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内容記述 |
The common morning glory (Ipomoea purpurea) , a native to Central America, is a common weed in the southeastern United States. The plant was introduced to Europe probably in the 17th century and to Japan in the 18th century. The cultivars with purple, red and white flowers were already recorded in the 18th century, and the most extensively characterized mutant displaying white flowers with pigmented spots and sectors was recorded in the early 19th century. The early genetic studies on the variegated flower line, named flaked, were conducted by Barker (1917) in the United States and followed by Imai and Tabuchi (1935) in Japan. The flaked mutation, also called anthocyaninflaked (aflaked or af) , was recently shown to be caused by the insertion of the transposable element Tip100 into the CHS-D gene for anthocyanin biosynthesis. The 3.9 kb Tip100 element belonging to the Ac/Ds family contains an open reading frame encoding a polypeptide of 808 amino acids. The patterns of the flower variegations in the mutable flaked (af) lines are determined by the frequency and timing of the excision of Tip100 from the CHS-D gene. In addition to the mutable af allele, there is a leaf variegation mutation yellow-green leafmutable, or yglm, conferring dark green spots and sectors on a yellow green background. A plant carrying the two alleles, af and yglm, tends to show a peculiar similarity in both timing and frequency of the variegations in flowers and leaves, and the patterns of the flower and leaf variegations are also generally heritable. Based on the observation, it has been proposed that there must be another genetic element termed Modulator acting on both af and yglm alleles and that the timing and frequency of the variegations in flowers and leaves are determined by the heritable state of the Modulator.<br />As an initial step to understand the complicated flower and leaf variegation system in the common morning glory, I examined whether Tip100 is an autonomous element carrying an active transposase gene within the element. Tip100 and its internal deletion derivative dTip100 were introduced into tobacco (Nicotiana tabacum) , and their capability to transpose in the transgenic tobacco plants was examined. The introduced Tip100 element was able to transpose from the integrated vector into the tobacco genome whereas dTip100 was not. The defective dTip100 could be transposed by introduction of Tip100, indicating that Tipl00 is the first autonomous element characterized in Ipomoea species. Based on these results, I have speculated the nature of the putative Modulator element affecting flower and leaf variegations in various mutable lines. <br />To characterize whether many different mutant alleles in the CHS-D gene are present in the common morning glory producing either white flowers or white flowers with pigmented spots and sectors, the structures of the CHS-D gene regions in the mutants grown as a weed in the southeastern United States. All of the mutants examined were found to be derivatives of the af mutant previously characterized, and some of them carry additional DNA rearrangements associated with Tip100. Based on the results, I have concluded that there had been a "founder" mutation in the CHS-D gene and that subsequent DNA rearrangements mediated by Tip100 and other mobile genetic elements had occurred in some of the geographically spread mutants originated from the founder. I have also discussed a possible evolutionary process to generate variants and mutants carrying sequence alterations in the CHS-D gene region of the common morning glory. |
要旨・審査要旨 / Abstract, Screening Result (279.6 kB)
