Low rates of X-Y recombination, not turnovers, account for homomorphic sex chromosomes in several diploid species of Palearctic green toads (Bufo viridis subgroup)


  • [Correction added after online publication 25 January 2013:typographical misspelling of Palearctic corrected.]

Correspondence: Nicolas Perrin, University of Lausanne, Department of Ecology and Evolution (DEE), Biophore, CH-1015 Lausanne, Switzerland.

Tel. +41 21 692 41 84; fax +41 21 692 41 65;

e-mail: nicolas.perrin@unil.ch


Contrasting with birds and mammals, most ectothermic vertebrates present homomorphic sex chromosomes, which might be due either to a high turnover rate or to occasional X-Y recombination. We tested these two hypotheses in a group of Palearctic green toads that diverged some 3.3 million years ago. Using sibship analyses of sex-linked markers, we show that all four species investigated share the same pair of sex chromosomes and a pattern of male heterogamety with drastically reduced X-Y recombination in males. Phylogenetic analyses of sex-linked sequences show that X and Y alleles cluster by species, not by gametolog. We conclude that X-Y homomorphy and fine-scale sequence similarity in these species do not stem from recent sex-chromosome turnovers, but from occasional X-Y recombination.