title: B chromosomes have a functional effect on female sex determination in Lake Victoria cichlid fishes.
abstract: The endemic cichlid fishes in Lake Victoria are a model system for speciation through adaptive radiation. Although the evolution of the sex-determination system may also play a role in speciation, little is known about the sex-determination system of Lake Victoria cichlids. To understand the evolution of the sex-determination system in these fish, we performed cytogenetic analysis in 11 cichlid species from Lake Victoria. B chromosomes, which are present in addition to standard chromosomes, were found at a high prevalence rate (85%) in these cichlids. In one species, B chromosomes were female-specific. Cross-breeding using females with and without the B chromosomes demonstrated that the presence of the B chromosomes leads to a female-biased sex ratio in this species. Although B chromosomes were believed to be selfish genetic elements with little effect on phenotype and to lack protein-coding genes, the present study provides evidence that B chromosomes have a functional effect on female sex determination. FISH analysis using a BAC clone containing B chromosome DNA suggested that the B chromosomes are derived from sex chromosomes. Determination of the nucleotide sequences of this clone (104.5 kb) revealed the presence of several protein-coding genes in the B chromosome, suggesting that B chromosomes have the potential to contain functional genes. Because some sex chromosomes in amphibians and arthropods are thought to be derived from B chromosomes, the B chromosomes in Lake Victoria cichlids may represent an evolutionary transition toward the generation of sex chromosomes.
authors.affiliation: Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, Yokohama, Japan.
authors.forename: Kohta
authors.initials: K
authors.lastname: Yoshida
authors.forename: Yohey
authors.initials: Y
authors.lastname: Terai
authors.forename: Shinji
authors.initials: S
authors.lastname: Mizoiri
authors.forename: Mitsuto
authors.initials: M
authors.lastname: Aibara
authors.forename: Hidenori
authors.initials: H
authors.lastname: Nishihara
authors.forename: Masakatsu
authors.initials: M
authors.lastname: Watanabe
authors.forename: Asato
authors.initials: A
authors.lastname: Kuroiwa
authors.forename: Hirohisa
authors.initials: H
authors.lastname: Hirai
authors.forename: Yuriko
authors.initials: Y
authors.lastname: Hirai
authors.forename: Yoichi
authors.initials: Y
authors.lastname: Matsuda
authors.forename: Norihiro
authors.initials: N
authors.lastname: Okada
mesh_terms: D000818:Animals; D002875:Chromosomes / Q000235:genetics*; D023681:Cichlids / Q000235:genetics*; D019143:Evolution, Molecular; D005260:Female; D060106:Lakes; D010802:Phylogeny; D012732:Sex Determination Analysis