Low genetic diversity, moderate local adaptation, and phylogeographic insights in Cornus nuttallii (Cornaceae)

Genetic knowledge is completely lacking for Pacific dogwood (Cornus nuttallii), a western North American temperate tree that is pollinated and dispersed by biological vectors. We investigated how history, geography, and climate have affected population genetic structure, local adaptation, and the phylogeography of this species.

We examined patterns and levels of diversity in nuclear microsatellites (SSRs) and cpDNA haplotypes in populations from across the species range. We compared these results to population differentiation and genetic clines in phenotypic traits in a common garden.

Genetic diversity was low for both nuclear SSRs and cpDNA. There was a lack of population structure (F(ST) = 0.090) in the coastal portion of the species range, with estimates of population genetic diversity in microsatellite markers decreasing with latitude from California to British Columbia. A disjunct interior population in Idaho 450 km from the coastal range had the lowest diversity but the highest divergence of all populations studied. Only a single nucleotide polymorphism was discovered after sequencing 5547 base pairs in seven noncoding regions of cpDNA. Both cpDNA haplotypes were widely distributed throughout the species range. Quantitative variation among populations was moderate (0.11 ≤ Q(ST) ≤ 0.63), and weak but significant adaptive clines were found between quantitative traits and population climatic variables (0.09 ≤ R(2) ≤ 0.34).

Cornus nuttallii likely faced a population bottleneck in a single southern refugium during the Last Glacial Maximum. Despite low genetic diversity, it is weakly to moderately locally adapted.