First Report of Sex Chromosomes in Night Lizards (Scincoidea: Xantusiidae)

Abstract

Squamate reptiles (lizards, snakes, and amphibians) are an outstanding group for studying sex chromosome evolution—they are old, speciose, geographically widespread, and exhibit myriad sex-determining modes. Yet, the vast majority of squamate species lack heteromorphic sex chromosomes. Cataloging the sex chromosome systems of species lacking easily identifiable, heteromorphic sex chromosomes, therefore, is essential before we are to fully understand the evolution of vertebrate sex chromosomes. Here, we use restriction site-associated DNA sequencing (RADseq) to classify the sex chromosome system of the granite night lizard, Xantusia henshawi. RADseq is an effective alternative to traditional cytogenetic methods for determining a species’ sex chromosome system (i.e., XX/XY or ZZ/ZW), particularly in taxa with non-differentiated sex chromosomes. Although many xantusiid lineages have been karyotyped, none possess heteromorphic sex chromosomes. We identified a ZZ/ZW sex chromosome system in X. henshawi—the first such data for this family. Furthermore, we report that the X. henshawi sex chromosome contains fragments of genes found on Gallus gallus chromosomes 7, 12, and 18 (which are homologous to Anolis carolinensis chromosome 2), the first vertebrate sex chromosomes to utilize this linkage group.

Conserved ZZ/ZW sex chromosomes in Caribbean croaking geckos (Aristelliger: Sphaerodactylidae)

“…We do not have sufficient sampling to analyse the A. expectatus clades individually but we suspect that there are many more lineage‐specific sex‐specific RAD markers that remain unidentified. However, we here show that this methodology can prove successful even with highly divergent samples (see also Hundt, Liddle, Nielsen, Pinto, & Gamble, 2019; Nielsen, Pinto, Guzmán‐Méndez, & Gamble, 2020). Although we would not expect to find shared sex‐specific RAD markers in all cases, these results should be encouraging in cases where samples are of uncertain taxonomic placement, not geographically proximate, or might have elevated genetic diversity.…”

“…We do not have sufficient sampling to analyse the A. expectatus clades individually but we suspect that there are many more lineage-specific sex-specific RAD markers that remain unidentified. However, we here show that this methodology can prove successful even with highly divergent samples (see also Hundt, Liddle, Nielsen, Pinto, & Gamble, 2019;Nielsen, Pinto, Guzmán-Méndez, & Gamble, 2020 Aristelliger expectatus is likely comprised of cryptic diversity.…”

Mixed‐sex offspring produced via cryptic parthenogenesis in a lizard

“…However, we were not able to identify sex chromosomes via cytogenetic examination after conventional and differential staining (Figure ), suggesting that if present, the sex chromosomes are only poorly differentiated, which is not uncommon in certain lineages of reptiles (Johnson Pokorná & Kratochvíl, 2016). Molecular evidence for ZZ/ZW sex chromosomes within xantusiid lizard was provided in Xantusia henshawi (Nielsen, Pinto, Guzmán‐Méndez, & Gamble, 2020). The divergence between the genera Xantusia and Lepidophyma is several dozen million years (Pyron, Burbrink, & Wiens, 2013); however, considering the large stability of sex chromosomes in most amniote lineages (Johnson Pokorná & Kratochvíl, 2016), it is quite possible that these genera share homologous sex chromosomes.…”

Poorly differentiated XX/XY sex chromosomes are widely shared across skink radiation

“…The orthologs of X-specific genes of the common sandfish showed a pseudoautosomal or autosomal pattern in the outgroups from the three families (Cordylidae, Gerrhosauridae and Xantusiidae) who along with skinks form the clade Scincoidea. ZZ/ZW sex chromosomes containing genes from other regions were reported in a night lizard from the family Xantusiidae [65], while sex determination is not known up to now in any cordylid or gerrhosaurid lizards.…”

“…However, other lineages such as the caenophidian and non-caenophidian snakes, the xantusiid lizard Xantusia henshawi, the pygopodid geckos and skinks [63][64][65][66]…”