Fine Mapping of HWC2, a Complementary Hybrid Weakness Gene, and Haplotype Analysis Around the Locus in Rice

Abstract

Hybrid weakness is a reproductive barrier. In rice, the hybrid weakness caused by two complementary genes--HWC1 and HWC2--has been surveyed extensively. However, their gene products and the molecular mechanism that causes hybrid weakness have remained unknown. We first performed fine mapping of HWC2, narrowing down the area of interest to 19 kb. We thereby identified five candidate genes. Second, we performed haplotype analysis around the HWC2 locus of 33 cultivars. With 15 DNA markers examined, all the 13 Hwc2-1 carriers share the same haplotype for consecutive 14 DNA markers. As for hwc2-2 carriers, five out of 20 have the haplotypes relatively similar to those of Hwc2-1 carriers. However, the other haplotypes differ remarkably from them. These results are useful to identify the HWC2 gene and to study rice varietal differentiation.

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

“…Having learned recently that all the plant materials used by Oka (1957) have been maintained in the National Institute of Genetics, Japan, we plan to use these cultivars to find Hwa1-1-or Hwa2-1-specific conserved haplotypes of DNA markers. Regarding HWC2 genes, we found that the DNA marker banding patterns of 14 markers surrounding the HWC2 locus were conserved completely among the 13 Hwc2-1 carriers (Kuboyama et al 2009). We will find potential carriers with the aid of DNA markers from our rice germplasm collection and make test crosses to determine whether they really carry the genes if such a haplotype is found in the Hwa1-1 or Hwa2-1 carriers.…”

“…According to one model, hybrid weakness results from interaction among non-allelic genes: the causal genes of another hybrid weakness in rice, HWC1 and HWC2, are located on the different chromosomes Kuboyama et al 2009). This case is true for the A. thaliana case ).…”

“…Reportedly, it encodes an NB-LRR protein ). We fine-mapped HWC2 gene in rice, and narrowed down the area of interest to 19 kb, identifying five candidate genes, one of which encodes an NB-LRR protein (Kuboyama et al 2009). Alcázar et al (2009) fine mapped a causal gene of incompatibility or hybrid breakdown to a cluster of TIR-NB-LRR genes in Arabidopsis.…”

“…The latter phenomenon results from the use of a set of complementary dominant genes: Hybrid weakness C1 (HWC1) and Hybrid weakness C2 (HWC2). We have detected the chromosomal locations of these genes and have performed fine mapping (Ichitani et al 2001;Ichitani et al 2007; Kuboyama et al 2009). However, the chromosomal locations of HWA1 and HWA2 have remained unknown.…”

Nicotiana debneyi has a single dominant gene causing hybrid lethality in crosses with N. tabacum

“…Hybrid lethality is a type of reproductive isolation mechanism that has been observed in several plant species, including Nicotiana species (Tezuka et al 2010;Yamada et al 1999), rice (Kuboyama et al 2009), wheat (Mizuno et al 2010), cotton (Song et al 2009), and Arabidopsis thaliana (Bomblies et al 2007). This mechanism often causes the death of hybrid plants and can be a barrier for the introduction of desirable genes into cultivated species by wide hybridization.…”