| Research Abstract |
This research project aims to reveal maintenance mechanism of DNA variation in natural populations of the wild plant Arbidopsis thaliana, by investigation relationship between DNA polymorphism and phenotypic variation. Our previous studies have shown two unexpected features of DNA polymorphism in nuclear gene regions of this plant species. One is high level of DNA polymorphism, despite selfing nature of this plant species. One is high level of DNA polymorphism, despite selfing nature of this plant species. The other is the presence of two divergent sequence types (dimorphism). This kind of pattern of DNA polymorphism has not been detected other organisms studied. Obviously, dimorphism causes the level of DNA polymorphism. There are two hypotheses concerning the origin and maintenance mechanism of dimorphism. The first is based on evolutionary history of this plant. In the ancient population of A. thaliana, population division occurred, which caused genetic differentiation between the i
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solated populations. Then, after the period of genetic isolation, fusion of isolated populations took place, resulting m dimorphism in the present population. The second requires natural selection, especially balancing selection, such as overdominance. This kind of natural selection favors coexistence of different alleles, here divergent sequence types. In this investigation, two different approaches were taken. One is to examine if the dimorphism is the general phenomenon in A. thaliana nuclear genes, by extending our effort on other nuclear regions. Newly studied regions were ChiB, PgiC, Atmyb2, 5' upstream region of Adh, Acl5, and Als. In Atmyb2, and 5' upstream region of Adh, dimorphism was not detected, and level of DNA polymorphism was low. But, in the other regions, dimorphism and high level of polymorphism were observed. These results indicates that dimorphism is a characteristic of DNA polymorphism in this plant species, causing the high level of polymorphic variation at the species level. All the results were published in scientific journals, or orally presented in meetings. The second was the investigation of correlation between DNA and phenotypic variations. We focused on Adh, using the Northern and SAGE analyses. Unfortunately, satisfactory results were not obtained. However, this kind of analysis will be important, especially in the era of post-genome project. We would like to perform this kind of research in future, utilizing our experience acquired in this investigation. Less
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