Genetic Cause of Hybrid Lethality Observed in Reciprocal Interspecific Crosses between and .

Hybrid lethality, a type of postzygotic reproductive isolation, is an obstacle to wide hybridization breeding. Here, we report the hybrid lethality that was observed in crosses between the cultivated tobacco, (section ), and the wild tobacco species, (section ). Reciprocal hybrid seedlings were inviable at 28 °C, and the lethality was characterized by browning of the hypocotyl and roots, suggesting that hybrid lethality is due to the interaction of nuclear genomes derived from each parental species, and not to a cytoplasmic effect. Hybrid lethality was temperature-sensitive and suppressed at 36 °C. However, when hybrid seedlings cultured at 36 °C were transferred to 28 °C, all of them showed hybrid lethality. After crossing between an monosomic line missing one copy of the Q chromosome and , hybrid seedlings with or without the Q chromosome were inviable and viable, respectively. These results indicated that gene(s) on the Q chromosome are responsible for hybrid lethality and also suggested that has the same allele at the () locus responsible for hybrid lethality as other species in the section . Haplotype analysis around the locus suggested that there are at least six and two haplotypes containing and alleles, respectively, in the section .