Molecular cloning and characterization of complemental causal genes of hybrid weakness of rice located in a homologous chromosomal region

研究成果の概要（和文）：岡らによって報告されたイネ雑種弱勢は第11染色体のほぼ相同な領域に座乗するHwa1-1とHwa2-1によって生じる．本課題はこれらの遺伝子の単離を目的として実施された．長鎖の配列を出力する次世代シーケンサーを用いた塩基配列解読結果を高密度連鎖解析によって絞り込まれた位置情報に基づき解析し、HWA1領域の配列の連なり(contig)を構築することができた．Hwa1-1領域は、日本晴と比較して20kbpの欠失が認められた．HWA2については1つのcontigにまとめることができず、3つのcontigに分かれた．Hwa2-1の領域は日本晴のゲノム配列とは構造が大きく異なっていることが明らかになった． 研究成果の概要（英文）：Oka reported that epistatic interaction between Hwa1-1and Hwa2-1 cause a hybrid weakness of rice and we have shown that these causal genes located on the almost the same region of chromosome 11. This research project aimed at molecular cloning of Hwa1-1and Hwa2-1. DNA sequence of the donor plants of these two genes were analyzed by PacBio DNA sequencers. DNA sequences of DNA markers used in fine mapping of these two causal genes were used as queries of homology search against DNA sequences derived from the donor plants using software BLAST (NCBI). As a result, a contig of Hwa1-1 was constructed and it had 20 kbp deletion in comparison with the Nipponbare genome sequence. On the other hand, the chromosomal structure of Hwa2-1region had large difference with that of Nipponbare homologous region, and we could not construct a contig covering the Hwa2-1interval

HWA1- and HWA2-Mediated Hybrid Weakness in Rice Involves Cell Death, Reactive Oxygen Species Accumulation, and Disease Resistance-Related Gene Upregulation

Hybrid weakness is a type of reproductive isolation in which F1 hybrids of normal parents exhibit weaker growth characteristics than their parents. F1 hybrid of the Oryza sativa Indian cultivars ‘P.T.B.7′ and ‘A.D.T.14′ exhibits hybrid weakness that is associated with the HWA1 and HWA2 loci. Accordingly, the aim of the present study was to analyze the hybrid weakness phenotype of the ‘P.T.B.7′ × ‘A.D.T.14′ hybrids. The height and tiller number of the F1 hybrid were lower than those of either parent, and F1 hybrid also exhibited leaf yellowing that was not observed in either parent. In addition, the present study demonstrates that SPAD values, an index correlated with chlorophyll content, are effective for evaluating the progression of hybrid weakness that is associated with the HWA1 and HWA2 loci because it accurately reflects degree of leaf yellowing. Both cell death and H2O2, a reactive oxygen species, were detected in the yellowing leaves of the F1 hybrid. Furthermore, disease resistance-related genes were upregulated in the yellowing leaves of the F1 hybrids, whereas photosynthesis-related genes tended to be downregulated. These results suggest that the hybrid weakness associated with the HWA1 and HWA2 loci involves hypersensitive response-like mechanisms