4.2 Phenotypic divergences between sympatric weedy rice
populations associated with local adaptation
Substantial divergence in vegetative and reproductive growth traits,
including plant height, number of tillers, flowering time and
reproductive traits, was detected between the early- and late-season
weedy rice populations during the early-season common garden.
Furthermore, the early-season populations had evident advantages in
growth and development, such as higher plant height, more tillers and
earlier flowering, compared to the corresponding late-season
populations. However, no such divergence was detected in the late-season
common garden experiment. Therefore, these findings demonstrate
considerable phenotypic divergence between the sympatric two-season
weedy rice populations in the stressful early rice-cultivation season.
Previous study had detected considerable genetic divergence between the
early- and late-season weedy rice populations (Kong et al., 2021), and
our findings provided solid evidence to support the genetic divergence
from phenotypes. In addition, evident local adaptation (LA) for these
divergent traits was only detected in the early-season weedy rice
populations in the stressful early-season common garden, based on the
method adopted from (Hereford, 2009). Altogether, these findings not
only provide evidence of growth and development divergence between the
sympatric two-season weedy rice populations, but also demonstrate the
occurrence of evident local adaptation in the early-season populations,
which may be caused by the stressful environment in the early season.
The common garden experiment is generally used to test for local
adaptation signal in traits of interest, such as life history traits and
physiology, because it enables to unravel the genetic basis of complex
phenotypes across various populations without the confounding effects of
the corresponding environment (de Villemereuil et al., 2016, 2020). For
example, physiological evidence for local adaptation to both freezing
and drought stress in closely related American live oaks species was
obtained from the common garden experiments (Cavender-Bares and
RamÃrez-Valiente, 2017). Additionally, van Boheemen et al. (2019) found
the rapid and repeated local adaptation to climate in an invasive plant
by examination of trait divergence in a common garden experiment. These
studies demonstrate that local adaptation generally occurs in plant
populations, when the plants expanded a new ecological niche or habitat
and experienced some environmental stresses, such as low temperature and
drought. Therefore, the evident local adaptation detected in the
early-season weedy rice populations suggested that the early-season
weedy rice may expand from the late season, and evolve in a rapid
adaptive evolution process in the early season because of the stressful
environment.
Plants often show differences in morphology within and between
populations because of different local environments, which cause
different selective pressures to shape adaptive genetic variation in
plant individuals, and lead to heritable differences in plant phenotype
(Groot et al., 2018). Generally, the phenotype of an individual is
determined by the interactions between the environment and its genotype,
which includes local adaptation (Des Marais et al., 2013). In previous
study, considerable genetic divergence between the early- and
late-season weedy rice populations in the same rice fields (Kong et al.
2021) was detected. Therefore, the phenotypic divergence between the
two-season weedy rice populations is closely associated with the genetic
divergence, which may be mainly caused by the local adaptation in the
early-season populations. In addition, local adaptation is also assumed
to occur under limited gene flow (Jacob et al., 2017), and it means the
barriers to gene flow between populations are conducive to the
maintenance of adaptive traits or alleles within population.
Interestingly, the limited gene flow between the two-season weedy rice
populations was also proposed by Kong et al. (2021), which was explained
to maintain genetic diversity within populations. The finding of limited
gene flow between the early- and late-season weedy rice populations
provided evidence of the existence of two cryptical populations in the
same rice field, and suggested that the weedy rice individuals, which
adapted to low temperature and long daylength conditions, can survive
and only retain in the early rice-cultivation season. Therefore, through
local adaptation in the stressful environment and limited gene flow
between the two seasons, the genetic and phenotypical divergence both
emerged in weedy rice populations occurring in the same rice fields,
which is a typical event of sympatric divergence in plant species.