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Correlation of parasite speciation and specificity with host evolutionary relationships
International Journal for Parasitology 17 "0887# 0462Ð0471
Correlation of parasite speciation and speci_city with host evolutionary relationships R[C[ Tinsley \ J[A[ Jackson School of Biological Sciences\ University of Bristol\ Bristol BS7 0UG\ U[K[ Received 09 November 0886^ received in revised form 1 February 0887^ accepted 09 March 0887
Abstract Protopolystoma "Monogenea\ Polystomatidae# is strictly speci_c to the anuran amphibian genus Xenopus[ The host group is characterised by a polyploid series in which chromosome numbers re~ect diploid\ tetraploid\ octoploid and dodecaploid constitutions^ the series is considered to have evolved through interspecies hybridisation and genome duplication[ This study correlates information on host evolutionary relationships with patterns of parasite speciation and host speci_city[ Protopolystoma is restricted to one subgenus "Xenopus# with multiples of 25 chromosomes\ and is absent from the subgenus Silurana "with multiples of 19 chromosomes#[ Molecular\ biochemical and karyotype evidence distinguishes three subgroups within Xenopus[ Representative species from each subgroup\ Xenopus muelleri\ Xenopus fraseri and Xenopus laevis\ have been examined for polystomatid infection[ Two species of Protopolystoma occur in each of these host species[ In X[ muelleri\ the two Protopolystoma species re~ect parasite co!speciation corresponding with the divergence of two sibling host species[ Xenopus fraseri and X[ laevis "both with 1n 25 chromosomes# are implicated in the hybrid origin of two octoploid species\ Xenopus wittei and Xenopus vestitus "both 1n 61#[ The relationships of the Protopolystoma species in these Xenopus taxa re~ect this presumed ancestry[ Xenopus wittei carries two species of Protopolystoma\ one shared with X[ fraseri and the other shared with X[ laevis[ Xenopus vestitus carries a single species of Protopolystoma which is shared with X[ laevis but there is no {{heirloom|| which re~ects its hybrid origin involving X[ fraseri[ In addition to these shared parasite species which may re~ect shared host genes\ X[ fraseri and X[ laevis each carry separate species!speci_c Protopolystoma which do not occur in other Xenopus species even where there is evidence of common genetic information "as in the allopolyploid wittei and vestitus#[ This case study may be interpreted as indicating a powerful in~uence of host genetic factors on susceptibility to infection\ host!speci_city\ and parasite speciation[ Þ 0887 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved Keywords] Co!speciation^ Monogenea^ Polystomatidae^ Protopolystoma^ Anura^ Pipidae^ Xenopus^ Host polyploidy^ Host!speci_city
0[ Introduction HostÐparasite co!evolution has occupied a cen! tral place in parasitology throughout the devel! opment of the discipline and has major importance
Corresponding author[
for interpretation of fundamental aspects including host!speci_city\ parasitic adaptation\ phylogeny\ distribution\ host immune defence and parasite counter!defence\ and the concept of an evol! utionary arms race[ Previous phylogenetic analyses of hostÐparasite assemblages have documented evidence for co!speciation "parallel cladogenesis# of some host and parasite associations "reviewed by Clayton and Moore ð0Ł\ including co!speciation of
S9919!6408:87:,08[99¦9[99 Þ 0887 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved PII] S9919!6408"87#99974!X
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cestodes and lice in birds ð1\ 2Ł#[ However\ there are few systems for which there are equally!resolved phylogenies for parasites and hosts[ The Mono! genea are traditionally regarded as highly host! speci_c and recent analyses of the evolutionary interactions of hosts and parasites have been inter! preted in relation to co!speciation and colonisation ð3Ð6Ł[ This paper presents a case study of species level comparisons involving an anuran amphibian\ for which there is now detailed phylogenetic and other biological information ð7Ł\ and a poly! stomatid monogenean\ for which species diversity has recently been documented ð8Ł[
0[0[ The host group Species of the genus Xenopus "Anura] Pipidae#* the African clawed toads*have been the subject of intensive laboratory studies for over 69 years\ initially for research in animal physiology\ bio! chemistry and developmental biology\ and more recently genetics\ immunology and molecular biology[ Incidentally\ these studies have generated comprehensive information enabling interpretation of systematic relationships and evolution ð7Ł[ Pipid anurans have a rich palaeontological record\ including species from the Cretaceous which can be assigned to present day groups within the genus Xenopus ð09Ł[ Fossil representatives occur in both Africa and South America\ and their dis! tribution indicates a wide geographical range before the Atlantic rift[ There are four extant genera of pipids] one of these*Pipa*occurs in South Amer! ica\ and the other three occur in Africa*Xenopus\ Hymenochirus and Pseudhymenochirus "of which the latter two are actually more closely related to Pipa than they are to Xenopus#[ Up to the early 0869s\ only six species of Xenopus were known and almost all research interest focused on just one of these\ Xenopus laevis[ Knowledge of the species diversity of Xenopus has increased greatly in recent years\ particularly as a result of _eldwork in rela! tively remote highland regions of Africa ð00Ł[ At present\ there are 06 formally described species and at least a further three are recognised but not yet named ð01\ 02Ł[ These species have a characteristic which is highly unusual amongst vertebrates] their
chromosome numbers form a polyploid series which represent diploid\ tetraploid\ octoploid and dodecaploid genetic constitutions ð03Ł "Table 0#[ The chromosome complements re~ect a major evol! utionary divergence within the genus] one lineage comprises species with 1n 19 and 39 chro! mosomes "diploid and tetraploid#\ the other has species with 1n 25\ 61 and 097 chromosomes in which the 1n 25 level is interpreted as tetraploid with respect to the original diploid composition of 07 "not found in any known species#[ These two lineages of Xenopus were regarded by Kobel et al[ ð02Ł as subgenera] Silurana with multiples of 19 chromosomes\ and Xenopus with multiples of 07 chromosomes[ A series of biochemical and molec! ular studies suggest that their divergence is ancient] Knochel et al[ ð04Ł\ using a calibrated timeÐdiver! gence curve of globin evolution\ estimated sep! aration to have occurred about 019 million years ago "see also ð05Ł#[ The key factor involved in the evolution of the higher ploidy levels is considered to be interspecies hybridisation accompanied by duplication of the genome ð03Ł[ The allopolyploid species which result from this event therefore com! bine the genetic adaptations of both parental spec! ies[ In some cases\ there is biochemical and genetic evidence for the identity of the ancestors of poly! ploid species[ For instance\ Xenopus fraseri!like and X[ laevis!like species "both 1n 25# are considered to have been involved in the origins of the two octoploid "1n 61# species Xenopus vestitus and Xenopus wittei ð7\ 06Ł "Fig[ 2#[ Electrophoretic com! parison of globin polypeptides suggests that Xen! opus ruwenzoriensis "1n 097# may be a product of hybridisation between ancestral species of the X[ fraseri "1n 25# and Xenopus amieti "1n 61# lineages ð05\ 07Ł[ Other relationships within the genus have been de_ned by a variety of techniques\ including cyto! genetic characters\ electrophoretic comparisons of certain enzymes\ albumins and other serum proteins\ globins\ and basic sperm nuclear proteins\ and molecular analyses using mitochondrial DNA and ribosomal DNA "reviewed by Graf ð05Ł#[ There is good agreement that the 25 chromosome species may be subdivided into three distinct assemblages] the fraseri group "including pygmaeus and fraseri#\ the laevis group "gilli\ largeni and the laevis ras!
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Table 0 Species of Xenopus] chromosome numbers 1n 19
39
25
61
097
tropicalis
epitropicalis
borealis clivii fraseri gilli laevis largeni muelleri pygmaeus
amieti andrei boumbaensis vestitus wittei
longipes ruwenzoriensis
senkreis#\ and the muelleri group "borealis\ clivii and muelleri#[ This well!studied vertebrate makes a good model for integrated studies of host and parasite evol! ution[ In particular\ the relatively exceptional occurrence of allopolyploidisation could have a profound e}ect on host!speci_city and the present patterns of parasite distribution within the Xenopus lineages[ There is comprehensive evidence that Xen! opus species have an immune response which is equivalent in all major characteristics to that of mammals\ but there is only sketchy information on the operation of host immunity against parasite infection ð08\ 19Ł[ Theoretically\ allopolyploid! isation could bring together genetic characteristics controlling susceptibility to infection and hence in~uence the capacity of parasite lineages to infect hybrid polyploids with a recombined genome[ This study aims to establish correlations between host and parasite speciation patterns which may provide evidence of the in~uence of host genetic factors on parasite speci_city and evolution[
0[1[ The parasite fauna of Xenopus The genus Xenopus harbours an exceptionally rich parasite fauna] for the best studied species\ X[ laevis\ there are records of 18 genera of metazoan and protozoan parasites ð19Ł[ Amongst the met! azoans "monogeneans\ digeneans\ cestodes\ nema! todes\ an acarine mite and a leech# for which there is detailed information\ the great majority of rep! resentatives are distinguished by specialisations
which are often unique within their respective para! site groups\ and almost all are placed in their own genera\ families or higher taxonomic categories\ strictly speci_c to Xenopus[ Recently!published studies reveal a very high degree of correspondence in the patterns of speciation and host!speci_city of the parasites with the known evolutionary relation! ships in the host genus[ Some recent publications ð10Ð16Ł have documented a series of case studies in which both hostÐparasite co!speciation and col! onisation are indicated by analysis of relationships[ Almost all of these studies concur in recognising the major evolutionary divergence between the Xen! opus clades based on multiples of 19 or 25 chro! mosomes[ Thus\ in the nematode Camallanus\ the cestode Cephalochlamys\ and the digeneans Oli! golecithus and Progonimodiscus\ distinct parasite species are represented in the two host lineages[ In other cases\ parasite genera well!represented in the 25 chromosome line are absent from 19:39 chro! mosome species "the subgenus Silurana# "for instance\ the digenean Dollfuschella and the mono! genean Protopolystoma\ see below#\ indicating either secondary loss\ or colonisation of Xenopus subsequent to the divergence of the 19 and 25 chro! mosome clades[ This pattern of host!speci_city is incomplete in only one genus so far studied] the nematode Batrachocamallanus has three species which are strictly speci_c to the 25 chromosome Xenopus clade and which are never found in 19:39 chromosome species^ however\ there is a fourth\ highly distinctive\ species which infects Xenopus tropicalis and Xenopus epitropicalis but which may co!occur in X[ fraseri!like hosts in lowland tropical
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forest where the two host groups overlap geo! graphically and ecologically[ Alongside this evidence that speciation patterns in the parasite fauna generally mirror the major division of the host group\ there is also _ner level detail that subgroups within the 25 chromosome lineage of Xenopus carry corresponding host!spec! i_c parasite taxa[ Tinsley ð16Ł summarised evidence that the three 25 chromosome Xenopus species groups fraseri\ laevis and muelleri are each {{recog! nised|| by a series of diverse parasite groups "includ! ing not only helminths but also the acarine mite Xenopacarus#[ 0[2[ Monogenean parasites of Xenopus Two monogenean genera infect the genus Xen! opus] Protopolystoma Bychowsky\ 0846 and Gyr! dicotylus Vercammen!Grandjean\ 0859[ Both share the features typical of the Xenopus parasite fauna in general] they are highly distinctive from a phylo! genetic viewpoint with morphological and life his! tory specialisations which suggest a long separation from nearest relatives[ These two monogeneans also illustrate the dual origin of the parasite assemblage associated with Xenopus[ Protopolystoma is related to genera\ including Polystoma\ which are typical parasites of the same infection site "the urinary bladder# of other anuran amphibians[ This suggests that it represents an {{heirloom|| from a common ancestry of monogeneans which have evolved with the anuran lineages[ Other helminths which con! stitute this group of {{anuran!typical|| parasites include the digeneans Dollfuschella\ Oligolecithus and Progonimodiscus whose relationships suggest that they have been inherited along parallel co! evolutionary lines during anuran evolution[ Despite this clear a.nity\ all show major divergence from their nearest relatives in other anurans\ suggesting a prolonged isolation with their pipid hosts[ By contrast\ Gyrdicotylus "found in the buccal cavity of Xenopus# exhibits the highly distinctive features of the Gyrodactylidae which are pre! dominantly ectoparasites\ on the skin\ _ns and gills\ of a wide range of _shes[ Gyrdicotylus is one of a number of Xenopus parasites representing transfers from _shes with which the fully!aquatic anuran shares habitats\ diet and other ecological niche par!
ameters[ This component of the Xenopus parasite fauna also includes the strigeatoid digeneans Diplo! stomulum\ Neascus and Tetracotyle\ the protozoan Trichodina\ the cestode Cephalochlamys\ and the camallanid nematodes Camallanus and Batra! chocamallanus\ most of which have diverged very considerably from their nearest relatives which are typical _sh parasites ð19Ł[ A preliminary analysis of the patterns of spe! ciation in Protopolystoma and Gyrdicotylus has been summarised by Tinsley ð16Ł[ For the latter genus\ formal descriptions of the species are cur! rently in progress[ For Protopolystoma\ taxonomic accounts of six species "_ve of them previously undescribed# are now in press ð8Ł[ This review con! siders the speciation of Protopolystoma against the background of evolutionary relationships recently established for the host genus "see above#[ 1[ Materials and methods Material has been obtained from throughout much of the geographical distribution of the genus Xenopus\ derived from _eldwork and from dis! section of preserved samples of host species in museum collections "see ð8\ 11Ð15Ł#[ Di}erentiation of the Protopolystoma species has been based on conventional morphological characters\ com! parable with those employed for other poly! stomatids "including the form and degree of branching of the gut\ the dimensions and shape of the haptoral hamuli and the male copulatory organ spines#[ The inter!speci_c di}erences are detailed by Tinsley and Jackson ð8Ł and are beyond the scope of this review[ As the designations are not yet for! mally published\ the taxa are referred to below as species D "which is the single previously!recognised species\ Protopolystoma xenopodis "Price# Bychow! sky#\ and species A\ B\ C\ E and F which are newly! recognised species[ 2[ Observations 2[0[ Correlation of speciation in Protopolystoma with host phylogeny 2[0[0[ Subgenus Silurana[ No polystomatids have been found in representatives of the subgenus
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Silurana\ X[ tropicalis "1n 19 chromosomes#\ X[ epitropicalis "1n 39#\ and other as yet undescribed forms with 39 chromosomes\ either in present studies ð8Ł or previously!published surveys "Fig[ 0#[ Experimental cross!infections of X[ tropicalis using oncomiracidia hatched from eggs of Proto! polystoma species from 1n 25 Xenopus "including X[ laevis# result in successful invasion\ but these never survive to reach sexual maturity in the urinary bladder] death invariably occurs during juvenile development ð8Ł "for life cycle details see ð17Ł#[ 2[0[1[ Subgenus Xenopus[ Extensive sampling\ including _eld collections of many thousands of Xenopus specimens\ has demonstrated that the genus Protopolystoma is restricted exclusively to the Xenopus clade which has multiples of 25 chro! mosomes "the subgenus Xenopus#[ Representatives of Protopolystoma have been found in all Xenopus species so far examined ð8Ł[ Amongst the Pro! topolystoma taxa di}erentiated\ there are distinct species corresponding with the three host groups distinguished by biochemical\ molecular and cyto! genetic characteristics] the muelleri group\ the fra! seri group\ and the laevis group[ This review considers the inter!relationships of the Pro! topolystoma species in one representative of each of these host groups\ in X[ muelleri\ X[ fraseri and X[ laevis respectively "Fig[ 0#\ and also in the allo! polyploid "1n 61# species X[ vestitus and X[ wittei which are presumed to have originated through hybridisation between laevis!like and fraseri!like ancestors[ 2[0[2[ Xenopus muelleri[ Two species of Pro! topolystoma are represented[ Field samples indicate
Fig[ 0[ Occurrence of Protopolystoma species amongst the main lineages of Xenopus[ Host chromosome number indicated above species or species group[ Note] the {{muelleri|| group is rep! resented in this study by two allopatric species previously included in X[ muelleri "see text#[
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that the muelleri Protopolystoma do not infect other Xenopus species even where these occur sym! patrically] X[ muelleri has frequent geographical overlap with laevis at the borders of their respective ranges\ and muelleri is also sympatric with X[ trop! icalis and X[ fraseri!group species at the boundary of tropical forest and savanna ð00Ł[ Laboratory experiments to test the outcome of reciprocal cross! infection\ using Protopolystoma from X[ laevis laevis and X[ muelleri "species D and E respectively# from a sympatric site in South Africa\ show that these parasites fail to develop patent infections in the {{foreign|| host species and are strictly host spec! ies!speci_c "studies in preparation#[ Xenopus muelleri has been recognised as a distinct species since 0733 ð18Ł[ However\ evidence has accumulated recently to indicate that it actually comprises two allopatric cryptic species\ one inhabiting the hot dry savannas of sub!Saharan Africa\ including the Sahel regions of western and central Africa\ the other distributed through the lowland\ largely coastal environments of east and south east Africa ð02Ł "see Fig[ 1#[ These geo! graphically!isolated forms are distinguished by a number of biological characteristics and their des! ignation as separate species is now in preparation[ Additionally\ they are distinguished unequivocally by morphologically!distinct representatives of Pro! topolystoma "Fig[ 2\ species E\ F#[ In this instance\ the divergence of a {{muelleri||!speci_c Pro! topolystoma into two species parallels the speciation of its host[ 2[0[3[ Xenopus fraseri[ Xenopus fraseri a}[ "includ! ing cryptic species morphologically!indistinguish! able from fraseri sensu stricto# harbours one Pro! topolystoma species which is strictly host!speci_c to the fraseri group "Fig[ 2\ species A#^ this is dis! tributed in the rainforests of the Congo Basin\ and occurs in no other Xenopus groups[ However\ fra! seri carries a second species of Protopolystoma "rec! orded in Cameroon and the Democratic Republic of Congo# which also occurs in X[ wittei in southern Rwanda "Fig[ 2\ species B#[ 2[0[4[ Xenopus laevis[ A comparable pattern occurs in X[ laevis[ This host harbours one species! speci_c Protopolystoma "species D# which infects
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Fig[ 1[ Geographical distributions of Xenopus species[ A[ X[ tropicalis group[ B[ X[ muelleri "western form#[ C[ X[ muelleri "eastern form#[ D[ X[ fraseri!like species[ E[ X[ laevis[ F[ X[ vestitus and X[ wittei "arrowed#[
Fig[ 2[ Speci_city of Protopolystoma species correlated with host evolution[ Host chromosome number indicated above species^ solid lines indicate observed hostÐparasite associations^ pre! sumed host genealogical relationships indicated by dotted lines[ The tetraploid toads X[ laevis and X[ fraseri are infected by 1 Protopolystoma species each] in both cases 0 parasite is narrowly speci_c\ while another also occurs in X[ wittei and:or X[ vestitus "the hybrid octoploid derivatives of X[ laevis and X[ fraseri! like lineages#[ The divergence of Protopolystoma spp[ E and F parallels the speciation of the eastern and western forms of X[ muelleri[
mous geographical range from the Cape\ South Africa\ to northern Cameroon "Fig[ 1#[ However\ it has not been recorded in any other Xenopus species examined\ even in well!de_ned areas of sympatry where the laevis subspecies overlap geographically with X[ muelleri\ X[ borealis\ X[ tropicalis\ X[ epi! tropicalis\ and the X[ fraseri!like species[ Xenopus laevis also harbours a second species of Pro! topolystoma "which is carried by X[ l[ victorianus and X[ l[ poweri in central Africa#^ however\ this species is shared with X[ vestitus and X[ wittei in the Rift Valley highlands bordering Rwanda\ Burundi\ Uganda and the Democratic Republic of Congo "Zaire# "Fig[ 2\ species C#[ 2[0[5[ Xenopus wittei and Xenopus vestitus[ The records above show that X[ wittei carries two species of Protopolystoma "B and C#\ 0 shared with X[ fraseri and 0 shared with X[ laevis and X[ vestitus^ X[ vestitus carries a single species "C# "shared with X[ laevis and X[ wittei#[ 3[ Interpretation
all the laevis subspecies so far examined\ Xenopus laevis laevis\ Xenopus l[ victorianus\ Xenopus l[ pow! eri and Xenopus l[ sudanensis\ throughout an enor!
The restriction of the genus Protopolystoma to the subgenus Xenopus and its absence from Silurana
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concurs with other evidence "above# of a major discontinuity in parasite distribution re~ecting the dichotomy between the 19 and 25 chromosome Xenopus clades[ As noted above\ the timing of this divergence has been estimated to be about 019 myr ago[ Paterson and Gray ð2Ł recognised three types of sorting event which may lead to absence of parasites from host lineages] extinction\ stochastic events at the time of host speciation "{{missed the boat||# and sampling error[ The latter can be excluded by the extensive population samples of Silurana species which have been examined[ However\ wider studies of pipid lineages in South America and Africa are necessary to establish the history of polystomatid parasitism in this host group[ The patterns of host!speci_city within species of the subgenus Xenopus are shown diagrammatically in Fig[ 2 "for host geographical distributions see Fig[ 1#[ Four species of Protopolystoma "A\ D\ E\ F# are strictly speci_c to single species of Xenopus] one in each of the two cryptic X[ muelleri species\ one in X[ fraseri and one in X[ laevis[ Two species infect more than one host species] one "B# occurs in two host species "fraseri and wittei#\ and one "C# occurs in three host species "laevis\ wittei and vesti! tus#[ With reference to the representatives of the three 25 chromosome Xenopus groups\ muelleri\ fra! seri and laevis\ Protopolystoma is represented by two distinct species in each of these groups[ The pattern of parasite divergence shown in {{X[ muelleri|| may be interpreted in terms of pairwise co!speciation[ The di}erentiation of the host into allopatric sibling species "distinguished by bio! chemical and other characters# is paralleled by di}erentiation of its two Protopolystoma rep! resentatives[ It is likely that the timing of parasite divergence may be correlated with the succession of climatic oscillations during the Quaternary\ which led to repeated expansion and contraction of the forest and savanna zones in Africa and hence to disjunction of the hosts| geographical ranges "review in ð00Ł#[ The pattern of hostÐparasite evolutionary relationships is more complicated in the other two 1n 25 chromosome host groups[ Xenopus wittei and X[ vestitus are allopolyploid species with 1n 61 chromosomes\ considered to have arisen by hybridisation of two 1n 25 species[
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A variety of evidence points in both cases to an involvement of X[ laevis!like and X[ fraseri!like ancestors in the origin of these octoploid descend! ants ð7\ 06Ł[ The links between wittei and the fraseri group are particularly robust "based on molecular\ biochemical and cytogenetic characters#\ whilst wit! tei is also linked to laevis by characters which include karyotype[ X[ vestitus has been interpreted\ in di}erent studies\ as showing equivalent links with both fraseri and laevis[ However\ precise relation! ships are still obscure] peptide mapping of serum albumins indicates that X[ vestitus has a relatively isolated position amongst the group of species "including X[ wittei# which are associated with X[ fraseri "reviewed by ð05Ł#[ Experimental hybrid! isation between X[ wittei and X[ vestitus suggests that these two 61 chromosome species share a com! mon chromosome set while the other half of the chromosome complement may have originated from di}erent parental species "see ð02Ł#[ The origin and relationships of the Pro! topolystoma species infecting the two octoploid Xenopus species can be interpreted in relation to the genetic information shared by these hosts with their presumed ancestors "Figs 3 and 4#[ In the case of X[ wittei\ the host genome may be considered to combine genetic information derived on the one hand from a fraseri!like ancestor and on the other hand from a laevis!like ancestor[ One species of Protopolystoma "B# is shared with X[ fraseri re~ect! ing one half of the host|s genetic background\ the other species of Protopolystoma "C# is shared with X[ laevis re~ecting the other half of the host|s ances! try "Fig[ 3#[ Where Protopolystoma species B and C occur in the present!day representatives of their ancestral host groups\ in X[ fraseri and X[ laevis respectively\ they are ecologically and geographically distinct from one another[ Xenopus fraseri is adapted to lowland tropical rainforest while X[ laevis occurs in cooler\ upland regions which are principally sav! anna and wooded savanna environments[ The two Xenopus species have been recorded in the same area only very occasionally\ for instance where there is gallery forest in grass savanna or where there are signi_cant clearings in forest\ but under these circumstances the two species are ecologically distinct with X[ fraseri principally inhabiting
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Fig[ 3[ Protopolystoma from hybrid octoploid Xenopus species] correlation between host genetic constitution and parasite speci! _city[ Presumed host genealogical relationships indicated by dotted lines^ shading in boxes represents host genetic con! stitution "chromosome number indicated above box#^ solid lines indicate observed hostÐparasite associations[ Protopolystoma species B from X[ fraseri a}[ and species C from X[ laevis occur in the octoploid hybrid X[ wittei[ Species C from X[ laevis occurs in the octoploid hybrid X[ vestitus\ but species B from X[ fraseri a}[ is absent[
Fig[ 4[ The occurrence of Protopolystoma species C in tetraploid and octoploid hosts] correlation between host genetic con! stitution and parasite speci_city[ Shading in boxes represents host genetic constitution "chromosome number indicated above box#^ solid lines indicate observed hostÐparasite associations[ Common X[ laevis!like genetic material may predispose X[ laevis and its hybrid octoploid derivatives to infection with species C[
swamps shaded by the tree canopy and X[ laevis principally in open swamps exposed to the sun ð00Ł[ Although the mobility of Xenopus "which is capable of overland migration between aquatic habitats# could permit some limited exposure of these Xen! opus species to invasion by parasites of the other host species\ it is likely that Protopolystoma species
B and C are largely isolated in fraseri and laevis respectively[ However\ Protopolystoma species B and C do occur together in populations in X[ wittei[ This host species occupies habitats which are gen! erally distinct from those of fraseri^ it occurs in the highlands of the western Rift\ bordering Uganda\ Rwanda and the Democratic Republic of Congo in biotypes\ including montane forest\ which are at much higher altitudes than those of lowland forest fraseri[ However\ there is a regular overlap between wittei and laevis in the complex topography of the mountains which border the Rift ð29Ł\ and\ in these circumstances\ the two hosts would be exposed to reciprocal cross!infection by parasites normally speci_c to the other[ Although Protopolystoma species B and C co!occur in X[ wittei populations\ they infect the same host individuals with a fre! quency lower than that expected by a random dis! tribution[ There is evidence of interference competition which precludes long!term co!exis! tence of these two species in the same host indi! vidual ð20Ł[ In the case of X[ vestitus\ although a range of evidence points to an equivalent combination of genetic information from ancestral species belong! ing to the laevis and fraseri species groups "Fig[ 3#\ only a single species of Protopolystoma is represented[ This taxon "C# re~ects the evolutionary and genetic relationships of its host with laevis "see above#[ However\ there is no {{heirloom|| rep! resenting the presumed involvement of a fraseri! like ancestor in the origin of X[ vestitus[ This absence may be a product of secondary loss\ or a failure of the fraseri!lineage parasite to capture "colonise# the allopolyploid vestitus[ Alternatively\ the lack of a fraseri!type parasite may be signi_cant in relation to current molecular evidence that the relationships of vestitus with the fraseri group are relatively more distant than in the case of X[ wittei "see above and ð05Ł#[ This analysis of inter!relationships within host and parasite lineages demonstrates that 0 species of Protopolystoma "C# occurs in three Xenopus species\ laevis\ wittei and vestitus[ Figure 4 correlates this co! occurrence in relation to host genetic constitution[ Each of the hosts of species C carries a common component of its genome which is {{laevis!like||\ suggesting that this facilitates\ perhaps through
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genetic susceptibility\ exploitation by this Pro! topolystoma representative[ In contrast to this evidence of shared genes and shared parasites\ X[ laevis harbours a second species of Protopolystoma "P[ xenopodis\ referred to as species D in Figs 0 and 2# which is strictly host! speci_c to laevis[ In the _eld\ in central Africa\ laevis\ wittei and vestitus regularly share con_ned habitats which would create circumstances for natural cross!infection experiments ð29\ 21Ł[ However\ no instances of species D infection in vestitus and wittei have been encountered "in many hundreds of specimens from natural populations#\ nor does species D infect any of the other Xenopus species which have so far been studied[ In lab! oratory cross!infection experiments\ Proto! polystoma species D from X[ laevis fails to produce patent infections in X[ wittei con_rming the _eld evidence of strict physiological host speci_city "ð8Ł\ Jackson and Tinsley\ unpublished#[ In this case\ the presence of a complete set of chromosomes which are more or less laevis!like in X[ vestitus and X[ wittei does not permit transfer of the laevis!speci_c Protopolystoma to the polyploid hybrid o}spring of laevis[ This exclusive parasite distribution parallels exactly the case of Protopolystoma species A which is known only from X[ fraseri\ this does not infect other Xenopus species including the allopolyploid X[ wittei and X[ vestitus even though these latter two species carry a component of fraseri!like genetic information "presumably modi_ed since the hybrid! isation event responsible for their origin#[ In other words\ the fraseri!like genetic information in X[ wittei which allows Protopolystoma species B to infect wittei does not equally permit species A to infect this polyploid species[ Comprehensive studies of biochemistry\ genetics and molecular biology demonstrate the close relationship of X[ wittei and X[ vestitus\ and this is supported by their sharing of Protopolystoma spec! ies C[ However\ it may be highly signi_cant that Protopolystoma species B infects X[ wittei and not X[ vestitus[ Again\ the frequent co!occurrence of these octoploid species in con_ned habitats in cen! tral Africa should facilitate reciprocal cross!infec! tion^ the failure of this parasite to transfer suggests that host genetic constitution may be the dominant
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in~uence in susceptibility to Protopolystoma species B[ Thus\ the ability of species B to infect X[ wittei and its failure to infect X[ vestitus may be deter! mined by the half of the host genome which re~ects relative a.nity to the fraseri group "alongside the set of 25 laevis!like chromosomes which they share "see above##[ The signi_cance of absence of infection in these hostÐparasite associations must be interpreted with caution\ particularly where experimental tests of host!speci_city have not been carried out[ Never! theless\ the apparent strict host!speci_city may re~ect a powerful e}ect of host genetic factors in determining susceptibility to infection[ The data presented in this review suggest that the XenopusÐProtopolystoma system represents a highly informative model for studies of host and parasite co!speciation\ using _eld data\ parasite morphological characteristics\ and some laboratory cross!infection experiments[ The immune capa! bilities of Xenopus are very well documented ð22\ 23Ł\ and therefore this hostÐparasite system could also provide a model for studies of the immu! nological factors which may determine host!speci! _city and may\ ultimately\ direct evolution[
Acknowledgements This work was supported by research grants from the Natural Environment Research Council "GR2:5550 and GR8:521#\ the Royal Society and the Systematics Association[
References ð0Ł Clayton DH\ Moore J[ HostÐparasite evolution[ Oxford] Oxford University Press\ 0886[ ð1Ł Hoberg EP\ Brooks DR\ Siegel!Causey D[ HostÐparasite co!speciation] history\ principles\ and prospects[ In] Clay! ton DH\ Moore J\ editors[ HostÐparasite evolution[ Oxford] Oxford University Press\ 0886]101Ð124[ ð2Ł Paterson AM\ Gray RD[ HostÐparasite co!speciation\ host switching\ and missing the boat[ In] Clayton DH\ Moore J\ editors[ HostÐparasite evolution[ Oxford] Oxford Uni! versity Press\ 0886]125Ð149[ ð3Ł Klassen GJ\ Beverley!Burton M[ Phylogenetic relationships of Ligictaluridus spp[ "Monogenea] Ancyrocephalidae# and
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R[C[ Tinsley\ J[A[ Jackson : International Journal for Parasitolo`y 17 "0887# 0462Ð0471 their ictalurid "Siluriformes# hosts] an hypothesis[ Proc Hel! minthol Soc Wash 0876^43]73Ð89[ Klassen GJ\ Beverley!Burton M[ North American fresh water ancyrocephalids "Monogenea# with articulating hap! toral bars] hostÐparasite coevolution[ Syst Zool 0877^26]068Ð078[ Boeger WA\ Kritsky DC[ Phylogeny\ coevolution\ and revision of the Hexabothriidae Price\ 0831 "Monogenea#[ Int J Parasitol 0878^08]314Ð339[ Wheeler TA\ Beverley!Burton M[ Systematics of Onch! ocleidus Mueller\ 0825 "Monogenea] Ancyrocephalidae#] phylogenetic relationships\ evolution\ and host associ! ations[ Can J Zool 0878^56]695Ð602[ Tinsley RC\ Kobel HR[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885[ Tinsley RC\ Jackson JA[ Speciation of Protopolystoma Bychowsky\ 0846 in hosts of the genus Xenopus "Anura#[ Syst Parasitol 0887\ in press[ Baez AM[ The fossil record of the Pipidae[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]218Ð236[ Tinsley RC\ Loumont C\ Kobel HR[ Geographical dis! tribution and ecology[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]24Ð48[ Tinsley RC[ A new species of Xenopus "Anura] Pipidae# from the highlands of Ethiopia[ AmphibiaÐReptilia 0884^05]264Ð277[ Kobel HR\ Loumont C\ Tinsley RC[ The extant species[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]8Ð22[ Kobel HR[ Allopolyploid speciation[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford Uni! versity Press\ 0885]280Ð390[ Knochel W\ Korge E\ Basner A\ Mayerhof W[ Globin evolution in the genus Xenopus] comparative analysis of cDNAs coding for adult globin polypeptides of Xenopus borealis and Xenopus tropicalis[ J Mol Evol 0875^12]100Ð 112[ Graf J!D[ Molecular approaches to the phylogeny of Xeno! pus[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]268Ð278[ Tymowska J[ Polyploidy and cytogenetic variation in frogs of the genus Xenopus[ In] Green DM\ Sessions SK\ editors[ Amphibian cytogenetics and evolution[ London] Academic Press\ 0880]148Ð186[ Burki E\ Fischberg M[ Evolution of globin expression in the genus Xenopus "Anura] Pipidae#[ Molec Biol Evol 0874^1]169Ð166[ Tinsley RC[ Parasitic disease in amphibians] control by the regulation of worm burdens[ Parasitology 0884^000]S042Ð S067[ Tinsley RC[ Parasites of Xenopus[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford Uni! versity Press\ 0885]122Ð150[
ð10Ł Fain A\ Tinsley RC[ A new Xenopacarus "Acari\ Erey! netidae# from the nasal cavities of Xenopus sp[ "fraseri group#\ with a discussion on the evolution hostÐparasite[ Rev Zool Afr 0882^096]402Ð406[ ð11Ł Jackson JA\ Tinsley RC[ Representatives of Batra! chocamallanus n[ g[ "Nematoda] Procamallaninae# from Xenopus spp[ "Anura] Pipidae#] geographical distribution\ host range and evolutionary relationships[ Syst Parasitol 0884^20]048Ð077[ ð12Ł Jackson JA\ Tinsley RC[ Evolutionary relationships\ host range and geographical distribution of Camallanus Railliet + Henry\ 0804 species "Nematoda] Camallaninae# from clawed toads of the genus Xenopus "Anura] Pipidae#[ Syst Parasitol 0884^21]0Ð10[ ð13Ł Jackson JA\ Tinsley RC[ The taxonomic status\ host range and geographical distribution of Dollfuschella Vercammen! Grandjean\ 0859 "Digenea] Halipeginae# from Xenopus species "Anura] Pipidae#[ Syst Parasitol 0886^25]0Ð00[ ð14Ł Tinsley RC\ Jackson JA[ The genus Oligolecithus Ver! cammen!Grandjean "Digenea] Telorchiidae# from Xenopus spp[ "Anura] Pipidae# with a description of O[ siluranae n[ sp[ from X[ tropicalis "Gray# in Ghana[ Syst Parasitol 0884^21]020Ð039[ ð15Ł Jackson JA\ Tinsley RC[ Paramphistome digeneans from Xenopus species "Pipidae# in Africa] taxonomy\ host!speci! _city and biogeography[ Syst Parasitol\ in press[ ð16Ł Tinsley RC[ Evolutionary inferences from host and parasite co!speciation[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]392Ð319[ ð17Ł Tinsley RC\ Owen RW[ Studies on the biology of Pro! topolystoma xenopodis "Monogenoidea#] the onco! miracidium and life!cycle[ Parasitology 0864^60]334Ð352[ ð18Ł Frost DR[ Amphibian species of the world[ Lawrence\ Kansas\ U[S[A[] Allen Press and The Association of Sys! tematics Collections\ 0874[ ð29Ł Tinsley RC\ Kobel HR\ Fischberg M[ The biology and systematics of a new species of Xenopus "Anura] Pipidae# from the highlands of central Africa[ J Zool\ Lond 0868^077]58Ð091[ ð20Ł Jackson JA\ Tinsley RC[ Mutual exclusion of congeneric monogenean species in a space!limited habitat[ Para! sitology\ in press[ ð21Ł Tinsley RC[ Interactions between Xenopus species "Anura Pipidae#[ Monitore Zool Ital "N[S# Suppl[ 0870^04]022Ð049[ ð22Ł Du Pasquier L\ Wilson M\ Robert J[ The immune system of Xenopus] with special reference to B cell development and immunoglobulin genes[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]290Ð202[ ð23Ł Horton JD\ Horton TL\ Ritchie P[ Immune system of Xen! opus] T cell biology[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]168Ð188[
Correlation of parasite speciation and speci_city with host evolutionary relationships R[C[ Tinsley \ J[A[ Jackson School of Biological Sciences\ University of Bristol\ Bristol BS7 0UG\ U[K[ Received 09 November 0886^ received in revised form 1 February 0887^ accepted 09 March 0887
Abstract Protopolystoma "Monogenea\ Polystomatidae# is strictly speci_c to the anuran amphibian genus Xenopus[ The host group is characterised by a polyploid series in which chromosome numbers re~ect diploid\ tetraploid\ octoploid and dodecaploid constitutions^ the series is considered to have evolved through interspecies hybridisation and genome duplication[ This study correlates information on host evolutionary relationships with patterns of parasite speciation and host speci_city[ Protopolystoma is restricted to one subgenus "Xenopus# with multiples of 25 chromosomes\ and is absent from the subgenus Silurana "with multiples of 19 chromosomes#[ Molecular\ biochemical and karyotype evidence distinguishes three subgroups within Xenopus[ Representative species from each subgroup\ Xenopus muelleri\ Xenopus fraseri and Xenopus laevis\ have been examined for polystomatid infection[ Two species of Protopolystoma occur in each of these host species[ In X[ muelleri\ the two Protopolystoma species re~ect parasite co!speciation corresponding with the divergence of two sibling host species[ Xenopus fraseri and X[ laevis "both with 1n 25 chromosomes# are implicated in the hybrid origin of two octoploid species\ Xenopus wittei and Xenopus vestitus "both 1n 61#[ The relationships of the Protopolystoma species in these Xenopus taxa re~ect this presumed ancestry[ Xenopus wittei carries two species of Protopolystoma\ one shared with X[ fraseri and the other shared with X[ laevis[ Xenopus vestitus carries a single species of Protopolystoma which is shared with X[ laevis but there is no {{heirloom|| which re~ects its hybrid origin involving X[ fraseri[ In addition to these shared parasite species which may re~ect shared host genes\ X[ fraseri and X[ laevis each carry separate species!speci_c Protopolystoma which do not occur in other Xenopus species even where there is evidence of common genetic information "as in the allopolyploid wittei and vestitus#[ This case study may be interpreted as indicating a powerful in~uence of host genetic factors on susceptibility to infection\ host!speci_city\ and parasite speciation[ Þ 0887 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved Keywords] Co!speciation^ Monogenea^ Polystomatidae^ Protopolystoma^ Anura^ Pipidae^ Xenopus^ Host polyploidy^ Host!speci_city
0[ Introduction HostÐparasite co!evolution has occupied a cen! tral place in parasitology throughout the devel! opment of the discipline and has major importance
Corresponding author[
for interpretation of fundamental aspects including host!speci_city\ parasitic adaptation\ phylogeny\ distribution\ host immune defence and parasite counter!defence\ and the concept of an evol! utionary arms race[ Previous phylogenetic analyses of hostÐparasite assemblages have documented evidence for co!speciation "parallel cladogenesis# of some host and parasite associations "reviewed by Clayton and Moore ð0Ł\ including co!speciation of
S9919!6408:87:,08[99¦9[99 Þ 0887 Australian Society for Parasitology[ Published by Elsevier Science Ltd[ All rights reserved PII] S9919!6408"87#99974!X
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cestodes and lice in birds ð1\ 2Ł#[ However\ there are few systems for which there are equally!resolved phylogenies for parasites and hosts[ The Mono! genea are traditionally regarded as highly host! speci_c and recent analyses of the evolutionary interactions of hosts and parasites have been inter! preted in relation to co!speciation and colonisation ð3Ð6Ł[ This paper presents a case study of species level comparisons involving an anuran amphibian\ for which there is now detailed phylogenetic and other biological information ð7Ł\ and a poly! stomatid monogenean\ for which species diversity has recently been documented ð8Ł[
0[0[ The host group Species of the genus Xenopus "Anura] Pipidae#* the African clawed toads*have been the subject of intensive laboratory studies for over 69 years\ initially for research in animal physiology\ bio! chemistry and developmental biology\ and more recently genetics\ immunology and molecular biology[ Incidentally\ these studies have generated comprehensive information enabling interpretation of systematic relationships and evolution ð7Ł[ Pipid anurans have a rich palaeontological record\ including species from the Cretaceous which can be assigned to present day groups within the genus Xenopus ð09Ł[ Fossil representatives occur in both Africa and South America\ and their dis! tribution indicates a wide geographical range before the Atlantic rift[ There are four extant genera of pipids] one of these*Pipa*occurs in South Amer! ica\ and the other three occur in Africa*Xenopus\ Hymenochirus and Pseudhymenochirus "of which the latter two are actually more closely related to Pipa than they are to Xenopus#[ Up to the early 0869s\ only six species of Xenopus were known and almost all research interest focused on just one of these\ Xenopus laevis[ Knowledge of the species diversity of Xenopus has increased greatly in recent years\ particularly as a result of _eldwork in rela! tively remote highland regions of Africa ð00Ł[ At present\ there are 06 formally described species and at least a further three are recognised but not yet named ð01\ 02Ł[ These species have a characteristic which is highly unusual amongst vertebrates] their
chromosome numbers form a polyploid series which represent diploid\ tetraploid\ octoploid and dodecaploid genetic constitutions ð03Ł "Table 0#[ The chromosome complements re~ect a major evol! utionary divergence within the genus] one lineage comprises species with 1n 19 and 39 chro! mosomes "diploid and tetraploid#\ the other has species with 1n 25\ 61 and 097 chromosomes in which the 1n 25 level is interpreted as tetraploid with respect to the original diploid composition of 07 "not found in any known species#[ These two lineages of Xenopus were regarded by Kobel et al[ ð02Ł as subgenera] Silurana with multiples of 19 chromosomes\ and Xenopus with multiples of 07 chromosomes[ A series of biochemical and molec! ular studies suggest that their divergence is ancient] Knochel et al[ ð04Ł\ using a calibrated timeÐdiver! gence curve of globin evolution\ estimated sep! aration to have occurred about 019 million years ago "see also ð05Ł#[ The key factor involved in the evolution of the higher ploidy levels is considered to be interspecies hybridisation accompanied by duplication of the genome ð03Ł[ The allopolyploid species which result from this event therefore com! bine the genetic adaptations of both parental spec! ies[ In some cases\ there is biochemical and genetic evidence for the identity of the ancestors of poly! ploid species[ For instance\ Xenopus fraseri!like and X[ laevis!like species "both 1n 25# are considered to have been involved in the origins of the two octoploid "1n 61# species Xenopus vestitus and Xenopus wittei ð7\ 06Ł "Fig[ 2#[ Electrophoretic com! parison of globin polypeptides suggests that Xen! opus ruwenzoriensis "1n 097# may be a product of hybridisation between ancestral species of the X[ fraseri "1n 25# and Xenopus amieti "1n 61# lineages ð05\ 07Ł[ Other relationships within the genus have been de_ned by a variety of techniques\ including cyto! genetic characters\ electrophoretic comparisons of certain enzymes\ albumins and other serum proteins\ globins\ and basic sperm nuclear proteins\ and molecular analyses using mitochondrial DNA and ribosomal DNA "reviewed by Graf ð05Ł#[ There is good agreement that the 25 chromosome species may be subdivided into three distinct assemblages] the fraseri group "including pygmaeus and fraseri#\ the laevis group "gilli\ largeni and the laevis ras!
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Table 0 Species of Xenopus] chromosome numbers 1n 19
39
25
61
097
tropicalis
epitropicalis
borealis clivii fraseri gilli laevis largeni muelleri pygmaeus
amieti andrei boumbaensis vestitus wittei
longipes ruwenzoriensis
senkreis#\ and the muelleri group "borealis\ clivii and muelleri#[ This well!studied vertebrate makes a good model for integrated studies of host and parasite evol! ution[ In particular\ the relatively exceptional occurrence of allopolyploidisation could have a profound e}ect on host!speci_city and the present patterns of parasite distribution within the Xenopus lineages[ There is comprehensive evidence that Xen! opus species have an immune response which is equivalent in all major characteristics to that of mammals\ but there is only sketchy information on the operation of host immunity against parasite infection ð08\ 19Ł[ Theoretically\ allopolyploid! isation could bring together genetic characteristics controlling susceptibility to infection and hence in~uence the capacity of parasite lineages to infect hybrid polyploids with a recombined genome[ This study aims to establish correlations between host and parasite speciation patterns which may provide evidence of the in~uence of host genetic factors on parasite speci_city and evolution[
0[1[ The parasite fauna of Xenopus The genus Xenopus harbours an exceptionally rich parasite fauna] for the best studied species\ X[ laevis\ there are records of 18 genera of metazoan and protozoan parasites ð19Ł[ Amongst the met! azoans "monogeneans\ digeneans\ cestodes\ nema! todes\ an acarine mite and a leech# for which there is detailed information\ the great majority of rep! resentatives are distinguished by specialisations
which are often unique within their respective para! site groups\ and almost all are placed in their own genera\ families or higher taxonomic categories\ strictly speci_c to Xenopus[ Recently!published studies reveal a very high degree of correspondence in the patterns of speciation and host!speci_city of the parasites with the known evolutionary relation! ships in the host genus[ Some recent publications ð10Ð16Ł have documented a series of case studies in which both hostÐparasite co!speciation and col! onisation are indicated by analysis of relationships[ Almost all of these studies concur in recognising the major evolutionary divergence between the Xen! opus clades based on multiples of 19 or 25 chro! mosomes[ Thus\ in the nematode Camallanus\ the cestode Cephalochlamys\ and the digeneans Oli! golecithus and Progonimodiscus\ distinct parasite species are represented in the two host lineages[ In other cases\ parasite genera well!represented in the 25 chromosome line are absent from 19:39 chro! mosome species "the subgenus Silurana# "for instance\ the digenean Dollfuschella and the mono! genean Protopolystoma\ see below#\ indicating either secondary loss\ or colonisation of Xenopus subsequent to the divergence of the 19 and 25 chro! mosome clades[ This pattern of host!speci_city is incomplete in only one genus so far studied] the nematode Batrachocamallanus has three species which are strictly speci_c to the 25 chromosome Xenopus clade and which are never found in 19:39 chromosome species^ however\ there is a fourth\ highly distinctive\ species which infects Xenopus tropicalis and Xenopus epitropicalis but which may co!occur in X[ fraseri!like hosts in lowland tropical
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forest where the two host groups overlap geo! graphically and ecologically[ Alongside this evidence that speciation patterns in the parasite fauna generally mirror the major division of the host group\ there is also _ner level detail that subgroups within the 25 chromosome lineage of Xenopus carry corresponding host!spec! i_c parasite taxa[ Tinsley ð16Ł summarised evidence that the three 25 chromosome Xenopus species groups fraseri\ laevis and muelleri are each {{recog! nised|| by a series of diverse parasite groups "includ! ing not only helminths but also the acarine mite Xenopacarus#[ 0[2[ Monogenean parasites of Xenopus Two monogenean genera infect the genus Xen! opus] Protopolystoma Bychowsky\ 0846 and Gyr! dicotylus Vercammen!Grandjean\ 0859[ Both share the features typical of the Xenopus parasite fauna in general] they are highly distinctive from a phylo! genetic viewpoint with morphological and life his! tory specialisations which suggest a long separation from nearest relatives[ These two monogeneans also illustrate the dual origin of the parasite assemblage associated with Xenopus[ Protopolystoma is related to genera\ including Polystoma\ which are typical parasites of the same infection site "the urinary bladder# of other anuran amphibians[ This suggests that it represents an {{heirloom|| from a common ancestry of monogeneans which have evolved with the anuran lineages[ Other helminths which con! stitute this group of {{anuran!typical|| parasites include the digeneans Dollfuschella\ Oligolecithus and Progonimodiscus whose relationships suggest that they have been inherited along parallel co! evolutionary lines during anuran evolution[ Despite this clear a.nity\ all show major divergence from their nearest relatives in other anurans\ suggesting a prolonged isolation with their pipid hosts[ By contrast\ Gyrdicotylus "found in the buccal cavity of Xenopus# exhibits the highly distinctive features of the Gyrodactylidae which are pre! dominantly ectoparasites\ on the skin\ _ns and gills\ of a wide range of _shes[ Gyrdicotylus is one of a number of Xenopus parasites representing transfers from _shes with which the fully!aquatic anuran shares habitats\ diet and other ecological niche par!
ameters[ This component of the Xenopus parasite fauna also includes the strigeatoid digeneans Diplo! stomulum\ Neascus and Tetracotyle\ the protozoan Trichodina\ the cestode Cephalochlamys\ and the camallanid nematodes Camallanus and Batra! chocamallanus\ most of which have diverged very considerably from their nearest relatives which are typical _sh parasites ð19Ł[ A preliminary analysis of the patterns of spe! ciation in Protopolystoma and Gyrdicotylus has been summarised by Tinsley ð16Ł[ For the latter genus\ formal descriptions of the species are cur! rently in progress[ For Protopolystoma\ taxonomic accounts of six species "_ve of them previously undescribed# are now in press ð8Ł[ This review con! siders the speciation of Protopolystoma against the background of evolutionary relationships recently established for the host genus "see above#[ 1[ Materials and methods Material has been obtained from throughout much of the geographical distribution of the genus Xenopus\ derived from _eldwork and from dis! section of preserved samples of host species in museum collections "see ð8\ 11Ð15Ł#[ Di}erentiation of the Protopolystoma species has been based on conventional morphological characters\ com! parable with those employed for other poly! stomatids "including the form and degree of branching of the gut\ the dimensions and shape of the haptoral hamuli and the male copulatory organ spines#[ The inter!speci_c di}erences are detailed by Tinsley and Jackson ð8Ł and are beyond the scope of this review[ As the designations are not yet for! mally published\ the taxa are referred to below as species D "which is the single previously!recognised species\ Protopolystoma xenopodis "Price# Bychow! sky#\ and species A\ B\ C\ E and F which are newly! recognised species[ 2[ Observations 2[0[ Correlation of speciation in Protopolystoma with host phylogeny 2[0[0[ Subgenus Silurana[ No polystomatids have been found in representatives of the subgenus
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Silurana\ X[ tropicalis "1n 19 chromosomes#\ X[ epitropicalis "1n 39#\ and other as yet undescribed forms with 39 chromosomes\ either in present studies ð8Ł or previously!published surveys "Fig[ 0#[ Experimental cross!infections of X[ tropicalis using oncomiracidia hatched from eggs of Proto! polystoma species from 1n 25 Xenopus "including X[ laevis# result in successful invasion\ but these never survive to reach sexual maturity in the urinary bladder] death invariably occurs during juvenile development ð8Ł "for life cycle details see ð17Ł#[ 2[0[1[ Subgenus Xenopus[ Extensive sampling\ including _eld collections of many thousands of Xenopus specimens\ has demonstrated that the genus Protopolystoma is restricted exclusively to the Xenopus clade which has multiples of 25 chro! mosomes "the subgenus Xenopus#[ Representatives of Protopolystoma have been found in all Xenopus species so far examined ð8Ł[ Amongst the Pro! topolystoma taxa di}erentiated\ there are distinct species corresponding with the three host groups distinguished by biochemical\ molecular and cyto! genetic characteristics] the muelleri group\ the fra! seri group\ and the laevis group[ This review considers the inter!relationships of the Pro! topolystoma species in one representative of each of these host groups\ in X[ muelleri\ X[ fraseri and X[ laevis respectively "Fig[ 0#\ and also in the allo! polyploid "1n 61# species X[ vestitus and X[ wittei which are presumed to have originated through hybridisation between laevis!like and fraseri!like ancestors[ 2[0[2[ Xenopus muelleri[ Two species of Pro! topolystoma are represented[ Field samples indicate
Fig[ 0[ Occurrence of Protopolystoma species amongst the main lineages of Xenopus[ Host chromosome number indicated above species or species group[ Note] the {{muelleri|| group is rep! resented in this study by two allopatric species previously included in X[ muelleri "see text#[
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that the muelleri Protopolystoma do not infect other Xenopus species even where these occur sym! patrically] X[ muelleri has frequent geographical overlap with laevis at the borders of their respective ranges\ and muelleri is also sympatric with X[ trop! icalis and X[ fraseri!group species at the boundary of tropical forest and savanna ð00Ł[ Laboratory experiments to test the outcome of reciprocal cross! infection\ using Protopolystoma from X[ laevis laevis and X[ muelleri "species D and E respectively# from a sympatric site in South Africa\ show that these parasites fail to develop patent infections in the {{foreign|| host species and are strictly host spec! ies!speci_c "studies in preparation#[ Xenopus muelleri has been recognised as a distinct species since 0733 ð18Ł[ However\ evidence has accumulated recently to indicate that it actually comprises two allopatric cryptic species\ one inhabiting the hot dry savannas of sub!Saharan Africa\ including the Sahel regions of western and central Africa\ the other distributed through the lowland\ largely coastal environments of east and south east Africa ð02Ł "see Fig[ 1#[ These geo! graphically!isolated forms are distinguished by a number of biological characteristics and their des! ignation as separate species is now in preparation[ Additionally\ they are distinguished unequivocally by morphologically!distinct representatives of Pro! topolystoma "Fig[ 2\ species E\ F#[ In this instance\ the divergence of a {{muelleri||!speci_c Pro! topolystoma into two species parallels the speciation of its host[ 2[0[3[ Xenopus fraseri[ Xenopus fraseri a}[ "includ! ing cryptic species morphologically!indistinguish! able from fraseri sensu stricto# harbours one Pro! topolystoma species which is strictly host!speci_c to the fraseri group "Fig[ 2\ species A#^ this is dis! tributed in the rainforests of the Congo Basin\ and occurs in no other Xenopus groups[ However\ fra! seri carries a second species of Protopolystoma "rec! orded in Cameroon and the Democratic Republic of Congo# which also occurs in X[ wittei in southern Rwanda "Fig[ 2\ species B#[ 2[0[4[ Xenopus laevis[ A comparable pattern occurs in X[ laevis[ This host harbours one species! speci_c Protopolystoma "species D# which infects
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Fig[ 1[ Geographical distributions of Xenopus species[ A[ X[ tropicalis group[ B[ X[ muelleri "western form#[ C[ X[ muelleri "eastern form#[ D[ X[ fraseri!like species[ E[ X[ laevis[ F[ X[ vestitus and X[ wittei "arrowed#[
Fig[ 2[ Speci_city of Protopolystoma species correlated with host evolution[ Host chromosome number indicated above species^ solid lines indicate observed hostÐparasite associations^ pre! sumed host genealogical relationships indicated by dotted lines[ The tetraploid toads X[ laevis and X[ fraseri are infected by 1 Protopolystoma species each] in both cases 0 parasite is narrowly speci_c\ while another also occurs in X[ wittei and:or X[ vestitus "the hybrid octoploid derivatives of X[ laevis and X[ fraseri! like lineages#[ The divergence of Protopolystoma spp[ E and F parallels the speciation of the eastern and western forms of X[ muelleri[
mous geographical range from the Cape\ South Africa\ to northern Cameroon "Fig[ 1#[ However\ it has not been recorded in any other Xenopus species examined\ even in well!de_ned areas of sympatry where the laevis subspecies overlap geographically with X[ muelleri\ X[ borealis\ X[ tropicalis\ X[ epi! tropicalis\ and the X[ fraseri!like species[ Xenopus laevis also harbours a second species of Pro! topolystoma "which is carried by X[ l[ victorianus and X[ l[ poweri in central Africa#^ however\ this species is shared with X[ vestitus and X[ wittei in the Rift Valley highlands bordering Rwanda\ Burundi\ Uganda and the Democratic Republic of Congo "Zaire# "Fig[ 2\ species C#[ 2[0[5[ Xenopus wittei and Xenopus vestitus[ The records above show that X[ wittei carries two species of Protopolystoma "B and C#\ 0 shared with X[ fraseri and 0 shared with X[ laevis and X[ vestitus^ X[ vestitus carries a single species "C# "shared with X[ laevis and X[ wittei#[ 3[ Interpretation
all the laevis subspecies so far examined\ Xenopus laevis laevis\ Xenopus l[ victorianus\ Xenopus l[ pow! eri and Xenopus l[ sudanensis\ throughout an enor!
The restriction of the genus Protopolystoma to the subgenus Xenopus and its absence from Silurana
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concurs with other evidence "above# of a major discontinuity in parasite distribution re~ecting the dichotomy between the 19 and 25 chromosome Xenopus clades[ As noted above\ the timing of this divergence has been estimated to be about 019 myr ago[ Paterson and Gray ð2Ł recognised three types of sorting event which may lead to absence of parasites from host lineages] extinction\ stochastic events at the time of host speciation "{{missed the boat||# and sampling error[ The latter can be excluded by the extensive population samples of Silurana species which have been examined[ However\ wider studies of pipid lineages in South America and Africa are necessary to establish the history of polystomatid parasitism in this host group[ The patterns of host!speci_city within species of the subgenus Xenopus are shown diagrammatically in Fig[ 2 "for host geographical distributions see Fig[ 1#[ Four species of Protopolystoma "A\ D\ E\ F# are strictly speci_c to single species of Xenopus] one in each of the two cryptic X[ muelleri species\ one in X[ fraseri and one in X[ laevis[ Two species infect more than one host species] one "B# occurs in two host species "fraseri and wittei#\ and one "C# occurs in three host species "laevis\ wittei and vesti! tus#[ With reference to the representatives of the three 25 chromosome Xenopus groups\ muelleri\ fra! seri and laevis\ Protopolystoma is represented by two distinct species in each of these groups[ The pattern of parasite divergence shown in {{X[ muelleri|| may be interpreted in terms of pairwise co!speciation[ The di}erentiation of the host into allopatric sibling species "distinguished by bio! chemical and other characters# is paralleled by di}erentiation of its two Protopolystoma rep! resentatives[ It is likely that the timing of parasite divergence may be correlated with the succession of climatic oscillations during the Quaternary\ which led to repeated expansion and contraction of the forest and savanna zones in Africa and hence to disjunction of the hosts| geographical ranges "review in ð00Ł#[ The pattern of hostÐparasite evolutionary relationships is more complicated in the other two 1n 25 chromosome host groups[ Xenopus wittei and X[ vestitus are allopolyploid species with 1n 61 chromosomes\ considered to have arisen by hybridisation of two 1n 25 species[
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A variety of evidence points in both cases to an involvement of X[ laevis!like and X[ fraseri!like ancestors in the origin of these octoploid descend! ants ð7\ 06Ł[ The links between wittei and the fraseri group are particularly robust "based on molecular\ biochemical and cytogenetic characters#\ whilst wit! tei is also linked to laevis by characters which include karyotype[ X[ vestitus has been interpreted\ in di}erent studies\ as showing equivalent links with both fraseri and laevis[ However\ precise relation! ships are still obscure] peptide mapping of serum albumins indicates that X[ vestitus has a relatively isolated position amongst the group of species "including X[ wittei# which are associated with X[ fraseri "reviewed by ð05Ł#[ Experimental hybrid! isation between X[ wittei and X[ vestitus suggests that these two 61 chromosome species share a com! mon chromosome set while the other half of the chromosome complement may have originated from di}erent parental species "see ð02Ł#[ The origin and relationships of the Pro! topolystoma species infecting the two octoploid Xenopus species can be interpreted in relation to the genetic information shared by these hosts with their presumed ancestors "Figs 3 and 4#[ In the case of X[ wittei\ the host genome may be considered to combine genetic information derived on the one hand from a fraseri!like ancestor and on the other hand from a laevis!like ancestor[ One species of Protopolystoma "B# is shared with X[ fraseri re~ect! ing one half of the host|s genetic background\ the other species of Protopolystoma "C# is shared with X[ laevis re~ecting the other half of the host|s ances! try "Fig[ 3#[ Where Protopolystoma species B and C occur in the present!day representatives of their ancestral host groups\ in X[ fraseri and X[ laevis respectively\ they are ecologically and geographically distinct from one another[ Xenopus fraseri is adapted to lowland tropical rainforest while X[ laevis occurs in cooler\ upland regions which are principally sav! anna and wooded savanna environments[ The two Xenopus species have been recorded in the same area only very occasionally\ for instance where there is gallery forest in grass savanna or where there are signi_cant clearings in forest\ but under these circumstances the two species are ecologically distinct with X[ fraseri principally inhabiting
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Fig[ 3[ Protopolystoma from hybrid octoploid Xenopus species] correlation between host genetic constitution and parasite speci! _city[ Presumed host genealogical relationships indicated by dotted lines^ shading in boxes represents host genetic con! stitution "chromosome number indicated above box#^ solid lines indicate observed hostÐparasite associations[ Protopolystoma species B from X[ fraseri a}[ and species C from X[ laevis occur in the octoploid hybrid X[ wittei[ Species C from X[ laevis occurs in the octoploid hybrid X[ vestitus\ but species B from X[ fraseri a}[ is absent[
Fig[ 4[ The occurrence of Protopolystoma species C in tetraploid and octoploid hosts] correlation between host genetic con! stitution and parasite speci_city[ Shading in boxes represents host genetic constitution "chromosome number indicated above box#^ solid lines indicate observed hostÐparasite associations[ Common X[ laevis!like genetic material may predispose X[ laevis and its hybrid octoploid derivatives to infection with species C[
swamps shaded by the tree canopy and X[ laevis principally in open swamps exposed to the sun ð00Ł[ Although the mobility of Xenopus "which is capable of overland migration between aquatic habitats# could permit some limited exposure of these Xen! opus species to invasion by parasites of the other host species\ it is likely that Protopolystoma species
B and C are largely isolated in fraseri and laevis respectively[ However\ Protopolystoma species B and C do occur together in populations in X[ wittei[ This host species occupies habitats which are gen! erally distinct from those of fraseri^ it occurs in the highlands of the western Rift\ bordering Uganda\ Rwanda and the Democratic Republic of Congo in biotypes\ including montane forest\ which are at much higher altitudes than those of lowland forest fraseri[ However\ there is a regular overlap between wittei and laevis in the complex topography of the mountains which border the Rift ð29Ł\ and\ in these circumstances\ the two hosts would be exposed to reciprocal cross!infection by parasites normally speci_c to the other[ Although Protopolystoma species B and C co!occur in X[ wittei populations\ they infect the same host individuals with a fre! quency lower than that expected by a random dis! tribution[ There is evidence of interference competition which precludes long!term co!exis! tence of these two species in the same host indi! vidual ð20Ł[ In the case of X[ vestitus\ although a range of evidence points to an equivalent combination of genetic information from ancestral species belong! ing to the laevis and fraseri species groups "Fig[ 3#\ only a single species of Protopolystoma is represented[ This taxon "C# re~ects the evolutionary and genetic relationships of its host with laevis "see above#[ However\ there is no {{heirloom|| rep! resenting the presumed involvement of a fraseri! like ancestor in the origin of X[ vestitus[ This absence may be a product of secondary loss\ or a failure of the fraseri!lineage parasite to capture "colonise# the allopolyploid vestitus[ Alternatively\ the lack of a fraseri!type parasite may be signi_cant in relation to current molecular evidence that the relationships of vestitus with the fraseri group are relatively more distant than in the case of X[ wittei "see above and ð05Ł#[ This analysis of inter!relationships within host and parasite lineages demonstrates that 0 species of Protopolystoma "C# occurs in three Xenopus species\ laevis\ wittei and vestitus[ Figure 4 correlates this co! occurrence in relation to host genetic constitution[ Each of the hosts of species C carries a common component of its genome which is {{laevis!like||\ suggesting that this facilitates\ perhaps through
R[C[ Tinsley\ J[A[ Jackson : International Journal for Parasitolo`y 17 "0887# 0462Ð0471
genetic susceptibility\ exploitation by this Pro! topolystoma representative[ In contrast to this evidence of shared genes and shared parasites\ X[ laevis harbours a second species of Protopolystoma "P[ xenopodis\ referred to as species D in Figs 0 and 2# which is strictly host! speci_c to laevis[ In the _eld\ in central Africa\ laevis\ wittei and vestitus regularly share con_ned habitats which would create circumstances for natural cross!infection experiments ð29\ 21Ł[ However\ no instances of species D infection in vestitus and wittei have been encountered "in many hundreds of specimens from natural populations#\ nor does species D infect any of the other Xenopus species which have so far been studied[ In lab! oratory cross!infection experiments\ Proto! polystoma species D from X[ laevis fails to produce patent infections in X[ wittei con_rming the _eld evidence of strict physiological host speci_city "ð8Ł\ Jackson and Tinsley\ unpublished#[ In this case\ the presence of a complete set of chromosomes which are more or less laevis!like in X[ vestitus and X[ wittei does not permit transfer of the laevis!speci_c Protopolystoma to the polyploid hybrid o}spring of laevis[ This exclusive parasite distribution parallels exactly the case of Protopolystoma species A which is known only from X[ fraseri\ this does not infect other Xenopus species including the allopolyploid X[ wittei and X[ vestitus even though these latter two species carry a component of fraseri!like genetic information "presumably modi_ed since the hybrid! isation event responsible for their origin#[ In other words\ the fraseri!like genetic information in X[ wittei which allows Protopolystoma species B to infect wittei does not equally permit species A to infect this polyploid species[ Comprehensive studies of biochemistry\ genetics and molecular biology demonstrate the close relationship of X[ wittei and X[ vestitus\ and this is supported by their sharing of Protopolystoma spec! ies C[ However\ it may be highly signi_cant that Protopolystoma species B infects X[ wittei and not X[ vestitus[ Again\ the frequent co!occurrence of these octoploid species in con_ned habitats in cen! tral Africa should facilitate reciprocal cross!infec! tion^ the failure of this parasite to transfer suggests that host genetic constitution may be the dominant
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in~uence in susceptibility to Protopolystoma species B[ Thus\ the ability of species B to infect X[ wittei and its failure to infect X[ vestitus may be deter! mined by the half of the host genome which re~ects relative a.nity to the fraseri group "alongside the set of 25 laevis!like chromosomes which they share "see above##[ The signi_cance of absence of infection in these hostÐparasite associations must be interpreted with caution\ particularly where experimental tests of host!speci_city have not been carried out[ Never! theless\ the apparent strict host!speci_city may re~ect a powerful e}ect of host genetic factors in determining susceptibility to infection[ The data presented in this review suggest that the XenopusÐProtopolystoma system represents a highly informative model for studies of host and parasite co!speciation\ using _eld data\ parasite morphological characteristics\ and some laboratory cross!infection experiments[ The immune capa! bilities of Xenopus are very well documented ð22\ 23Ł\ and therefore this hostÐparasite system could also provide a model for studies of the immu! nological factors which may determine host!speci! _city and may\ ultimately\ direct evolution[
Acknowledgements This work was supported by research grants from the Natural Environment Research Council "GR2:5550 and GR8:521#\ the Royal Society and the Systematics Association[
References ð0Ł Clayton DH\ Moore J[ HostÐparasite evolution[ Oxford] Oxford University Press\ 0886[ ð1Ł Hoberg EP\ Brooks DR\ Siegel!Causey D[ HostÐparasite co!speciation] history\ principles\ and prospects[ In] Clay! ton DH\ Moore J\ editors[ HostÐparasite evolution[ Oxford] Oxford University Press\ 0886]101Ð124[ ð2Ł Paterson AM\ Gray RD[ HostÐparasite co!speciation\ host switching\ and missing the boat[ In] Clayton DH\ Moore J\ editors[ HostÐparasite evolution[ Oxford] Oxford Uni! versity Press\ 0886]125Ð149[ ð3Ł Klassen GJ\ Beverley!Burton M[ Phylogenetic relationships of Ligictaluridus spp[ "Monogenea] Ancyrocephalidae# and
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ð4Ł
ð5Ł
ð6Ł
ð7Ł ð8Ł
ð09Ł
ð00Ł
ð01Ł
ð02Ł
ð03Ł
ð04Ł
ð05Ł
ð06Ł
ð07Ł
ð08Ł
ð19Ł
R[C[ Tinsley\ J[A[ Jackson : International Journal for Parasitolo`y 17 "0887# 0462Ð0471 their ictalurid "Siluriformes# hosts] an hypothesis[ Proc Hel! minthol Soc Wash 0876^43]73Ð89[ Klassen GJ\ Beverley!Burton M[ North American fresh water ancyrocephalids "Monogenea# with articulating hap! toral bars] hostÐparasite coevolution[ Syst Zool 0877^26]068Ð078[ Boeger WA\ Kritsky DC[ Phylogeny\ coevolution\ and revision of the Hexabothriidae Price\ 0831 "Monogenea#[ Int J Parasitol 0878^08]314Ð339[ Wheeler TA\ Beverley!Burton M[ Systematics of Onch! ocleidus Mueller\ 0825 "Monogenea] Ancyrocephalidae#] phylogenetic relationships\ evolution\ and host associ! ations[ Can J Zool 0878^56]695Ð602[ Tinsley RC\ Kobel HR[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885[ Tinsley RC\ Jackson JA[ Speciation of Protopolystoma Bychowsky\ 0846 in hosts of the genus Xenopus "Anura#[ Syst Parasitol 0887\ in press[ Baez AM[ The fossil record of the Pipidae[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]218Ð236[ Tinsley RC\ Loumont C\ Kobel HR[ Geographical dis! tribution and ecology[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]24Ð48[ Tinsley RC[ A new species of Xenopus "Anura] Pipidae# from the highlands of Ethiopia[ AmphibiaÐReptilia 0884^05]264Ð277[ Kobel HR\ Loumont C\ Tinsley RC[ The extant species[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]8Ð22[ Kobel HR[ Allopolyploid speciation[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford Uni! versity Press\ 0885]280Ð390[ Knochel W\ Korge E\ Basner A\ Mayerhof W[ Globin evolution in the genus Xenopus] comparative analysis of cDNAs coding for adult globin polypeptides of Xenopus borealis and Xenopus tropicalis[ J Mol Evol 0875^12]100Ð 112[ Graf J!D[ Molecular approaches to the phylogeny of Xeno! pus[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]268Ð278[ Tymowska J[ Polyploidy and cytogenetic variation in frogs of the genus Xenopus[ In] Green DM\ Sessions SK\ editors[ Amphibian cytogenetics and evolution[ London] Academic Press\ 0880]148Ð186[ Burki E\ Fischberg M[ Evolution of globin expression in the genus Xenopus "Anura] Pipidae#[ Molec Biol Evol 0874^1]169Ð166[ Tinsley RC[ Parasitic disease in amphibians] control by the regulation of worm burdens[ Parasitology 0884^000]S042Ð S067[ Tinsley RC[ Parasites of Xenopus[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford Uni! versity Press\ 0885]122Ð150[
ð10Ł Fain A\ Tinsley RC[ A new Xenopacarus "Acari\ Erey! netidae# from the nasal cavities of Xenopus sp[ "fraseri group#\ with a discussion on the evolution hostÐparasite[ Rev Zool Afr 0882^096]402Ð406[ ð11Ł Jackson JA\ Tinsley RC[ Representatives of Batra! chocamallanus n[ g[ "Nematoda] Procamallaninae# from Xenopus spp[ "Anura] Pipidae#] geographical distribution\ host range and evolutionary relationships[ Syst Parasitol 0884^20]048Ð077[ ð12Ł Jackson JA\ Tinsley RC[ Evolutionary relationships\ host range and geographical distribution of Camallanus Railliet + Henry\ 0804 species "Nematoda] Camallaninae# from clawed toads of the genus Xenopus "Anura] Pipidae#[ Syst Parasitol 0884^21]0Ð10[ ð13Ł Jackson JA\ Tinsley RC[ The taxonomic status\ host range and geographical distribution of Dollfuschella Vercammen! Grandjean\ 0859 "Digenea] Halipeginae# from Xenopus species "Anura] Pipidae#[ Syst Parasitol 0886^25]0Ð00[ ð14Ł Tinsley RC\ Jackson JA[ The genus Oligolecithus Ver! cammen!Grandjean "Digenea] Telorchiidae# from Xenopus spp[ "Anura] Pipidae# with a description of O[ siluranae n[ sp[ from X[ tropicalis "Gray# in Ghana[ Syst Parasitol 0884^21]020Ð039[ ð15Ł Jackson JA\ Tinsley RC[ Paramphistome digeneans from Xenopus species "Pipidae# in Africa] taxonomy\ host!speci! _city and biogeography[ Syst Parasitol\ in press[ ð16Ł Tinsley RC[ Evolutionary inferences from host and parasite co!speciation[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]392Ð319[ ð17Ł Tinsley RC\ Owen RW[ Studies on the biology of Pro! topolystoma xenopodis "Monogenoidea#] the onco! miracidium and life!cycle[ Parasitology 0864^60]334Ð352[ ð18Ł Frost DR[ Amphibian species of the world[ Lawrence\ Kansas\ U[S[A[] Allen Press and The Association of Sys! tematics Collections\ 0874[ ð29Ł Tinsley RC\ Kobel HR\ Fischberg M[ The biology and systematics of a new species of Xenopus "Anura] Pipidae# from the highlands of central Africa[ J Zool\ Lond 0868^077]58Ð091[ ð20Ł Jackson JA\ Tinsley RC[ Mutual exclusion of congeneric monogenean species in a space!limited habitat[ Para! sitology\ in press[ ð21Ł Tinsley RC[ Interactions between Xenopus species "Anura Pipidae#[ Monitore Zool Ital "N[S# Suppl[ 0870^04]022Ð049[ ð22Ł Du Pasquier L\ Wilson M\ Robert J[ The immune system of Xenopus] with special reference to B cell development and immunoglobulin genes[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]290Ð202[ ð23Ł Horton JD\ Horton TL\ Ritchie P[ Immune system of Xen! opus] T cell biology[ In] Tinsley RC\ Kobel HR\ editors[ The biology of Xenopus[ Oxford] Oxford University Press\ 0885]168Ð188[