Ellegren H, Moore S, Robinson N, Byrne K, Ward W, Sheldon B C
Department of Animal Breeding and Genetics, Swedish University of Agricultural Sciences, Uppsala.
Mol Biol Evol. 1997 Aug;14(8):854-60. doi: 10.1093/oxfordjournals.molbev.a025826.
The application of microsatellites in evolutionary studies requires an understanding of the patterns governing their evolution in different species. The finding that homologous microsatellite loci are longer, i.e., containing more repeat units, in human and in other primates has been taken as evidence for directional microsatellite evolution and for a difference in the rate of evolution between species. However, it has been argued that this finding is an inevitable consequence of biased selection of longer-than-average microsatellites in human, because cloning procedures are adopted to generate polymorphic and, hence, long markers. As a test of this hypothesis, we conducted a reciprocal comparison of the lengths of microsatellite loci in cattle and sheep using markers derived from the bovine genome as well as the ovine genome. In both cases, amplification products were longer in the focal species, and loci were also more polymorphic in the species from which they were originally cloned. The crossing pattern that we found suggests that interspecific length differences detected at homologous microsatellite loci are the result of biased selection of loci associated with cloning procedures. Hence, comparisons of microsatellite evolution between species are flawed unless they are based on reciprocal analyses or on genuinely random selection of loci with respect to repeat length.
微卫星在进化研究中的应用需要了解不同物种中控制其进化的模式。在人类和其他灵长类动物中，同源微卫星位点更长，即包含更多重复单元，这一发现被视为微卫星定向进化以及物种间进化速率存在差异的证据。然而，有人认为这一发现是人类中对长度超过平均水平的微卫星进行偏向性选择的必然结果，因为采用克隆程序来产生多态性标记，从而产生长标记。作为对这一假设的检验，我们使用源自牛基因组和羊基因组的标记，对牛和羊的微卫星位点长度进行了相互比较。在这两种情况下，在作为研究重点的物种中扩增产物更长，并且在最初克隆这些位点的物种中，位点的多态性也更高。我们发现的交叉模式表明，在同源微卫星位点检测到的种间长度差异是与克隆程序相关的位点偏向性选择的结果。因此，除非基于相互分析或对重复长度进行真正随机的位点选择，否则物种间微卫星进化的比较是有缺陷的。
