Metapopulation patterns of additive and nonadditive genetic variance in the sea bass (Dicentrarchus labrax)

Abstract

Describing and explaining the geographic within‐species variation in phenotypes (“phenogeography”) in the sea over a species distribution range is central to our understanding of a variety of eco‐evolutionary topics. However, phenogeographic studies that have a large potential to investigate adaptive variation are overcome by phylogeographic studies, still mainly focusing on neutral markers. How genotypic and phenotypic data could covary over large geographic scales remains poorly understood in marine species. We crossed 75 noninbred sires (five origins) and 26 dams (two origins; each side of a hybrid zone) in a factorial diallel cross in order to investigate geographic variation for early survival and sex ratio in the metapopulation of the European sea bass (Dicentrarchus labrax), a highly prized marine fish species. Full‐sib families (N = 1,950) were produced and reared in a common environment. Parentage assignment of 7,200 individuals was performed with seven microsatellite markers. Generalized linear models showed significant additive effects for both traits and pleiotropy between traits. A significant nonadditive genetic effect was detected. Different expression of traits and distinct relative performances were found for reciprocal crosses involving populations located on each side of the main hybrid zone located at the Almeria‐Oran front, illustrating asymmetric reproductive isolation. The poor fitness performance observed for the Western Mediterranean population of sea bass is discussed as it represents the main source of seed hatchery production, but also because it potentially illustrates nonadaptive introgression and maladaptation.

Population-specific variations of the genetic architecture of sex determination in wild European sea bass Dicentrarchus labrax L.

“…The percentage of females was slightly higher in Atlantic/West-Med populations compared to Eastern Mediterranean populations, suggesting a possible different tendency in sex-ratio related to the origin of the individuals under aquaculture conditions. This is consistent with the study by Guinand et al (2017), where the ♀WEM×♂Atlantic and ♀WEM×♂WEM crosses showed a higher mean proportion of females compared to the ♀WEM×♂NEM and ♀WEM×♂SEM crosses.…”

“…In this study we explored the genetic basis of the The ancestral architecture of the sex determination in sea bass might have evolved differently during the 300 000 years of divergence between Atlantic and Mediterranean lineages, explaining the origin of the variation that now has population-specific influences on sexual determination. Indeed, we did not find any linkage group common to all populations with groups of SNPs explaining more than 2% of the variance, which support the hypothesis put forward by Guinand et al (2017) that the most important genes affecting sex may differ between sea bass populations.…”

“…As previously reported by Guinand et al (2017), sex-ratio can show nonadditive components of genetic variance, and we have no possibility to disentangle additive and non-additive genetic effects in our case.…”

“…Another possibility is that some of the QTLs observed are not linked to additive genetic variation but to dominance. The higher number of QTLs in the ♀WEM×♂NAT cross could be indicative of dominant alleles involved in sex determination (especially since some heterosis in sex-ratio has been shown by Guinand et al 2017 when mating Atlantic and Mediterranean individuals).…”

“…In the classical view, polygenic sex determination is thought to be unstable, and should evolve either towards GSD in a fluctuating Introduction ancestral polygenic sex determination system, we may hypothesize that there could be a population-specific evolution of the sex determination mechanism, in other words the components of the model may balance differently between sub-populations of the same species that are exposed to different environmental conditions (Vandeputte et al 2012;Guinand et al 2017).…”