Lack of early laboratory postzygotic reproductive isolation between two ecotypes of Littorina saxatilis (Mollusca, Gastropoda) showing strong premating sexual isolation

Abstract

Two sympatric and divergent adaptive ecotypes of Littorina saxatilis (RB and SU) are known to hybridize showing partial premating isolation in the wild. Previous studies have revealed that morphological intermediate forms (presumably hybrids) present fitness (viability, sexual selection and fecundity) similar to that from pure ecotypes at the mid-shore. However, the absence of postzygotic isolation due to genetic incompatibility cannot be ruled out unless it is measured directly on true F ₁ hybrids. In this study, we overcome this problem and present data on 56 individual crosses including the four possible mating combinations (RB/RB, RB/SU, SU/RB and SU/SU) to compare fertilization and fecundity rates (including young progeny viability) between the four type crosses. Pooled RB female crosses showed apparently larger fertility and fecundity than pooled SU female crosses, probably because of differences in fecundity and laboratory survivorship between ecotypes. However, similar fertilization and fecundity rates were found for both RB and SU females when mated with different male types, supporting the idea that genetic-incompatibility-based postzygotic isolation can be ignored as a major determinant of this polymorphism in nature.