Speciation of Protopolystoma Bychowsky, 1957 (Monogenea: Polystomatidae) in hosts of the genus shape Xenopus (Anura: Pipidae)

The taxonomy, geographical distribution and hostrange of the polystomatid genus ProtopolystomaBychowsky, 1957 are reviewed. P. xenopodis(Price, 1943) and five new species are recognised,which occur in clawed toads ( Xenopus spp.)throughout subsaharan Africa. Of the two clawed toadsubgenera, Xenopus and Silurana, only theformer is infected. Protopolystoma spp. aredifferentiated by morphological variation of the gut,large hamulus and penis armature. P.xenopodis is found in Xenopus laevis subspeciesin South Africa, Transkei, Zimbabwe, DemocraticRepublic of Congo (D.R.C.), Rwanda, Uganda, Kenya andCameroon ( X. l. poweri and X. l.sudanensis are new host records). It also occurs inintroduced populations of X. l. laevis in theUnited States (southern California) and United Kingdom(South Wales). In subsaharan Africa the speciesdisplays significant, but continuous, geographicalvariation of penis spine size between southernpopulations in X. l. laevis and those in morenortherly host subspecies. Data on the natural hostrange of this parasite were complemented by anexperimental study of host-specificity in the southernform. This can produce patent infections in X.l. victorianus and X. gilli, but not X.wittei nor X. (Silurana) tropicalis. P.simplicis n. sp. is endemic to central and eastAfrican areas, infecting X. laevis subspecies ineastern D.R.C., Rwanda, Uganda and western Kenya, X. wittei-like hosts in eastern D.R.C., westernUganda, Rwanda and Burundi, X. vestitus inwestern Uganda and Xenopus sp. at Nairobi,Kenya. P. ramulosus n. sp. occurs in X.fraseri-like toads in eastern D.R.C. (Gabon andCameroon are also possible literature records), and P. fissilis n. sp. is found in X. fraseri-and X. wittei-like species in Cameroon andeastern D.R.C., and in southern Rwanda, respectively. Two Protopolystoma taxa are found in X.muelleri populations now suspected to representdistinct species: P. occidentalis n. sp. occursin X. muelleri (western form) in Ghana, Togo,Nigeria and Cameroon, while P. orientalis n. sp.is found in X. muelleri (eastern form) in SouthAfrica, Zimbabwe and Tanzania. The allopatricallydistributed species P. ramulosus, P.simplicis, P. occidentalis and P.orientalis form a relatively homogenous grouping withsome interspecific morphological overlap. These taxaare distinguished from P. xenopodis by penisspine morphology and from P. fissilis by hamulusroot form and aspects of gut morphology. Unidentified Protopolystoma sp. have been recorded in X. clivii in Ethiopia, X. fraseri aff. inCameroon and Xenopus sp. in Kenya and Tanzania. At some localities, single host species were infectedby two representatives of Protopolystoma. P. fissilis was recorded in eastern D.R.C. with P. ramulosus, with Protopolystoma sp. inCameroon in X. fraseri-like hosts and with P. simplicis in X. wittei-like hosts in Rwanda. P. xenopodis co-occurred with P. simplicisin X. laevis subspecies through central and eastAfrica.     

Paramphistome digeneans from shape Xenopus species (Pipidae) in Africa: taxonomy,host-specificity and biogeography

The taxonomy, host range and geographical distribution of paramphistome digeneans from Xenopus spp. in sub-Saharan Africa are reviewed. Two representatives of Progonimodiscus Vercammen-Grandjean, 1960 are recognised, both of which are narrowly or primarily specific to Xenopus. An analysis of morphometric and meristic characters indicated geographical variation in Progonimodiscus doyeri (Ortlepp, 1926), with two allopatric forms showing significant, but continuous, variation in testis size and vitelline follicle number. P. colubrifer n. sp. is distinguished from P. doyeri by the form of muscular elevations on the acetabulum accessory peduncle. It infects Xenopus ( Silurana) tropicalis-like toads from lowland tropical rain forest zones in Nigeria, Togo and the Ivory Coast, while P. doyeri occurs in hosts of the subgenus Xenopus from a wide variety of biotypes. Previous literature records indicate the presence of the southern P. doyeri morphological variant in X. laevis laevis in South Africa and Zimbabwe and the northern variant in X. l. victorianus, X. fraseri aff. and X. muelleri in the Democratic Republic of Congo, X. wittei in Uganda and the Democratic Republic of Congo, and the ranid Conraua crassipes in Cameroon (the only record of Progonimodiscus from a non-pipid host). New host and/or geographical records for this species are of the northern form in X. l. victorianus, X. l. bunyoniensis and X. vestitus in Uganda, X. l. sudanensis in Cameroon, X. borealis in Kenya, X. pygmaeus in the Democratic Republic of Congo, X. fraseri aff. in Cameroon, X. wittei aff. in the Democratic Republic of Congo and X. muelleri in Cameroon and Nigeria. While the geographical limits of the two P. doyeri variants are not known with precision, existing data are consistent with a turnover in the region of 15°S, where a notable discontinuity occurs in the distributions of other Xenopus parasites. Species of Diplodiscus Diesing, 1836 in Xenopus hosts are rare. Diplodiscus peregrinator n. sp. was recovered from X. tropicalis at a single locality in the Ivory Coast and distinguished by a combination of body size, egg size, genital pore position and acetabulum morphology. D. fischthalicus Meskal, 1970 was not found during the present study.     

The morphology and distribution of Xenopus vestitus (Anura: Pipidae) in Central Africa

Studies on the collections of the Musée Royal de l'Afrique Centrale and the Institut Royal des Sciences Naturelles de Belgique have shown that Xenopus kigesiensis Tinsley, 1973 is synonymous with X. vestitus Laurent, 1972. Previously unrecognized representatives of the species have been found in the museum collections and these considerably enlarge the known geographical distribution of X. vestitus. Samples have been recorded principally from highland areas adjacent to the western Rift in Zaire, Uganda and Ruanda; comprehensive morphological analysis demonstrates relatively limited intraspecific variation throughout the known range.
The distribution of X. vestitus overlaps with that of three other Xenopus taxa: X. laevis victorianus, X. bunyoniensis and a new Xenopus species. There is evidence from a large number of collections from widely separated localities that X. vestitus is commonly sympatric with X. l. victorianus. On the other hand, it is postulated that X. vestitus has recently invaded Lake Mutanda, Uganda, and has replaced the original X. bunyoniensis population during the past 30 years. The museum record provides a potential origin of this invasion: X. vestitus populations are well documented at Rutshuru, only 25 km downstream from Lake Mutanda.
A comparison of the morphological characters of X. vestitus, X. l. victorianus and X. bunyoniensis is presented to provide information on the systematic relationships and possible ecological adaptations of the overlapping species.     

The biology and systematics of a new species of Xenopus (Anura: Pipidae) from the highlands of Central Africa

A new species of clawed toad, Xenopus wittei sp. nov. is described from highland areas overlapping the borders of Rwanda, Uganda and Zaire. A comprehensive morphological analysis of the species has been carried out. X. wittei sp. nov. is very closely related to X. vestitus and both have a tetraploid chromosome complement (2 n = 72), the only examples within the genus. Nevertheless, the two species may be distinguished by easily observed features, including head morphology and dorsal body colours and patterns, and are also identified by their karyotypes and mating calls.
Data on distribution, ecology and parasite infection are derived from both museum collections and fieldwork in Central Africa. X. wittei sp. nov. is sympatric in parts of its range with X. vestitus, X. laevis victorianus and X. I. bunyoniensis , with up to three of the taxa sharing the same habitats. The implications of potential species interactions are discussed. Experimental hybridization has confirmed the reproductive isolation of the two tetraploid species. Parasitological studies show that they may be distinguished from one another by host specific species of the nematode Chitwoodchabaudia and from other Xenopus species by the absence of the tapeworm genus Cephalochlamys . Large scale population movements of the Xenopus species have apparently occurred in the Central African highlands during the period of available records. Sympatry observed amongst the present day representatives of Xenopus may have been an important factor in the evolution of these toads since X. wittei sp. nov. and X. vestitus are interpreted as allopolyploids, the products of interspecies hybridization.     

The genusOligolecithus Vercammen-Grandjean (Digenea: Telorchiidae) fromXenopus spp. (Anura: Pipidae), with a description ofO. siluranae n. sp. fromX. tropicalis (Gray) in Ghana

The taxonomy, host range and geographical distribution ofOligolecithus Vercammen-Grandjean, 1960, a genus of telorchiid digeneans fromXenopus spp. in Africa, is reviewed.O. jonkershoekensis Pritchard, 1964 is established as a junior synonym ofO. elianae Vercammen-Grandjean, 1960, andO. siluranae n. sp. is described fromX. tropicalis in Ghana. These two species are differentiated by variations in body length, testicular arrangement and ventral sucker width in proportion to body size.O. elianae occurs inX. laevis laevis from South Africa and Zimbabwe (new locality record),X. l. poweri from Zaire,X. l. victorianus from Zaire, Uganda and Rwanda (new locality record),X. l. bunyoniensis (new host record) from Uganda andX. l. sudanensis from Sudan (new host and locality record). It is also found inX. wittei from Uganda and Zaire andX. vestitus (new host record) from Uganda. The host ofO. siluranae belongs to a separate species group within the genusXenopus from the hosts ofO. elianae; this tropicalis group is phylogenetically isolated, but it also occurs in a different biotype, lowland tropical rain forest, ecologically distinct from the other known host species.     

Heterogeneous interspecific interactions in a host-parasite system

Macroparasites of vertebrates usually occur in multi-species communities, producing infections whose outcome in individual hosts or host populations may depend on the dynamics of interactions amongst the different component species. Within a single co-infection, competition can occur between conspecific and heterospecific parasite individuals, either directly or via the host's physiological and immune responses. We studied a natural single-host, multi-parasite model infection system (polystomes in the anuran Xenopus laevis victorianus) in which the parasite species show total interspecific competitive exclusion as adults in host individuals. Multi-species infection experiments indicated that competitive outcomes were dependent on infection species composition and strongly influenced by the intraspecific genetic identity of the interacting organisms. Our results also demonstrate the special importance of temporal heterogeneity (the sequence of infection by different species) in competition and co-existence between parasite species and predict that developmental plasticity in inferior competitors, and the induction of species-specific host resistance, will partition the within-host-individual habitat over time. We emphasise that such local (within-host) context-dependent processes are likely to be a fundamental determinant of population dynamics in multi-species parasite assemblages.     

A revision of the family Dissonidae Kurtz, 1924 (Copepoda: Siphonostomatoida)

Two new species of the parasitic copepod genus Dissonus Wilson, 1906 are described: D. excavatus n. sp. from the gills of a labrid, Bodianus perditio, and a lutjanid, Macolor niger, collected off New Caledonia and Taiwan, and D. inaequalis n. sp. from a hemiscylliid elasmobranch, Chiloscyllium punctatum, collected off Sarawak (Malaysia) and the Philippines. Material of D. heronensis Kabata, 1966 is described from a balistid host, Pseudobalistes fuscus, off New Caledonia, and this constitutes a new host record for this parasite. D. manteri Kabata, 1966 was collected from four serranid host species off New Caledonia and from one of the same hosts off Taiwan. Two of the hosts from New Caledonia, Plectropomus laevis and Epinephelus cyanopodus, represent new host records. D. pastinum Deets & Dojiri, 1990 was recognised as a new synonym of D. nudiventris Kabata, 1966, so the total number of valid species is now twelve. Material from museum collections of D. nudiventris, D. similis Kabata, 1966 and D. spinifer Wilson, 1906 was re-examined and provided new information which is utilised in a key to all valid species of Dissonus.     

The mating calls of 12 species and sub-species of the genus Xenopus (Amphibia : Anura)

The principal acoustic characteristics of the mating calls of 12 species and sub-species of the genus Xenopus were determined: X. laevis laevis, X. laevis petersi, X. laevis victorianus, X. gilli, X. muelleri, X. borealis, X. clivii, X. fraseri, X. ruwenzoriensis, X. wittei, X. vestitus and X. tropicalis . These calls are very specific, especially among species whose hybrids are particularly viable in the laboratory. Twin species have not been discovered. Most of the species emit calls with high harmonic frequencies of 16 kHz; these frequencies reach 80 and 150 kHz in X. I. laevis and X. ruwenzoriensis , the ultra-sound level. This was a previously unknown phenomenon in the Batrachians.     

Evolutionary relationships,host range and geographical distribution of Camallanus Railliet & Henry, 1915 species (Nematoda: Camallaninae) from clawed toads of the genus Xenopus (Anura: Pipidae)

Representatives of the genus Camallanus Railliet & Henry, 1915 occur mainly in teleost fishes, although a significant number of species have also been recorded from anuran amphibians. The taxonomy, host range, geographical distribution and phylogenetic relationships of Camallanus spp. from African clawed toads (Xenopus spp.) are reviewed. Besides C. kaapstaadi Southwell & Kirshner, 1937, which shows a widespread distribution in sub-Saharan Africa and occurs in X. laevis subspecies, X. wittei, X. fraseri-like toads, X. borealis and X. muelleri, three new species were found: C. siluranae n. sp. from X. tropicalis in west Africa, C. macrocephalus n. sp. from X. borealis in Kenya, and C. xenopodis n. sp. from X. laevis laevis in South Africa and X. borealis in Kenya. C. johni Yeh, 1960 described from Xenopus sp. in Tanzania is considered a species inquirenda. C. kaapstaadi and C. macrocephalus are very closely related and both occur in the oesophagus of their hosts, unlike other Camallanus spp. which are found in the intestine or more rarely the stomach. Some of the unusual morphological features of these species may be an adaptation to attachment in the oesophagus. The host of C. siluranae, X. tropicalis, belongs to a separate species group (as has been established by recent molecular and cytological studies) to those of C. kaapstaadi, C. macrocephalus and C. xenopodis. Morphological affinities suggest that Camallanus spp. from clawed toads are not monophyletic with those from other amphibians and that C. siluranae is distantly related to, and probably not monophyletic with the remaining species from clawed toads. The Camallanus fauna of Xenopus spp. may thus be derived from at least two independent colonisations, of different host clades, by parasite lineages occurring in teleost fishes.     

A redescription of Chabaudus leberrei (Bain & Philippon, 1969) (Nematoda: Seuratoidea) from Xenopus spp. in Swaziland

Chabaudus leberrei (Bain & Philippon, 1969) is redescribed from the pipid anurans Xenopus muelleri (Peters) and X. laevis laevis (Daudin) (new host records) in northern Swaziland, based on light and scanning electron microscope studies. The six anterior protuberances characteristic of the genus Chabaudus Inglis & Ogden, 1965, are, in C. leberrei, formed by bipartite lamellae associated with the internal margins of the three lips. Intraspecific variation in the number and disposition of male caudal papillae and in the development of the cephalic vesicle is documented.     

Parasite infectivity to hybridising host species: a link between hybrid resistance and allopolyploid speciation?

Variation in host-specific infectivity was studied in monogenean polystome parasites (Protopolystoma spp.) of the interfertile, parapatric anurans Xenopus laevis laevis and Xenopus muelleri. Laboratory-raised host F1 hybrids were resistant to parasites respectively specific to each parent taxon in nature. This resistance occurred against parasite isolates from both inside and outside a host hybrid/sympatric zone (and no isolate was compatible with the foreign host species under experimental conditions). Geographical Protopolystoma xenopodis isolates showed variable infectivity to a single full-sib group of their usual host, X. l. laevis, and strains with high or low infectivity to these sibs co-occurred in spatially distant local areas (separated by 1,700 km). The host compatibility of P. xenopodis was also subject to host genotypexparasite genotype interactions. Refractoriness to some parasites or pathogens, as a consequence of hybridisation, may have conferred a selective advantage on the allopolyploid pathway by which most Xenopus spp. are believed to have evolved.     

Studies on the ecology and systematics of a new species of clawed toad, the genus Xenopus, from western Uganda

A new species of African clawed toad, Xenopus kigesiensis sp.n., has been recorded from Lake Mutanda in South-west Kigezi, Uganda. Information on the biology and morphological variation of the species has been compiled from a sample preserved in the field and from a collection of living specimens maintained in the laboratory. An analysis of the characters of the species has been conducted and its relationships with the six known species of the genus assessed.
In the early part of this century the clawed toad populations occupying the lakes of South-West Kigezi exclusively comprised a Xenopus laevis subspecies. It appears that X. kigesiensis sp.n. has recently migrated into Lake Mutanda and has replaced the previous form. Hybridization experiments have demonstrated the reproductive isolation of X. kigesiensis sp.n. from two subspecies of X. laevis. There is evidence of a change in the ecology of the lakes during the period of apparent invasion.
The locality of X. kigesiensis sp.n. lies close to one of the main watersheds of Central Africa; taxonomically the toad forms an important link in the series of known Xenopus species.     

Comparative molecular phylogeography of two Xenopus species, X. gilli and X. laevis, in the south-western Cape Province, South Africa

Xenopus gilli is a vulnerable anuran with a patchy distribution along the south-western coast of the Cape Province, South Africa. This species is sympatric with Xenopus laevis laevis , a widespread relative found over much of southern Africa. We examined the molecular phylogeography and population structure of the contact zone between these species to obtain information about historical biogeography and conservation management of this region. Analyses of the distribution, frequency, and cladistic and phenetic relationships among mitochondrial DNA haplotypes indicate that population subdivision is present in both taxa but that long-term isolation of sets of populations has occurred in X. gilli only. Haplotype and nucleotide diversity are also considerably higher within and among X. gilli ponds than X. l. laevis ponds in this region. We attribute the genetic segregation of X. gilli populations to ancient habitat fragmentation by ocean transgression into X. gilli habitat and to continued habitat alteration by human activity. The lower level of genetic diversity in X. l. laevis in this region is likely a result of a recent arrival of this taxon to the south-western Cape region relative to X. gilli . Population structure in X. l. laevis may be a result of isolation by distance. Clear evidence exists for at least two management units within X. gilli and strongly supports the establishment of protective measures east of False Bay in order to conserve a substantial portion of this species' extant genetic diversity.     

Host factors limiting monogenean infections: a case study

Comprehensive field data on polystomatid monogeneans record low prevalence and intensity of infection and suggest that worm burdens in this group are strongly regulated: thus, in the majority of Polystoma species infecting anuran amphibians mean abundance is typically less than one parasite/host. There is circumstantial evidence that the dominant control is attributable to host factors which over-ride variations in transmission success. This review provides a brief summary of information on Pseudodiplorchis americanus, a parasite of the desert toad, Scaphiopus couchii, and then focuses in detail on the spectrum of factors regulating infrapopulations of Protopolystoma xenopodis, a parasite of the aquatic Xenopus laevis. Infection levels of adult worms and their contribution to transmission are regulated by external environmental factors (especially temperature), by host factors (including behaviour and population density), and by a range of parasite factors including intra- and inter-specific competitive interactions and variations in intrinsic characters, especially survivorship and reproductive output. In addition to these factors whose primary effect is to modulate transmission rates, there is a major attrition in parasite numbers between invasion and maturity (3 months post-infection). Long-term laboratory experiments on the Xenopus laevis/Protopolystoma xenopodis interaction demonstrate a powerful acquired immune response. Primary infection is characterised by a high prevalence of established adult worms but the success of subsequent challenge infection is greatly reduced, leading to low prevalence and extended pre-patent period. In the small proportion of hosts supporting a second infection of adult parasites, surviving burdens are small (one to two worms/host) and show reduced egg production. These results provide an explanation for the low burdens encountered in field studies: a majority of adult X. laevis in natural populations are likely to exhibit strong, relatively long-term, post-infection immunity after the loss of a previous infection.     

Morphology and life history of Petasiger variospinosus (trematoda: echinostomatidae) in the Free State, South Africa

Specimens of the freshwater snail Bulinus tropicus (Krauss, 1848) collected in the Free State, South Africa shed cercariae with an oral collar bearing 27 spines. Tadpoles of the African clawed toad Xenopus laevis laevis Daudin, 1802 collected from the same waters harbored metacercariae with a similar collar of spines. Adults were obtained after feeding infected tadpoles to laboratory-reared reed cormorants, Phalacrocorax africanus (Gmelin, 1789). The parasite was identified as Petasiger variospinosus (Odhner, 1910), the life cycle was experimentally completed, and stages described by the use of light and scanning electron microscopy.     

Observations on the distribution, specificity and pathogenicity of the acanthocephalan Pomphorhynchus laevis (Müller)

The distribution, specificity and pathogenicity of the acanthocephalan Pomphorhynchus laevis (Müller) were studied in the River Avon, Hampshire. The parasite occurred in every fish species, but three categories of hosts could be distinguished. Using growth and maturation as criteria of specificity, only chub Leuciscus cephalus (L.) and barbel Barbus barbus (L.) were recognized as its preferred hosts. Parasites occasionally matured in trout Salmo trutta (L.) and dace Leuciscus leuciscus (L.), but none grew or matured in other host species. Changes in the abundance of P. laevis along the river were related to changes in the abundance of both the intermediate host, Gammarus pulex , and the preferred hosts, and its occurrence in fish to the importance of G. pulex in their diet. In the upper reaches of the River Avon and in other localities populations could be maintained at a low level by parasites maturing in trout, and presence and abundance at any site depended upon the presence and abundance of both intermediate and definitive hosts especially and upon the dietary preferences of the latter. The absence of P. laevis from many parts of Britain is nevertheless still inexplicable. The parasite caused local damage to the intestinal wall of fish, the extent of which varied from species to species, but did not affect host growth rate or cause direct mortality and P. laevis cannot be regarded as an important pathogen in the River Avon or any other British River.     

Chromosome complements of the genus Xenopus

The oytogenetic analysis of the genus Xenopus shows that X. laevis laevis, X. laevis petersi, X. laevis victorianus, X. (laevis) borealis, X. gilli, X. muelleri, and X. fraseri have chromosome numbers 2n=36; X. tropicalis has 20 (2n), X. (laevis) bunyoniensis 72 and X. ruwenzoriensis 108. This heterogeneity of the chromosome numbers is interesting as it represents new examples of polyploidy among Anurans. There are no big morphological differences among the karyotypes of the divers species, only the chromosomes with secondary constrictions vary considerably.     

Histopathology in Leuciscus cephalus (Pisces: Cyprinidae) resulting from infection with Pomphorhynchus laevis (Acanthocephala).

The histopathology present in Leuciscus cephalus Risso 1826 from the River Po, naturally infected with the acanthocephalan Pomphorhynchus laevis Müller 1776, was studied and described. The parasite was only found within the host's digestive tract, and a density of 10 P. laevis per square centimeter was common. On histological examination it was noticed that both male and female parasites penetrated deeply through all the layers of the host digestive tract by means of their slender neck, bulb and proboscis; thus the bulb and proboscis become encapsulated beneath the serosa and mesentery. As a result, the presence of a capsule was noticed on the external surface of the host digestive tract. P. laevis caused local damage to the intestinal wall, and the parasite elicited an intense host reaction. The response of L. cephalus at the attachment site of the acanthocephalan was hyperplasia of host connective tissue, with cellular infiltration. In a few hosts, the presence of a fibrous nodule on the outer surface of the alimentary canal was observed.     

Geographic and within-population structure in variable resistance to parasite species and strains in a vertebrate host

Host resistance to parasites and parasite infectivity may be subject to significant genetically determined variation within species. However, relatively little is known of how this variability is structured in natural vertebrate populations and their macroparasites. A laboratory experiment on host susceptibility-parasite infectivity variation in a wildlife host-parasite system (subspecies of the anuran X. laevis and their polystome flatworms), including 33 pairwise allopatric and sympatric host-parasite combinations (three parasite geographical isolates x 11 host full-sibling families, n=600), revealed a complex pattern of infection success. Results amongst host sibships from different localities suggested that infection success was subject to a highly significant locality x parasite isolate interaction. Within localities, a highly significant sibship x isolate interaction also occurred in one of two groups of sibships examined. The existence of such interactions suggests a potential for frequency-dependent, Red Queen-like selection. Interaction between locality and isolate was partly due to higher infection levels in sympatric combinations, consistent with a general pattern of host-specific adaptation. However, some allopatric combinations produced unpredictably high infection levels, resulting in very asymmetrical cross-infectivity patterns (where the reciprocal cross-infections produced negligible infection). This phylogeographically structured host-parasite system may, therefore, sometimes generate local parasite strains with high infectivity to allopatric hosts. Secondary contact between populations could thus result in significant, and unequal, transfer of parasites.     

Genetic and morphometric variation in the Holarctic helminth parasite Andrya arctica (Cestoda, Anoplocephalidae) in relation to the divergence of its lemming hosts (Dicrostonyx spp.)

Andrya arctica is a cestode parasite of the family Anoplocephalidae (Cyclophyllidea), parasitizing lemmings of the genus Dicrostonyx throughout the Holarctic region. The population structure of this intestinal parasite was studied from eight different regions, six of which represented different genetic entities of lemming hosts. Molecular sequence tagged site markers and minisatellite fingerprints as well as morphology and morphometries were used to reveal the population structure of A. arctica in the Holarctic region. The results suggest that the evolutionary history of this cestode species has included different processes acting on different geographical regions. On the Siberian mainland (host D. torquatus), the division of the parasites into different genetic entities agreed perfectly with the chromosomal races of the lemming hosts that points towards a shared evolutionary history between the host and the parasite ('cospeciation'). The main phylogenetic split of Dicrostonyx between Eurasia and North America was not, however, observed in A. arctica. This suggests that in the Nearctic (host D. groenlandicus) the parasite has remained relatively unmodified because of the large cohesive populations ('coadaptation'). The uniqueness of the Greenland population, and possibly also that of the Wrangel Island, can be explained by peripheral isolation, refugial effects or founder effects.