Abstract

Populus , a genus of deciduous trees distributed throughout the northern-hemisphere comprising aspens, poplars and cottonwoods, has recently become a model for forest trees and subject of many interdisciplinary studies in higher plants. However, due to a high level of morphological diversity, extensive inter-species hybridization, and a low level of DNA sequence variation among species in this group, the taxonomy and phylogeny of the genus still remains controversial and incongruent phylogenetic relationships among poplar species have challenged evolutionary biologists for more than a decade. Nuclear genomic data based upon inter simple sequence repeat (ISSR) as well as six nuclear genome based Sequence Characterized Inter Simple Sequence Repeat (SCISSR) data were used to assess genetic relationships among species of the genus Populus . The direction of natural hybridization between two sympatric Populus species in North America ( P. deltoides and P. balsamifera ) was also investigated using species-specific single nucleotide polymorphism (SNP) markers in both the nuclear and chloroplast genomes. Overall, the genetic relatedness estimates based on nuclear ISSR data were congruent with phylogenetic trees based on other molecular (RFLP and DNA sequence) and morphological data, but provided better resolution in assessing the genetic relatedness among closely related taxa, and provided genetic evidence for previously suspected introgressions. The resulting SCISSR based tree was also mainly congruent with the genetic relatedness estimates based on other molecular (RFLP, ISSR, and DNA sequence) and morphological data, but showed better resolution of the internal nodes. This result would suggest the need for larger sequence data sets for precise reconstruction of phylogenetic relationships among closely related species of Populus . Although hybridization is generally considered to be symmetrical, with both hybridizing species being equally likely to be the male or female parent, several studies have demonstrated the presence of asymmetrical hybridization and introgression from one species to the other. All natural hybrid individuals, identified based on morphological traits, had nuclear alleles corresponding to both parental species, while the chloroplast genotypes showed similarity to P. deltoides , indicating asymmetrical hybridization with P. deltoides as the maternal and P. balsamifera as the paternal donor species. This observed asymmetrical hybridization may be attributable to cytonuclear interactions.