title: The mechanisms underlying sexual differentiation of behavior and physiology in mammals and birds: relative contributions of sex steroids and sex chromosomes.
abstract: From a classical viewpoint, sex-specific behavior and physiological functions as well as the brain structures of mammals such as rats and mice, have been thought to be influenced by perinatal sex steroids secreted by the gonads. Sex steroids have also been thought to affect the differentiation of the sex-typical behavior of a few members of the avian order Galliformes, including the Japanese quail and chickens, during their development in ovo. However, recent mammalian studies that focused on the artificial shuffling or knockout of the sex-determining gene, Sry, have revealed that sex chromosomal effects may be associated with particular types of sex-linked differences such as aggression levels, social interaction, and autoimmune diseases, independently of sex steroid-mediated effects. In addition, studies on naturally occurring, rare phenomena such as gynandromorphic birds and experimentally constructed chimeras in which the composition of sex chromosomes in the brain differs from that in the other parts of the body, indicated that sex chromosomes play certain direct roles in the sex-specific differentiation of the gonads and the brain. In this article, we review the relative contributions of sex steroids and sex chromosomes in the determination of brain functions related to sexual behavior and reproductive physiology in mammals and birds.
authors.affiliation: Molecular Toxicology Section, Center for Environmental Health Sciences, National Institute for Environmental Studies Tsukuba, Japan.
authors.forename: Fumihiko
authors.initials: F
authors.lastname: Maekawa
authors.affiliation: Division of Life Science, Graduate School of Science and Engineering, Saitama University Saitama, Japan.
authors.forename: Shinji
authors.initials: S
authors.lastname: Tsukahara
authors.affiliation: Ecological Genetics Research Section, Center for Environmental Biology and Ecosystem Studies, National Institute for Environmental Studies Tsukuba, Japan.
authors.forename: Takaharu
authors.initials: T
authors.lastname: Kawashima
authors.affiliation: Molecular Toxicology Section, Center for Environmental Health Sciences, National Institute for Environmental Studies Tsukuba, Japan.
authors.forename: Keiko
authors.initials: K
authors.lastname: Nohara
authors.affiliation: College of Liberal Arts and Sciences, Kitasato University Sagamihara, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Ohki-Hamazaki
keywords: birds; chimera; neurosteroids; rodents; sexual differentiation of the brain