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

Abstract

The weak growth occurring in hybrids derived from crosses between two normal strains is referred to as hybrid weakness. F 1 hybrid weakness (F 1 HW) in a cross between a Peruvian rice cultivar 'Jamaica' and Japanese rice cultivars has been reported and was considered to be controlled by a set of complementary genes, Hwc1 and Hwc2. We observed the development of F 1 plants between Nipponbare and Jamaica (NJF 1 s). NJF 1 s were characterized by short roots, rolled leaves and a short stature. Although the NJF 1 embryos were normal, the growth of the primary roots was arrested and in most cases, they could not emerge from seeds of the hybrids. Histological observation of the root apical meristem (RAM) at 10 days after sowing revealed that the zone of cell division in the NJF 1 plants was lost, and that the cells were not short enough to explain the reduced root length of the NJF 1 plants. Thus, it was suggested that the shorter roots the NJF 1 plants might be due to reduced cell division. Although no distinct morphological abnormalities in the shoot apical meristem were observed, leaf primordium production was delayed in the NJF 1 plants. Inhibition of root elongation in this type of hybrid weakness was alleviated under high temperature (34°C) conditions in diverse genetic backgrounds. The threshold temperature required for the recovery of root growth from hybrid weakness seemed to range from 29 to 30°C. Among 85 NJF 1 plants that germinated and grew at 34°C for 10 days, and then outdoors, 18 plants eventually flowered. These plants were dwarf and their flowering time was delayed compared with that of their parents. In spite of their abnormal development, they were fertile. Thus, it is suggested that high temperature treatment was also effective to overcome hybrid weakness and to obtain next generation in this cross combination.

Genetic and Cytological Analysis of a Novel Type of Low Temperature-Dependent Intrasubspecific Hybrid Weakness in Rice

“…And the photosynthetic rate of the HW F 1 was significantly lower than those of the parents. Currently, several hybrid weakness or necrosis cases in some plants were previously reported to be temperature sensitive [8] and high temperature could rescue hybrid weakness [17]. Autoactivation of immune responses associated with Arabidopsis hybrid breakdown at low temperature has been described [8].…”

“…Hybrid weakness induced by low temperature will affect the application of heterosis in hybrid rice, to some extent, especially in the double-cropping rice areas of South China, although the frequency of hybrid weakness in rice is apparently much lower than that of intraspecific crosses in Arabidopsis [8]. Hybrid weakness in rice was reported more than four decades ago and its occurrence was sometimes conditioned by low temperature [17]. These hybrid weakness cases showed similar symptoms, including stunted growth, fewer tillers and abnormal root development.…”

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

“…The phenotype differs considerably from that of hybrids carrying both Hwc1-1 and Hwc2-1, of which the symptoms are characterized by root growth inhibition appearing just 5 days after germination (Ichitani et al 2001;Saito et al 2007).…”

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

“…The genotypes of each recombinant were estimated to be Hwc2-1/Hwc2-1 if all hybrid plants showed inhibition of root elongation (usually less than 1 cm), hwc2-2/hwc2-2 if they showed normal root elongation (usually more than 5 cm), and Hwc2-1/hwc2-2 if hybrid plants segregated. A few intermediate phenotypes were transplanted in pots and grown for 1 month to check if hybrid weakness symptom as reported in our previous paper (Saito et al 2007) was observed or not. The genotypes of internal DNA markers between the two initial encompassing markers of HWC2 (Table 3) were tested to narrow down the area of interest.…”