High-fertility individuals observed in the progeny of low-fertility Nicotiana F1 interspecific hybrid and mechanistic insights into their appearance.

Interspecific hybrids with different genomes from their parents often result in hybrid sterility due to meiotic failure. This is a typical example of reproductive isolation that limits interspecific hybridization. Although a few progenies can be obtained in such cases, the inheritance pattern of fertility has not yet been studied in detail. Therefore, using Nicotiana as a model, the fertility of hybrids derived from crosses between species with different genomes was investigated up to the F₃ generation. In the F₂ population, which developed from selfing seeds obtained from the low-fertility F₁ generation, extremely large variation in pollen viability (1.2-82.9%, average 43.2%), fruit set rate (0-100%, average 69.3%), and seed number (16-185, average 63.5) were observed among individuals. In addition, some of the F₃ individuals from F₂ individuals with low fertility were high-fertility. Furthermore, it was suggested that not only pollen viability, but also pistil length and number of ovules may be involved in the variation in fertility. Genome-wide genotyping using GRAS-Di technology revealed that the genomes of high-fertility F₂ individuals had fewer heterozygous genomic regions, leading to a hypothesis regarding the association between the proportion of heterozygous regions in the genome and pollen viability. To our knowledge, this study represents the first experimental observation that high-fertility F₂ individuals can be obtained from interspecific F₁ hybrids carrying different genomes with extremely low fertility without change of ploidy. These findings can be considered basic insights that enhance our understanding of plant speciation and adaptation.