Genetic variation of growth and sex ratio in the European sea bass (Dicentrarchus labrax L.) as revealed by molecular pedigrees

Abstract : The European sea bass (Dicentrarchus labrax) is a major species of Mediterranean aquaculture, the production of which rose from almost nothing in 1985 to more than 100.000 tonnes annually at present. In many cases, wild-caught broodstock is still used to produce juveniles for aquaculture, and farmed population are predominantly male - which unfortunately perform less than females. The aim of the present research was first to quantify the genetic variation of growth and sex ratio between families of sea bass produced by artificial fertilization and reared in a "common garden" approach, using the genotyping of microsatellite markers to reconstruct the pedigrees. In a second phase, we also studied the response in terms of growth and sex ratio to an experimental selection applied on body length.We first could show that the experimental technique chosen (artificial fertilization, common garden rearing and pedigree reconstruction by genotyping) was efficient and could be applicable not only to conduct experiments but also to set up breeding programmes in sea bass.Growth is a heritable trait in sea bass, with a high heritability for body weight at commercial size (h²=0.38-0.44 around 400 g mean weight), but a lower value for growth rate from 35 to 400g (0.16-0.34), showing the importance of the highly heritable (h²=0.61) early growth in the building of the performance at commercial size. Additionally, we showed that sea bass growth was not significantly impacted by dominance or non genetic maternal effects. We estimated genotype by environment interactions for growth between highly divergent ongrowing sites, showing that although interactions were moderate for body weight at commercial size (rA= 0.70-0.99 between sites), they were much higher for growth rate (rA=0.21-0.61 between sites). Although we purposely chose very divergent ongrowing environments, this highlights the importance of conducting breeding programs in environments resembling the production environment.We showed that the sex ratio of natural populations in the wild did not differ from 50-50 on average, although some age classes could have a biased sex ratio, probably due to environmental effects. In a farmed population, sex ratios were shown to differ between families and to be equally influenced by the sire and the dam. No purely genetic model could account for the distributions observed, which could fit either to a model with a minimum of two bi-allelic loci plus micro-environmental variance, or to a polygenic threshold model with h²=0.62 for sex tendency on the underlying scale. This last model also revealed a positive genetic correlation (rA=0.50) between sex tendency and growth. This allowed us to predict that domestication should tend towards a balancing of the sex ratio at 50-50, while selection for faster growth should bias population sex ratios towards females. This is precisely what we observed later on in our selection response experiment, which also confirmed the potential of the species to be selected for faster growth, with a 23% gain in body weight in the first generation.The polygenic (or at least polyfactorial) model of sex determination is considered rare in Vertebrates. After developing its possible use to tend towards monosex female farmed populations of sea bass, we assessed its position in the theory of sex determination in ectotherm Vertebrates, where it seems that it could well be more frequent as initially thought. Polygenic sex determination could be a means for species and populations to move along the ESD-GSD continuum (Environmental or Genetic Sex Determination).
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Marc Vandeputte. Genetic variation of growth and sex ratio in the European sea bass (Dicentrarchus labrax L.) as revealed by molecular pedigrees. Animal genetics. AgroParisTech, 2012. English. ⟨NNT : 2012AGPT0057⟩. ⟨pastel-00957623⟩

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