The heredity of sex determination in tilapias

Abstract

Evidence for the genetic mechanism of sex determination in tilapias has been generated from interspecific hybridization, chromosome manipulations, sex inversion and intraspecific investigations. In some interspecific hybrids and hybrid derivatives, Mendelian segregations are found in sex ratio, i.e., 0 : 1, 1 : 1 and 1 : 3 segregations. Attempts were made to explain these ratios by a dual system of sex chromosomes whereby, in some species, the female is homogametic and the male heterogametic, while in another group of species the opposite is the case. This model, alone, is not capable of explaining further sets of empirical data, since in other crosses and gynogenetic broods sex ratios are variable and non-Mendelian. Series of single-pair, intraspecific spawns showed that sex ratio is a continuously variable trait, with a mean of about 50% and, in at least one species, a range of 0–100%. Sex ratio was also shown to be a stable trait and to respond to selection, behaving like a quantitative trait. Based on these facts, we hypothesize that some sex-determining factors are concentrated on one pair of chromosomes which thereby become the sex chromosomes. Presumably crossing-over takes place between these sex chromosomes, as demonstrated in other fish species. Further genetic determinants of sex are situated on other chromosomes, the autosomes. Segregation of sex chromosomes brings about Mendelian sex segretions in the absence of variation in autosomally carried sex determinants. Non-Mendelian sex segregations, continuous variation in sex ratios and response to selection could result from variation in autosomal sex determinants and crossing-over between sex chromosomes. We suggest that the genetic mechanism of sex determination in tilapias is analogous to that in platyfish, depending on variation in both sex chromosome and autosomally carried factors. Empirical evidence for this is not available for tilapias, due to the absence of sex-linked markers. In such a complex system, the chances of producing “super males”, which generate all-male broods with any female, in either interor intraspecific combination, appear small. We suggest that intraspecific investigations have a better chance than interspecific ones of advancing our knowledge of sex determination in tilapias, as well as producing stable broodstocks generating sex ratios tending to all-male progenies.