Department of Organismic and Evolutionary Biology, Harvard University, 16 Divinity Avenue, Cambridge, MA, 02138, USA,
Dev Genes Evol. 2014 Mar;224(2):65-77. doi: 10.1007/s00427-013-0463-7. Epub 2014 Jan 10.
In several metazoans including flies of the genus Drosophila, germ line specification occurs through the inheritance of maternally deposited cytoplasmic determinants, collectively called germ plasm. The novel insect gene oskar is at the top of the Drosophila germ line specification pathway, and also plays an important role in posterior patterning. A novel N-terminal domain of oskar (the Long Oskar domain) evolved in Drosophilids, but the role of this domain in oskar functional evolution is unknown. Trans-species transgenesis experiments have shown that oskar orthologs from different Drosophila species have functionally diverged, but the underlying selective pressures and molecular changes have not been investigated. As a first step toward understanding how Oskar function could have evolved, we applied molecular evolution analysis to oskar sequences from the completely sequenced genomes of 16 Drosophila species from the Sophophora subgenus, Drosophila virilis and Drosophila immigrans. We show that overall, this gene is subject to purifying selection, but that individual predicted structural and functional domains are subject to heterogeneous selection pressures. Specifically, two domains, the Drosophila-specific Long Osk domain and the region that interacts with the germ plasm protein Lasp, are evolving at a faster rate than other regions of oskar. Further, we provide evidence that positive selection may have acted on specific sites within these two domains on the D. virilis branch. Our domain-based analysis suggests that changes in the Long Osk and Lasp-binding domains are strong candidates for the molecular basis of functional divergence between the Oskar proteins of D. melanogaster and D. virilis. This molecular evolutionary analysis thus represents an important step towards understanding the role of an evolutionarily and developmentally critical gene in germ plasm evolution and assembly.
在包括果蝇属的几种后生动物中，生殖系的特化是通过母体细胞质决定因素的遗传来实现的，这些因素统称为生殖质。新型昆虫基因 oskar 位于果蝇生殖系特化途径的顶端，并且在后部模式形成中也起着重要作用。 Oskar 的一个新的 N 端结构域（长 Oskar 结构域）在果蝇目中进化，但该结构域在 Oskar 功能进化中的作用尚不清楚。跨物种转基因实验表明，来自不同果蝇物种的 Oskar 同源基因在功能上已经分化，但尚未研究潜在的选择压力和分子变化。作为了解 Oskar 功能如何进化的第一步，我们应用分子进化分析对来自 Sophophora 亚属的 16 种完全测序的果蝇物种的 Oskar 序列进行了分析，包括 Drosophila virilis 和 Drosophila immigrans。我们表明，总体而言，该基因受到纯化选择的影响，但个别预测的结构和功能域受到异质选择压力的影响。具体而言，两个域，即果蝇特有的长 Oskar 结构域和与生殖质蛋白 Lasp 相互作用的区域，比 Oskar 的其他区域进化得更快。此外，我们提供的证据表明，在 D. virilis 分支上，这些两个域中的特定位点可能受到正选择的作用。我们的基于结构域的分析表明，在 Long Osk 和 Lasp 结合结构域中的变化是 D. melanogaster 和 D. virilis 的 Oskar 蛋白之间功能分化的分子基础的强有力候选者。因此，这种分子进化分析代表了理解在生殖质进化和组装中起关键作用的关键基因的重要一步。
