Langley C H, MacDonald J, Miyashita N, Aguadé M
Center for Population Biology, University of California, Davis 95616.
Proc Natl Acad Sci U S A. 1993 Mar 1;90(5):1800-3. doi: 10.1073/pnas.90.5.1800.
Levels of DNA sequence polymorphism at the suppressor of forked [su(f)] region in natural populations of Drosophila melanogaster and Drosophila simulans are estimated by restriction map analysis. su(f) is located at the base of the euchromatic portion of the X chromosome where the level of crossing-over per physical length is extremely low. In a survey of 55 alleles from three natural populations of D. melanogaster, only 2 restriction sites of 27 hexanucleotide and 108 tetranucleotide restriction sites scored are polymorphic. Among 103 alleles from three natural populations of D. simulans, just one polymorphic restriction site is found in 109 tetranucleotide-recognizing restriction sites scored. The few polymorphisms in these surveys yield estimates of per site heterozygosities (0.00, 0.0002, and 0.0005, respectively) at least a factor of 10 less than the average observed at loci located in regions of the genome with normal levels of crossing-over. Because under a broad category of models of molecular evolution (including the neutral theory) a correlation between levels of polymorphism and interspecific divergence is expected, the DNA sequence divergence is examined for the su(f) region. Contrary to the predicted correlation, the estimated divergence (0.12 substitution per silent site) is, in fact, greater than that observed at loci in regions of normal crossing-over. According to an alternative hypothesis (hitchhiking effect model) intraspecific polymorphism is swept out of the population in regions of the genome closely linked to rare but selectively favored variants as they quickly go to fixation; the rate of divergence is, however, unaffected by these rare hitchhiking events. Thus, the observed paucity of polymorphism and lack of correlation with divergence are in accord with the theory of the hitchhiking effect and several recent reports of polymorphism and divergence in other genomic regions with reduced crossing-over per physical length.
通过限制性图谱分析，估计了黑腹果蝇和拟果蝇自然种群中叉状抑制因子[su(f)]区域的DNA序列多态性水平。su(f)位于X染色体常染色质部分的基部，该区域每物理长度的交叉水平极低。在对来自黑腹果蝇三个自然种群的55个等位基因的调查中，在检测的27个六核苷酸和108个四核苷酸限制性位点中，只有2个限制性位点是多态的。在来自拟果蝇三个自然种群的103个等位基因中，在检测的109个识别四核苷酸的限制性位点中仅发现一个多态性限制性位点。这些调查中发现的少数多态性产生了每个位点杂合度的估计值（分别为0.00、0.0002和0.0005），至少比在基因组中交叉水平正常的区域中观察到的平均杂合度低10倍。因为在广泛的分子进化模型类别（包括中性理论）下，预计多态性水平与种间分歧之间存在相关性，所以对su(f)区域的DNA序列分歧进行了研究。与预测的相关性相反，估计的分歧（每个沉默位点0.12个替换）实际上大于在交叉正常区域的位点处观察到的分歧。根据另一种假设（搭便车效应模型），种内多态性在与罕见但受到选择青睐的变体紧密连锁的基因组区域中被从种群中清除，因为它们迅速固定；然而，分歧率不受这些罕见搭便车事件的影响。因此，观察到的多态性稀少以及与分歧缺乏相关性与搭便车效应理论以及最近关于其他每物理长度交叉减少的基因组区域中的多态性和分歧的几份报告一致。
