Data from: The genetic contribution to sex determination and number of sex chromosomes vary among populations of common frogs (Rana temporaria)

The patterns of sex determination and sex differentiation have been shown to differ among geographic populations of common frogs. Notably, the association between phenotypic sex and linkage group 2 (LG2) has been found to be perfect in a northern Swedish population, but weak and variable among families in a southern one. By analyzing these populations with markers from other linkage groups, we bring two new insights: (1) the variance in phenotypic sex not accounted for by LG2 in the southern population could not be assigned to genetic factors on other linkage groups, suggesting an epigenetic component to sex determination; (2) a second linkage group (LG7) was found to co-segregate with sex and LG2 in the northern population. Given the very short timeframe since post-glacial colonization (in the order of 1000 generations) and its seemingly localized distribution, this neo-sex chromosome system might be the youngest one described so far. It does not result from a fusion, but more likely from a reciprocal translocation between the original Y chromosome (LG2) and an autosome (LG7), causing their co-segregation during male meiosis. By generating a strict linkage between several important genes from the sex-determination cascade (Dmrt1, Amh and Amhr2), this neo-sex chromosome possibly contributes to the ‘differentiated sex race’ syndrome (strictly genetic sex determination and early gonadal development) that characterizes this northern population.现有研究显示，普通青蛙（common frogs）不同地理种群的性别决定与性别分化模式存在差异。值得关注的是，在瑞典北部种群中，表型性别与连锁群2（linkage group 2，LG2）之间的关联完全吻合；而在南部种群中，二者的关联则较弱且在家系间存在变异。本研究通过使用来自其他连锁群的分子标记对上述种群进行分析，获得两项全新的研究发现：（1）南部种群中未被LG2解释的表型性别变异，无法归因于其他连锁群上的遗传因素，这提示性别决定存在表观遗传组分；（2）研究发现北部种群中第二个连锁群（LG7）与性别及LG2发生共分离。鉴于该新性染色体系统（neo-sex chromosome system）自冰期后拓殖以来的时间跨度极短（仅约1000代）且分布范围看似局限，它可能是目前已报道的最年轻的新性染色体系统。该系统并非由染色体融合演化而来，更可能源于原始Y染色体（LG2）与一条常染色体（LG7）之间的相互易位，进而导致二者在雄性减数分裂过程中共分离。通过使性别决定级联中的数个关键基因（Dmrt1、Amh及Amhr2）之间形成严格的连锁关系，这一新性染色体可能促成了该北部种群特有的"分化性宗综合征（differentiated sex race syndrome）"——即具备严格遗传性别决定与早期性腺发育的特征。