Department of Molecular and Cellular Biology, Harvard University, Cambridge, MA, USA.
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
Mol Biol Evol. 2021 Dec 9;38(12):5491-5513. doi: 10.1093/molbev/msab284.
Germ line specification is essential in sexually reproducing organisms. Despite their critical role, the evolutionary history of the genes that specify animal germ cells is heterogeneous and dynamic. In many insects, the gene oskar is required for the specification of the germ line. However, the germ line role of oskar is thought to be a derived role resulting from co-option from an ancestral somatic role. To address how evolutionary changes in protein sequence could have led to changes in the function of Oskar protein that enabled it to regulate germ line specification, we searched for oskar orthologs in 1,565 publicly available insect genomic and transcriptomic data sets. The earliest-diverging lineage in which we identified an oskar ortholog was the order Zygentoma (silverfish and firebrats), suggesting that oskar originated before the origin of winged insects. We noted some order-specific trends in oskar sequence evolution, including whole gene duplications, clade-specific losses, and rapid divergence. An alignment of all known 379 Oskar sequences revealed new highly conserved residues as candidates that promote dimerization of the LOTUS domain. Moreover, we identified regions of the OSK domain with conserved predicted RNA binding potential. Furthermore, we show that despite a low overall amino acid conservation, the LOTUS domain shows higher conservation of predicted secondary structure than the OSK domain. Finally, we suggest new key amino acids in the LOTUS domain that may be involved in the previously reported Oskar-Vasa physical interaction that is required for its germ line role.
生殖系的特化对于有性繁殖的生物体至关重要。尽管它们具有重要作用，但特化动物生殖细胞的基因的进化历史是异质和动态的。在许多昆虫中， Oskar 基因是生殖系特化所必需的。然而， Oskar 基因的生殖系作用被认为是从祖先的体节作用中共同进化而来的衍生作用。为了解决蛋白质序列的进化变化如何导致 Oskar 蛋白功能的变化，使其能够调节生殖系特化，我们在 1565 个公开的昆虫基因组和转录组数据集搜索了 Oskar 的同源基因。我们在最早分化的谱系中鉴定出 Oskar 的直系同源物是 Zygentoma 目（银鱼和火虫），这表明 Oskar 起源于有翅昆虫出现之前。我们注意到 Oskar 序列进化中的一些特定目趋势，包括全基因复制、分支特异性缺失和快速分化。所有已知的 379 个 Oskar 序列的比对揭示了新的高度保守残基，这些残基可能促进 LOTUS 结构域的二聚化。此外，我们确定了 OSK 结构域中具有保守 RNA 结合潜力的区域。此外，我们表明，尽管总体氨基酸保守性较低，但 LOTUS 结构域的预测二级结构比 OSK 结构域具有更高的保守性。最后，我们提出了 LOTUS 结构域中可能参与先前报道的 Oskar-Vasa 物理相互作用的新关键氨基酸，该相互作用是 Oskar 发挥其生殖系作用所必需的。
