Chromosomal Location of HWA1 and HWA2, Complementary Hybrid Weakness Genes in Rice

Abstract

Hybrid weakness phenomena in rice reportedly have two causes: those of HWC1 and HWC2 genes and those of HWA1 and HWA2 genes. No detailed study of the latter has been reported. For this study, we first produced crosses among cultivars carrying the weakness-causing allele on the HWA1 and HWA2 loci to confirm the phenotype of the hybrid weakness and the genotypes of the cultivars on the two loci, as reported earlier. We then confirmed that these cultivars belong to Indica. Subsequent linkage analysis of HWA1 and HWA2 genes conducted using DNA markers revealed that both genes are located in the 1,637-kb region, surrounded by the same DNA markers on the long arm of chromosome 11. The possibility of allelic interaction inducing hybrid weakness is discussed.

An overview on reproductive isolation inOryza sativacomplex

“…The phenotype of hybrid weakness is similar to the symptoms associated with disease responses ( Bomblies and Weigel 2007 ). Hybrid weakness has been reported in many crosses involving O. rufipogon × O. sativa , O. barthii × O. glaberrima as well as between the subspecies of O. sativa ( indica × indica and japonica × japonica hybridizations) ( Ichitani et al 2011 ; Zhang et al 2012 ; Chen et al 2013 , 2014 ). Early embryogenesis is normal, and normal seedlings are established ( Chen et al 2013 ).…”

“…Although, many cases of hybrid weakness have been reported in rice, the exact molecular mechanism underlying hybrid weakness is still not well known. Previous studies have suggested that hybrid weakness is under the control of complementary gene either dominant or recessive ( Amemiya and Akemine 1963 ; Chu and Oka 1970; Ichitani et al 2007 , 2011 ; Saito et al 2007; Kuboyama et al 2009 ; Chen et al 2014 ). Some studies have suggested that hybrid weakness is due to the activity of defence-related genes which was further confirmed by the high activity of F 1 hybrids against pathogens ( Bombiles and Weigel 2007 ; Chen et al 2014 ).…”

“…Almost all kinds of reproductive barriers so far reported in plants have been found in O. sativa complex and these include reduced cross-fertility, low germinability of F 1 seeds, F 1 inviability, F 1 pollen and embryo sac sterility, and a sporophytic sterility and weakness in F 2 generations also known as hybrid breakdown ( Fig. 2 ) ( Amemiya and Akemine 1963 ; Chu and Oka 1969 , 1970; Ichtiani et al 2007, 2011 ; Chen et al 2008, 2014 ). Despite the occurrence of many barriers to hybridization, introgression among these Oryza species is common.…”

A novel discovery of a long terminal repeat retrotransposon-induced hybrid weakness in rice

“…Hybrid weakness in Asian rice ( Oryza sativa L.) was first described in the 1950s (Oka, 1957). Subsequently, many genetic sources of hybrid weakness have been reported in rice crosses (Amemiya and Akemine, 1963; Chu and Oka, 1972; Okuno and Fukuoka, 1999; Ichitani et al ., 2007, 2011; Fu et al , 2013; Chen et al , 2013, 2014). Hybrid weakness in monocots, especially in rice, is frequently reported (Koide et al , 2008).…”

“…However, the detailed mechanisms of hybrid weakness in rice are not yet fully understood. Few genes have been narrowed down to sufficiently small genomic intervals (Ichitani et al , 2007, 2011; Kuboyama et al , 2009). Two loci, Hwi1 and Hwi2 , consisting of three genes that cause hybrid weakness in an interspecific F 1 hybrid between O. sativa and a wild rice Oryza rufipogon have been identified and cloned (Chen et al , 2013, 2014).…”