Summary from the authors: The rise and fall of differentiated sex chromosomes in geckos

Link to the paper: https://onlinelibrary.wiley.com/doi/full/10.1111/mec.15126

The Madagascar ground gecko (Paroedura picta) is a member of one of the few vertebrate lineages suspicious of the loss of highly differentiated sex chromosomes. Photo credit: Petr Jan Juračka.

Differentiated sex chromosomes such as XX/XY chromosomes of viviparous mammals and ZZ/ZW sex chromosomes of birds with highly degenerated Y and W, respectively, evolved in animals multiple times. Their noteworthy convergent characteristic is the evolutionary stability, documented among amniotes for dozens of millions of years in mammals, birds, and some lineages of lizards, snakes and turtles. The differentiation of sex chromosomes stemming from the cessation of recombination between them is assumed to be largely a one-way process. We found that the differentiated ZZ/ZW sex chromosomes with highly degenerated W of the Madagascan geckos of the genus Paroedura were likely present in the common ancestor of the genus. However, the subclade of the genus seems to reverse the for a considerable evolutionary time highly differentiated ZZ/ZW sex chromosomes back to poorly differentiated state and thus represents a rare case of the loss of once highly differentiated sex chromosomes. Notably, the differentiated ZZ/ZW sex chromosomes of these geckos share genes with the XX/XY sex chromosomes of viviparous mammals and the ZZ/ZW sex chromosomes of lacertid lizards, as well as with the XX/XY sex chromosomes of iguanas and ZZ/ZW sex chromosomes of softshell turtles. Along with other analogous cases which we summarize in our contribution, this finding reinforces the observation that particular chromosomes are repeatedly co -opted for the function of sex chromosomes in amniotes.

The reconstruction of the evolutionary history of sex chromosomes in the gecko genus Paroedura as revealed by the distribution of the sexual differences in copy numbers of genes linked to differentiated ancestral Z chromosome of the genus. Note that these genes were originally likely autosomal, i.e. they had the same number of copies in males and females (yellow). In the common ancestor of the genus, these genes had twice as many copies in males (ZZ) than in females (ZW) as a consequence of their loss from the degenerated W (violet). Still later in a subclade of the genus, the same genes turned back to the same copy numbers in both sexes (light blue) suggesting a reversal of the ancestral differentiated sex chromosomes back to poorly differentiated state.

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