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Mol Biol Evol. 2015 Sep;32(9):2328-37. doi: 10.1093/molbev/msv113. Epub 2015 May 8.

Sex-Chromosome Homomorphy in Palearctic Tree Frogs Results from Both Turnovers and X-Y Recombination.

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Department of Ecology & Evolution, Biophore Building, University of Lausanne, Lausanne, Switzerland
Laboratory of Behavioral and Population Ecology, School of Biological Sciences, Seoul National University, Seoul, Republic of Korea.
Department of Ecology & Evolution, Biophore Building, University of Lausanne, Lausanne, Switzerland.
Leibniz-Institute of Freshwater Ecology and Inland Fisheries-IGB, Berlin, Germany.
Institute of Cytology, Russian Academy of Sciences, St. Petersburg, Russia.
Department of Life Sciences and Division of EcoScience, Ewha Womans University, Seoul, Republic of Korea.


Contrasting with birds and mammals, poikilothermic vertebrates often have homomorphic sex chromosomes, possibly resulting from high rates of sex-chromosome turnovers and/or occasional X-Y recombination. Strong support for the latter mechanism was provided by four species of European tree frogs, which inherited from a common ancestor (∼ 5 Ma) the same pair of homomorphic sex chromosomes (linkage group 1, LG1), harboring the candidate sex-determining gene Dmrt1. Here, we test sex linkage of LG1 across six additional species of the Eurasian Hyla radiation with divergence times ranging from 6 to 40 Ma. LG1 turns out to be sex linked in six of nine resolved cases. Mapping the patterns of sex linkage to the Hyla phylogeny reveals several transitions in sex-determination systems within the last 10 My, including one switch in heterogamety. Phylogenetic trees of DNA sequences along LG1 are consistent with occasional X-Y recombination in all species where LG1 is sex linked. These patterns argue against one of the main potential causes for turnovers, namely the accumulation of deleterious mutations on nonrecombining chromosomes. Sibship analyses show that LG1 recombination is strongly reduced in males from most species investigated, including some in which it is autosomal. Intrinsically low male recombination might facilitate the evolution of male heterogamety, and the presence of important genes from the sex-determination cascade might predispose LG1 to become a sex chromosome.


DMRT1; Hyla; fountain of youth hypothesis; recombination; sex-chromosome transitions

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