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: i) 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; ii) 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.

已有研究表明，普通青蛙不同地理种群的性别决定与性别分化模式存在差异。值得注意的是，在瑞典北部种群中，表型性别（phenotypic sex）与连锁群2（linkage group 2，LG2）之间的关联极强且完全匹配；而在南部种群中，该关联则较弱且在不同家系（family）间存在差异。本研究通过使用其他连锁群的分子标记（marker）对这两类种群开展分析，获得两项全新发现：其一，南部种群中未被连锁群2解释的表型性别变异，无法归因于其他连锁群上的遗传因子（genetic factor），这表明性别决定存在表观遗传组分（epigenetic component）；其二，在北部种群中，第二条连锁群（LG7）被发现与性别及连锁群2存在共分离（co-segregate）现象。考虑到该种群自冰期后拓殖（post-glacial colonization）的时间极短（仅约1000代）且分布范围看似局限，这一新性染色体系统（neo-sex chromosome system）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（chromosome fusion）产生，更可能源于原始Y染色体（original Y chromosome，LG2）与常染色体（autosome，LG7）之间的相互易位（reciprocal translocation），进而导致二者在雄性减数分裂（male meiosis）过程中发生共分离。通过在性别决定级联反应（sex-determination cascade）中的数个关键基因（Dmrt1、Amh与Amhr2）之间建立严格的连锁关系，这一新性染色体可能促成了该北部种群特有的“分化性性别种群综合征”——即严格遗传性别决定（strictly-genetic sex determination）与早期性腺发育（early gonadal development）表型。