The discovery of sex chromosome systems in non-model organisms has elicited growing recognition that sex chromosomes evolved via diverse paths that are not fully elucidated. Lineages with labile sex determination, such as turtles, hold critical cues, yet data are skewed toward hide-neck turtles (suborder Cryptodira) and scant for side-neck turtles (suborder Pleurodira). Here, we used classic and molecular cytogenetics to investigate Emydura subglobosa (ESU), an unstudied side-neck turtle with genotypic sex determination from the family Chelidae, where extensive morphological divergence exists among XX/XY systems. Our data represent the first cytogenetic description for ESU. Similarities were found between ESU and E. macquarii (EMA), such as identical chromosome number (2n = 50), a single and dimorphic nucleolus organizer region (NOR) localized in a microchromosome pair (ESU14) of both sexes (detected via FISH of 18S rDNA). Only the larger NOR is active (detected by silver staining). As in EMA, comparative genome hybridization revealed putative macro XX/XY chromosomes in ESU (the 4th largest pair). Our comparative analyses and revaluation of previous data strongly support the hypothesis that Emydura's XX/XY system evolved via fusion of an ancestral micro-Y (retained by Chelodina longicollis) onto a macro-autosome. This evolutionary trajectory differs from the purported independent evolution of XX/XY from separate ancestral autosomes in Chelodina and Emydura that was previously reported. Our data permit dating this Y-autosome fusion to at least the split of Emydura around 45 Mya and add critical information about the evolution of the remarkable diversity of sex-determining mechanisms in turtles, reptiles, and vertebrates.

中文翻译：

---

推定的XX / XY男性Heterogamety的发现在澳龟属subglobosa的海龟暴露性染色体演进的新型轨迹在澳龟属

非模式生物中性染色体系统的发现引起了人们越来越多的认识，即性染色体是通过尚未完全阐明的多种途径进化而来的。具有不稳定性别决定的谱系，例如海龟，持有关键线索，但数据偏向于隐藏颈龟（Cryptodira 亚目）和侧颈龟（Pleurodira 亚目）。在这里，我们使用经典和分子细胞遗传学来研究 Emydura subglobosa (ESU)，这是一种未经研究的侧颈龟，具有来自 Chelidae 家族的基因型性别决定，其中 XX/XY 系统之间存在广泛的形态差异。我们的数据代表了 ESU 的第一个细胞遗传学描述。在 ESU 和 E. macquari (EMA) 之间发现了相似性，例如相同的染色体数 (2n = 50)，位于两性微染色体对 (ESU14) 中的单个和二形核仁组织区 (NOR)（通过 18S rDNA 的 FISH 检测）。只有较大的 NOR 是活跃的（通过银染检测）。与 EMA 一样，比较基因组杂交揭示了 ESU 中假定的宏 XX/XY 染色体（第四大对）。我们对先前数据的比较分析和重新评估强烈支持了以下假设：Emydura 的 XX/XY 系统是通过将祖先微 Y（由 Chelodina longicollis 保留）融合到宏常染色体上进化而来的。这种进化轨迹不同于先前报道的 XX/XY 与 Chelodina 和 Emydura 中单独的祖先常染色体的独立进化。