Rice Genomics and Genetics 2024, Vol.15, No.3, 94-105 http://cropscipublisher.com/index.php/rgg 99 4 Geographical Distribution and Migration Patterns 4.1 Historical biogeography of Oryza The genus Oryza, which includes both wild and cultivated rice species, has a complex evolutionary history marked by significant biogeographical events. The origin of Oryza is traced back to approximately 24 million years ago (MYA) during the Miocene epoch, with the deepest split within the genus occurring around 15 MYA (Tang et al., 2010). The early diversification of the AA-genome species, a major group within Oryza, is estimated to have occurred around 2.93 MYA (Zhu et al., 2014). This period of diversification was characterized by rapid radiation events, which have posed challenges in resolving the phylogenetic relationships within the genus. The distribution of Oryza species is widespread, encompassing various continents and regions. The AA-genome species, for instance, are distributed worldwide, with significant populations in Asia, Africa, and Australia. The biogeographical history of Oryza suggests that long-distance dispersal played a crucial role in the genus's diversification. This is supported by evidence of trans-oceanic dispersal events, which facilitated the spread of Oryza species across different continents (Tang et al., 2010). The genus's ability to adapt to diverse ecological niches has further contributed to its extensive geographical distribution. 4.2 Contemporary migration and gene flow Natural dispersal mechanisms have significantly influenced the contemporary migration patterns of Oryza species. These mechanisms include water-mediated seed dispersal, which is particularly effective in wetland habitats where many Oryza species thrive. Additionally, the presence of specific morphological traits, such as awns and buoyant seeds, enhances the ability of these species to disperse over long distances (Tang et al., 2010). The role of natural dispersal is evident in the genetic diversity observed within and between Oryza populations, which reflects historical and ongoing gene flow. Human activities have also played a pivotal role in the migration and distribution of Oryza species. The domestication of rice, particularly Oryza sativa, has led to extensive human-mediated migration, resulting in the widespread cultivation of rice across various continents. This process has facilitated gene flow between wild and cultivated rice species, contributing to the genetic diversity observed in contemporary Oryza populations (Stein et al., 2018). The introduction of rice to new regions through trade and agricultural practices has further expanded the geographical range of the genus, highlighting the significant impact of human intervention on the distribution patterns of Oryza (Spano et al., 2018; Torke et al., 2021). The geographical distribution and migration patterns of Oryza species are shaped by a combination of historical biogeographical events, natural dispersal mechanisms, and human-mediated migration. These factors have collectively contributed to the extensive and diverse distribution of Oryza species across the globe. 5 Domestication and Evolution of Cultivated Rice 5.1 Domestication centers and early cultivation The domestication of rice, one of the most significant agricultural developments in human history, has been a subject of extensive research and debate. The origins of cultivated rice (Oryza sativa) are traced back to specific regions in Asia. Genetic evidence suggests that Oryza sativa japonica rice was first domesticated from a population of O. rufipogon in the middle area of the Pearl River in southern China. Subsequently, Oryza sativa indica rice developed from crosses between japonica rice and local wild rice as the initial cultivars spread into South East and South Asia (Huang et al., 2012). Additionally, African rice (Oryza glaberrima) was independently domesticated along the Niger River, indicating a separate domestication event from its Asian counterpart (Wang et al., 2014). 5.2 Genetic evidence of domestication The domestication of rice involved the selection of specific genes and traits that were advantageous for cultivation. Studies have identified numerous domestication-associated traits through high-resolution genetic mapping. These traits include changes in plant architecture, seed shattering, and flowering time, which were crucial for the transition from wild to cultivated forms (Huang et al., 2012). Comparative genomic analyses have revealed that
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