Functional and evolutionary genomic inferences in <i>Populus</i> through genome and population sequencing of American and European aspen

The <i>Populus</i> genus is one of the major plant model systems, but genomic resources have thus far primarily been available for poplar species, and primarily <i>Populus trichocarpa</i> (Torr. & Gray), which was the first tree with a whole-genome assembly. To further advance evolutionary and functional genomic analyses in <i>Populus</i>, we produced genome assemblies and population genetics resources of two aspen species, <i>Populus tremula</i> L. and <i>Populus tremuloides</i> Michx. The two aspen species have distributions spanning the Northern Hemisphere, where they are keystone species supporting a wide variety of dependent communities and produce a diverse array of secondary metabolites. Our analyses show that the two aspens share a similar genome structure and a highly conserved gene content with <i>P. trichocarpa</i> but display substantially higher levels of heterozygosity. Based on population resequencing data, we observed widespread positive and negative selection acting on both coding and noncoding regions. Furthermore, patterns of genetic diversity and molecular evolution in aspen are influenced by a number of features, such as expression level, coexpression network connectivity, and regulatory variation. To maximize the community utility of these resources, we have integrated all presented data within the PopGenIE web resource (PopGenIE.org).