Academic journal article Genetics

A Polygenic Hypothesis for Sex Determination in the European Sea Bass Dicentrarchus Labrax

Academic journal article Genetics

A Polygenic Hypothesis for Sex Determination in the European Sea Bass Dicentrarchus Labrax

Article excerpt

ABSTRACT

Polygenic sex determination, although suspected in several species, is thought to be evolutionarily unstable and has been proven in very few cases. In the European sea bass, temperature is known to influence the sex ratio. We set up a factorial mating, producing 5.893 individuals from 253 full-sib families, all reared in a single batch to avoid any between-families environmental effects. The proportion of females in the offspring was 18.3%, with a large variation between families. Interpreting sex as a threshold trait, the heritability estimate was 0.62 ± 0.12. The observed distribution of family sex ratios was in accordance with a polygenic model or with a four-sex-factors system with environmental variance and could not be explained by any genetic model without environmental variance. We showed that there was a positive genetic correlation between weight and sex (r^sub A^ = 0.50 ± 0.09), apart from the phenotypic sex dimorphism in favor of females. This supports the hypothesis that a minimum size is required for sea bass juveniles to differentiate as females. An evolution of sex ratio by frequency-dependent selection is expected during the domestication process of Dicentrarchus labrax populations, raising concern about the release of such fish in the wild.

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IN gonochoric species with genetically determined sex, a one-to-one sex ratio is known to be optimal in an infinite population of diploid individuals with random mating and Mendelian segregation (Fisher 1930; Charnov 1975). The observation of skewed sex ratios may imply, among other things, non-Mendelian segregation as in Drosophila (Vaz and Carvalho 2004), nonrandom mating (Hamilton 1967), or environmental sex determination (ESD; Bull 1985). In the latter case, the sex of an individual is not fixed at conception, but is influenced by environmental conditions during its early life. ESD is expected to be favored when the offspring lives in patchy environments, which may confer advantages to being male or female, and neither the offspring nor the parent have control and/or predictive ability on the type of patch in which the offspring will live (Charnov and Bull 1977). Temperature (e.g., Bull and Vogt 1979; Baroiller and D'Cotta 2001) seems to be the main environmental factor implied, but density (Ellenby 1954) and social status (Francis and Barlow 1993) have been shown to be possible sex-determining environmental factors. In species with ESD, in many cases there is also a genetic variation (Bull et al. 1982; Conover and Heins 1987a; Janzen 1992; Baroiller and D'Cotta 2001), which in some cases has been described as polygenic (Bull et al. 1982; Janzen 1992). Polygenic sex determination, however, is considered to be evolutionarily unstable (Rice 1986) and its maintenance is still poorly understood. It is thought by some authors to be the ancestral type of sex determinism in fish (Kirpichnikov 1981), but organisms where it is accepted that sex has a polygenic component are indeed very few: the parasitic wasp Nasonia vitripennis (Orzack and Gladstone 1994), the turtles Graptemys ouachitensis (Bull et al. 1982) and Chelydra serpentina ( Janzen 1992), and probably the swordtail fish Xiphophorus helleri (Kosswig 1964).

The European sea bass (Dicentrarchus labrax) is a gonochoristic teleost fish distributed in the northeastern Atlantic, the Mediterranean, and the Black Sea (Pickett and Pawson 1994). They live in shallow, coastal waters, estuaries, lagoons, and harbors, moving to deeper waters (up to 100 m deep) as they grow. Although they can live in waters <5°, they seek temperatures >10°, and even 15° in their first year (Kelley 1988). They spawn in open waters from late winter to early spring, depending on the latitude. The eggs hatch in 4-9 days, and the young fish move inshore in their first month toward the warmest waters, especially in estuaries (Pickett and Pawson 1994). …

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