The complete chloroplast genome and phylogenetic relationship of Apocynum pictum (Apocynaceae), a Central Asian shrub and second-class national protected species of western China.

Apocynum pictum of the dogbane family, Apocynaceae, is a perennial semi-shrub species of ecological, medicinal, and economic value. It is mainly distributed in semi-arid, saline-alkaline, and desert regions of Xinjiang, Qinghai, and Gansu of western China and adjacent regions from Kazakhstan and Mongolia. Here, we reported the complete chloroplast (cp) genome of A. pictum for the first time, and we found that it had a circular structure with an estimated length of 150,749 bp and a GC content of 38.3%. The cp genome was composed of a large single copy (LSC), a single small single copy (SSC), and two inverted repeat (IR) regions, which were 81,888 bp, 17,251 bp and 25,805 bp long, respectively. The cp genome of A. pictum encoded 134 genes and contained 66 simple sequence repeats (SSRs). A comparative analysis with other cp genomes from Apocynaceae indicated that the cp genome of A. pictum was very conserved, except for subtle differences occurring in the protein-coding genes accD, ndhF, rpl22, rpl32, rpoC2, ycf1 and ycf2. A phylogenetic reconstruction showed that A. pictum and A. venetum were sister species, forming a strongly supported clade with Trachelospermum. Interestingly, nucleotide substitution ratios (Ka/Ks) between A. pictum and A. venetum on accD and ndhF were >1.0, suggesting positive selective pressure on these genes. Our result enriches the genomic resources for the diverse dogbane family and provides critical molecular resources to develop future studies on ecological adaptation to desert habitats in Apocynum.