Civetta Alberto, Rajakumar Sujeetha A, Brouwers Barb, Bacik John P
Department of Biology, University of Winnipeg, Winnipeg, Manitoba, Canada.
Mol Biol Evol. 2006 Mar;23(3):655-62. doi: 10.1093/molbev/msj074. Epub 2005 Dec 15.
Hybrid males resulting from crosses between closely related species of Drosophila are sterile. The F1 hybrid sterility phenotype is mainly due to defects occurring during late stages of development that relate to sperm individualization, and so genes controlling sperm development may have been subjected to selective diversification between species. It is also possible that genes of spermatogenesis experience selective constraints given their role in a developmental pathway. We analyzed the molecular evolution of three genes playing a role during the sperm developmental pathway in Drosophila at an early (bam), a mid (aly), and a late (dj) stage. The complete coding region of these genes was sequenced in different strains of Drosophila melanogaster and Drosophila simulans. All three genes showed rapid divergence between species, with larger numbers of nonsynonymous to synonymous differences between species than polymorphisms. Although this could be interpreted as evidence for positive selection at all three genes, formal tests of selection do not support such a conclusion. Departures from neutrality were detected only for dj and bam but not aly. The role played by selection is unique and determined by gene-specific characteristics rather than site of expression. In dj, the departure was due to a high proportion of neutral synonymous polymorphisms in D. simulans, and there was evidence of purifying selection maintaining a high lysine amino acid protein content that is characteristic of other DNA-binding proteins. The earliest spermatogenesis gene surveyed, which plays a role in both male and female gametogenesis, was bam, and its significant departure from neutrality was due to an excess of nonsynonymous substitutions between species. Bam is degraded at the end of mitosis, and rapid evolutionary changes among species might be a characteristic shared with other degradable transient proteins. However, the large number of nonsynonymous changes between D. melanogaster and D. simulans and a phylogenetic comparative analysis among species confirms evidence of positive selection driving the evolution of Bam and suggests an yet unknown germ cell line developmental adaptive change between these two species.
由亲缘关系较近的果蝇物种杂交产生的杂种雄性是不育的。F1杂种不育表型主要是由于发育后期与精子个体化相关的缺陷，因此控制精子发育的基因可能在物种间经历了选择性多样化。考虑到精子发生相关基因在发育途径中的作用，它们也有可能受到选择性限制。我们分析了果蝇精子发育途径中在早期（bam）、中期（aly）和晚期（dj）发挥作用的三个基因的分子进化。在黑腹果蝇和拟果蝇的不同品系中对这些基因的完整编码区进行了测序。所有这三个基因在物种间都显示出快速分化，物种间非同义与同义差异的数量多于多态性。尽管这可以解释为这三个基因都受到正选择的证据，但正式的选择测试并不支持这一结论。仅在dj和bam中检测到偏离中性的情况，而aly没有。选择所起的作用是独特的，由基因特异性特征而非表达位点决定。在dj中，偏离是由于拟果蝇中高比例的中性同义多态性，并且有证据表明纯化选择维持了高赖氨酸氨基酸蛋白质含量，这是其他DNA结合蛋白的特征。所研究的最早的精子发生基因bam在雄性和雌性配子发生中都起作用，其显著偏离中性是由于物种间非同义替换过多。Bam在有丝分裂末期降解，物种间快速的进化变化可能是与其他可降解瞬时蛋白共有的特征。然而，黑腹果蝇和拟果蝇之间大量的非同义变化以及物种间的系统发育比较分析证实了正选择驱动Bam进化的证据，并表明这两个物种之间存在尚未知的生殖细胞系发育适应性变化。
