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Sex determination and sex chromosomes can be very diverse between teleost species. The group of tilapias shows a polymorphism in sex determination not only between closely related species but also between domestic strains within a species. In the Nile tilapia, the major effect genes and therefore the Y chromosome have been located on either linkage group 1 (LG1) or LG23 depending on the strains. In a Japanese strain, the sex determinant of LG23 (the  amhY  gene) has been identified as a duplicated  amh  (anti-Müllerian hormone) gene, with its gametolog found on the X chromosome ( amhX ).  AmhY  is located in tandem with the  amhΔY  gene (a truncated form) on the Y chromosome. X and Y chromosome markers based on the  amh  genes have been validated only on a few domestic strains but not in wild populations. Here, we used four of these markers in order to examine (1) the possible variation in sex determination of a wild population of Nile tilapia living in Lake Kou (Burkina Faso), (2) putative polymorphisms for these  amh  copies and (3) the existence of sex reversed individuals in the wild. Our genotyping of 91 wild Kou individuals with the  amh  sex-diagnostic markers of LG23 showed that while phenotypic females were all XX, phenotypic males were either XY or XX. Progeny testing of eight of these XX males revealed that one of these males consistently sired all-female progenies, suggesting that it is a wild sex reversed male (which could result from high temperature effects). The other XX males gave balanced sex ratios, suggesting that sex is controlled by another locus (possibly on another LG) which may be epistatically dominant over the LG23 locus. Finally, identification of unexpected  amh  genotypes was found for two individuals. They produced either balanced or female-biased sex ratios, depending on the breeder with whom they were crossed, suggesting possible recombination between the X and the Y chromosomes.

Mismatches between the genetic and phenotypic sex in the wild Kou population of Nile tilapia Oreochromis niloticus