Conservation of Sex-Linked Markers among Conspecific Populations of a Viviparous Skink, Niveoscincus ocellatus, Exhibiting Genetic and Temperature-Dependent Sex Determination

Abstract

Sex determination systems are exceptionally diverse and have undergone multiple and independent evolutionary transitions among species, particularly reptiles. However, the mechanisms underlying these transitions have not been established. Here, we tested for differences in sex-linked markers in the only known reptile that is polymorphic for sex determination system, the spotted snow skink, Niveoscincus ocellatus, to quantify the genomic differences that have accompanied this transition. In a highland population, sex is determined genetically, whereas in a lowland population, offspring sex ratio is influenced by temperature. We found a similar number of sex-linked loci in each population, including shared loci, with genotypes consistent with male heterogamety (XY). However, population-specific linkage disequilibrium suggests greater differentiation of sex chromosomes in the highland population. Our results suggest that transitions between sex determination systems can be facilitated by subtle genetic differences.

Fig. 1.

—Hamming’s proportional distance among Niveoscincus ocellatus individuals of highland (H) and lowland (L) populations for sex-linked loci unique to the highland (left panel) and lowland (right panel) populations. Presence/absence (PA; lower segment) and SNP (upper segment). Highland PA n = 16, lowland PA n = 20, highland SNPs n = 12, lowland SNPs n = 16.

Fig. 2.

—Linkage disequilibrium network plot of sex-linked clusters from the highland and lowland populations of Niveoscincus ocellatus. Green circles indicate sex-linked SNPs (n = 32 in the highland, n = 34 in the lowland populations); “a”–“u” denote 21 loci sex-linked in both the highland and lowland population. The perfectly sex-linked locus for each cluster is in black. Red solid edges have an R² > 0.99, black dashed 0.99 > R² > 0.80, gray dotted R² < 0.80.