Key Roles of De-Domestication and Novel Mutation in Origin and Diversification of Global Weedy Rice

Crop-Weed Introgression Plays Critical Roles in Genetic Differentiation and Diversity of Weedy Rice: A Case Study of Human-Influenced Weed Evolution

As an important driving force, introgression plays an essential role in shaping the evolution of plant species. However, knowledge concerning how introgression affects plant evolution in agroecosystems with strong human influences is still limited. To generate such knowledge, we used InDel (insertion/deletion) molecular fingerprints to determine the level of introgression from japonica rice cultivars into the indica type of weedy rice. We also analyzed the impact of crop-to-weed introgression on the genetic differentiation and diversity of weedy rice, using InDel (insertion/deletion) and SSR (simple sequence repeat) molecular fingerprints. Results based on the STRUCTURE analysis indicated an evident admixture of some weedy rice samples with indica and japonica components, suggesting different levels of introgression from japonica rice cultivars to the indica type of weedy rice. The principal coordinate analyses indicated indica-japonica genetic differentiation among weedy rice samples, which was positively correlated with the introgression of japonica-specific alleles from the rice cultivars. In addition, increased crop-to-weed introgression formed a parabola pattern of dynamic genetic diversity in weedy rice. Our findings based on this case study provide evidence that human activities, such as the frequent change in crop varieties, can strongly influence weed evolution by altering genetic differentiation and genetic diversity through crop-weed introgression in agroecosystems.

Sympatric genetic divergence between early- and late-season weedy rice populations

Sympatric genetic divergence is the most appealing and controversial pattern in the theory of ecological speciation. Examples that support sympatric genetic divergence in plant species are extremely rare. Solid evidence of sympatric genetic divergence will provide deep insights for revealing the underlying mechanisms of ecological speciation. We analysed the total genomic DNA sequences of 120 weedy rice (WR; Oryza sativa f. spontanea) plants, representing three WR population pairs separately from three early- and late-season rice fields, in comparison with those of the co-occurring rice cultivars and other rice materials. We detected substantial genetic divergence within the pairs of the sympatric early- and late-season WR populations, although genetic divergence was unevenly distributed across the genomes. Restricted gene flow was determined between the sympatric WR populations, resulting in their distinct genetic structures. We also detected relatively low genetic diversity that was likely to be associated with stronger selection in early-season WR populations. Our findings provide strong evidence for sympatric genetic divergence between the WR populations in the same fields but in different seasons. We conclude that temporal isolation plays an important role in creating genetic divergence between sympatric populations/species in plants.

The complete chloroplast genome of japonica type weedy rice (Oryza sativa f. spontanea)

As a noxious weed, weedy rice (Oryza sativa f. spontanea Roshev. 1931) has threatened global food security and sustainable crop production. On the other hand, weedy rice has a strong tolerance for abiotic stresses and the potential to provide rich resources for rice genetic improvement. Thus, for a more comprehensive understanding of its speciation, we sequenced and assembled the first complete chloroplast genome of Oryza sativa f. spontanea (japonica type). The complete chloroplast genome was 134,555 bp in length and encoded 133 genes, including 83 protein-coding genes, 42 tRNA genes and 8 rRNA genes. Phylogenetic analysis revealed that the indica-japonica differentiation of weedy rice was closely related to cultivated rice, and Oryza sativa f. spontanea (japonica type) was genetically more closely clustered with cultivated rice O. sativa (japonica type) than O. nivara or other wild rice.