Adaptive evolution of the Populus tremula photoperiod pathway

Perennial plants monitor seasonal changes through changes in environmental conditions such as the quantity and quality of light and genes in the photoperiodic pathway are known to be involved in controlling these processes. Here, we examine 25 of genes from the photoperiod pathway in Populus tremula (Salicaceae) for signatures of adaptive evolution. Overall, levels of synonymous polymorphism in the 25 genes are lower than at control loci selected randomly from the genome. This appears primarily to be caused by lower levels of synonymous polymorphism in genes associated with the circadian clock. Natural selection appears to play an important role in shaping protein evolution at several of the genes in the photoperiod pathways, which is highlighted by the fact that approximately 40% of the genes from the photoperiod pathway have estimates of selection on nonsynonymous polymorphisms that are significantly different from zero. A surprising observation we make is that circadian clock-associated genes appear to be over-represented among the genes showing elevated rates of protein evolution; seven genes are evolving under positive selection and all but one of these genes are involved in the circadian clock of Populus.