Résumé
The genetic basis and evolutionary implications of local adaptation in
high gene flow marine organisms are still poorly understood. In several
Mediterranean fish species, alternative migration patterns exist between
individuals entering coastal lagoons that offer favorable conditions for
growth and those staying in the sea where environmental conditions are
less subject to rapid and stressful change. Whether these coexisting
strategies are phenotypically plastic or include a role for local
adaptation through differential survival needs to be determined. Here, we
explore the genetic basis of alternate habitat use in western
Mediterranean populations of the gilthead sea bream (Sparus aurata).
Samples from lagoonal and open sea habitats were typed for 3 candidate
gene microsatellite loci, 7 anonymous microsatellites and 44 AFLP markers
to test for genotype-environment associations. While anonymous markers
globally indicated high levels of gene flow across geographic locations
and habitats, non-neutral differentiation patterns correlated with habitat
type were found at two candidate microsatellite loci located in the
promoter region of the Growth hormone and Prolactin genes. Further
analysis of these two genes revealed that a mechanism based on habitat
choice alone could not explain the distribution of genotype frequencies at
a regional scale, thus implying a role for differential survival between
habitats. We also found an association between allele size and habitat
type, which, in the light of previous studies, suggests that polymorphisms
in the proximal promoter region could influence gene expression by
modulating transcription factor binding, thus providing a potential
explanatory link between genotype and growth phenotype in nature.