Salient Biological Features, Systematics, and Genetic Variation of Populus

The genus Populus includes morphologically diverse species of deciduous, relatively short-lived, and fast-growing trees. Most species have wide ranges of distribution but tend to occur primarily in riparian or mountainous habitats. Trees from this genus are typically dioecious, flower before leaf emergence, and produce large amounts of wind-dispersed pollen or seeds. Seedlings are droughtand shade-intolerant, and their establishment depends on disturbance and high soil moisture. Asexual reproduction is common and occurs via root sprouting and/or rooting of shoots. Fossil records suggest that the genus appeared in the late Paleocene or early Eocene (i.e., 50–60 million years BP). According to one commonly used classification, the genus is comprised of 29 species divided into six sections, but a number of phylogenetic inconsistencies remain. Natural hybridization both within and among sections is extensive and is believed to have played a major role in the evolution of extant species of Populus. Both neutral molecular markers and adaptive traits reveal high levels of genetic variation within populations. Deviations from Hardy–Weinberg equilibrium are commonly detected in molecular marker studies. These deviations typically have small to moderate magnitudes and tend to be caused by heterozygote deficiency, indicating the possible existence of population substructure. Genetic differentiation among populations is much stronger for adaptive traits than for neutral markers, which suggests that divergent selection has played a dominant role in shaping patterns of adaptive genetic variation. Molecular and bioinformatic resources are actively being developed for multiple species of Populus, which makes this genus an excellent system for studying tree genetics and genomics. G.T. Slavov (B) Department of Biology, West Virginia University, Morgantown, WV 26506, USA e-mail: [email protected] 15 S. Jansson et al. (eds.), Genetics and Genomics of Populus, Plant Genetics and Genomics: Crops and Models 8, DOI 10.1007/978-1-4419-1541-2_2, C © Springer Science+Business Media, LLC 2010 16 G.T. Slavov and P. Zhelev 1 Dendrological Overview