Population genetic perturbations of intermediate hosts, often a consequence of human pressure on environmental resources, can precipitate unexpectedly severe disease outbreaks. Such disturbances are set to become increasingly common following range changes concomitant with climate shifts, dwindling natural resources and major infrastructure changes such as hydroprojects. Construction of the Diama dam in the Senegal River Basin (SRB) reduced river salinity, enabling the freshwater snail intermediate host Biomphalaria pfeifferi to rapidly expand its distribution. A serious public health problem ensued, with an epidemic of intestinal schistosomiasis occurring in the previously schistosome-free Richard-Toll region within 2 years. The current study aimed to assess the population variability of B. pfeifferi in the SRB, and speculate upon its subsequent impact on host-parasite interactions following such engineered ecological change. Genetic variation at nine polymorphic microsatellite loci revealed little population differentiation in SRB snails compared with those from natural habitats in Zimbabwe, where Schistosoma mansoni transmission is much lower. 'Open' SRB habitats are associated with greater water contact, smaller population sizes and less genetic diversity, with sites downstream of Richard-Toll showing greater inter- and intrapopulation variation, concomitant with less frequent human contact. These observations may be explained by rapid expansion into pristine habitat selecting for high fecundity genotypes at the expense of schistosome resistance, presenting S. mansoni with genetically homogenous highly fecund susceptible populations around the focal point, promoting development of a highly compatible host-parasite relationship. Longitudinal study of such systems may prove important in predicting public health risks engendered by future environmental engineering projects.

The striped-mouse, Rhabdomys pumi/io, is widely distributed throughout southern Africa. It prefers grasslands but is also found in vlei areas and dry river beds and its attraction to cultivated land has resulted in extensive damage to plants. An allozyme study of R. pumi/io populations in different regions of southern Africa was undertaken to evaluate the genetic structure wilhin and between 23 populations and to draw conclusions about the taxonomic status of populations within this species. Fifteen of the 26 loci examined were polymorphic. The mean heterozygosity (0.073) was high for a mammal, although relatively low heterozygosities (0.036-0.054) were recorded for three localities from the peninsular region of the Western Cape. The high mean value for local genetic differentiation (F 51) of 0.459 and the low mean value for the effective number of migrants (Nm) of 0.179 indicated low levels of gene flow between the different localities of R. pumilio. The negative, near zero F, 5 value of-0.01 indicated a balance between heterozygotes and homozygotes. Rogers (1972) genetic similarity ranged between 0.796 and 0.988, and Nei's (1978) unbiased genetic distance varied between 0.000 and 0.189 between the samples of R. pumilio. The phenogram based on Nei's (1978) unbiased genetic distance showed some degree of geographical subgrouping. The Mantel test indicated a Significant relationship between the F. 1 values and the geographical distances between sample pairs, supporting an isolation by distance model for R. pumilio. Although the genetic evidence for geographical divergence does suggest the possible existence of subspecies, this remains to be substantiated.

Results of chromosome G- and C-banding studies on 12 specimens of the ice rat, Otomys sloggetti robertsi, from three localities and allozyme analyses of seven individuals from two localities are reported. The subspecies karyotype contains 42, mostly acrocentric, chromosomes. No chromosomal variation was detected among animals from these localities. Allozyme analysis tentatively suggested a limited gene flow between the two populations examined.

This study examines inter-population variation in the subspecies of water rat Dasymys incomtus in eastern and extreme southwestern South Africa. Dasymys incomtus is widespread but its distribution is limited to water sources. Two techniques, protein electrophoresis and traditional cranial morphometrics, were used to investigate South African D. /. incomtus populations with known differences in chromosome numbers (2n = 38 from the KwaZulu-Natal and 2n = 46 from the Northern Province). Both electrophoretic and morphometric data confirmed the cytogenetic results, with specimens from the Northern Province forming one group and specimens from the KwaZulu-Natal forming another. A high F CT value (0.424), low Nm value (0.068) and the presence of three fixed alleles (Aspartate Aminotransferase-1, Glycerol-3-phosphate and Purine-nucleoside) across the populations indicated the genetic separation of the KwaZulu-Natal and Northern Province localities. Morphological separation between the populations occurred in five cranial characters: condylobasal length, palatal length, greatest mandible length, incisor to condyle length and ZAE, as evidenced by their high F-values and their high loadings on the first principal component. Carson's population flush theory, Wright's isolation by distance theory and the role of paleoclimates in conjunction with habitat constraints on population movement were considered as possible explanations for the patterns seen in this study.Resume. -Cette e*tude a permis d'examiner des variations entre populations du rat d'eau Dasymys incomtus. Les individus appartenant a cette espece sont tres ropandus en Afrique mais ont une ^partition qui se limite aux zones humides. Deux techniques, Felectrophorese des proteines et la morphomotrie classique cränienne ont £te utilises pour examiner d'eVentuelles differences entre des specimens pr&entant des nombres chromosomiques distincts selon Gordon (1991) (2n = 38 pour la sous-espece du KwaZulu-Natal et 2n = 46 pour celle de la Northern Province). D'une maniere g£n6rale, les donne*es d'ölectrophorese et de morphometrie confirment les r£sultats cytt^notiques, les specimens de la Northern Province formant un groupe, ceux du KwaZulu-Natal un autre. Une valeur F^ haute (0.424), une valeur Nm basse (0.068) et la prosence de trois alleles fixes (AAT -1, GDP et PNP) dans les populations ont montro une sopara- Mammalia, t. 66, n° 3, 2002 : 381-404. Brought to you by | University of Georgia Libraries Authenticated Download Date | 5/30/15 10:35 PM 382 MAMMALIA tion genetique des localites du KwaZulu-Natal et de la Northern Province. Au niveau morphologique, ces deux populations se distinguent par cinq caracteres craniens: CBL, PAL, GML, ITC et ZAE, grace ä leurs hautes valeurs de F et leurs poids £leves sur le premier axe de Fanalyse en composantes principales. La throne de "flush" de population de Carson, la theorie d'isolement par la distance de Wright et le role des paleoclimats en liaison avec les contraintes d'habitat sur les mouvements de population, ont ete consi...