Moriyama E N, Hartl D L
Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138.
Genetics. 1993 Jul;134(3):847-58. doi: 10.1093/genetics/134.3.847.
The nuclear genes of Drosophila evolve at various rates. This variation seems to correlate with codon-usage bias. In order to elucidate the determining factors of the various evolutionary rates and codon-usage bias in the Drosophila nuclear genome, we compared patterns of codon-usage bias with base compositions of exons and introns. Our results clearly show the existence of selective constraints at the translational level for synonymous (silent) sites and, on the other hand, the neutrality or near neutrality of long stretches of nucleotide sequence within noncoding regions. These features were found for comparisons among nuclear genes in a particular species (Drosophila melanogaster, Drosophila pseudoobscura and Drosophila virilis) as well as in a particular gene (alcohol dehydrogenase) among different species in the genus Drosophila. The patterns of evolution of synonymous sites in Drosophila are more similar to those in the prokaryotes than they are to those in mammals. If a difference in the level of expression of each gene is a main reason for the difference in the degree of selective constraint, the evolution of synonymous sites of Drosophila genes would be sensitive to the level of expression among genes and would change as the level of expression becomes altered in different species. Our analysis verifies these predictions and also identifies additional selective constraints at the translational level in Drosophila.
果蝇的核基因以不同速率进化。这种变异似乎与密码子使用偏好相关。为了阐明果蝇核基因组中不同进化速率和密码子使用偏好的决定因素，我们将密码子使用偏好模式与外显子和内含子的碱基组成进行了比较。我们的结果清楚地表明，同义（沉默）位点在翻译水平上存在选择性限制，另一方面，非编码区域内长段核苷酸序列具有中性或近中性特征。这些特征在特定物种（黑腹果蝇、拟暗果蝇和粗壮果蝇）的核基因之间以及果蝇属不同物种中的特定基因（乙醇脱氢酶）之间的比较中均有发现。果蝇同义位点的进化模式与原核生物的更为相似，而与哺乳动物的不同。如果每个基因表达水平的差异是选择性限制程度差异的主要原因，那么果蝇基因同义位点的进化将对基因间的表达水平敏感，并会随着不同物种中表达水平的改变而变化。我们的分析验证了这些预测，并还确定了果蝇在翻译水平上的其他选择性限制。
