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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.

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.

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ABSTRACT. Seven of 92 lactating Holstein cows on a dairy farm developed urticaria with alopecia and decreased milk production, and three of the seven died over the course of 7 to 18 days. Pathologic examination of the three cows, including the two dead and one euthanized cow, revealed that the skin, liver, spleen, kidneys, heart, salivary glands, pancreas, adrenal glands, mammary glands, lymph nodes, and trigeminal ganglia had lymphocytic to lymphogranulomatous inflammation. Inflammation predominated by lymphocytic infiltration was prominent in the heart, pancreas, mammary glands, adrenal gland, and trigeminal ganglia. Severe granulomatous inflammation with multinucleated giant cells was present in the spleen and kidneys. These lesions and their distributions were most similar to those seen in suspected cases of citrus pulp toxicosis and hairy vetch toxicosis. The outbreak of this disease resolved with the elimination of Citrus pulp from the feed. Immunohistochemical detection of lymphocytes and macrophage markers confirmed dramatic hyperplasia of CD3-positive T lymphocytes in these lesions. This strongly suggested that a type 4 hypersensitivity reaction played a role in the development of the lesions.