Rice Genomics and Genetics 2024, Vol.15, No.3, 94-105 http://cropscipublisher.com/index.php/rgg 101 Hybridization and introgression have played significant roles in the evolution of cultivated rice. The domestication of Oryza sativa indica involved hybridization between japonica rice and local wild rice, leading to the development of new cultivars as the initial domesticated varieties spread into new regions (Huang et al., 2012). Additionally, the study of whole chloroplast genome sequences has provided a well-resolved phylogeny of the AA genome Oryza species, revealing the evolutionary relationships and hybridization events among wild and domesticated rice species (Wambugu et al., 2015). These findings underscore the importance of hybridization and introgression in the diversification and adaptation of cultivated rice. 6 Genetic Diversity and Conservation 6.1 Levels and patterns of genetic diversity Intraspecific variation within Oryza species has been extensively studied using various molecular markers. For instance, a study utilizing inter simple sequence repeat (ISSR) polymorphism revealed significant genetic diversity within Oryza species, identifying 87 putative genome/species-specific molecular markers (Joshi et al., 2000). Another study focused on nucleotide variation in Oryza officinalis and its close relatives, showing that these species harbor relatively low levels of nucleotide diversity compared to other plant species, which could be attributed to their smaller historic effective population sizes (Zhang and Ge, 2006). Additionally, a detailed study of molecular diversity in Oryza sativa and its wild relatives using 176 SSR markers demonstrated significant genetic discrimination among accessions, with a mean of 16 alleles per SSR marker (Agrama and Eizenga, 2008). Interspecific variation among Oryza species has been highlighted through various phylogenetic and genomic studies. For example, the analysis of 13 reference genomes spanning the Oryza species tree revealed rapid species diversification and the emergence of novel genetic elements, including transposons and new coding and noncoding genes (Stein et al., 2018). Another study using ISSR markers suggested that the genus Oryza may have evolved following a polyphyletic pathway, with Oryza brachyantha being the most divergent species (Joshi et al., 2000). Furthermore, phylogenetic analyses based on nuclear genes and intergenic regions have successfully reconstructed the relationships among AA-genome species, indicating that Oryza meridionalis is the earliest divergent lineage (Zhu et al., 2014). 6.2 Conservation strategies for wildOryza species In situ conservation strategies are crucial for preserving the genetic diversity of wild Oryza species in their natural habitats. A study on the endangered wild species Oryza granulata in Yunnan, China, highlighted the importance of rescuing and conserving core populations for in situ conservation. The field survey showed that 44% of natural populations have become extinct over the last 30 years, emphasizing the need for immediate conservation efforts (Wu et al., 2004). Additionally, the genetic diversity analysis within populations of Oryza granulata revealed higher genetic variation among populations than within populations, suggesting that in situ conservation should focus on maintaining multiple populations to preserve overall genetic diversity (Wu et al., 2004). Ex situ conservation strategies involve preserving genetic material outside of its natural habitat, which is essential for safeguarding genetic resources against habitat loss and other threats. The study on Oryza granulata proposed sampling and preserving more populations with fewer individuals from each population for ex situ conservation (Wu et al., 2004). Moreover, the establishment of genomic databases, such as OryzaGenome, which integrates genotype and phenotype information, provides a valuable resource for ex situ conservation efforts. This database includes genotype information for various Oryza species, facilitating the preservation and study of genetic diversity (Ohyanagi et al., 2015). Additionally, the release of a complete long-read assembly of IR 8 'Miracle Rice' marks a significant milestone in modern rice research, contributing to the ex situ conservation of important genetic resources (Stein et al., 2018). By combining in situ and ex situ conservation strategies, we can ensure the preservation of genetic diversity within the genus Oryza, which is vital for future crop protection and the sustainable use of these genetic resources.
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