The European sea bass (Dicentrarchus labrax) is a temperate-zone euryhaline teleost of prime importance for aquaculture and fisheries. This species is subdivided into two naturally hybridizing lineages, one inhabiting the north-eastern Atlantic Ocean and the other the Mediterranean and Black seas. Here we provide a high-quality chromosome-scale assembly of its genome that shows a high degree of synteny with the more highly derived teleosts. We find expansions of gene families specifically associated with ion and water regulation, highlighting adaptation to variation in salinity. We further generate a genome-wide variation map through RAD-sequencing of Atlantic and Mediterranean populations. We show that variation in local recombination rates strongly influences the genomic landscape of diversity within and differentiation between lineages. Comparing predictions of alternative demographic models to the joint allele-frequency spectrum indicates that genomic islands of differentiation between sea bass lineages were generated by varying rates of introgression across the genome following a period of geographical isolation.

Animal conflicts are influenced by social experience such that a previous winning experience increases the probability of winning the next agonistic interaction, whereas a previous losing experience has the opposite effect. Since androgens respond to social interactions, increasing in winners and decreasing in losers, we hypothesized that socially induced transient changes in androgen levels could be a causal mediator of winner/loser effects. To test this hypothesis, we staged fights between dyads of size-matched males of the Mozambique tilapia (Oreochromis mossambicus). After the first contest, winners were treated with the anti-androgen cyproterone acetate and losers were supplemented with 11-ketotestosterone. Two hours after the end of the first fight, two contests were staged simultaneously between the winner of the first fight and a naive male and between the loser of first fight and another naive male. The majority (88%) of control winners also won the second interaction, whereas the majority of control losers (87%) lost their second fight, thus confirming the presence of winner/loser effects in this species. As predicted, the success of anti-androgen-treated winners in the second fight decreased significantly to chance levels (44%), but the success of androgenized losers (19%) did not show a significant increase. In summary, the treatment with anti-androgen blocks the winner effect, whereas androgen administration fails to reverse the loser effect, suggesting an involvement of androgens on the winner but not on the loser effect.