INTRASPECIFIC AND INTERSPECIFIC PATTERNS OF ALLOZYME VARIATION AMONG SPECIES OF BIOMPHALARIA PRESTON, 1910 (GASTROPODA, PLANORBIDAE)

Freshwater snails in the genus Biomphalaria transmit Schistosoma manso ni, a causative agent of human schistosomiasis in Africa and the Neotr opical region. Twenty-five populations representing 11 species of Biom phalaria from both Africa and the Neotropical region were examined usi ng starch gel electrophoresis for 22 loci. The percentage of polymorph ic loci observed per population varied from 4.5 to 63.6%, with a mean of 28.7%; observed heterozygosity levels varied from 0.002 to 0.126, w ith an average of 0.052. Observed heterozygosity and polymorphism valu es fell within the range of values reported in previous studies of the genus, but observed heterozygosity in most cases was less than expect ed under Hardy-Weinberg equilibrium. The genetic variance was partitio ned among species, populations and individuals using hierarchical G-st atistics. For all of the populations considered together, the variance among species was 58.4%, among populations within species 24.9%, and among individuals within a population 16.7%. When only the Neotropical species were considered, the percentages of the total variance attrib utable to variation among species, among populations within species, a nd among individuals were 18.1%, 29.2% and 52.7%, respectively. For th e African species, 27.8% of the variance was among species, 42.3% amon g populations, and 29.9% among individuals. Intraspecific Roger's gene tic distances ranged from 0.01 to 0.38, and interspecific distances fr om 0.1 to 0.84. Nei's unbiased distances ranged from 0.00 to 0.44 with in a nominal species, and from 0.02 to 1.82 between species. Non-metri c multidimensional scaling of Rogers' distances revealed a clear separ ation of African species from Neotropical species and of the Neotropic al species from each other. The African species formed overlapping clu sters.