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

In species pairs able to produce fertile hybrids, the amount of introgressi on can be limited by the frequency of interspecific copulations. We investi gated the frequency of interspecific copulations between Viviparus ater and V. contectus and the proportion of hybrid offspring of V. ater females in natural populations in northern Italy. Although there was significant assor tative 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 Ii contectus was involved, were interspecific. At sites w here both species were abundant 14.8% of all matings were interspecific. Ma le-male matings occurred in approximately 3% of copulations. In 1993, 12 ou t of 325 V. ater females collected from 10 sympatric populations and held i n 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 hyb rid offspring (ranging from 0 to 2.3% depending on the site) was positively related to the proportion of V. contectus in the population. Since intersp ecific copulations are frequent and hybrid offspring are produced in nature , we conclude that assortative mating is not an effective isolating mechani sm between V. ater and V. contectus. (C) 2000 The Linnean Society of London .