Fitch D H, Bailey W J, Tagle D A, Goodman M, Sieu L, Slightom J L
Department of Anatomy, Wayne State University School of Medicine, Detroit, MI 48201.
Proc Natl Acad Sci U S A. 1991 Aug 15;88(16):7396-400. doi: 10.1073/pnas.88.16.7396.
Regions surrounding the single gamma-globin gene of galago and the duplicated gamma 1- and gamma 2-globin genes of gibbon, rhesus monkey, and spider monkey were sequenced and aligned with those from humans. Contrary to previous studies, spider monkey was found to have not one but two gamma-globin genes, only one of which (gamma 2) is functional. The reconstructed evolutionary history of the gamma-globin genes and their flanking sequences traces their origin to a tandem duplication of a DNA segment approximately 5.5 kilobases long that occurred before catarrhine primates (humans, apes, and Old World monkeys) diverged from platyrrhines (New World monkeys), much earlier than previously thought. This reconstructed molecular history also reveals that the duplication resulted from an unequal homologous crossover between two related L1 long interspersed repetitive elements, one upstream and one downstream of the single ancestral gamma-globin gene. Perhaps facilitated by the redundancy resulting from the duplication, the gamma-globin genes escaped the selective constraints of embryonically functioning genes and evolved into fetally functioning genes. This view is supported by the finding that a burst of nonsynonymous substitutions occurred in the gamma-globin genes while they became restructured for fetal expression in the common ancestor of platyrrhines and catarrhines.
对婴猴单个γ-珠蛋白基因以及长臂猿、恒河猴和蜘蛛猴的重复γ1-和γ2-珠蛋白基因周围区域进行了测序，并与人类的相应区域进行了比对。与之前的研究相反，发现蜘蛛猴有两个而非一个γ-珠蛋白基因，其中只有一个（γ2）具有功能。γ-珠蛋白基因及其侧翼序列的重建进化史将它们的起源追溯到一个约5.5千碱基长的DNA片段的串联重复，该重复发生在狭鼻猿（人类、猿类和旧世界猴）与阔鼻猿（新世界猴）分化之前，比之前认为的要早得多。这个重建的分子历史还表明，这种重复是由两个相关的L1长散在重复元件之间的不等位同源交叉导致的，一个在单个祖先γ-珠蛋白基因的上游，一个在下游。也许是由于重复产生的冗余，γ-珠蛋白基因摆脱了胚胎功能基因的选择限制，进化成了胎儿功能基因。这一观点得到了以下发现的支持：在阔鼻猿和狭鼻猿的共同祖先中，γ-珠蛋白基因在为胎儿表达进行重组时发生了一阵非同义替换。
