title: Genetic consequences of fragmentation in "arbor vitae," eastern white cedar (Thuja occidentalis L.), toward the northern limit of its distribution range.
abstract: We tested the hypothesis that marginal fragmented populations of eastern white cedar (EWC) are genetically isolated due to reduced pollen and gene flow. In accordance with the central-marginal model, we predicted a decrease in population genetic diversity and an increase in differentiation along the latitudinal gradient from the boreal mixed-wood to northern coniferous forest. A total of 24 eastern white cedar populations were sampled along the north-south latitudinal gradient for microsatellite genotyping analysis. Positive F(is) values and heterozygote deficiency were observed in populations from the marginal (F(is) = 0.244; P(HW) = 0.0042) and discontinuous zones (F(is) = 0.166; P(HW) = 0.0042). However, populations from the continuous zone were in HW equilibrium (F(is) = -0.007; P(HW) = 0.3625). There were no significant latitudinal effects on gene diversity (H(s)), allelic richness (AR), or population differentiation (F(st)). Bayesian and NJT (neighbor-joining tree) analyses demonstrated the presence of a population structure that was partly consistent with the geographic origins of the populations. The impact of population fragmentation on the genetic structure of EWC is to create a positive inbreeding coefficient, which was two to three times higher on average than that of a population from the continuous zone. This result indicated a higher occurrence of selfing within fragmented EWC populations coupled with a higher degree of gene exchange among near-neighbor relatives, thereby leading to significant inbreeding. Increased population isolation was apparently not correlated with a detectable effect on genetic diversity. Overall, the fragmented populations of EWC appear well-buffered against effects of inbreeding on genetic erosion.
authors.affiliation: Northwest A&F University Shaanxi, 712100, China ; Chaire Industrielle CRSNG-UQAT-UQAM en Aménagement Forestier Durable, Institut de recherche sur les forêts, Université du Québec en Abitibi-Témiscamingue Québec, J9X 5E4, Canada.
authors.forename: Huaitong
authors.initials: H
authors.lastname: Xu
authors.forename: Francine
authors.initials: F
authors.lastname: Tremblay
authors.forename: Yves
authors.initials: Y
authors.lastname: Bergeron
authors.forename: Véronique
authors.initials: V
authors.lastname: Paul
authors.forename: Cungen
authors.initials: C
authors.lastname: Chen
keywords: Boreal forest; distribution limit; genetic diversity; latitudinal gradient; microsatellite genotyping; northern edge