Sex chromosome differentiation and the W- and Z-specific loci in Xenopus laevis.
Genetic sex-determining systems in vertebrates include two basic types of heterogamety; XX (female)/XY (male) and ZZ (male)/ZW (female) types. The African clawed frog Xenopus laevis has a ZZ/ZW-type sex-determining system. In this species, we previously identified a W-specific sex (female)-determining gene dmw, and specified W and Z chromosomes, which could be morphologically indistinguishable (homomorphic). In addition to dmw, we most recently discovered two genes, named scanw and ccdc69w, and one gene, named capn5z in the W- and Z-specific regions, respectively. In this study, we revealed the detail structures of the W/Z-specific loci and genes. Sequence analysis indicated that there is almost no sequence similarity between 278kb W-specific and 83kb Z-specific sequences on chromosome 2Lq32-33, where both the transposable elements are abundant. Synteny and phylogenic analyses indicated that all the W/Z-specific genes might have emerged independently. Expression analysis demonstrated that scanw and ccdc69w or capn5z are expressed in early differentiating ZW gonads or testes, thereby suggesting possible roles in female or male development, respectively. Importantly, the sex-determining gene (SDG) dmw might have been generated after allotetraploidization, thereby indicating the construction of the new sex-determining system by dmw after species hybridization. Furthermore, by direct genotyping, we confirmed that diploid WW embryos developed into normal female frogs, which indicate that the Z-specific region is not essential for female development. Overall, these findings indicate that sex chromosome differentiation has started, although no heteromorphic sex chromosomes are evident yet, in X. laevis. Homologous recombination suppression might have promoted the accumulation of mutations and transposable elements, and enlarged the W/Z-specific regions, thereby resulting in differentiation of the W/Z chromosomes.
Kitasato Institute for Life Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641, Japan.
Shuuji
S
Mawaribuchi
Institute for Amphibian Biology, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan.
Shuji
S
Takahashi
Department of Biosciences, School of Science, Kitasato University, 1-15-1 Minamiku, Sagamihara 252-0373, Japan.
Mikako
M
Wada
Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan.
Yoshinobu
Y
Uno
Department of Applied Molecular Biosciences, Graduate School of Bioagricultural Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601, Japan.
Yoichi
Y
Matsuda
Misaki Marine Biological Station (MMBS), Graduate School of Science, The University of Tokyo, 1024 Koajiro, Misaki, Miura, Kanagawa 238-0225, Japan.
Mariko
M
Kondo
Laboratory of Tissue and Polymer Sciences, Faculty of Advanced Life Science, Hokkaido University, N10W8, Kita-ku, Sapporo 060-0810, Japan.
Akimasa
A
Fukui
Department of Biosciences, School of Science, Kitasato University, 1-15-1 Minamiku, Sagamihara 252-0373, Japan.
Nobuhiko
N
Takamatsu
Department of Biological Sciences, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Masanori
M
Taira
Department of Biosciences, School of Science, Kitasato University, 1-15-1 Minamiku, Sagamihara 252-0373, Japan. Electronic address: ito@sci.kitasato-u.ac.jp.
Michihiko
M
Ito
D000818:Animals; D005075:Biological Evolution; D007446:Chromosome Inversion; D004251:DNA Transposable Elements / Q000235:genetics; D004171:Diploidy; D019143:Evolution, Molecular; D005260:Female; D020440:Gene Duplication; D005796:Genes; D006238:Haploidy; D017404:In Situ Hybridization, Fluorescence; D008297:Male; D010802:Phylogeny; D060888:Real-Time Polymerase Chain Reaction; D012730:Sex Chromosomes / Q000235:genetics*; D019849:Sex Determination Processes / Q000235:genetics; D012733:Sex Differentiation / Q000235:genetics*; D014982:Xenopus laevis / Q000235:genetics*
D004251:DNA Transposable Elements
Allotetraploidization; Homologous recombination; Sex chromosome; Sex-determining gene; Transposable element; Whole genome duplication