雑草イネの遺伝的特性とその起源に関する考察  [in Japanese] Genetic Characterization of Weedy Rices and the Inference on their Origins  [in Japanese]

雑草イネとは稲田の中や周辺に雑草として定着しているイネで,直播田で多く報告されている。世界各地で収集された雑草イネ24系統の各種形質およびアイソザイム変異を調査し,その遺伝的特性を明らかにしようとした。繁殖体系に関しては,自然脱粒・自然発芽する自生型と,形態・生態が栽培イネと非常に似ているためイネに混入したまま収穫・播種される作物擬態型の2つのタイプが認められた。また,インド型・日本型への分化が明瞭に認められた。供試系統はインド型的作物擬態型(I群),インド型的自生型(II群),日本型的自生型(III群)に大別されたが,これらは異なる起源を持つと考えられる。作物擬態型は,古い在来品種が持っていた多様な遺伝変異の中から雑草的なものが選抜されて残ったのであろう。野生イネの分布する熱帯の水田地帯で見出される自生型は,野生イネと栽培イネの自然交雑に由来するものと考えられる。野生イネの分布していない地域で見出される自生型系統の起源についてはよくわからないが,目印交雑のような遠縁品種間交雑の分離後代に由来する可能性や,過去に存在した野生イネと栽培イネとの自然交雑の結果生じた可能性などが考えられる。中国長江下流域に自生していた雑草イネ(III群)の成立には,この地域に存在していた可能性の高い日本型的野生イネが関与したと考えることもできる。

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, KCI0_3 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.