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

  • ️Fri Jan 01 2021

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

Alexander Kostmann et al. Proc Biol Sci. 2021.

Abstract

Differentiated sex chromosomes are believed to be evolutionarily stable, while poorly differentiated sex chromosomes are considered to be prone to turnovers. With around 1700 currently known species forming ca 15% of reptile species diversity, skinks (family Scincidae) are a very diverse group of squamates known for their large ecological and morphological variability. Skinks generally have poorly differentiated and cytogenetically indistinguishable sex chromosomes, and their sex determination was suggested to be highly variable. Here, we determined X-linked genes in the common sandfish (Scincus scincus) and demonstrate that skinks have shared the same homologous XX/XY sex chromosomes across their wide phylogenetic spectrum for at least 85 million years, approaching the age of the highly differentiated ZZ/ZW sex chromosomes of birds and advanced snakes. Skinks thus demonstrate that even poorly differentiated sex chromosomes can be evolutionarily stable. The conservation of sex chromosomes across skinks allows us to introduce the first molecular sexing method widely applicable in this group.

Keywords: genome; molecular sexing; qPCR; sex chromosomes; sex determination; vertebrates.

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Conflict of interest statement

We declare we have no competing interests.

Figures

Figure 1.
Figure 1.

Karyogram reconstruction from metaphase after FISH with the probe for rDNA loci in male individuals of Scincus scincus (a) and Tropidophorus baconi (b). Signal of rDNA loci was detected in a single chromosome from the 9th pair in males of S. scincus, but in both chromosomes of the 12th pair in T. baconi. CGH revealed differences between sexes neither in males, nor in females in both S. scincus (c,d) and T. baconi (e,f). (Online version in colour.)

Figure 2.
Figure 2.

Log2-transformed male to female ratios of DNA-seq read coverage per gene in Scincus scincus. The genes are illustrated according to the position of their orthologues in the chromosomes of Podarcis muralis. Note that the region homologous to PMU10 (indicated by a red arrow) possesses half male to female ratio in read coverage depth, demonstrating that this genome part is X-specific in Scincus scincus. (Online version in colour.)

Figure 3.
Figure 3.

Average relative gene dose ratios between males and females across 13 species of skinks (Scincidae) and four outgroup species from the families Cordylidae, Gerrhosauridae and Xantusiidae. Blue bars represent autosomal control genes. The genes from the X-specific region of S. scincus have either ratio around 0.5 consistent with the X-specificity (red bars), or around 1.0 consistent with pseudoautosomal or autosomal position (orange bars). Numbers of successfully tested genes in each category are presented. All data are presented in electronic supplementary material, table S3. (Online version in colour.)

Figure 4.
Figure 4.

Although sex determination was traditionally assumed to be highly unstable in squamate reptiles, around 60% of squamates are members of five clades with sex chromosomes conserved for dozens of millions of years [current study; –18,20,21]. A molecular sexing method based on qPCR was developed for each of these clades, allowing the accurate identification of the sex in approximately 6000 species of squamates. (Online version in colour.)

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