Fine mapping and allelic dosage effect of Hwc1, a complementary hybrid weakness gene in rice

Abstract

Hybrid weakness is a reproductive barrier that is found in many plant species. In rice, the hybrid weakness caused by two complementary genes, Hwc1 and Hwc2, has been surveyed intensively. However, their gene products and the molecular mechanism that causes hybrid weakness have remained unknown. We performed linkage analyses of Hwc1, narrowed down the area of interest to 60 kb, and identified eight candidate genes. In the F(2) population, in which both Hwc1 and Hwc2 genes were segregated, plants were separable into four classes according to their respective phenotypes: severe type, semi-severe type, F(1) type, and normal type. Severe type plants show such severe symptoms that they could produce only tiny shoot-like structures; they were unable to generate roots. Genetic analyses using closely linked DNA markers of the two genes showed that the symptoms of the F(2) plants were explainable by the genotypes of Hwc1 and Hwc2. Weakness was observed in plants that have both Hwc1 and Hwc2. In Hwc1 homozygote, the symptoms worsened and severe type or semi-severe type plants appeared. Consequently, Hwc1 should have a gene dosage effect and be a semi-dominant gene. The dosage effect of Hwc2 was recognizable, but it was not so severe as that in Hwc1. These results are useful to elucidate the mechanism that causes the hybrid weakness phenomenon and the role of each causal gene in hybrid weakness.

Developmental Observation and High Temperature Rescue from Hybrid Weakness in a Cross between Japanese Rice Cultivars and Peruvian Rice Cultivar 'Jamaica'

“…This reduced activity of the SAM should be one of the reasons for the short stature of the NJF 1 plants. It was reported that homozygotes for Hwc1 and Hwc2 showed a severe weakness and very small shoots and did not develop roots (Ichitani et al 2007). Thus, the genes responsible for this type of hybrid weakness may also be involved in the SAM maintenance and not only in the RAM maintenance.…”

“…To reveal the molecular aspect of F 1 HW, the causal genes should be cloned and the 'mutation' by which the genes cause F 1 HW should be determined. Hwc1 and Hwc2 have already been fine-mapped (Ichitani et al 2007) and they are expected to be cloned. When these causal genes of F 1 HW are fully identified, it may become possible to elucidate the mechanisms where by high temperature treatment enabled to rescue the hybrids from weakness.…”

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

“…Genetic analysis for hybrid lethality has been conducted in some plant species, including rice (Ichitani et al 2007;Kuboyama et al 2009), wheat (Mizuno et al 2010), cotton (Song et al 2009), and A. thaliana (Bomblies et al 2007). In these species, hybrid lethality is caused by the interaction between two dominant complementary genes from parental plants.…”

Hybrid Weakness Found in the Backcross Progeny of an Inter-Specific Cross of <em>Oryza Sativa / O. Glumaepatula</em>

“…Hybrid in-viability (or weakness) and hybrid sterility have been observed in inter-specific as well as in intra-specific crosses [2,3,4,5,6,7,8,9,10], while hybrid breakdown has been reported only in intra-specific crosses [2,11,12,13,14]. By using molecular linkage maps, some of the genes controlling reproductive barriers have been located on several chromosomes [8,9,10,12,13,15].…”