Abstract
The majority of lizards classified in the superfamily Iguanoidea have an XX/XY sex-determination system in which sex-chromosomal linkage shows homology with chicken (Gallus gallus) chromosome 15 (GGA15). However, the genomics of sex chromosomes remain largely unexplored owing to the presence of homomorphic sex chromosomes in majority of the species. Recent advances in high-throughput genome complexity reduction sequencing provide an effective approach to the identification of sex-specific loci with both single-nucleotide polymorphisms (SNPs) and restriction fragment presence/absence (PA), and a better understanding of sex chromosome dynamics in Iguanoidea. In this study, we applied Diversity Arrays Technology (DArTseqTM) in 29 phenotypic sex assignments (14 males and 15 females) of green iguana (Iguana iguana). We confirmed a male heterogametic (XX/XY) sex determination mode in this species, identifying 29 perfectly sex-linked SNP/PA loci and 164 moderately sex-linked SNP/PA loci, providing evidence probably indicative of XY recombination. Three loci from among the perfectly sex-linked SNP/PA loci showed partial homology with several amniote sex chromosomal linkages. The results support the hypothesis of an ancestral super-sex chromosome with overlaps of partial sex-chromosomal linkages. However, only one locus among the moderately sex-linked loci showed homology with GGA15, which suggests that the specific region homologous to GGA15 was located outside the non-recombination region but in close proximity to this region of the sex chromosome in green iguana. Therefore, the location of GGA15 might be further from the putative sex-determination locus in green iguana. This is a paradigm shift in understanding linkages on homomorphic X and Y sex chromosomes. The DArTseq platform provides an easy-to-use strategy for future research on the evolution of sex chromosomes in Iguanoidea, particularly for non-model species with homomorphic or highly cryptic sex chromosomes.
Highlights
The genetic framework for sex organ differentiation is highly conserved, but a diversity of sex determination is observed in amniotes (O’Meally et al, 2012; Ezaz et al, 2017)
Comparison of the dosages of four genes (TMEM132D, CCDC92, ATP2A2, and PEBP1) among five males and five females by quantitative polymerase chain reaction (qPCR) showed that the green iguana exhibited non-sex-specific dosages (Table 1, Figure 1 and Supplementary Tables S4–S7) for ACA X-linked genes, sex-chromosomal homology was confirmed from the similarity to ACAX and correspondence to GGA15 (Rovatsos et al, 2014b)
Recent evidence based on comparison of partial gene content using a qPCR approach has shown that sex chromosomes of green iguana are highly conserved with many lizards of Iguanoidea (Rovatsos et al, 2014b)
Summary
The genetic framework for sex organ differentiation is highly conserved, but a diversity of sex determination is observed in amniotes (O’Meally et al, 2012; Ezaz et al, 2017). Studies of the partially comparative gene constituents of sex chromosomes between males and females using quantitative polymerase chain reaction (qPCR) analysis have revealed a pattern of different gene dosages in ACAX-linked regions in the majority of iguanoid lizards, which probably exhibit the XX/XY sex-determination system (Alföldi et al, 2011; Gamble et al, 2014; Rovatsos et al, 2014a,b; Altmanová et al, 2017) These results suggest that sex-chromosomal linkage homology is highly conserved in lizards of Iguanoidea
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