Comparative analysis of the complete chloroplast genomes of seven Populus species: Insights into alternative female parents of Populus tomentosa

Populus tomentosa, of section Populus, is distributed mainly in northern China. This species has high resistance to many diseases and insects, and it plays key roles in shelterbelts and urban afforestation in northern China. It has long been suspected to be a hybrid, but its parents remain unknown. In the present study, we report four newly sequenced complete cp genomes from section Populus and comparative genomic analyses of these new sequences and three published cp genome sequences. The seven cp genomes ranged from 155,853 bp (P. tremula var. davidiana) to 156,746 bp (P. adenopoda) in length, and their gene orders, gene numbers and GC contents were similar. We analyzed SNPs, indels, SSRs and repeats among the seven cp genomes, and eight small inversions were detected in the ndhC-trnV, rbcL-accD, petA-psbJ, trnW-trnP, rpl16-rps3, trnL-ycf15, ycf15-trnL, and ndhF-trnL intergenic regions. Furthermore, seven divergent regions (trnH-psbA, matK, psbM-psbD, ndhC-trnV, ycf1, ndhF-ccsA and ccsA-ndhD) were found in more highly variable regions. The phylogenetic tree reveals that P. tomentosa is closely related to P. alba and P. alba var. pyramidalis. Hence, P. alba was involved in the formation of P. tomentosa.