Sequencing and analyses on chloroplast genomes of Tetrataenium candicans and two allies give new insights on structural variants, DNA barcoding and phylogeny in Apiaceae subfamily Apioideae

Background. Tetrataenium candicans is a traditional Chinese folk herbal medicine used in the treatment of asthma and rheumatic arthritis. Alongside several Tordyliinae species with fleshy roots, it is also regarded as a substitute for a Chinese material medicine called 'Danggur'. However, a lack of sufficient sampling and genomic information has impeded species identification and the protection of wild resources. Methods. The complete chloroplast genomes of T. candicans from two populations, Tetrataenium yunnanense and Semenovia transilliensis, were assembled from two pipelines using data generated from next generation sequencing (NGS). Pseudogenes, inverted repeats (IRs) and hyper-variable regions were located by Geneious 11.1.5. Repeat motifs were searched using MISA and REPuter. DNA polymorphism and segment screening were processed by DNAsp5, and PCR product was sequenced with Sanger's sequencing method. Phylogeny was inferred by MEGA 7.0 and PhyML 3.0. Results. The complete chloroplast genomes of T. candicans from two populations, T. yunnanense and S. transilliensis, were 142,261 bp, 141,985 bp, 142,714 bp and 142,145 bp in length, respectively, indicating conservative genome structures and gene categories. We observed duplications of trnH and psbA caused by exceptional contractions and expansions of the IR regions when comparing the four chloroplast genomes with previously published data. Analyses on DNA polymorphism located 29 candidate cp DNA barcodes for the authentication of `Danggue counterfeits. Meanwhile, 34 hypervariable markers were also located by the five Tordyliinae chloroplast genomes, and 11 of them were screened for population genetics of T. candicans based on plastome information from two individuals. The screening results indicated that populations of T.candicans may have expanded. Phylogeny inference on Apiaceae species by CDS sequences showed most lineages were well clustered, but the five Tordyliinae species failed to recover as a monophyletic group, and the phylogenetic relationship between tribe Coriandreae, tribe Selineae, subtribe Tordyliinae and Sinodielsia Glade remains unclear. Discussion. The four chloroplast genomes offer valuable information for further research on species identification, cp genome structure, population demography and phylogeny in Apiaceae subfamily Apioideae.