Comparison of phylogenic trees of the Amel-X and Amel-Y fragments inferred (a) with the insertion (b) without the insertion. (a) The phylogenic tree of the complete fragment shows trans-specific clustering by sex chromosome in Cetartiodactyla. Tip labels are haplotypes as deposited in the EMBL database; Y and X are for Amel-Y and Amel-X haplotypes respectively. Stenella cœruleoalba haplotypes were named according to population origin (YA/Group 1, YB/Group 2, see Methods). (b) The inferred phylogeny after removing the insertion gives a slightly different picture: trans-specific clustering by sex-chromosome is lost except in Cetaceans.