Comparison of the chloroplast genomics of nine endangered Habenaria species and phylogenetic analysis.

BACKGROUND

Habenaria, a genus in the family Orchidaceae, are the nearly cosmopolitan orchids, and most species have significant medicinal and ornamental values. Despite the morphological and molecular data that have been studied in recent years, the phylogenetic relationship is still unclear.

RESULTS

We sequenced, assembled, and annotated the chloroplast (cp) genomes of two species (Habenaria aitchisonii Rchb.f. and Habenaria tibetica Schltr.ex Limpricht) of Habenaria grown on the Qinghai-Tibetan Plateau (QTP), and compared them with seven previously published cp genomes which may aid in the genomic profiling of these species. The two genomes ranged from 155,259-155,269 bp in length and both included 132 genes, encoding 86 proteins, 38 tRNAs and 8 rRNAs. In the cp genomes, the tandem repeats (797), SSRs (2195) and diverse loci (3214) were identified. Comparative analyses of codon usage, amino frequency, microsatellite, oligo repeats and transition and transversion substitutions revealed similarities between the species. Moreover, we identified 16 highly polymorphic regions with a nucleotide diversity above 0.02, which may be suitable for robust authentic barcoding and inferring in the phylogeny of Habenaria species. Among the polymorphic regions, positive selection was significantly exerted on several genes, such as cemA, petA, and ycf1. This finding may suggest an important adaptation strategy for the two Habenaria species on the QTP. The phylogenetic relationship revealed that H. aitchisonii and H. tibetica were more closely related to each other than to the other species, and the other seven species were clustered in three groups. In addition, the estimated divergence time suggested that the two species separated from the others approximately 0.39 Mya in the Neogene period. Our findings also suggest that Habenaria can be divided into different sections.

CONCLUSIONS

The results of this study enriched the genomics resources of Habenaria, and SSR marker may aid in the conservation management of two endangered species.