Hudson R R
Department of Ecology and Evolutionary Biology, University of California, Irvine 92717.
Proc Natl Acad Sci U S A. 1994 Jul 19;91(15):6815-8. doi: 10.1073/pnas.91.15.6815.
Different regions of the Drosophila genome have very different rates of recombination. For example, near centromeres and near the tips of chromosomes, the rates of recombination are much lower than in other regions. Several surveys of polymorphisms in Drosophila have now documented that levels of DNA polymorphism are positively correlated with rates of recombination; i.e., regions with low rates of recombination tend to have low levels of DNA polymorphism within populations of Drosophila. Three hypotheses are reviewed that might account for these observations. The first hypothesis is that regions of low recombination have low neutral mutation rates. Under this hypothesis between-species divergences should also be low in regions of low recombination. In fact, regions of low recombination have diverged at the same rate as other regions of the genome. On this basis, this strictly neutral hypothesis is rejected. The second hypothesis is that the process of fixation of favorable mutations leads to the observed correlation between polymorphism and recombination. This occurs via genetic hitchhiking, in which linked regions of the genome are swept along with the selectively favored mutant as it increases in frequency and eventually fixes in the population. This hitchhiking model with fixation of favorable mutations is compatible with major features of the data. By assuming this model is correct, one can estimate the rate of fixation of favorable mutations. The third hypothesis is that selection against continually arising deleterious mutations results in reduced levels of polymorphism at linked loci. Analysis of this background selection model shows that it can produce some reduction in levels of polymorphism but cannot explain some extreme cases that have been observed. Thus, it appears that hitchhiking of favorable mutations and background selection against deleterious mutations must be considered together to correctly account for the patterns of polymorphism that are observed in Drosophila.
果蝇基因组的不同区域具有非常不同的重组率。例如，在着丝粒附近和染色体末端附近，重组率比其他区域低得多。现在，对果蝇多态性的几项调查已经证明，DNA多态性水平与重组率呈正相关；也就是说，在果蝇种群中，重组率低的区域往往具有低水平的DNA多态性。本文综述了三种可能解释这些观察结果的假说。第一种假说是，重组率低的区域具有低的中性突变率。在这个假说下，低重组率区域的物种间差异也应该较低。事实上，低重组率区域与基因组的其他区域以相同的速率发生了分化。基于此，这个严格的中性假说被否定。第二种假说是，有利突变的固定过程导致了观察到的多态性与重组之间的相关性。这是通过遗传搭便车发生的，在遗传搭便车过程中，随着选择性有利突变在频率上增加并最终在种群中固定下来，基因组的连锁区域也随之被带动。这种有利突变固定的搭便车模型与数据的主要特征是相符的。假设这个模型是正确的，就可以估计有利突变的固定率。第三种假说是，针对不断出现的有害突变的选择导致连锁位点的多态性水平降低。对这种背景选择模型的分析表明，它可以使多态性水平有所降低，但无法解释一些已观察到的极端情况。因此，似乎必须同时考虑有利突变的搭便车和针对有害突变的背景选择，才能正确解释在果蝇中观察到的多态性模式。
