Barker Michael S, Demuth Jeffery P, Wade Michael J
Department of Biology, Indiana University, Bloomington, Indiana, United States of America.
PLoS Genet. 2005 Nov;1(5):e57. doi: 10.1371/journal.pgen.0010057. Epub 2005 Nov 11.
The origin of evolutionary novelty is believed to involve both positive selection and relaxed developmental constraint. In flies, the redesign of anterior patterning during embryogenesis is a major developmental innovation and the rapidly evolving Hox gene, bicoid (bcd), plays a critical role. We report evidence for relaxation of selective constraint acting on bicoid as a result of its maternal pattern of gene expression. Evolutionary theory predicts 2-fold greater sequence diversity for maternal effect genes than for zygotically expressed genes, because natural selection is only half as effective acting on autosomal genes expressed in one sex as it is on genes expressed in both sexes. We sample an individual from ten populations of Drosophila melanogaster and nine populations of D. simulans for polymorphism in the tandem gene duplicates bcd, which is maternally expressed, and zerknüllt (zen), which is zygotically expressed. In both species, we find the ratio of bcd to zen nucleotide diversity to be two or more in the coding regions but one in the noncoding regions, providing the first quantitative support for the theoretical prediction of relaxed selective constraint on maternal-effect genes resulting from sex-limited expression. Our results suggest that the accelerated rate of evolution observed for bcd is owing, at least partly, to variation generated by relaxed selective constraint.
进化新特征的起源被认为涉及正向选择和宽松的发育限制。在果蝇中，胚胎发育过程中前部模式的重新设计是一项重大的发育创新，快速进化的同源异型基因双尾（bcd）起着关键作用。我们报告了由于双尾基因的母系基因表达模式，作用于双尾基因的选择限制有所放松的证据。进化理论预测，母系效应基因的序列多样性比合子表达基因高两倍，因为自然选择对单性表达的常染色体基因的作用效率只有对两性表达基因的一半。我们从十个黑腹果蝇种群和九个拟果蝇种群中各选取一个个体，检测母系表达的串联基因重复体双尾（bcd）和合子表达的零尾（zen）的多态性。在这两个物种中，我们发现双尾基因与零尾基因在编码区的核苷酸多样性之比为二或更高，但在非编码区为一，这为因性别限制表达而对母系效应基因的选择限制放松的理论预测提供了首个定量支持。我们的结果表明，观察到的双尾基因加速进化速率至少部分归因于选择限制放松所产生的变异。
