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 Ramírez-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.