Prezygotic and Postzygotic Reproductive Incompatibilities Complement Each Other in the Formation of a Cryptic Amphipod Species: The Example of a Lake Baikal Species Complex Eulimnogammarus verrucosus

Reproductive barriers to gene flow play a key role in speciation. However, as it is not always feasible to study them directly, most studies rely on genetic divergence to infer species delimitation. In order to correlate genetic distances to reproductive incompatibilities, compact groups of closely related species are needed. In this work, we explored a species complex of Baikal amphipods (Crustacea: Amphipoda: Gammaroidea), Eulimnogammarus verrucosus. Three biological species (W, S, and E), geographically isolated in Baikal, had been found to have interspecific differences exceeding the patristic distance threshold of 0.16, and a postzygotic incompatibility had been confirmed for the closest pair, W and S. Here, we expanded our knowledge on geographical distribution of the species, discovering that secondary contact between the W and S species already occurs in natural conditions near the source of the Angara River. Our experiments have shown that the three species within the E. verrucosus species complex are separated by both prezygotic and postzygotic barriers. While neither of these barriers is absolute, their combination can ensure reproductive isolation upon secondary contact of the species. The experimental system we have developed in this and previous works can provide support for testing species delimitation hypotheses based on sequencing data and further extend these results to related species for which such experiments are unfeasible.