title: Ever-young sex chromosomes in European tree frogs.
abstract: Non-recombining sex chromosomes are expected to undergo evolutionary decay, ending up genetically degenerated, as has happened in birds and mammals. Why are then sex chromosomes so often homomorphic in cold-blooded vertebrates? One possible explanation is a high rate of turnover events, replacing master sex-determining genes by new ones on other chromosomes. An alternative is that X-Y similarity is maintained by occasional recombination events, occurring in sex-reversed XY females. Based on mitochondrial and nuclear gene sequences, we estimated the divergence times between European tree frogs (Hyla arborea, H. intermedia, and H. molleri) to the upper Miocene, about 5.4-7.1 million years ago. Sibship analyses of microsatellite polymorphisms revealed that all three species have the same pair of sex chromosomes, with complete absence of X-Y recombination in males. Despite this, sequences of sex-linked loci show no divergence between the X and Y chromosomes. In the phylogeny, the X and Y alleles cluster according to species, not in groups of gametologs. We conclude that sex-chromosome homomorphy in these tree frogs does not result from a recent turnover but is maintained over evolutionary timescales by occasional X-Y recombination. Seemingly young sex chromosomes may thus carry old-established sex-determining genes, a result at odds with the view that sex chromosomes necessarily decay until they are replaced. This raises intriguing perspectives regarding the evolutionary dynamics of sexually antagonistic genes and the mechanisms that control X-Y recombination.
authors.affiliation: Department of Ecology and Evolution, University of Lausanne, Lausanne, Switzerland.
authors.forename: Matthias
authors.initials: M
authors.lastname: Stöck
authors.forename: Agnès
authors.initials: A
authors.lastname: Horn
authors.forename: Christine
authors.initials: C
authors.lastname: Grossen
authors.forename: Dorothea
authors.initials: D
authors.lastname: Lindtke
authors.forename: Roberto
authors.initials: R
authors.lastname: Sermier
authors.forename: Caroline
authors.initials: C
authors.lastname: Betto-Colliard
authors.forename: Christophe
authors.initials: C
authors.lastname: Dufresnes
authors.forename: Emmanuel
authors.initials: E
authors.lastname: Bonjour
authors.forename: Zoé
authors.initials: Z
authors.lastname: Dumas
authors.forename: Emilien
authors.initials: E
authors.lastname: Luquet
authors.forename: Tiziano
authors.initials: T
authors.lastname: Maddalena
authors.forename: Helena Clavero
authors.initials: HC
authors.lastname: Sousa
authors.forename: Iñigo
authors.initials: I
authors.lastname: Martinez-Solano
authors.forename: Nicolas
authors.initials: N
authors.lastname: Perrin
mesh_terms: D000818:Animals; D001001:Anura / Q000235:genetics*; D005260:Female; D005787:Gene Frequency; D050172:Genes, X-Linked; D050173:Genes, Y-Linked; D008040:Genetic Linkage; D005819:Genetic Markers; D049810:Genetic Speciation; D016013:Likelihood Functions; D008297:Male; D008957:Models, Genetic; D010802:Phylogeny; D011995:Recombination, Genetic; D019849:Sex Determination Processes; D014960:X Chromosome / Q000235:genetics*; D014998:Y Chromosome / Q000235:genetics*
chemical_list: D005819:Genetic Markers