雑草イネの遺伝的特性とその起源に関する考察

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タイトル別名
  • Genetic Characterization of Weedy Rices and the Inference on their Origins.
  • ザッソウ イネ ノ イデンテキ トクセイ ト ソノ キゲン ニ カンスル コウ

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Weedy rice is defined as rice plants growing as weeds in and around the arable lands. Various forms of weedy rice were observed in many rice growing countries, but are decreasing under intensive cultivation system. Origin of weedy form and its significance in rice domestication are the interesting but unsolved problems for rice evolutionists. Further, high adaptability to adverse environments found in weedy rice might be a target trait for rice breeders. In the present study, 24 weedy rice strains collected from various countries were examined. Firstly, variation between wild and cultivated types was assessed for each strain by three characters (degree of shedding, degree of seed dormancy and grain weight). The strains examined were largely grouped into two types, those similar to wild and cultivated rice, respectively (Fig. l). The former is a naturally propagating type characterized by high seed shedding and strong seed dormancy. While the latter is a crop mimic type and probably has survived by being sown and harvested together with cultivars unconsciously. Secondly, variation between Indica and Japonica types was assessed by three diagnostic characters (phenol reaction, KCI03 resistance and apiculus hair length), and also by 11 diagnostic isozyme loci. The strains examined were either Indica- or Japonica-like type in characters as well as in allozymes (Fig. 2). Based on the above mentioned two axes of variation, the strains examined were classified into three groups (Fig. 3) ; Indica-like crop mimics (I), Indica-like self-propagating type (II), and Japonica-like self-propagating type (III) . Crop mimics (GroupI) may be the remnants of old cultivars. Self-propagating type found in tropical regions where wild rices are abundant (Group II) is most probably the products of natural hybridization between wild and cultivated rices. Origin of self-propagating type distributed outside the wild-rice area (Group III and a part of Group II) is most problematic. One possibility is that they are hybrid derivatives of the crosses between distantly related cultivars. Another possibility is that they are the derivatives of natural hybridization between formerly existintg but now extinct wild rice and cultivated rice. A possibility that weedy forms in lower Yangtze valley (Group III) are the product of natural hybridization between Japonica cultivars and Japonica-like wild rice which is now extinct can not be ruled out.

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