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Three strains of mirror carp originating from important regions of carp culture in France (Brenne, Dombes, Forez) were compared for growth at 5 weeks, 1 summer and 2 summers of age in ponds using a paternal cross on the same Dombes dams. An internal scaly control was produced by crossing an homozygous inbred strain on the same dams and mixed with the tested strains in all ponds in a proportion of 1:3. No between-strain differences in weight were seen at any time when taking into account the weight of the internal control. However, the internal control weight at 2 summers of age was lower than that of the tested strains (P<0.05). The use of the internal control divides the number of replicates needed to have a good statistical power by 5 at 5 weeks and by 8 at 2 summers, when compared to separate testing without control. No genotype by environment or strain–control interactions were detected. The validity and optimisation of the use of an internal control in strain testing are discussed. [Copyright &y& Elsevier]

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(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.

Group feeding pattern is a response correlated to selection for growth in brown trout. Previous results suggest that the feeding pattern of two lines (NL and SY), selected with the same process at two different fish farms, differ. This study addresses the following questions: (i) are the patterns differentially fixed by the selection process and if yes (ii) is this explained by the different genetic background of the lines? In a first experiment, we tested if the group feeding behavior can be inherited. We produced and reared under the same experimental conditions selected hybrids (NLS x SYS) and pure SY selected line (SYS). We compared their behavior on self-feeders when reared single or mixed (groups of 500 fish; mean initial BW = 6 g, 90-day period). Growth rates were comparable among the groups. In the lines reared single, NLS x SYS exhibited greater morning feeding activity than SYS. Feeding activity of SYS was higher later in the day. The inter-day variation of the morning peak was less variable for the hybrid than for the SYS line. In the mixed group, the pattern was intermediate between that of the hybrid and the pure line. In a second experiment we compared the feeding pattern of control hybrids (NLC x SYC) to pure control lines (SYC and NLC) reared single or mixed (groups of 500 fish; mean initial BW = 12 g, 92-day period). Feeding activity patterns were similar among the control groups. These results show that group feeding pattern is a heritable trait that can be indirectly selected with growth, which shape is less influenced by the growth performance and the genetic background than by the environmental conditions applied during the selection process.