Repeated turnovers keep sex chromosomes young in willows

背景回顾：Salicaceae species have diverse sex determination systems and frequent sex chromosome turnovers. 

提出问题：However, compared with poplars, the diversity of sex determination in willows is poorly understood, and little is known about the evolutionary forces driving their turnover. 

主要发现：Here, we characterized the sex determination in two Salix species, S. chaenomeloides and S. arbutifolia, which have an XY system on chromosome 7 and 15, respectively. 

结果1-杨柳性别决定系统：Based on the assemblies of their sex determination regions, we found that the sex determination mechanism of willows may have underlying similarities with poplars, both involving intact and/or partial homologs of a type A cytokinin respons e regulator (RR) gene. 

结果2-XY—ZW：Comparative analyses suggested that at least two sex turnover events have occurred in Salix, one preserving the ancestral pattern of male heterogamety, and the other changing heterogametic sex from XY to ZW, which could be partly explained by the “deleterious mutation load” and “sexually antagonistic selection” theoretical models. 

结果3-提出假设：We hypothesize that these repeated turnovers keep sex chromosomes of willow species in a perpetually young state, leading to limited degeneration. 

结论：Our findings further improve the evolutionary trajectory of sex chromosomes in Salicaceae species, explore the evolutionary forces driving the repeated turnovers of their sex chromosomes, and provide a valuable reference for the study of sex chromosomes in other species.

Hypothetical model for two sex system turnover events (7XY→15XY; 15XY→15ZW) in Salix.

杨柳科物种具有不同的性别决定系统以及频繁的性染色体转换。但是，与杨树相比，我们对于柳树性别决定系统的多样性还所知不多，尤其驱动其性染色体转换的演化因素更是不清楚。本文中，作者研究了两个柳树物种，即腺柳和钻天柳的性别决定系统，其XY染色体分别为7号和15号染色体。基于对其性别决定区域的组装，作者发现柳树的性别决定机制与杨树的存在相似性，二者都与完整和片段化的细胞分裂素A型响应调控因子RR基因有关。比较分析显示，柳属植物至少发生了两次性别转换事件，一次保留了祖先的雄性异形配子模式，另外一次则将XY型转变成了ZW型，这可以用“有害突变负荷”和“性别对抗选择”两种理论模型来进行部分解释。作者假设，这些重复出现的性别转换保持柳树的性染色体处于一个相对比较“年轻”的状态，从而使得衰退得以减弱。本文的研究进一步揭示了杨柳科物种性染色体的演化轨迹，探索了驱动性染色体重复转换的演化因素，为其他物种的性染色体研究提供了有益参考。

p.s. 

有害突变负荷，即由于基因的有害或致死突变而降低了适合度，给群体带来的负荷。

性别对抗选择，即在此性别中有益的基因在彼性别中是有害的。

** 马 涛 **