Hybrid seedlings from reciprocal interspecific crosses between Nicotiana nudicaulis and N. tabacum were not viable when cultured at 28°C; this an example of hybrid lethality. Characteristically, shoot apices and root tips of these hybrids began to brown immediately after germination at 28°C. However, hybrid seedlings did not exhibit any symptoms of lethality at 34°C, and their growth was normal at this higher temperature. When hybrid seedlings were cultured at 34°C and then transferred to 28°C, lethal symptoms emerged rapidly after the transfer. Therefore, hybrid lethality associated with reciprocal interspecific crosses between N. nudicaulis and N. tabacum was due to the interaction of coexisting heterologous genomes and not to a cytoplasmic effect. Furthermore, as the N. nudicaulis×N. tabacum seedlings die, features of apoptotic cell death, including chromatin condensation, nuclear fragmentation, and fragmentation of DNA, were evident. These observations indicated that hybrid lethality of N. nudicaulis×N. tabacum seedlings is accompanied by apoptotic cell death. This report is the first to demonstrate that hybrid lethality with apoptotic cell death results from N. nudicaulis×N. tabacum hybridization.
We found a number of male-sterile plants in a wild beet ( Beta vulgaris L. subsp. maritima) accession line, FR4-31. The inheritance study of the male sterility indicated the trait to be of the cytoplasmic type. The mitochondrial genome of FR4-31 proved to lack the male-sterility-associated genes preSatp6 and orf129, which are characteristic of the Owen CMS and I-12CMS(3) cytoplasms of beets, respectively. Instead, the truncated cox2 gene involved in G CMS originating from wild beets was present in the FR4-31 mitochondrial genome. In Southern hybridization using four mitochondrial gene probes, the FR4-31 cytoplasm showed patterns similar to those typical of the G cytoplasm. It is thus likely that the FR4-31 cytoplasm has a different CMS mechanism from both Owen CMS and I-12CMS(3), and that the FR4-31 and G cytoplasms resemble each other closely. A restriction map of the FR4-31 mitochondrial DNA was generated and aligned with those published for the Owen and normal fertile cytoplasms. The FR4-31 mitochondrial genome was revealed to differ extensively in arrangement from the Owen and normal genomes, and the male-sterile Owen and FR4-31 genomes seem to be derived independently from an ancestral genome.
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