Legume Genomics and Genetics 2024, Vol.15, No.3, 118-125 http://cropscipublisher.com/index.php/lgg 119 2 Genomic Characteristics of Robinia pseudoacacia 2.1 Genome structure and organization Robinia pseudoacacia, commonly known as black locust, exhibits a typical chloroplast genome structure that includes two single copy regions (large and small single copy sections) and a pair of inverted repeats (IRs). The genome sizes range from 155 364 bp to 155 655 bp, with variations observed among different varieties (Yu et al., 2019). The chloroplast genomes contain 124~130 protein-coding genes, with some genes containing introns, such as the rpoC1 gene with the longest introns at 2 828 bp. Additionally, the genome-wide pattern of DNA methylation in R. pseudoacacia reveals significant methylation in CG, CHG, and CHH contexts, with intergenic regions showing the highest methylation levels (Zhang et al., 2021). 2.2 Gene content and functional annotation The gene content of R. pseudoacacia includes a variety of functional genes, with a significant number of simple sequence repeats (SSRs) identified. A study identified 170 SSR loci distributed in 162 non-redundant sequences, with dinucleotide repeats being the most predominant (Dong et al., 2019). Furthermore, the development of EST-SSR markers has facilitated the genetic diversity analysis and DNA fingerprinting of R. pseudoacacia cultivars, revealing a high level of polymorphism and genetic differentiation among different cultivars. The functional annotation of these genes indicates their involvement in various biological processes, including catalytic activity, metabolic processes, and cellular processes (Zhang et al., 2021). 2.3 Comparative genomics with related species Comparative genomic analysis of R. pseudoacacia with related species such as Sophora japonica has highlighted the superior performance of R. pseudoacacia in terms of functional traits. R. pseudoacacia exhibits greater photosynthetic capacity, higher leaf nitrogen concentration, and lower carbon-to-nitrogen ratio compared to S. japonica, which contributes to its superior ecological strategies and invasive potential (Luo et al., 2016). Additionally, phylogenetic analysis using chloroplast genomes has shown that R. pseudoacacia is closely related to Lotus japonicus, while being more distantly related to Acacia ligulata. This comparative analysis provides insights into the evolutionary relationships and adaptive traits of R. pseudoacacia, which are crucial for its management and utilization in silviculture. 3 Evolutionary History and Genetic Diversity 3.1 Phylogenetic relationships Robinia pseudoacacia, commonly known as black locust, has been the subject of various phylogenetic studies to understand its evolutionary relationships. Research has shown that the symbiotic genes of R. pseudoacacia rhizobia, derived from different geographical regions such as Poland and Japan, exhibit significant sequence conservation. This suggests that the symbiotic apparatus of R. pseudoacacia rhizobia might have evolved under strong host plant constraints, indicating a vertical transmission of these genes (Mierzwa et al., 2010). Additionally, studies on the phylogenetic structure of R. pseudoacacia forests in the Loess Plateau, China, have highlighted the influence of habitat on phylogeny, showing that species richness is closely correlated with phylogenetic diversity (Zhao et al., 2020). 3.2 Population genetics and genetic variation The genetic diversity and population structure of R. pseudoacacia have been extensively studied using various molecular markers. For instance, simple sequence repeat (SSR) markers have revealed vast genetic differentiation among different populations in China (Figure 1), with an average of 8.352 alleles per locus and a mean Shannon's index of 1.302. This study also found that 93% of the genetic variation was within collection sites, while 7% was among sites, indicating a high level of genetic diversity within populations (Guo et al., 2021). Similarly, inter-simple sequence repeat (ISSR) markers have shown that genetic differentiation among populations is relatively small, whereas within populations it is greater, suggesting a high level of genetic diversity at the species level.
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