The chloroplast genome characteristics, comparative genomics and gene resource mining of Celtis sinensis (Persoon, 1805)

Celtis is a large genus in Cannabaceae family, with more than 70 species in the world. However, the intraspecific variabilities of morphological features make it difficult for some species to be distinguished based on their morphological characteristics. To supply the chloroplast (cp) genome resources of Celtis for species identification, the plastome of Celtis sinensis Persoon 1805 was newly sequenced and comparative genomics was analyzed. The chloroplast genome was 159,085 bp in length and had a quadripartite structure consisting of two inverted repeats (IRs) separated by a small single copy (SSC) and a large single copy (LSC) region. A total of 133 genes were annotated, including 88 protein-coding genes, eight rRNA genes, and 37 tRNA genes. Among the protein-coding genes, the frequency of the leucine codon is the highest and that of the cysteine codon is the lowest. Comparative genomic analysis showed that the IR(S) region was more conservative than the LSC and SSC regions, with most sequence variations located in the intergenic spacer rather than the protein-coding region. Moreover, sixteen highly divergent hotspots were identified. The ML phylogenetic tree showed that all involved Celtis species were clustered together, and the plastome reported in this paper has high enough resolution to distinguish C. sinensis (Pers.) from other Celtis plants. This study provides useful genetic resources for the identification of C. sinensis (Pers.) and is also of great significance for the phylogeny study of Celtis plants in the future.