Turn Over of Sex-Determining Genetics as well as Sex Chromosomes
Turn Over of Sex-Determining Genetics as well as Sex Chromosomes

Sex chromosomes in most amphibians are homomorphic (undifferentiated) in both sexes and are characterized by frequent turnover. This is in sharp contrast to sex chromosomes in 2 major vertebrate groups, the mammals and birds, where they are heteromorphic in one sex and are highly conserved. Sex-determining mechanisms in anuran amphibians, particularly in relation to the turnover of sex-determining genes and sex chromosomes, are summarized and their evolution is discussed.

The heteromorphy or homomorphy (differentiated or undifferentiated) of sex chromosomes has a close correlation with the conservation of the sex-determining genes. If sex chromosomes are heteromorphic in one sex, the sex-determining gene is evolutionally conserved. For example, therian mammals share the heteromorphic sex chromosome pair, a large X chromosome and a tiny Y chromosome, on which the male-determining gene Sry has been conserved for 166 million years (MY) (Fig. 1). Similarly, in all birds, the ZW sex chromosomes are heteromorphic in females, and the male-determining gene Dmrt1 on the Z chromosome has been conserved for 100 MY Romanov et al., 2014 On the other hand, if JAV Online Streaming is homomorphic in both sexes, for example as in 96% species of amphibians , the sex-determining gene on the sex chromosome is replaced easily and rapidly by another gene of a different chromosome. This replacement is called “turnover of sex-determining genes and sex chromosomes” Schartl, 2004 and is observed in abundant species of poikilothermic vertebrates. Turnover occurs between different taxa or even between geographic populations within a single species.

Turn Over of Sex-Determining Genetics as well as Sex Chromosomes

Sex chromosomes of humans and frogs. The X and Y chromosomes are heteromorphic in human males, while the sex chromosome 1 is homomorphic in both sexes in frogs. Photographs of the human sex chromosomes are provided by Chizuko Nishida.

Turnover is well studied in the teleost fish medaka (Oryzias). The sex-linkage analysis in medaka indicated the existence of 7 independent linkage groups, which implies that 7 genes are expected to be candidate sex-determining genes: 2 genes for female and 5 for male sex determination, of which 3 male-determining genes have already been identified In all vertebrates.