Sex-determining systems, sex chromosomes, and sex-determining genes in frogs.
All of the anuran amphibians examined so far have genetic sex-determining systems, which include female heterogametic ZZ/ZW and male heterogametic XX/XY types. For example, the Japanese wrinkled frog Glandirana rugosa has both types. Most of frog species including the African clawed frog Xenopus laevis possess homomorphic sex chromosomes, while most mammalian and avian species have heteromorphic sex chromosomes. Thus, there should be a variety of sex-determining genes and sex chromosomes in frogs, although only X. laevis W-linked gene dm-W has been reported as a sex-determining gene. Interestingly, estrogen or androgen can induce sex reversal in many frog species, suggesting a vital role of sex steroid hormones on sex identity. In other words, frogs in the same order are good examples for the understanding of diversity of sex-determining systems. In this chapter, I summarize the diversity of frog sex-determining systems and discuss why sex-determining genes and systems have been unstable in frogs.
- sex determination
- sex chromosome
- sex-determining gene
- sex steroid
- default sex
Sexual reproduction is the most common life cycle in animals and plants. Meiotic recombination mediated through sexual reproduction is believed to allow genetic variation for survival of some populations against environmental changes. Thus, sex systems are very important for life evolution and biodiversity. In vertebrates, female and male sexes could be mainly defined by the property of gonads, ovaries producing eggs and testes producing sperm, respectively. Importantly, undifferentiated gonads in most vertebrate species have potential to differentiate into ovaries and testes. Then sex determination could be defined as the decision of bipotential gonads to develop as either ovaries or testes in vertebrates.
There are a variety of sex-determining systems in organisms. In vertebrates, they could be classified roughly into two types: genetic and environmental types. Endothermic vertebrates exclusively have the former system, which includes female (ZW) and male (XY) heterogametic sex chromosomes. Most mammalian and avian species have the XX/XY and ZZ/ZW systems, respectively, while there are both ZZ/ZW- and XX/XY-type systems in teleost fish, amphibians, and reptiles . In addition, ectothermic vertebrates including reptiles and fish have not only the genetic sex-determining systems but also environmental sex-determining systems, such as temperature- and social-dependent types. Remarkably, all amphibian species possess the genetic systems, although they have ectothermic traits like reptiles and fish .
In the chapter, I introduce sex-determining systems, sex chromosomes, and sex-determining genes in amphibian frogs and discuss the relationships among them.
2. Sex-determining systems and sex chromosomes in frogs
As described in the above section, all anuran amphibians examined so far have the genetic sex-determining systems including the ZZ/ZW and XX/XY types ( Table 1 ). For examples, the (African bullfrog)
|Species||Sex-determining type||Morphology of sex chromosomes||Sex-determining gene|
|ZZ/ZW or XX/XY||Heteromorphic/homomorphic|
The XX/XY and ZZ/ZW systems in most mammals and all birds examined have been maintained for more than a 100 million years, which is greatly connected with the monophyletic and heteromorphic sex chromosomes among most species of therian mammals or avians: the monophyly of the Z or Y sex chromosomes is closely related to the maintainability of the sex-determining gene
3. Discovery of a female sex-determining gene
dm-Win the African clawed frog
In 1990, human
The DM domain of DM-W has about 90% amino acid sequence identity with those of DMRT1.L and DMRT1.S. However, the DM-W C-terminal region shares almost no similarity with those of DMRT1s. The last fourth exon of
4. Sex reversal and sex chromosome differentiation
Although all frog species might genetically determine sex as mentioned above, most frog species could accept male-to-female or female-to-male sex reversals by treatment of sex steroids, estrogen, or androgen, respectively, during tadpole development . Importantly, many frogs of them have homomorphic sex chromosomes. For example,
Moreover, we recently analyzed detail structures of the sex chromosomes on 2Lq32-33 in
5. Conclusions and perspective
All frogs examined possess genetic sex-determining systems, and most of them have homomorphic sex chromosomes. The genetic systems could be easy to change during species diversity, that is, the instability of the systems, maybe because of homomorphic sex chromosomes, which could have a potential to convert a sex-determining gene into a new one on another chromosome, resulting in the change of sex chromosomes. Then I propose a “GENE-eat-GENE” model for turnover of sex-determining genes: there has been battles among the present sex-determining gene and candidates of new sex-determining genes for king/queen ship in some populations holding homomorphic sex chromosomes ( Figure 1 ). Accordingly, I predict that there are great many sex-determining genes in frogs, although only one