The rise and fall of differentiated sex chromosomes in geckos.

Amniotes possess variability in sex determination, ranging from environmental sex determination to genotypic sex determination with differentiated sex chromosomes. Differentiated sex chromosomes have emerged independently several times. Their noteworthy convergent characteristic is the evolutionary stability, documented among amniotes in mammals, birds, and some lineages of lizards, snakes and turtles. Combining the analysis of multiple partial transcriptomes with the comparison of copy gene numbers between male and female genomes, we uncovered partial gene content of the highly differentiated ZZ/ZW sex chromosomes in the gecko genus Paroedura. The differentiated ZZ/ZW sex chromosomes of these geckos share genes with the part of the chicken chromosome 4 homologous with the XX/XY sex chromosomes of viviparous mammals and the ZZ/ZW sex chromosomes of lacertid lizards, as well as with the chicken chromosome 15, homologous with the XX/XY sex chromosomes of iguanas and ZZ/ZW sex chromosomes of softshell turtles. Along with other analogous cases, this finding reinforces the observation that particular chromosomes are repeatedly coopted for the function of sex chromosomes in amniotes. Notably, according to the phylogenetic distribution, the subclade of the genus Paroedura represents a rare case of the reversal of the for a considerable evolutionary time highly differentiated ZZ/ZW sex chromosomes back to poorly differentiated state.

Sex determination in Madagascar geckos of the genus Paroedura (Squamata: Gekkonidae): are differentiated sex chromosomes indeed so evolutionary stable?

Among amniote vertebrates, geckos represent a clade with exceptional variability in sex determination; however, only a minority of species of this highly diverse group has been studied in this respect. Here, we describe for the first time a female heterogamety in the genus Paroedura, the group radiated in Madagascar and adjacent islands. We identified homomorphic ZZ/ZW sex chromosomes with a highly heterochromatic W chromosome in Paroedura masobe, Paroedura oviceps, Paroedura karstophila, Paroedura stumpffi, and Paroedura lohatsara. Comparative genomic hybridization (CGH) revealed that female-specific sequences are greatly amplified in the W chromosome of P. lohatsara and that P. gracilis seems to possess a derived system of multiple sex chromosomes. Contrastingly, neither CGH nor heterochromatin visualization revealed differentiated sex chromosomes in the members of the Paroedura picta-Paroedura bastardi-Paroedura ibityensis clade, which is phylogenetically nested within lineages with a heterochromatic W chromosome. As a sex ratio consistent with genotypic sex determination has been reported in P. picta, it appears that the members of the P. picta-P. bastardi-P. ibityensis clade possess homomorphic, poorly differentiated sex chromosomes and may represent a rare example of evolutionary loss of highly differentiated sex chromosomes. Fluorescent in situ hybridization (FISH) with a telomeric probe revealed a telomere-typical pattern in all species and an accumulation of telomeric sequences in the centromeric region of autosomes in P. stumpffi and P. bastardi. Our study adds important information for the greater understanding of the variability and evolution of sex determination in geckos and demonstrates how the geckos of the genus Paroedura provide an interesting model for studying the evolution of the sex chromosomes.