Comparative Chloroplast Genomics of Four Pilea Species (Urticaceae): High Levels of Sequences Divergence Provides New Insight into Interspecific Diversity in Pilea

Abstract

Background Pilea is a genus of perennial herbs from the family Urticaceae, which are used for courtyard ornamental. For some species, they are used as medicinal plants in traditional Chinese medicine as well. The morphological characteristics of medicinal species from Pilea are similar, and it is difficult to accurately distinguish them based only on morphological characteristics. Besides, the species classification of Pilea are still controversial. The classification of many species are still in an unresolved state. At present, there is no information about the chloroplast genomes of Pilea, which limits our further understanding of this genus. Here, we first reported 4 chloroplast genomes of Pilea taxa (P. mollis, P. glauca, P. peperomioides and P. serpyllacea), and performed comprehensive comparative analysis. Results The four chloroplast genomes have similar structural characteristics and gene order with other angiosperms. These genomes all have a typical quartile structure, which contains 113 unique genes, including 79 protein-coding genes, 4 rRNA genes, and 30 tRNA genes. Besides, we detected SSRs and repeat sequences, and analyzed the expansion/contraction of IR regions. In particular, the comparative analysis showed a rather level of sequence divergence in the non-coding regions, even in the protein-coding regions of the four genome sequences, suggesting a high level of genetic diversity in Pilea. Moreover, we identified eight hypervariable regions, including petN-psbM; psbZ-trnG-GCC; trnT-UGU-trnL-UAA; accD-psbI; ndhF-rpl32; rpl32-trnL-UAG; ndhA-intron and ycf1, are proposed for use as DNA barcode regions. Phylogenetic analysis showed that four Pilea species form a monophyletic cluster with a 100% bootstrap value. Conclusion The results obtained here could provide abundant information for the phylogenetic position of Pilea and further species identification. High levels of sequences divergence promote our understanding of the interspecific diversity of this genus, also provide reference for the rational classification of unsolved species in the future.