4.3 Shift in flowering time contributes to sympatric divergence in weedy rice
Differences in flowering time were observed between the early- and late-season weedy rice populations, and the early-season populations showed early flowering phenotype in the early rice-cultivation season. In addition, evident local adaptation for early flowering time was also detected in the early-season weedy rice populations. These results suggested a divergent performance of flowering time between the sympatric two-season weedy rice populations, most likely caused by the local adaptation in the stressful early rice-cultivation season.
Generally, shift in flowering time is usually accompanied with local adaptation in flowering plants (Waser & Campbell, 2004), and have the potential to act as strong prezygotic reproductive barriers in plants. In some species, such as the grasses Agrostis tenuis and Anthoxanthum odoratum , heavy metals tolerant and intolerant races differ in seasonal time of flowering, and so are partially isolated reproductively at a pre-pollination stage (McNeilly & Antonovics, 1968). In addition, flowering time divergence was also found in some compelling examples of sympatric divergence. For instance, the two palm species (Howea ) in Lord Howe Island segregate according to the acidity of the soil, and obvious disjunctions in flowering time were found between the two palms (Savolainen et al., 2006). In addition, two sister species of mountain rose (Metrosideros ) endemic to Lord Howe Island also exhibited apparent divergence in flowering time because of their divergent ecological niches (Osborne et al., 2020). These studies provide reliable evidence for shifts in flowering time caused by local adaptation between sympatric plant populations, and demonstrate its important role in sympatric divergence of flowering plants. Therefore, the findings of local adaptation associated disjunctions in flowering time indicate a possible rapid adaptive evolution in weedy rice populations occurring in the same rice fields, and probably generate genetic and phenotypic divergence between the sympatric two-season weedy rice populations.
In addition, for agricultural weeds, the early flowering phenotype would likely result in the evolution of weed populations that display a shorter life cycle, allowing plants to set and shed seed prior to crop harvest (Ashworth et al., 2015). Therefore, we propose two possible reasons for the early-season weedy rice evolved an early flowering phenotype which causing divergent flowering time between the two-season weedy rice populations. First, the genes response to the stressful environment in the early rice-cultivation season are closely linked to the genes regulate flowering in weedy rice, or pleiotropy (Nosil et al., 2009), most likely caused by genetic variations. Second, to ensure reproduction success in the new environment, weedy rice shortened its growth period to complete its life cycle as soon as possible, most likely caused by epigenetic modification (Cabej, 2019). In other words, flowering time is crucial in determining the adaptation of weedy rice in different ecological environments, and may contribute to the reproductive isolation between sympatric plant populations because of genetic or epigenetic variations in certain adaptive genes. Therefore, it is necessary to identify the adaptive genes associated with divergent performances, such as flowering time, and investigate their mechanisms underlie local adaptation in the early-season weedy rice populations in the future work.