Reptile sex determination is attracting much attention because the great diversity of sex-determination and dosage compensation mechanisms permits us to approach fundamental questions about mechanisms of sex chromosome turnover. Recent studies have made significant progress in better understanding diversity and conservation of reptile sex chromosomes, with however no reptile master sex determination genes identified. Here we describe an integrated genomics and cytogenetics pipeline, combining probes generated from the microdissected sex chromosomes with transcriptome and genome sequencing to explore the sex chromosome diversity in non-model Australian reptiles. We tested our pipeline on a turtle, two species of geckos, and a monitor lizard. Genes identified on sex chromosomes were compared to the chicken genome to identify homologous regions among the four species. We identified candidate sex determining genes within these regions, including conserved vertebrate sex-determining genes pdgfa, pdgfra amh and wt1, and demonstrated their testis or ovary-specific expression. All four species showed gene-by-gene rather than chromosome-wide dosage compensation. Our results imply that reptile sex chromosomes originated by independent acquisition of sex-determining genes on different autosomes, as well as translocations between different ancestral macro- and microchromosomes. We discuss the evolutionary drivers of the slow differentiation and turnover of reptile sex chromosomes.
爬行动物的性别决定一直备受关注，其性别决定和剂量补偿机制的多样性是我们理解复杂的性染色体转换机制的绝佳材料。近年来，关于爬行动物性染色体多样性和保护的研究取得了一系列重要进展，但目前爬行动物的主效性别决定基因被报道和研究地极少。该文发展了一套将基因组学和细胞遗传学结合起来的研究性染色体的方法。我们首先通过显微解剖获得性染色体特异的探针，再与转录组和基因组测序相结合，从而探索研究了多个澳大利亚非模式爬行动物的性染色体多样性。我们在一种龟、两种壁虎和一种巨蜥中测试了我们的流程。通过将性染色体上的基因与鸡的基因组相比较，我们推断了这四个物种与鸡的同源的候选性别决定区域。我们在这些区域中发现了候选的性别决定基因，包括在脊椎动物中比较保守的性别决定基因 pdgfa、 pdgfra、 amh和 wt1，并证明了它们在睾丸或卵巢内具有组织特异性的基因表达。所有四个物种都表现出基因水平的剂量补偿，而不是染色体水平内的剂量补偿。我们的结果表明，爬行动物的性染色体起源于不同常染色体上独立获得的性别决定基因，以及来源于祖先中不同的大染色体和小染色体之间的易位。我们进一步探讨了爬行动物性染色体缓慢分化和相互之间发生颠换的演化驱动因素。.
Keywords: Cytogenetics; Genomics; Reptiles; Sex chromosome turnover; Sex determination.