Comparative chloroplast genome analyses of Amomum: insights into evolutionary history and species identification

Background Species in genus Amomum always have important medicinal and economic values. Classification of Amomum using morphological characters has long been a challenge because they exhibit high similarity. The main goals of this study were to mine genetic markers from cp genomes for Amomum species identification and discover their evolutionary history through comparative analysis. Results Three species Amomum villosum, Amomum maximum and Amomum longipetiolatum were sequenced and annotated for the complete chloroplast (cp) genomes, and the cp genomes of A. longipetiolatum and A. maximum were the first reported. Three cp genomes exhibited typical quadripartite structures with 163,269-163,591 bp in length. Each genome encodes 130 functional genes including 79 protein-coding, 26 tRNAs and 3 rRNAs genes. 113-152 SSRs and 99 long repeats were identified in the three cp genomes. By designing specific primers, we amplified the highly variable loci and the mined genetic marker ccsA exhibited a relatively high species identification resolution in Amomum. The nonsynonymous and synonymous substitution ratios (Ka/Ks) in Amomum and Alpinia showed that most genes were subjected to a purifying selection. Phylogenetic analysis revealed the evolutionary relationships of Amomum and Alpinia species and proved that Amomum is paraphyletic. In addition, the sequenced sample of A. villosum was found to be a hybrid, becoming the first report of natural hybridization of this genus. Meanwhile, the high-throughput sequencing-based ITS2 analysis was proved to be an efficient tool for interspecific hybrid identification and with the help of the chloroplast genome, the hybrid parents can be also be determined. Conclusion The comparative analysis and mined genetic markers of cp genomes were conducive to species identification and evolutionary relationships of Amomum.