Phylogenetic analysis of eleven species of Biomphalaria Preston, 1910 (Gastropoda: Planorbidae) based on comparisons of allozymes

Freshwater snails in the genus Biomphalaria transmit Schistosoma mansoni in Africa, South America and the Caribbean region. Although considerable attention has been given to the identification of species, little is known of evolutionary relationships among the species. A phylogenetic analysis of 25 populations representing 11 species was performed on 25 enzyme loci examined using starch gel electrophoresis. A phylogenetic analysis of the individual populations produced 60 trees of equal length. The 60 trees have a consistency index value of 75.9% and a retention index value of 76.5%. The phylogenetic analysis provided strong support for the monophyly of Biomphalaria with either 14 or 15 synapomorphies uniting all of the species included and separating them from the outgroup, two species of Helisoma. Four nominal species represented by multiple populations formed monophyletic groups. Populations of B. sudanica, B. choanomphala, and B. alexandrina were interspersed. Ten arrangements were obtained for the populations of these three species. A variety of ingroup taxa were used to root the trees, and all provided support for the use of Helisoma species as an outgroup. In all of the trees obtained, the African species together formed a monophyletic group. In none of the trees obtained did the neotropical species form a monophyletic group. A constrained analysis requiring the monophyly of the neotropical species as well as the African species resulted in 90 trees just two steps longer than the shortest trees. Analysis of the species from either hemisphere alone resulted in decreased resolution, as measured by an increase in the number of trees obtained. This finding suggests that further comparisons of species from the two hemispheres will be of considerable value. Finally, two species which are resistant to infection with S. mansoni were included among the eleven studied. Neither of these species formed the sister group to all of the other species included, indicating that susceptibility is the plesiomorphic state, and that resistance is derived. Similarly, in none of the trees obtained did the two resistant species fall out as sister taxa, indicating that resistance arose independently twice.     

Bank voles in linear habitats show restricted gene flow as revealed by mitochondrial DNA (mtDNA)

Genetic structure of bank vole populations in linear river bank habitat in southeast Norway was determined from analyses of DNA sequences for the mitochondrial D-loop. Animals were sampled at sites separated by 1 km, along two forested river banks separated by ≈ 100 m of open water. Twenty-six distinct haplotypes were found among 120 voles. The voles showed significant deviation from panmixis on both sides of the river. Animals from the same site or from sites 1 km apart were more likely to share haplotypes than animals 2 km apart or more. Common haplotypes were widespread on both river banks, and had a wider distribution than relatively rare haplotypes. Some rare haplotypes were found on both banks, but most were restricted to a single bank. The results suggest that short-term gene flow may be restricted for female bank voles in linear habitats. Female territorial behaviour may vary with habitat geometry. In the linear habitat described here, females defend only two territorial borders and may effectively limit female dispersal. Results were compared to a previous study of bank voles from this region in a two-dimensional habitat. Gene flow in the linear habitat was much more restricted than gene flow in the two-dimensional habitat. Probable mechanisms underlying this difference are discussed.     

Biogeography in the Death Valley region: evidence from springsnails (Hydrobiidae: Tryonia)

Allozyme and mitochondrial DNA variation were analysed to examine evolution of the nine species of springsnails (genus Tryonia) living in the Death Valley system (Owens and Amargosa basins) of southeastern California and southwestern Nevada. Both allozyme and mtDNA evidence indicate that this highly endemic fauna is non-monophyletic. Species from the upper Amargosa basin comprise a clade most closely related to snails living in the Colorado basin. Snails from the lower Amargosa basin (Death Valley trough) reflect a complex evolutionary history and two of these species are more closely related to an estuarine species from western California than to other snails of the region. These results indicate a commonality of pattern with the well-studied Death Valley pupfishes (Cyprinodon), which also are non-monophyletic and include species that are most closely related to Colorado basin congeners. These biogeographic patterns are interpreted within the context of a recently proposed model for the early history of the lower Colorado River.     

Population structure and taxonomic discrimination among three species of Biomphalaria Preston, 1910 (Gastropoda: Planorbidae) from Kenya

The genetic basis for species designations for African species of Biomphalaria has received little attention. Populations of B. choanomphala, B. sudanica and B. pfeifferi from Kenya were examined using starch gel electrophoresis. Analysis of population structure as measured using Wright's F -statistics for B. sudanica and B. choanomphala revealed that both species show substantial departures from expectations under random mating. Populations of B. choanomphala show the least evidence of inbreeding, and populations of A sudanica are intermediate between B. choanomphala and previously reported levels of inbreeding for B. pfeifferi. Syn topic populations of B. pfeifferi and B. sudanica were genetically distinct. Syntopic populations of B. choanomphala and B. sudanica also showed evidence of separate gene pools, although the evidence is not as conclusive as for B. pfeifferi and B. sudanica. Possible explanations for the failure to find clear differences between B. sudanica and B. choanomphala are considered.     

GENETIC STRUCTURE OF KENYAN POPULATIONS OF BIOMPHALARIA PFEIFFERI (GASTROPODA: PLANORBIDAE)

Biomphalaria Pfeifferi is widespread in Africa, tolerates adiverse array of habitats, and is highly susceptible to schistosomeinfection. As such, it is the most important host of Schistosomamansoni in the Old World. Representatives of Biomphalaria pfeifferifrom twelve localities in southern Kenya were examined usingstarch gel electrophoresis in order to describe the populationstructure. Ten to fifteen loci were resolved in each population.Genotypic frequencies obtained for 10 loci have been used tocalculate Nei' genetic distances. Nine eastern populations wererelatively similar to each other (D 0.016), but divergent fromwestern populations (D 0.178). Two of three western populationswere quite similar (D = 0.001), the third being more divergent(D 0.258). Genotypic frequencies showed a substantial departurefrom Hardy-Weinberg equilibrium, due to a marked deficiencyof heterozygotes. Calculation of F-statistics revealed evidenceof non-random mating as well as considerable differentiationamong localities. Biomphalaria pfeifferi differs from otherBiomphalaria species studied to date in exhibiting evidenceof non-random mating. Higher levels of inbreeding could alterthe pattern of response to selection imposed by parasites suchas Schistosoma mansoni. (Received 18 May 1989; accepted 30 October 1989)     

GENETIC AND CONCHOLOGICAL COMPARISON OF SNAILS (HELIX ASPERSA) DIFFERING IN SHELL DEPOSITION OF LEAD

Populations of snails inhabiting areas with different historiesof Pb contamination differed in their deposition of Pb in shellrelative to soft tissues. Genetic variation, measured usingisozymes, was not related to Pb history nor geographic distancebetween populations. Shell characteristics were significantlydifferent among sites; shell dry weight was strongly relatedto soil calcium levels. Shells of snails from areas with longhistories of Pb contamination were significantly more robust(greater shell width/shell height ratio) than snails from otherlocations. H. asprsa adaptation to Pb contamination may involvesignificant changes in shell characteristics but these do notcorrelate with genetic traits assessed with allozymes (Received 29 December 1994; accepted 15 October 1995)