Data from: Recurrent losses and rapid evolution of the condensin II complex in insects

Condensins play a crucial role in the organization of genetic material by compacting and disentangling chromosomes. Based on studies in a few model organisms, the condensin I and condensin II complexes are considered to have distinct functions, with the condensin II complex playing a role in meiosis and somatic pairing of homologous chromosomes in Drosophila. Intriguingly, the Cap-G2 subunit of condensin II is absent in Drosophila melanogaster, and this loss may be related to the high levels of chromosome pairing seen in flies. Here, we find that all three non-SMC subunits of condensin II (Cap-G2, Cap-D3, and Cap-H2) have been repeatedly and independently lost in taxa representing multiple insect orders, with some taxa lacking all three. We also find that all non-Dipteran insects display near-uniform low pairing levels regardless of their condensin II complex composition, suggesting that some key aspects of genome organization are robust to condensin II subunit losses. Finally, we observe consistent signatures of positive selection in condensin subunits across flies and mammals. These findings suggest that these ancient complexes are far more evolutionarily labile than previously suspected, and are at the crossroads of several forms of genomic conflicts. Our results raise fundamental questions about the specific functions of the two condensin complexes in taxa that have experienced subunit losses, and open the door to further investigations to elucidate the diversity of molecular mechanisms that underlie genome organization across various life forms.黏连蛋白（Condensins）通过压缩并解缠染色体，在遗传物质的组织过程中发挥关键作用。基于对少数模式生物的研究，黏连蛋白I与黏连蛋白II复合物被认为功能迥异，其中黏连蛋白II复合物在果蝇（Drosophila）的减数分裂与同源染色体体细胞配对中发挥功能。值得注意的是，黑腹果蝇（Drosophila melanogaster）中缺失黏连蛋白II的Cap-G2亚基，这一缺失或许与果蝇体内高水平的染色体配对现象相关。本研究发现，黏连蛋白II的三个非SMC亚基（Cap-G2、Cap-D3与Cap-H2）在涵盖多个昆虫目的类群中发生了多次独立丢失，部分类群甚至完全缺失这三个亚基。我们同时观察到，所有非双翅目昆虫的染色体配对水平均近乎一致地处于较低水平，且与其黏连蛋白II复合物的组成并无关联，这提示基因组组织的部分关键特征对黏连蛋白II亚基的丢失具有鲁棒性。最后，我们在果蝇与哺乳动物的黏连蛋白亚基中检测到了一致的正选择信号。上述研究结果表明，这类古老的复合物在进化上的可塑性远高于此前的推测，且处于多种基因组冲突的交汇节点。本研究对发生了亚基丢失的类群中两种黏连蛋白复合物的特定功能提出了基础性问题，同时为进一步阐明不同生命形式中基因组组织背后的分子机制多样性开辟了研究路径。