Sex determination and sex chromosome evolution in the medaka, Oryzias latipes, and the platyfish, Xiphophorus maculatus

Abstract

The first vertebrate master sex-determining gene different from Sry has been very recently discovered in a small aquarium fish, the medaka (Oryzias latipes). In this fish, the X and Y chromosomes apparently differ only by a 250-kb Y-specific region containing dmrt1bY (also called DMY and dmrt1Y), a male-specific copy of the autosomal gene dmrt1. Dmrt1 is a putative transcription factor probably involved in testis formation in different vertebrate lineages. Dmrt1bY is the only gene having escaped the drastic process of degeneration that devastated the small Y-specific region of the medaka. Mutations leading to truncation or lower expression of dmrt1bY result in XY male-to-female sex reversal. Hence, both genetic and functional evidences converge in making dmrt1bY an outstanding candidate for the function of a master sex-determining gene in fish. Nevertheless, dmrt1bY could not be detected in certain other Oryzias species or in more divergent fishes. Phylogenetic analysis revealed that the duplication of the autosomal dmrt1 that formed dmrt1bY is young in evolutionary terms. Hence, dmrt1bY is not the universal master sex-determining gene in fish. Because the classical fish models, such as zebrafish and pufferfish, are not very adequate to study the basis of genetic sex determination, alternative models, such as the platyfish (Xiphophorus maculatus), are re-emerging. In this fish, which is a well-suited laboratory organism, gene loci involved in pigmentation, melanoma formation, and sexual maturity have been mapped close to the master sex-determining gene. Interestingly, the platyfish can harbor three different sex chromosomes (W, X, and Y) in certain natural populations. Bacterial artificial chromosome contigs covering the sex-determining region of the platyfish are already available, and the positional cloning of the master sex-determining gene(s) should provide new insights into sex determination and sex chromosome evolution in fish and other vertebrates.