Rice Genomics and Genetics 2024, Vol.15, No.4, 178-189 http://cropscipublisher.com/index.php/rgg 185 8 Case Studies of Geographical Migration and Domestication 8.1 Oryza sativa inAsia The migration of Oryza sativa within Asia is a complex process that has been extensively studied. The domestication of O. sativa is believed to have occurred in multiple regions within Asia, with significant centers of origin identified in China and India. The first known center of rice cultivation in China generated the O. sativa subspecies japonica, while the indica subspecies arose from the second center of domestication in the Ganges River plains of India (Chatterjee et al., 2020). This dual-origin hypothesis is supported by genetic evidence showing that Asian rice originated from multiple wild progenitor subpopulations, although de novo domestication appears to have occurred only once, with domestication alleles being transferred between rice subpopulations through introgression (Lam et al., 2019). The domestication of Oryza sativa involved a series of genetic changes that were selected for their advantageous traits. Comparative analysis of domestication genes between Asian and African rice has revealed that the two species underwent independent but convergent evolution processes (Thierry et al., 2022). Key domestication traits such as non-shattering seeds, erect plant architecture, and increased yield were selected during the domestication process. For instance, the gene PROG1, associated with an erect plant architecture phenotype, showed convergent selection in both O. sativa and O. glaberrima (Chatterjee et al., 2020). The domestication of O. sativa was a complex process influenced by both natural and human selection, leading to the development of high-yielding and adaptable rice varieties. 8.2 Oryza glaberrima inAfrica The migration patterns of Oryza glaberrima, the African rice, are less well understood compared to its Asian counterpart. However, recent genomic studies have provided insights into its geographical diversification. Population structure analysis revealed five genetic clusters localizing to different geographic regions, suggesting a non-centric or polycentric origin of African rice domestication (Veltman et al., 2019). The inner Niger delta has been suggested as a significant center of origin, but molecular data to support this hypothesis is still emerging. The genetic structure within O. glaberrima shows a geographical association, indicating that different regions contributed key alleles for various domestication traits (Singh et al., 2018). The domestication of Oryza glaberrima is characterized by a severe bottleneck caused by domestication, with signatures of recent and strong positive selection not unequivocally pointing to candidate domestication genes (Veltman et al., 2019). This suggests that domestication proceeded differently than in Asian rice, either by selection on different alleles or different modes of selection. The domestication of African rice was influenced by significant climatic changes, such as the drying of the Sahara, which triggered the domestication process (Zhang et al., 2022). The genetic and developmental basis of some domestication traits are conserved between Asian and African rice, indicating a convergent evolution process (Thierry et al., 2022). Despite the challenges in identifying the exact domestication center, it is clear that African rice underwent a complex domestication process influenced by both environmental and human factors. 9 Challenges and Future Directions in Research 9.1 Gaps in current knowledge Despite significant advancements in understanding the domestication and migration of Oryza species, several gaps remain. One major gap is the incomplete understanding of the genetic basis of domestication traits across different Oryza species. For instance, while the domestication of Asian rice (Oryza sativa) has been extensively studied, the evolutionary history of African rice (Oryza glaberrima) is less clear, with evidence suggesting a multiregional origin rather than a single centric domestication event (Fujino et al., 2019). Additionally, the role of introgression from wild relatives into domesticated species, such as the introgression of the Pi-cd locus fromOryza meridionalis into O. sativa, is not fully understood. There is also a need for more comprehensive phylogenetic analyses to resolve the complex history of introgression and diversification within the Oryza genus (Stein et al., 2018).
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