Fine mapping of a gene responsible for pollen semi-sterility in hybrids between Oryza sativa L. and O. glaberrima Steud

Abstract

Asian cultivated rice (Oryza sativa L.) and African cultivated rice (Oryza glaberrima Steud.) are the two main cultivated rice species in the world, with strong heterosis in their F1 hybrids. However, hybrid sterility is a major barrier, although significant heterosis has been observed. In this study, an F1 pollen semi-sterility locus, S19, was identified on rice chromosome 3 by using near-isogenic lines derived from repeated backcross and marker-assisted selection. The typical pollen semi-sterility was observed in F1 hybrids between S19-NIL and Dianjingyou 1. Cytological study of pollen developmental stages indicated that pollen abortion occurred at the late binucleate stage because of a starch accumulation obstacle in some pollen grains. Molecular analysis revealed that the semi-sterility was caused by the abortion of most male gametophytes carrying the S19 allele from the japonica variety Dianjingyou 1. In a population of 12,780 F2 plants derived from S19-NIL/Dianjingyou 1, the S19 locus was fine-mapped to a chromosomal region of 54 kb based on BAC clones of cv. Nipponbare. Interestingly, an addition of a DNA fragment of about 89 kb to the 54-kb region was found in S19-NIL based on BAC clones of O. glaberrima. Gene prediction analysis identified 12 open reading frames (ORF) based on the region of Dianjingyou 1, while 32 ORFs were predicted in S19-NIL. Map-based cloning of this gene will help us to understand the underlying mechanism of hybrid sterility between the two cultivated rice species.