Hollocher H, Templeton A R, DeSalle R, Johnston J S
Department of Biology, Washington University, St. Louis, Missouri 63130.
Genetics. 1992 Feb;130(2):355-66. doi: 10.1093/genetics/130.2.355.
Natural populations of Drosophila mercatorum are polymorphic for a phenotypic syndrome known as abnormal abdomen (aa). This syndrome is characterized by a slow-down in egg-to-adult developmental time, retention of juvenile abdominal cuticle in the adult, increased early female fecundity, and decreased adult longevity. Previous studies revealed that the expression of this syndrome in females is controlled by two closely linked X chromosomal elements: the occurrence of an R1 insert in a third or more of the X-linked 28S ribosomal genes (rDNA), and the failure of replicative selection favoring uninserted 28S genes in larval polytene tissues. The expression of this syndrome in males in a laboratory stock was associated with the deletion of the rDNA normally found on the Y chromosome. In this paper we quantify the levels of genetic variation for these three components in a natural population of Drosophila mercatorum found near Kamuela, Hawaii. Extensive variation is found in the natural population for both of the X-linked components. Moreover, there is a significant association between variation in the proportion of R1 inserted 28S genes with allelic variation at the underreplication (ur) locus such that both of the necessary components for aa expression in females tend to cosegregate in the natural population. Accordingly, these two closely linked X chromosomal elements are behaving as a supergene in the natural population. Because of this association, we do not believe the R1 insert to be actively transposing to an appreciable extent. The Y chromosomes extracted from nature are also polymorphic, with 16% of the Ys lacking the Y-specific rDNA marker. The absence of this marker is significantly associated with the expression of aa in males. Hence, all three of the major genetic determinants of the abnormal abdomen syndrome are polymorphic in this natural population.
在黑腹果蝇的自然种群中，存在一种名为异常腹部（aa）的表型综合征的多态性。这种综合征的特征是从卵到成虫的发育时间延长、成虫保留幼虫腹部表皮、早期雌蝇繁殖力增加以及成虫寿命缩短。先前的研究表明，雌性中这种综合征的表达受两个紧密连锁的X染色体元件控制：在三分之一或更多的X连锁28S核糖体基因（rDNA）中出现R1插入，以及在幼虫多线组织中有利于未插入28S基因的复制选择失败。在实验室品系中，雄性中这种综合征的表达与通常在Y染色体上发现的rDNA缺失有关。在本文中，我们对在夏威夷卡姆埃拉附近发现的黑腹果蝇自然种群中这三个成分的遗传变异水平进行了量化。在自然种群中，两个X连锁成分都存在广泛的变异。此外，R1插入的28S基因比例的变异与复制不足（ur）位点的等位基因变异之间存在显著关联，使得雌性中aa表达的两个必要成分在自然种群中倾向于共分离。因此，这两个紧密连锁的X染色体元件在自然种群中表现为一个超基因。由于这种关联，我们认为R1插入在相当程度上不会活跃转座。从自然界提取的Y染色体也是多态的，16%的Y染色体缺乏Y特异性rDNA标记。这种标记的缺失与雄性中aa的表达显著相关。因此，异常腹部综合征的所有三个主要遗传决定因素在这个自然种群中都是多态的。
