Comparative and phylogenetic analysis of the complete chloroplast genomes of ten Pittosporum species from East Asia.

Pittosporum (Pittosporaceae) is famous as the ornamental and medical values, which is distributed tropical and subtropical regions of Eastern Hemisphere. The few phylogenetic studies have included samples from the Pacific Island, but the phylogenetic relationships of Asian species has not been studied. Here, the complete chloroplast (cp) genomes of ten Pittosporum species from East Asia were first sequenced and compared with those of the published species of this genus. Our results indicated that cp genomes of these species had a typical and conserved quadripartite structure. 131 genes were identical in order and orientation and no changes of inverted repeat (IR) occurred. However, the comparative analysis of cp genomes suggested that sequence divergence mainly appeared in non-coding or intergenic regions, in which several divergence hotspots were identified. By contrast, protein-coding genes showed the lowest variance under strong purifying selection. Phylogenetic analysis based on the cp genome sequences showed that the tested Pittosporum species were clustered into two major clades, in which the Asian species formed Clade I and the remaining species from Australia and New Zealand formed Clade II with high support values, which was consistent with the results of ITS data with low support values. These results suggested that cp genome is a robust phylogenetic indicator for deep nodes in the phylogeny of Pittosporum. Meanwhile, these results will provide the valuable information to better understand the phylogeny and biogeography of Pittosporum.