The chloroplasts genomic analyses of four specific Caragana species.

Background: Many species of the genus Caragana have been used as wind prevention and sand fixation plants. They are also important traditional Chinese medicine, and ethnic medicine resource plant. Thus, chloroplast genomes (cp-genome) of some of these important species must be studied.

Methods: In this study, we analyzed the chloroplast genomes of C. jubata, C. erinacea, C. opulens, and C. bicolor, including their structure, repeat sequences, mutation sites, and phylogeny.

Results: The size of the chloroplast genomes was between 127,862 and 132,780 bp, and such genomes contained 112 genes (30 tRNA, 4 rRNA, and 78 protein-coding genes), 43 of which were photosynthesis-related genes. The total guanine + cytosine (G+C) content of four Caragana species was between 34.49% and 35.15%. The four Caragana species all lacked inverted repeats and can be classified as inverted repeat-lacking clade (IRLC). Of the anticipated genes of the four chloroplast genomes, introns were discovered in 17 genes, most of which were inserted by one intron. A total of 50 interspersed repeated sequences (IRSs) were found among them, 58, 29, 61, and 74 simple sequences repeats were found in C. jubata, C. bicolor, C. opulens, and C. erinacea, respectively. Analyses of sequence divergence showed that some intergenic regions (between trnK-UUU and rbcl; trnF-GAA and ndhJ; trnL-CAA and trnT-UGU; rpoB and trnC-GCA; petA and psbL; psbE and pebL; and sequences of rpoC, ycf1, and ycf2) exhibited a high degree of variations. A phylogenetic tree of eight Caragana species and another 10 legume species was reconstructed using full sequences of the chloroplast genome.

Conclusions: (1) Chloroplast genomes can be used for the identification and classification of Caragana species. (2) The four Caragana species have highly similar cpDNA G+C content. (3) IRS analysis of the chloroplast genomes showed that these four species, similar to the chloroplast genome of most legumes, lost IRLC regions. (4) Comparative cp-genomic analysis suggested that the cp genome structure of the Caragana genus was well conserved in highly variable regions, which can be used to exploit markers for the identification of Caragana species and further phylogenetic study. (5) Results of phylogenetic analyses were in accordance with the current taxonomic status of Caragana. The phylogenetic relationship of Caragana species was partially consistent with elevation and geographical distribution.