Genetic variation in sympatric and allopatric populations of hybridizing freshwater snail species (Viviparus ater and V. Contectus)

Abstract

To estimate the geographical extent of introgression, we studied the genetic structure of sympatric and allopatric populations of hybridizing freshwater snail species Viviparus ater and V. contectus in central Europe. Six allozyme loci which were variable in Lake Garda, Italy in a previous study (five nearly diagnostic loci between the two species and one highly polymorphic locus in V. contectus) were analyzed from ten sympatric locations and four allopatric populations each for the two species. Presumably introgressed genes (low allele frequencies) were found from at least one locus in seven out of the ten sympatric sites. These seven sites covered most of northern Italy. The data indicate that introgression has occurred from Viviparus contectus to V. ater and vice versa. Therefore, there is a possibility of widespread introgression or mosaic zones in nature. However, we cannot rule out that the observed patterns are due to the shared ancestry. V. ater possessed low genetic variation (the jackknifed mean of Wright's F(SI) ± S.E. over four loci was 0.041 ± 0.004). On the other hand, V. contectus showed high genetic differentiation (the jackknifed mean of F(ST) ± S.E. over six loci was 0.546 ± 0.166). Although introgression may have caused evolutionary, changes in V. ater and V. contectus, it was not strong enough to level out the genetic differences between the two species, which may have originated from isolation among populations in V. contectus and a past bottleneck event in V. ater.