Comparative Population Genetics of Poplars to Examine Causes of Differing Levels of Nucleotide Diversity

Abstract

Species can differ dramatically in the amount of DNA variation contained within the genomes of different individuals. Generally, the magnitude of nucleotide diversity for a species is described by the equation:

θ = 4Nem

where θ is the amount of diversity in DNA sequence among individuals of a species, Ne is the population size, and m is the baseline mutation rate. Different types of genomes or taxonomic groups often show orders of magnitude variation in mutation rate. However, when closely related species differ in θ, it is generally assumed this reflects differences in population size (Ne), since mutation rates tend to be evolutionarily conserved over shorter time scales. This assumption is almost never empirically tested. Poplar trees (genus Populus) are an ecologically and genetically diverse group that provides an ideal system to study this issue. According to previous work, nucleotide diversity differs remarkably among poplar species in North America and Europe. This could reflect a difference in average Ne among species, perhaps because past glacial histories impacted species’ population sizes differently on each continent. However, most European and North American species are not closely related; thus the difference in diversity could also reflect an inherited difference in mutation rate between divergent sections of the genus.