The dragon lizard Pogona vitticeps has ZZ/ZW micro-sex chromosomes

Abstract

The bearded dragon, Pogona vitticeps (Agamidae: Reptilia) is an agamid lizard endemic to Australia. Like crocodilians and many turtles, temperature-dependent sex determination (TSD) is common in agamid lizards, although many species have genotypic sex determination (GSD). P. vitticeps is reported to have GSD, but no detectable sex chromosomes. Here we used molecular cytogenetic and differential banding techniques to reveal sex chromosomes in this species. Comparative genomic hybridization (CGH), GTG- and C-banding identified a highly heterochromatic microchromosome specific to females, demonstrating female heterogamety (ZZ/ZW) in this species. We isolated the P. vitticeps W chromosome by microdissection, re-amplified the DNA and used it to paint the W. No unpaired bivalents were detected in male synaptonemal complexes at meiotic pachytene, confirming male homogamety. We conclude that P. vitticeps has differentiated previously unidentifable W and Z micro-sex chromosomes, the first to be demonstrated in an agamid lizard. Our finding implies that heterochromatinization of the heterogametic chromosome occurred during sex chromosome differentiation in this species, as is the case in some lizards and many snakes, as well as in birds and mammals. Many GSD reptiles with cryptic sex chromosomes may also prove to have micro-sex chromosomes. Reptile microchromosomes, long dismissed as non-functional minutiae and often omitted from karyotypes, therefore deserve closer scrutiny with new and more sensitive techniques.

Females Have Larger Ratio of Second‐to‐Fourth Digits Than Males in Four Species of Salamandridae, Caudata

“…As far as we know, there is no information about the sex chromosomes of Dendrobatidae, Craugastoridae, Polychrotidae, and Tropiduridae in the literature. Dactyloidae members have homomorphic sex chromosomes (Gamble et al, 2014), but Lacertidae, Agamidae, and the majority of Gekkonidae members have female heterogametic systems (Olmo et al, 1987;Ezaz et al, 2005Ezaz et al, , 2009Kupriyanova et al, 2008;Molavi et al, 2014), while Scincidae and Iguanidae members have male systems (Ezaz et al, 2009). The above-mentioned information about chromosomal sex-determination systems confirmed our statement that species with female heterogametic systems (such as Lacertidae and Agamidae) are characterized by higher digit ratios in males, whereas, species with male heterogametic systems (such as humans and all species of newts which we studied) are characterized by higher digit-ratio in females.…”

Did sex chromosome turnover promote divergence of the major mammal groups?

“…There are phylogenies of closely related agamid lizards that reveal at least four flip‐flops through genetic sex determination (GSD) and temperature sex determination (TSD), generating novel sex chromosomes 27. Some reptile species have sex chromosome systems that can be overridden by temperature 28, 29; these dual systems are extremely labile, able to change rapidly to TSD systems and even between ZW and XY 30, 31. Similar GSD‐TSD transitions may be common in some fish and may also facilitate changes in sex determination 32, 33, 34.…”

Extension, single-locus conversion and physical mapping of sex chromosome sequences identify the Z microchromosome and pseudo-autosomal region in a dragon lizard, Pogona vitticeps

“…Female heterogametic GSD (ZZ males/ZW females) is apparently ubiquitous in snakes (Beçak et al, 1964;Ohno, 1967;Matsubara et al, 2006) whereas turtles and lizards with GSD have either female or male heterogamety (XY males/XX females), in some cases with multiple sex chromosomes (Solari, 1994). The heterogametic sex of many reptiles is yet to be determined because the sex chromosomes are homomorphic or cryptic (Ezaz et al, 2005(Ezaz et al, , 2006b.…”