Article Sex-Chromosome Homomorphy in Palearctic Tree Frogs Results from Both Turnovers and X–Y Recombination Christophe Dufresnes,* ,1 Ama € el Borz  ee, 2 Agne `s Horn, 1 Matthias St € ock, 3 Massimo Ostini, 1 Roberto Sermier, 1 J  er ^ ome Wassef, 1 Spartak N. Litvinchuck, 4 Tiffany A. Kosch, 2 Bruce Waldman, 2 Yikweon Jang, 5 Alan Brelsford, 1 and Nicolas Perrin 1 1 Department of Ecology & Evolution, Biophore Building, University of Lausanne, Lausanne, Switzerland 2 Laboratory of Behavioral and Population Ecology, School of Biological Sciences, Seoul National University, Seoul, Republic of Korea 3 Leibniz-Institute of Freshwater Ecology and Inland Fisheries—IGB, Berlin, Germany 4 Institute of Cytology, Russian Academy of Sciences, St. Petersburg, Russia 5 Department of Life Sciences and Division of EcoScience, Ewha Womans University, Seoul, Republic of Korea *Corresponding author: E-mail: christophe.dufresnes@unil.ch. Associate editor: Patricia Wittkopp Abstract 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. Key words: Hyla, recombination, DMRT1, sex-chromosome transitions, fountain of youth hypothesis Introduction Sex chromosomes are evolving along drastically different tra- jectories, depending on lineages. Most mammals and birds, for instance, present strongly heteromorphic sex chromo- somes with highly degenerated Y and W chromosomes, re- spectively. This heteromorphy results from a long history of recombination arrest, initiated some 170 and 130 Ma, respec- tively (Charlesworth D and Charlesworth B 2000; Bachtrog 2013). In sharp contrast, many groups of fish, amphibians and reptiles present undifferentiated sex chromosomes, testifying to a very distinct history (Bachtrog et al. 2014). Homomorphy may result from a high rate of turnovers, during which sex chromosomes are replaced before they had time to decay (Schartl 2004; Volff et al. 2007). Sex-determina- tion mechanisms seem particularly labile in amphibians and fishes; comparative mapping shows that sex-determining sys- tems can switch rapidly (Kikuchi and Hamaguchi 2013; Malcom et al. 2014), leading to different sex chromosome pairs between closely related species (Mank and Avise 2009; Kitano and Peichel 2011) or even conspecific populations (Miura 2007). Different forces may drive turnovers, including sexually antagonistic selection (van Doorn and Kirkpatrick 2007, 2010), sex-ratio selection (Grossen et al. 2011), and del- eterious mutations accumulating on nonrecombining chro- mosomes (Blaser et al. 2013). Despite substantial theoretical consideration, the relative contribution of these mechanisms in natural systems remains poorly understood. Comparative analyses suggest that, in this context of high turnover, some genomic regions are repeatedly and independently co-opted for sex determination, likely because they carry important genes from the sex-determining cascade (Graves and Peichel 2010; O’Meally et al. 2012; Brelsford et al. 2013). Alternatively, homomorphy may result from occasional X– Y recombination, occurring either in males or in sex-reversed XY females (the “fountain-of-youth” model; Perrin 2009). Theoretical studies suggest that X–Y recombination should evolve toward very low but nonzero values, under the oppos- ing forces of sexually antagonistic selection (which favors re- combination arrest) and the load of deleterious mutations that accumulate on nonrecombining regions (Grossen et al. 2012). Very low recombination rates seem sufficient to purge this load and prevent X–Y differentiation over evolutionary times (Grossen et al. 2012). This latter model has received support from studies on a group of European tree frog ß The Author 2015. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 2328 Mol. Biol. Evol. 32(9):2328–2337 doi:10.1093/molbev/msv113 Advance Access publication May 8, 2015 at Universite & EPFL Lausanne on August 28, 2015 http://mbe.oxfordjournals.org/ Downloaded from