Frequency of interspecific matings and of hybrid offspring in sympatric populations of Vtviparus ater and V. contectus (Mollusca: Prosobranchia)

In species pairs able to produce fertile hybrids, the amount of introgression can be limited by the frequency of interspecific copulations. We investigated the frequency of interspecific copulations between Kviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assortative mating, the frequency of interspecific copulations was substantial. At sites where V. contectus was rare, 5.9% of all matings, and up to 73% of matings in which V. contectus was involved, were interspecific. At sites where both species were abundant 14.8% of all matings were interspecific. Male-male matings occurred in approximately 3% of copulations. In 1993, 12 out of 325 V. ater females collected from 10 sympatric populations and held in isolation for two to five months gave birth to 25 hybrid offspring, out of a total of 2548 offspring produced by all females. The proportion of hybrid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since interspecific copulations are frequent and hybrid offspring are produced in nature, we conclude that assortative mating is not an effective isolating mechanism between V. ater and V. contectus.