Sex Determination and Male Differentiation in Southern Swordtail Fishes: Evaluation from an Evolutionary Perspective

Abstract

Southern swordtail fishes, which belong to the viviparous teleosts called Xiphophorus, are unique models for studies of evolution of sex chromosomes. Monofactorial sex-determining systems, with either the male or the female being the heterogametic sex, as well as sex determination involving more than two sex chromosomes, are found in swordtails and related species. Some swordtail species seem to have originated by crossing between two closely related species. Although the sword has disappeared in many Xiphophorus species during evolution, females of non-sworded species still prefer sworded males, demonstrating a discrepancy between natural and sexual selection. Natural sex change has not been documented sufficiently convincingly in swordtails, but, at least in some subspecies, two or more male phenotypes exist. In a laboratory experiment performed for over 30 years, it has been observed that sex-determining genes may be translocated from one chromosome to another in hybrids of these evolutionary young species. While the factors suggested to play central roles in sex determination and differentiation, e.g., Dmrt1 and AMH, are highly conserved during evolution, several master determining factors have been detected in teleosts. Endocrine-induced sex reversal has been demonstrated in the guppy Poecilia reticulata, another viviparous fish. In swordtails (X. helleri), endocrine disruptors such as nonylphenol and bisphenol A may cause testis cell degeneration and the inhibition of spermatogenesis. Furthermore, swordtails are very easy to breed in freshwater aquaria and, therefore, may be good models for studying the factors influencing sex determination and male differentiation.