Molecular evolution of LRR type R-genes in sexual and asexual Eupatorium

尽管许多理论家提出了各种各样的观点，但性别的适应功能仍然不确定。其中，现存的两种理论分别是：(1)抗突变理论（Muller's rachet and Kondrashov's hatchet）和(2)抗寄生虫理论（parasite Red Queen model）。本研究的目的是通过比较有性和无性泽兰群体LRR - r基因的分子进化，来验证后者的一些预测。一个具体的预测是，在有性宿主种群中，抗病基因的变异性比在无性宿主种群中更高。以往对紫茎泽兰的研究表明，在无性群体中，双病毒感染的发生率要高得多，并且双病毒感染肯定会降低寄主植物的适合度。在感染有性宿主种群的双病毒中，在宿主范围决定因子orfc4中发现了加速替代。所有这些发现都支持了寄生红皇后模型。在本研究中，我重点研究了植物中NBS-LRR型抗病基因（R基因）。我成功地确定了PCR产物的序列，这些PCR产物是通过设计用于扩增含有部分r基因区域的DNA的引物集获得的。测定了6株有性植物和6株无性植物各5个无性系的序列。这些序列的比较表明，无性植物的等位基因多样性高于有性植物。这一发现支持了与预测相反的观点。这一意想不到的结果可能表明协同进化在无性植物中较少发生。因此，无性植物的协同进化能力较弱。在本研究中，我比较了泽兰r基因的5′区序列。已知病原体的无毒基因与r基因的3'区相互作用。需要使用r基因的3'区序列进行进一步的比较研究，以进一步详细地测试红皇后模型的预测。

The adaptive function of sex remains uncertain in spite that many theorists developed various ideas. Among them, two theories still alive are : (1) anti-mutation theory (Muller's rachet and Kondrashov's hatchet) and (2) anti-parasite theory (parasite Red Queen model). The purpose of this study is to test some predictions of the latter by comparing molecular evolution of LRR R-genes between sexual and asexual populations of Eupatorium. A specific prediction tested is that variability of disease resistance genes is higher in sexual host populations than in asexual host populations. Previous studeis in Eupatorium spp. revealed that incidence of geminivirus infection in the field was much higher in asexual populations, and geminivirus infection definitively decreased fitness of host plants. In geminiviruses that infected sexual host populations accelerated replacement substitutions wee found in ORF C4, a host-range determinant. All of these findingse supported the parasite Red Queen model. In this study, I focused on NBS-LRR type disease resistance genes in plants (R genes). I successfully determined sequences of PCR products that were obtained by using primer sets designed to amplify DNA containing parts of R-gene regions. Sequences of five clones for each of six sexual and six asexual plants were deterined. Comparison of these sequences revealed that allelic diversity of an indivudual is higher in asexual plants than in sexual plants. This finding supported the reverse of the prediction. This unexpected result may show that concerted evulution is less frequent in asexual plants. Then, asexual plants may be disadvantageous in lower ability of concerted evulution. In this study, I compared 5' region of R-gene sequences in Eupatorium. Avirulent genes of pathogens are known to interact with 3' region of R-genes. Further comparative studies using 3' region sequences of R-genes are needed to test predictions of the Red Queen model in further detail.