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Stickleback fishes show a remarkable diversity of sex chromosomes. At left is a phylogeny for 7 species. The first column to the right of the species names shows the type of sex chromosome system. X1X2Y indicates that an autosome has fused with the ancestral Y, which results in a neo-X (the former partner of the fused autosome) that is unlinked to but segregates with the ancestral X. The second column shows the linkage group that determines sex. After Hendry et al. (2009).
