Comparative analysis of chloroplast genome structure and molecular dating in Myrtales.

BACKGROUND

Myrtales is a species rich branch of Rosidae, with many species having important economic, medicinal, and ornamental value. At present, although there are reports on the chloroplast structure of Myrtales, a comprehensive analysis of the chloroplast structure of Myrtales is lacking. Phylogenetic and divergence time estimates of Myrtales are mostly constructed by using chloroplast gene fragments, and the support for relationships is low. A more reliable method to reconstruct the species divergence time and phylogenetic relationships is by using whole chloroplast genomes. In this study, we comprehensively analyzed the structural characteristics of Myrtales chloroplasts, compared variation hotspots, and reconstructed the species differentiation time of Myrtales with four fossils and one secondary calibration point.

RESULTS

A total of 92 chloroplast sequences of Myrtales, representing six families, 16 subfamilies and 78 genera, were obtained including nine newly sequenced chloroplasts by whole genome sequencing. Structural analyses showed that the chloroplasts range in size between 152,214-171,315 bp and exhibit a typical four part structure. The IR region is between 23,901-36,747 bp, with the large single copy region spanning 83,691-91,249 bp and the small single copy region spanning 11,150-19,703 bp. In total, 123-133 genes are present in the chloroplasts including 77-81 protein coding genes, four rRNA genes and 30-31 tRNA genes. The GC content was 36.9-38.9%, with the average GC content being 37%. The GC content in the LSC, SSC and IR regions was 34.7-37.3%, 30.6-36.8% and 39.7-43.5%, respectively. By analyzing nucleotide polymorphism of the chloroplast, we propose 21 hypervariable regions as potential DNA barcode regions for Myrtales. Phylogenetic analyses showed that Myrtales and its corresponding families are monophyletic, with Combretaceae and the clade of Onagraceae + Lythraceae (BS = 100%, PP = 1) being sister groups. The results of molecular dating showed that the crown of Myrtales was most likely to be 104.90 Ma (95% HPD = 87.88-114.18 Ma), and differentiated from the Geraniales around 111.59 Ma (95% HPD = 95.50-118.62 Ma).

CONCLUSIONS

The chloroplast genome structure of Myrtales is similar to other angiosperms and has a typical four part structure. Due to the expansion and contraction of the IR region, the chloroplast genome sizes in this group are slightly different. The variation of noncoding regions of the chloroplast genome is larger than those of coding regions. Phylogenetic analysis showed that Combretaceae and Onagraceae + Lythraceae were well supported as sister groups. Molecular dating indicates that the Myrtales crown most likely originated during the Albian age of the Lower Cretaceous. These chloroplast genomes contribute to the study of genetic diversity and species evolution of Myrtales, while providing useful information for taxonomic and phylogenetic studies of Myrtales.