Heritability estimates for male proportion in hybrids between Nile tilapia females (Oreochromis niloticus) and blue tilapia males (Oreochromis aureus)

Abstract

Abstract Estimates of the genetic variation in male proportion (MP) were obtained from a total of 83 hybrid families, produced by crossing 82 Nile tilapia females ( Oreochromis niloticus ) with 35 blue tilapia males ( Oreochromis aureus ), which were stocked in 132 hapas in a common pond. The parents of each species originated from three different countries (Israel, Taiwan, China), but only seven of the nine possible sire-dam origin combinations (crosses) were produced. After on average 53 days in the hapas (April–July) a random sample of approximately 94 fish per hapa was slaughtered and sexed by examining gonad tissues under the microscope. Overall MP was 77%, and thus below the expected value of 100% assuming a major sex determination system (XX/XY in Nile tilapia and WZ/ZZ in blue tilapia). The effect of cross on MP was highly significant, but not very reliable as some of the crosses were represented with a limited number of families. The variation in MP among the families was substantial (from 0% to 100%) and within-cross heritability estimates for MP on the observed scale were 0.38 ± 0.07 and 0.42 ± 0.09 when assuming either equal or different additive genetic sire-dam variances for the parental species, respectively; and, as expected, higher on the underlying liability scale (0.79 ± 0.11 and 0.82 ± 0.13). The effect common to full-sibs other than additive genetics was significant, but explained a low proportion of the total variance (0.04 ± 0.01 and 0.08 ± 0.02 on the observed and liability scales, respectively). The magnitude of the estimated heritabilities suggests that MP in hybrids is partly under additive (polygenic) genetic control. However, these estimates may be biased since parental strains may not be pure and since the unknown genetic sex of all parents (determined by the major genetic XX/XY and WZ/ZZ sex determination system) may not necessarily match the phenotypic sex. Possibilities for selection for increased MP in hybrids to commercially required levels (> 95% males) are discussed.