Incipient post-zygotic barrier in a model system of ecological speciation with gene flow

The role of post-zygotic isolation in nonallopatric ecological speciation is still mostly unknown and information on the nature and strength of these barriers in well-known speciation models is essential for a deeper understanding of such processes. The Galician ecotypes of the marine snail Littorina saxatilis represent one of the best studied cases of nonallopatric ecological speciation. Here, we test the existence of incipient post-zygotic isolation by comparing the fertility of male hybrids with that of both pure forms [ridged and banded (RB) and smooth and unbanded (SU) ecotypes]. We analysed the degree of sperm DNA fragmentation (SDF) of individuals morphologically classified as RB, SU and hybrids, sampled from two locations. SDF analyses were chosen to study sperm quality because, in other animal species, SDF rates correlate with important parameters for speciation research, such as fertilization and abortion rates and viability of adult progeny. In the present work, hybrids showed significantly higher SDF rates than RB and SU males in one location and significantly higher variances in both locations. These results suggest the existence of an incipient post-zygotic barrier, the strength of which may vary across the Galician shore, and highlight the potential of SDF analyses for speciation research.