Selection footprint at the first intron of the Prl gene in natural populations of the flathead mullet (Mugil cephalus, L. 1758)

The knowledge about mechanisms of adaptive evolution in highly dispersive species such as marine fishes largely improved over the past years. However, studies reporting consistent phenotype–genotype relationship that aim to understand eco-physiological performance of fish in natural environments still concerned only few species and few candidate genes. In order to understand how salinity may shape genetic variation in fish, we analysed six anonymous microsatellite loci and the first intron of the candidate prolactin-A (Prl) gene in environmentally-contrasted populations of the euryhaline flathead mullet (Mugil cephalus; Mugilidae). Prolactin is commonly reported as “the” freshwater adaptive hormone in marine fishes, and differentiation was expected among populations from marine, estuarine, and lagoon environments and individuals transferred from marine to freshwater conditions (reservoir) after their first year. Indeed, no genetic structure was found at microsatellite loci, consistent with the dispersion ability of this species. In contrast, significant genetic differentiation was observed at locus Prl (θ̂ = 0.022, P = 0.013). “Neutrality tests” reported significant support (P < 0.01) for putative selection at locus Prl when comparing allele frequency distributions of the translocated freshwater population to other populations; the shortest Prl allele was found more frequent in the reservoir than in other populations. To further investigate this selective process, correlation among individual otolith growth rates of reservoir's fish and Prl genotypes was investigated. While just below the conventional level of significance, individuals homozygous for the shortest Prl allele presented lower then higher growth rates during their first (P = 0.060) and second (P = 0.087) years than other genotypes, respectively. Results suggest that local selective advantage for growth, and differential energy costs are associated to Prl's genotypes depending on the fish habitat.