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

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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.