Transcriptomic analysis of codon usage patterns and gene expression characteristics in leafy spurge

Leafy spurge (Euphorbia esula) is an important herb and potential energy source with medicinal value. Codon usage bias (CUB) is a static feature of genes and genomes that results from adaptation and selection during long-term evolution and facilitates molecular breeding in transgenic plants. Here, we used TransDecoder to identify candidate coding regions from the downloaded leafy spurge transcriptome and generate coding region annotation files based on reference genomes. The whole genome showed A/T bias, especially at terminal positions, and seven high-frequency codons were identified. We compared codon usage frequencies to identify candidate exogenous expression receptor systems for leafy spurge. The identified factors affecting leafy spurge CUB included natural selection and other factors, mutation pressure and base composition, with natural selection and other factors being dominant. The observed CUB was significantly positively correlated with the gene expression levels. Systematic analysis of whole-genome leafy spurge revealed that highly expressed protein-coding genes presented greater CUB than did less expressed protein-coding genes. Furthermore, the highly expressed genes tended to have terminal G/C bases. In summary, we conducted a series of related studies based on the leafy spurge whole-genome sequence and laid a foundation for selecting suitable exogenous expression receptor systems and improving gene expression levels.