Species Origin of Genomic Factors in Nicotiana nudicaulis Watson Controlling Hybrid Lethality in Interspecific Hybrids between N. nudicaulis Watson and N. tabacum L

Hybrid lethality is expressed at 28°C in the cross Nicotiana nudicaulis × N. tabacum . The S subgenome of N. tabacum has been identified as controlling this hybrid lethality. To clarify the responsible genomic factor(s) of N. nudicaulis , we crossed N. trigonophylla (paternal progenitor of N. nudicaulis ) with N. tabacum , because hybrids between N. sylvestris (maternal progenitor of N. nudicaulis ) and N. tabacum are viable when grown in a greenhouse. In the cross N. trigonophylla × N. tabacum , approximately 50% of hybrids were vitrified, 20% were viable, and 20% were nonviable at 28°C. To reveal which subgenome of N. tabacum was responsible for these phenotypes, we crossed N. trigonophylla with two progenitors of N. tabacum , N. sylvestris (SS) and N. tomentosiformis (TT). In the cross N. sylvestris × N. trigonophylla , we confirmed that over half of hybrids of N. sylvestris × N. trigonophylla were vitrified, and none of the hybrids of N. trigonophylla × N. tomentosiformis were. The results imply that the S subgenome, encoding a gene or genes inducing hybrid lethality in the cross between N. nudicaulis and N. tabacum , has one or more genomic factors that induce vitrification. Furthermore, in vitrified hybrids of N. trigonophylla × N. tabacum and N. sylvestris × N. trigonophylla , we found that nuclear fragmentation, which progresses during expression of hybrid lethality, was accompanied by vitrification. This observation suggests that vitrification has a relationship to hybrid lethality. Based on these results, we speculate that when N. nudicaulis was formed approximately 5 million years ago, several causative genomic factors determining phenotypes of hybrid seedlings were inherited from N. trigonophylla . Subsequently, genome downsizing and various recombination-based processes took place. Some of the causative genomic factors were lost and some became genomic factor(s) controlling hybrid lethality in extant N. nudicaulis .