Ingvarsson Pär K
Umeå Plant Science Centre, Department of Ecology and Environmental Science, University of Umeå, SE-891 87 Umeå, Sweden.
Mol Biol Evol. 2005 Sep;22(9):1802-12. doi: 10.1093/molbev/msi171. Epub 2005 May 25.
Plants defend themselves against the attack of natural enemies by using an array of both constitutively expressed and induced defenses. Long-lived woody perennials are overrepresented among plant species that show strong induced defense responses, whereas annual plants and crop species are underrepresented. However, most studies of plant defense genes have been performed on annual or short-lived perennial weeds or crop species. Here I use molecular population genetic methods to survey six wound-inducible protease inhibitors (PIs) in a long-lived woody, perennial plant species, the European aspen (Populus tremula), to evaluate the likelihood of either recurrent selective sweeps or balancing selection maintaining amino acid polymorphisms in these genes. The results show that none of the six PI genes have reduced diversities at synonymous sites, as would be expected in the presence of recurrent selective sweeps. However, several genes show some evidence of nonneutral evolution such as enhanced linkage disequilibrium and a large number of high-frequency-derived mutations. A group of at least four Kunitz trypsin inhibitor genes appear to have experienced elevated levels of nonsynonymous substitutions, indicating allelic turnover on an evolutionary timescale. One gene, TI1, has enhanced levels of intraspecific polymorphism at nonsynonymous sites and also has an unusual haplotype structure characterized by two divergent haplotypes occurring at roughly equal frequencies in the sample. One haplotype has very low levels of intraallelic nucleotide diversity, whereas the other haplotype has levels of diversity comparable to other genes in P. tremula. Patterns of sequence diversity at TI1 do not fit a simple model of either balancing selection or recurrent selective sweeps. This suggests that selection at TI1 is more complex, possibly involving allelic cycling.
植物通过一系列组成型表达和诱导型防御来抵御天敌的攻击。在表现出强烈诱导防御反应的植物物种中，长寿的木本多年生植物占比过高，而一年生植物和作物物种的占比则过低。然而，大多数关于植物防御基因的研究都是在一年生或短命多年生杂草或作物物种上进行的。在这里，我使用分子群体遗传学方法，对一种长寿的木本多年生植物欧洲山杨（Populus tremula）中的六种创伤诱导型蛋白酶抑制剂（PI）进行了调查，以评估反复发生的选择性清除或平衡选择维持这些基因中氨基酸多态性的可能性。结果表明，六个PI基因中没有一个在同义位点上的多样性降低，而这在反复发生选择性清除的情况下是可以预期的。然而，几个基因显示出一些非中性进化的证据，如增强的连锁不平衡和大量高频衍生突变。一组至少四个Kunitz胰蛋白酶抑制剂基因似乎经历了非同义替换水平的升高，这表明在进化时间尺度上存在等位基因更替。一个基因TI1在非同义位点上的种内多态性水平有所提高，并且具有不寻常的单倍型结构，其特征是在样本中出现频率大致相等的两种不同单倍型。一种单倍型的等位基因内核苷酸多样性水平非常低，而另一种单倍型的多样性水平与欧洲山杨中的其他基因相当。TI1的序列多样性模式不符合平衡选择或反复发生选择性清除的简单模型。这表明TI1的选择更为复杂，可能涉及等位基因循环。
