Intervarietal and intravarietal genetic structure in Douglas‐fir: nuclear SSRs bring novel insights into past population demographic processes, phylogeography, and intervarietal hybridization

Abstract

Douglas-fir (Pseudotsuga menziesii) is one of numerous wide-range forest tree species represented by subspecies/varieties, which hybridize in contact zones. This study examined the genetic structure of this North American conifer and its two hybridizing varieties, coastal and Rocky Mountain, at intervarietal and intravarietal level. The genetic structure was subsequently associated with the Pleistocene refugial history, postglacial migration and intervarietal hybridization/introgression. Thirty-eight populations from the USA and Canada were genotyped for 13 nuclear SSRs and analyzed with simulations and traditional population genetic structuring methods. Eight genetic clusters were identified. The coastal clusters embodied five refugial populations originating from five distinct refugia. Four coastal refugial populations, three from California and one from western Canada, diverged during the Pleistocene (56.9–40.1 ka). The three Rocky Mountain clusters reflected distinct refugial populations of three glacial refugia. For Canada, ice covered during the Last Glacial Maximum, we present the following three findings. (1) One refugial population of each variety was revealed in the north of the distribution range. Additional research including paleodata is required to support and determine whether both northern populations originated from cryptic refugia situated south or north of the ice-covered area. (2) An interplay between intravarietal gene flow of different refugial populations and intervarietal gene flow by hybridization and introgression was identified. (3) The Canadian hybrid zone displayed predominantly introgressants of the Rocky Mountain into the coastal variety. This study provides new insights into the complex Quaternary dynamics of this conifer essential for understanding its evolution (outside and inside the native range), adaptation to future climates and for forest management.

Genetic diversity and adaptive traits of European versus American Douglas-fir seedlings

“…Provenances from coastal British Columbia, the Western Cascade Mountains and Oregon with a mean annual temperature ranging from 6 to 9.5°C are predicted to be the most productive for Central Europe under both current and future climate conditions (Chakraborty et al 2016). They are also characterized by the highest genetic diversity for the Douglas-fir (Table S1, Figure S1 in the supplementary materials), with a peak in Northern Oregon and Southern Washington west of the Cascades and decreasing trends towards south and north of the distribution range (Li and Adams 1989;Neophytou et al 2016;van Loo et al 2015).…”

“…The extracted DNA samples were amplified and genotyped with 13 nuSSRs. The 13 nuSSRs are identical to those used by van Loo et al (2015). Details on the PCR procedure are given in van Loo et al (2015).…”

“…Both the variety composition and the potential native origin were assessed using multilocus genotype data of the studied seedlings and the reference data set developed by van Loo et al (2015). Genotypes were not corrected for null alleles.…”

“…We further calculated the allelic diversity parameters (6) allelic richness (As) and (7) private allelic richness As(p) (the number of distinct alleles private (specific) to a population based on a standardized population size by rarefaction using ADZE v. 1.0) to allow a comparison of populations with different sample sizes (Szpiech et al 2008). We applied a standardized population size of 8 individuals, which is the same as used in the reference data set (van Loo et al 2015).…”

“…The interior variety (also called Rocky Mountain variety) grows further east of British Columbia, across the Rocky Mountains to New Mexico (USA) (Eckenwalder 2009). In the contact zone of the two varieties, hybridization is evident (Gugger et al 2010;van Loo et al 2015;Wei et al 2011).…”