Tiffin Peter
Department of Plant Biology, University of Minnesota, St. Paul, Minnesota 55108, USA.
Genetics. 2004 Jul;167(3):1331-40. doi: 10.1534/genetics.104.026856.
Patterns of DNA sequence diversity vary widely among genes encoding proteins that protect plants against pathogens and herbivores. Comparative studies may help determine whether these differences are due to the strength of selection acting on different types of defense, in different evolutionary lineages, or both. I analyzed sequence diversity at three chitinases, a well-studied component of defense, in two species of Zea and several Poaceae taxa. Although the Zea species are closely related and these genes code for proteins with similar biochemical function, patterns of diversity vary widely within and among species. Intraspecific diversity at chiB, chiI, and Z. mays ssp. parviglumis chiA are consistent with a neutral-equilibrium model whereas chiA had no segregating sites within Z. diploperennis--consistent with a recent and strong selective sweep. Codons identified as having diverged among Poaceae taxa in response to positive selection were significantly overrepresented among targets of selection in Arabis, suggesting common responses to selection in distantly related plant taxa. Divergence of the recent duplicates chiA and chiB is consistent with positive selection but relaxed constraint cannot be rejected. Weak evidence for adaptive divergence of these duplicated downstream components of defense contrasts with strong evidence for adaptive divergence of genes involved in pathogen recognition.
在编码保护植物免受病原体和食草动物侵害的蛋白质的基因中，DNA序列多样性模式差异很大。比较研究可能有助于确定这些差异是由于作用于不同类型防御的选择强度不同，在不同的进化谱系中，还是两者兼而有之。我分析了玉米两个物种和几个禾本科类群中三种几丁质酶（一种研究充分的防御成分）的序列多样性。尽管玉米物种亲缘关系密切，且这些基因编码具有相似生化功能的蛋白质，但物种内和物种间的多样性模式差异很大。chiB、chiI和玉米小颖亚种chiA的种内多样性与中性平衡模型一致，而chiA在多年生玉米中没有分离位点——这与近期强烈的选择性清除一致。在禾本科类群中因正选择而分化的密码子在南芥的选择靶点中显著富集，这表明在远缘植物类群中对选择有共同反应。近期复制的chiA和chiB的分化与正选择一致，但不能排除放松的约束。这些重复的下游防御成分适应性分化的证据薄弱，这与病原体识别相关基因适应性分化的有力证据形成对比。
