Abstract

Species of the genus Populus , collectively known as poplars, are widely distributed over the northern hemisphere, and well known for their ecological, economical, and evolutionary importance. The extensive inter-species hybridization and high morphological diversity in this group pose difficulties in identifying taxonomic units for comparative evolutionary studies and systematics. In order to understand the evolutionary relationships among poplars and to provide a framework for biosystematic classification, I reconstructed the phylogeny of the genus Populus based on nucleotide sequences of three non-coding regions of the chloroplast DNA (intron of trnL , and intergenic regions of trnT-trnL and trnL-trnF ) and ITS1 and ITS2 of the nuclear rDNA. The resulting phylogenetic trees showed polyphyletic relationships among species in the sections Tacamahaca and Aigeiros . The chloroplast DNA sequence of P. nigra was similar to that of the species of section Populus , whereas nuclear DNA sequence data suggested a close affinity of P. nigra to species of the section Aigeiros , suggesting a possible hybrid origin for P. nigra . Similarly, the chloroplast DNA sequences of P. tristis and P. szechuanica were similar to that of the species of section Aigeiros , while the nuclear sequences revealed a close affinity to species of the section Tacamahaca , suggesting a hybrid origin for these two Asiatic balsam poplars. The incongruities between nuclear- and chloroplast-DNA based phylogenetic trees suggest reticulate evolution in the genus Populus .