Long awns are important for seed dispersal in wild rice (Oryza rufipogon), but are absent in cultivated rice (Oryza sativa). The genetic mechanism involved in loss-of-awn in cultivated rice remains unknown. We report here the molecular cloning of a major quantitative trait locus, An-1, which regulates long awn formation in O. rufipogon. An-1 encodes a basic helix-loop-helix protein, which regulates cell division. The nearly-isogenic line (NIL-An-1) carrying a wild allele An-1 in the genetic background of the awnless indica Guangluai4 produces long awns and longer grains, but significantly fewer grains per panicle compared with Guangluai4. Transgenic studies confirmed that An-1 positively regulates awn elongation, but negatively regulates grain number per panicle. Genetic variations in the An-1 locus were found to be associated with awn loss in cultivated rice. Population genetic analysis of wild and cultivated rice showed a significant reduction in nucleotide diversity of the An-1 locus in rice cultivars, suggesting that the An-1 locus was a major target for artificial selection. Thus, we propose that awn loss was favored and strongly selected by humans, as genetic variations at the An-1 locus that cause awn loss would increase grain numbers and subsequently improve grain yield in cultivated rice.
長芒對于野生稻種子散播十分重要，但是栽培稻往往無芒。對于栽培稻芒缺失的遺傳機(jī)理知道的還不多。本文報道了一個主效QTL An-1的分子克隆，其參與野生稻長芒的形成。An-1編碼了一個調(diào)控細(xì)胞分裂的bHLH蛋白。以秈稻無芒品種廣陸矮4號為背景插入一個野生稻An-1等位基因的近等基因系（NIL-An-1）能夠增加芒長和籽粒長度，但是每穗粒數(shù)較廣陸矮4號顯著減少。轉(zhuǎn)基因研究證實(shí)An-1正向調(diào)控芒的伸長，但是負(fù)向調(diào)控每穗粒數(shù)。還發(fā)現(xiàn)在An-1位點(diǎn)的遺傳變異與栽培稻的芒缺失相關(guān)聯(lián)。對野生稻和栽培稻的群體遺傳分析發(fā)現(xiàn)在栽培稻中An-1的核苷酸多樣性顯著降低，說明An-1位點(diǎn)是一個人工選擇的主要靶標(biāo)。因此，我們推測（馴化過程中）水稻芒的丟失是優(yōu)先且強(qiáng)烈受到人為選擇，因為在An-1位點(diǎn)發(fā)生的遺傳變異會導(dǎo)致芒丟失，從而增加籽粒數(shù)和栽培稻產(chǎn)量。