Population genetics of Viviparus (Mollusca: Prosobranchia): Homogeneity of V. ater and apparent introgression into v. contectus

Abstract

In Lake Garda at Lazise, Italy, the sympatric snails Viviparus ater and V contectus intermate promiscuously; controlled crosses have previously shown that many F1 offspring are viable and fertile and that backcross offspring are viable at least to the age of maturity. Here we report the results of an allozyme study of these taxa documenting introgression. Viviparus ater at Lake Garda is fixed for single alleles at the 10 loci we studied and comparison with V. ater populations in Switzerland confirms this genetic homogeneity. We estimate the neighbourhood size of V ater in Lake Garda as Nc ≈ 93400 using published demographic data; at equilibrium this would support a mean number of 1.37 detectable alleles/locus. V. ater is thus not at equilibrium and we suggest it has expanded its range significantly within the last several thousand years. In contrast, V contectus maintains normal levels of genetic variability despite the fact that V ater is more dense than V contectus by a ratio of about 14:1. We detected six loci differing between V ater and V. contectus and three of these show frequencies consistent with introgression from V. ater into V. contectus. The levels of introgression we found are consistent with information about the population densities and intermating rates at Lake Garda and with the fitnesses of hybrids bred under controlled conditions. Ultimately, if competitive exclusion does not take place, all the neutral and deleterious alleles in the V. contectus population should be replaced by V. ater alleles. © The Genetical Society of Great Britain.