Cretaceous park of sex determination: sex chromosomes are conserved across iguanas

Abstract

Many poikilothermic vertebrate lineages, especially among amphibians and fishes, possess a rapid turnover of sex chromosomes, while in endotherms there is a notable stability of sex chromosomes. Reptiles in general exhibit variability in sex-determining systems; as typical poikilotherms, they might be expected to have a rapid turnover of sex chromosomes. However, molecular data which would enable the testing of the stability of sex chromosomes are lacking in most lineages. Here, we provide molecular evidence that sex chromosomes are highly conserved across iguanas, one of the most speciesrich clade of reptiles. We demonstrate that members of the New World families Iguanidae, Tropiduridae, Leiocephalidae, Phrynosomatidae, Dactyloidae and Crotaphytidae, as well as of the family Opluridae which is restricted to Madagascar, all share homologous sex chromosomes. As our sampling represents the majority of the phylogenetic diversity of iguanas, the origin of iguana sex chromosomes can be traced back in history to the basal splitting of this group which occurred during the Cretaceous period. Iguanas thus show a stability of sex chromosomes comparable to mammals and birds and represent the group with the oldest sex chromosomes currently known among amniotic poikilothermic vertebrates.

Evolutionary stability of sex chromosomes in snakes

“…We designed specific primers for putative snake Z-specific genes and tested their sex linkage using qPCR. This approach provides a reliable comparison of the number of gene copies between sexes [5,17,[22][23][24]. We recently applied it to iguanas, revealing a high conservation of sex chromosomes across most of their families and confirming X-linkage of hundreds of genes in the model species ACA [5,17].…”

“…However, taking into account the recent evidence for evolutionary turnovers or losses of differentiated sex chromosomes in non-vertebrates [14,15] and reptiles [16,17], as well the fact that cytogenetically similar sex chromosomes may not be homologous [14,15], we cannot exclude a turnover of sex chromosomes even in this highly species-rich lineage. Moreover, it is not known whether snake lineages outside this clade also share homologous sex chromosomes.…”

“…The qPCR was carried out in a LightCycler II 480 (Roche Diagnostics) with all samples run in triplicate. The detailed qPCR protocol and the description of the calculation of the relative gene dose between sexes are available in our previous reports [5,17,22]. Briefly, the gene dosage of each target gene was calculated from crossing point values and was subsequently normalized to the dose of the single-copy reference gene eef1a1 from the same DNA sample.…”

Mammalian X homolog acts as sex chromosome in lacertid lizards

“…The qPCR with DNA template was carried out in a LightCycler II 480 (Roche Diagnostics, Basel, Switzerland) with all samples run in triplicate. The detailed qPCR protocol and the formula for the calculation of the relative gene dose between sexes have been presented in our previous papers (Rovatsos et al, 2014a(Rovatsos et al, , b, 2015. A relative female-to-male gene dosage ratio (r) of 0.5 is expected for Z-specific genes, that is, the genes lacking in the degenerated part of the W chromosome, and 1.0 for (pseudo)autosomal genes.…”

“…This difference in copy number between sexes can be determined by qPCR, allowing the reliable identification of sex-specific genes (Rovatsos et al, 2014a(Rovatsos et al, -c, 2015; for similar application of qPCR see also Nguyen et al, 2013;Gamble et al, 2014;Literman et al, 2014).…”