Sex Determination

**10**

*Gene Expression and Phenotypic Traits*

[1] Natale F, Vivo M, Falco G, Angrisano T. Deciphering DNA methylation signatures of pancreatic cancer and pancreatitis. Clinical Epigenetics. 2019;**1**(1):132. DOI: 10.1186/s13148-019-0728-8

**References**

[2] Saenen ND, Martens DS, Neven KY, Alfano R, Bové H, Janssen BG, et al. Air pollution-induced placental alterations:

Encyclopedia of the Social & Behavioral Sciences. 2nd ed. Elsevier, London, UK. 2015. pp. 683-702. DOI: 10.1016/ B978-0-08-097086-8.82051-6

An interplay of oxidative stress, epigenetics, and the aging phenotype? Clinical Epigenetics. 2019;**11**(1):124. DOI: 10.1186/s13148-019-0688

[3] Tiphaine CM, Jordana TB, Timothy DS. Twin studies and epigenetics. In: International

**13**

**Chapter 2**

*Michihiko Ito*

default sex, ectothermy

**1. Introduction**

**Abstract**

Systems in Frogs

Instability of Sex-Determining

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 sexdetermining 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

**Keywords:** sex determination, sex chromosome, sex-determining gene, sex steroid,

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

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 [1]. 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 [1].

bipotential gonads to develop as either ovaries or testes in vertebrates.

sex-determining genes and systems have been unstable in frogs.

## **Chapter 2**
