Comparison of chloroplast genomes of Gynura species: sequence variation, genome rearrangement and divergence studies

BACKGROUND: Some Gynura species have been reported to be natural anti-diabetic plants. Improvement of their traits towards application relies on hybridization. Clearly, phylogenetic relationships could optimize compatible hybridizations. For flowerings plants, chloroplast genomes have been used to solve many phylogenetic relationships. To date, the chloroplast genome sequences of 4 genera of the tribe Senecioneae have been uploaded to GenBank. The internal relationships within the genus Gynura and the relationship of the genus Gynura with other genera in the tribe Senecioneae need further research. RESULTS: The chloroplast genomes of 4 Gynura species were sequenced, assembled and annotated. In comparison with those of 12 other Senecioneae species, the Gynura chloroplast genome features were analysed in detail. Subsequently, differences in the microsatellite and repeat types in the tribe were found. From the comparison, it was found that IR expansion and contraction are conserved in the genera Gynura, Dendrosenecio and Ligularia. Compared to other regions on the chloroplast genome, the region from 25,000 to 50,000 bp was not conserved. Seven ndh genes in this region are under purifying selection, with small changes in amino acids. The whole chloroplast genome sequences of 16 Senecioneae species were used to build a phylogenetic tree. Based on the oldest Artemisia pollen fossil, the divergence time was estimated. CONCLUSIONS: Sequencing the chloroplast genomes of 4 Gynura species helps us to solve many problems. The phylogenetic relationships and divergence time among 4 Gynura and 16 Senecioneae species were evaluated by comparing their chloroplast genomes. The phylogenetic relationship of the genera Gynura and Ligularia was different from that observed previous work. In a previous phylogenetic tree, the genus Ligularia belonged to the Tussilagininae subtribe, which was in a lineage that diverged earlier than other genera. Further morphology and genome-wide analyses are needed to clarify the genus relationships.