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.