lorn [8]. In this ciliate, stomatogenesis is not telokinetal because the posterior extremes of the parental perizonal zone are involved and not the anterior extremes of the somatic kineties. Furthermore, it is not parakinetal because the stomatogenic kineties are not postoral neither they are all ventral and proliferation is not lateral but longitudinal. In Caenomorpha medusula the union of the conjugants occurs in the anterior and middle ventral zone of each cell. This process has been studied in only a few species of heterotrichs in which the conjugant union takes place at the peristomial areas, which can be unciliated as in Blepharisma [22] and Spirostomum (personal observations) or ciliated as in Stentor [4]. With regard to the nuclear phenomena present during conjugation, the heterotrichous ciliates are not a homogeneous group; there are differences even amongst the species of the same genus as can be expected if one considers their high variety of nuclear number. In C. medusula the number of macronuclei does not seem to bear relationship with the mating type; consequently we have observed pairs of conjugants in which both cells have the same number of nodes and others with an unequal number of macronuclear nodes. In our species, as in Spirostomum ambiguum [35], Blepharisma americanum [1], Stentor polymorphus and S. coeruleus [23] and many other species of ciliates, there are three maturation divisions but only one nuclear derivate of the second pregamic division undergoes the third maturation division. However, in other heterotrichs such as Blepharisma tropicum [36] and Fabrea salina [12] two or three to six nuclei undergo this third division so that the pronuclei of each conjugant can not be genetically identical. The reconstruction of the nuclear apparatus does not occur in any of the subtypes described by Raikov [29] when the synkaryon divides three times. In C. medusula one of the derivates from the second postzygotic division degenerates and the number of macronuclear anlagen varies. The variability of nuclear phenomena in the exconjugant has been previously described in other species of ciliated protozoa, for instance Didinium nasutum [27] and

Fabrea salina [12]. Oral reorganisation during conjugation was reported by Maupas (1889). This reorganisation begins when the conjugants are still united as in Paramecium tetraurelia [25] and Urocentrum turbo [34]. In these species oral reorgani-

sation begins soon after the pairs unite and finishes when the exconjugants are formed, but in C. medusula the process begins during the exchange of pronuclei and concludes in the exconjugants. In Nyctotheroides cordiformis (= Nyctotherus cordiformis) [41] only the adoral zone of membranelles degenerates and disappears during the first maturation division in order to be substituted by another of new formation. It must be pointed out that in C. medusula the fibrillar derivates associated to the kinetosomes disappear during the bipartition and conjugation processes. In both stages of the biological cycle this phenomenon seems to be a necessary and preceding step to the beginning of the longitudinal proliferation of the kinetosomes, and it bears with the stop of the ciliary movement. According to Grain [18] the fibrillar and microtubular derivates provide mechanical anchorage for the kinetosomes, but other authors [24,39] consider that these structures determine the direction of the effective stroke of ciliary beat. The shortening or overlapping of fibrillar derivates as the kinetodesmal fibers during the bipartition in ciliates have been observed in Paramecium [11, 14, 15]. Fernandez-Galiano [14] thinks that the kinetodesmal fibers might be a protein reserve that is mobilised during division whilst Cohen et al. [5] suggest that the likely role of these fibers is skeletal and this role is assumed by the cytospindle during bipartition of the cell.

Some considerations on the systematic position of the order Heterotrichida In the classifications made by Corliss [7] and by Levine et al. [21], the order Heterotrichida is included within the orders Odontostomatida, Oligotrichida and Hypotrichida in the subclass Spirotrichia in the class Polyhymenophorea. The order Heterotrichida comprises the suborders Heterotrichina, Clevelandellina, Armophorina, Coliphorina, Plagiotomina and Licnophorina. Small and Lynn [37], accepting in part the hypothesis by Seravin and Gerassimova [33], divide the phylum Ciliophora into three subphyla: Postciliodesmatophora, Rhabdophora and Cyrtophora. The first includes the classes Karyorelictea and Spirotrichea, and the class Spirotrichea comprises the orders Heterotrichida, Armophorida, Plagiotomida, Licnophorida, Clevelandellida and Odontostomatida. The suborder Coliphorina is included in the order Heterotrichida. In 1985, Small and Lynn divided the class Spirotrichea into three different subclasses: Heterotrichia, Choreotrichia and Stichotrichia and included in the first one the orders aforementioned, the order Odontostornatida and the new order Phacodiniida. Taking into account our results on the general morphology, morphogenesis of bipartition and conjugation in C. medusula as well as the ultrastructural data of this species [30] we are inclined to agree with Small and Lynn [37, 38] that the suborder Armorphorina (including C. medusula) should be considered as an order. According to Fernandez-Leborans [17] the members of suborder Armophorina come from the suborder Heterotrichina. This evolutionary transition is caused by several

120 . A. Martin-Gonzales, S. Serrano and D. Fernandez-Galiano

types of processes (Blepharisma ~ Bothrostoma ~ Metopus ~ Brachonella ~ Cirranter ~ Caenomorpha). In the first place there would have to be a process of perizonation [19]; in other words, several specialized somatic kineties or kinetal fragments would contribute to the oral infraciliature in the process of nutrition. In the second place, there is a torsion on the oral structures which would lead to a progressive horizontal location of the peristome, and finally a reduction of the posterior somatic infraciliature would take place. Unfortunately, there are no ultrastructural and stomatogenic data on the intermediate genera between Blepharisma and Caenomorpha supporting this hypothesis, and therefore it is not possible to establish comparative relations at these two essential levels. Consequently, the studies by Bohatier [3] and Pelvat and de Haller [26] on other heterotrichous ciliates which conclude that the phylogenetical line Blepharisma ~ Climacostomum ~ Stentor [19] is doubtful if one bears in mind the results obtained on morphogenesis, and it is highly probable that these ciliates come from different evolutionary lines. Acknowledgements This work was supported by the "Comision Asesora de Investigaci6n Cientffica y Tecnica" (Proyecto 0754/81).

References 1 Bhandary A. V. (1960): Conjugation in Blepharisma undulans americanum. ]. Protozool., 7, 250-255. 2 Bohatier]. (1978): Contribution a l'etude de la morphogenese de regeneration chez les protozoaires cilies des genres Dileptus et Condylostoma. These de l'Universite de Clermont-Ferrand. 3 Bohatier]., Tuffrau M. and Tuffrau H. (1976): Morphogenese de regeneration dans Ie genre Condylostoma (cilies heterotriches). Protistologica, 12, 295-306. 4 Burchill B. R. (1967): Conjugation in Stentor coeruleus. J. Protozool., 14, 683-687. 5 Cohen]., Adoutte A., Grandchamp S., Houdebine L. M. and Beisson]. (1982): Immunocytochemical study of microtubular structures throughout the cell cycle of Paramecium. Biol. Cell., 44, 35-44. 6 Corliss J. O. (1973): Evolutionary trends in patterns of stomatogenesis in the ciliated protozoa. ]. Protozool., 20 (suppl.), 506. 7 Corliss]. O. (1979): The ciliated protozoa: characterization, classification and guide to the literature. 2. ed., p. 455. Pergamon Press. 8 Corliss J. O. and Lorn ]. (1985): An annoted glossary of protozoological terms. In: Hutner S. H., Lee j. j. and Bovee E. C. (eds.): An illustrated guide to the protozoa, pp. 393-575. Allen Press, Kansas, USA. 9 Dryl S. (1959): Antigenic transformation in Paramecium aurelia after homologous antiserum treatment during autogamy and conjugation.]. Protozool., 6 (suppl.), 25. 10 Dubochet C. F., Peck R. K. and Haller G. de. (1979): Morphogenesis in the heterotrich ciliate Climacostomum virens. I. Oral development during cell division. J. Protozool., 26, 218-226. 11 Ehret C. F. and MacArdle E. W. (1974): The structure of Paramecium as viewed from its constituent levelsof organiza-

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Key words: Caenomorpha medusula - Ciliophora - Morphogenesis - Conjugation - Oral reorganisation Ana Martin-Gonzalez, Depto. de Microbiologia, Facultad de Biologfa, Universidad Complutense, 28040 Madrid, Espana