Phenological and morphological variations of Oryza rufipogon and O. nivara in Sri Lanka and their evolutionary implications

Abstract

Phenological and morphological variation are widely viewed as a pivotal driver of ecological adaptation and speciation. Oryza rufipogon and O. nivara, the wild progenitors of cultivated rice, are most closely related and provide a unique system for investigating ecological adaption and speciation. Here, we investigate variation patterns of flowering phenology and morphological traits within and between O. rufipogon and O. nivara populations in Sri Lanka by incorporating the in situ observation in natural habitats and manipulative experiments in common gardens. First, we observed varying degrees of phenological variation under different temporal and environmental conditions, suggesting that flowering phenology of two Oryza species is highly plastic and vary depending on both environment and management practices. Particularly, the Sri Lankan O. nivara exhibits high plasticity in flowering phenology with water availability being a primary determinant, implying that O. nivara might not be an annual in the strict sense. Second, we unexpectedly found that flowering time of the two species in Sri Lanka overlapped to a large extent, which suggests that the primary factor to maintain the species divergence in Sri Lanka may not be flowering time but rather environments. Third, we uncovered two distinct morphological groups, corresponding to O. rufipogon and O. nivara and show that habitat difference rather than spatial isolation is the main explanation for the phenotypic divergence between species. Moreover, our results indicate a substantial link between morphological variation and several major climatic factors including altitude, temperature and rainfall. Finally, our selection analysis suggests that interspecific divergence in the traits related to reproduction and habitat preference is most likely driven by natural selection. Together, our case study on the Sri Lankan O. rufipogon and O. nivara enhances the understanding of the roles of phenotypic plasticity and environmental factors in the processes of adaptation and speciation.