Data from: Drastic shift in flowering phenology of F1 hybrids explains the population structure of Imperata cylindrica in Japan

Hybridization is a major source of phenotypic variation and a driving force for evolution. On the other hand, these novel traits can often disrupt adaptive relationships between the parental phenotypes and their environments. However, it remains unclear how new hybrid traits disrupt local adaptation. Here, we report how a new phenotype of hybrids between two ecotypes of Imperata cylindrica contributes to rapid reproductive isolation from their parents and affects hybrid fitness. We analyzed 350 accessions of I. cylindrica collected from the 1980s to the 2010s throughout Japan to explore the genetic population structure of the hybrids. We surveyed flowering periods, seed sets, and germination of two ecotypes and their hybrids in both natural habitats and common gardens. Genetic analyses of population structure revealed that the hybrid populations consisted of only F1 individuals, without post-F1 hybrids. The flowering phenology of the F1 plants was delayed to autumn, 5–6 months later than the parental ecotypes. The drastic shift in flowering phenology prevents F1s from backcrossing. In addition, it changes their seed dispersal time to winter. Germination is inhibited by low temperatures, and the seeds likely decay before the next spring, resulting in the absence of an F2 generation. For the first time in the field, we found environmental mismatch of F1 as a specific mechanism for the maintenance of only F1 populations. Synthesis. We have demonstrated that this flowering phenology mismatch promotes reproductive isolation between the parents and F1s and affects various temporal components of the hybrids, resulting in a unique hybrid population consisting only of F1s. This system sheds light on the importance of hybrid traits in terms of rapid reproductive isolation.