Mate choice and male-male competition among morphologically cryptic but genetically divergent amphipod lineages

Mate preference for conspecifics does not necessarily lead to assortative mating in cases where mating outcomes also depend on preferences based on mate quality and on individual competitiveness. We tested how such traits affected mate choice among genetically divergent lineages (called molecular operational taxonomic units; MOTU) of the amphipod Gammarus fossarum. We presented males with two females, including one from its own MOTU. Females also potentially differed in body size, and therefore fecundity, and in time before reproduction, two traits previously recognized as important in male mate choice. Males generally preferred females from their own MOTU when females originated from highly divergent MOTUs (17 % genetic divergence), but not when they were more closely related (3.5 % genetic divergence). Contrary to expectations, they did not prefer larger females, but they consistently paired with the female closest to reproduction. A second experiment involving duos of males of different MOTUs in competition for a female also revealed that males consistently won the competition over pairing with females of their own MOTU. Overall, these results reveal a strong influence of genetic divergence on mate recognition and reproductive isolation between sympatric MOTUs. However, male preference for females that are close to being available for reproduction also potentially results in hybridization among closely related MOTUs. We examine these results in the light of field mating patterns observed in a previous study of G. fossarum and discuss the importance of considering competitiveness and preferences for mate quality signals when studying evolutionary consequences of secondary contact between divergent lineages.