Supplementary Material for: Extraordinary Diversity in the Origins of Sex Chromosomes in Anurans Inferred from Comparative Gene Mapping

Sex determination in frogs (anurans) is genetic and includes both male and female heterogamety. However, the origins of the sex chromosomes and their differentiation processes are poorly known. To investigate diversity in the origins of anuran sex chromosomes, we compared the chromosomal locations of sex-linked genes in 4 species: the African clawed frog (Xenopus laevis), the Western clawed frog (Silurana/X. tropicalis), the Japanese bell-ring frog (Buergeria buergeri), and the Japanese wrinkled frog (Rana rugosa). Comparative mapping data revealed that the sex chromosomes of X. laevis, X. tropicalis and R. rugosa are different chromosome pairs; however, the sex chromosomes of X. tropicalis and B. buergeri are homologous, although this may represent distinct evolutionary origins. We also examined the status of sex chromosomal differentiation in B. buergeri, which possesses heteromorphic ZW sex chromosomes, using comparative genomic hybridization and chromosome painting with DNA probes from the microdissected W chromosome. At least 3 rearrangement events have occurred in the proto-W chromosome: deletion of the nucleolus organizer region and a paracentric inversion followed by amplification of non-W-specific repetitive sequences.

蛙类（无尾目Anurans）的性别决定由遗传机制调控，且兼具雄性异配与雌性异配两种形式。然而，学界对其性染色体的起源及分化进程仍了解有限。为探究无尾目性染色体起源的多样性，本研究对4个物种的性别连锁基因染色体定位开展了比较分析，涉及物种包括非洲爪蟾（Xenopus laevis）、西方爪蟾（Silurana/X. tropicalis）、日本铃鸣蛙（Buergeria buergeri）以及日本皱蛙（Rana rugosa）。比较定位数据显示，非洲爪蟾、热带爪蟾与日本皱蛙的性染色体分属不同的染色体对；但热带爪蟾与日本铃鸣蛙的性染色体具有同源性，尽管这一现象可能对应着各自独立的进化起源。本研究还借助比较基因组杂交（comparative genomic hybridization）技术，以及基于显微切割W染色体制备的DNA探针的染色体涂染（chromosome painting）技术，对拥有异形ZW性染色体的日本铃鸣蛙的性染色体分化状态进行了检测。该物种的原W染色体至少经历了3次重排事件：核仁组织者区的缺失、一次臂内倒位，以及随后发生的非W特异性重复序列的扩增。