Allozyme variation of Thuja occidentalis L. in Northwestern Ontario

Genetic consequences of fragmentation in "arbor vitae," eastern white cedar (Thuja occidentalis L.), toward the northern limit of its distribution range

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.

Genetic diversity and differentiation of core vs. peripheral populations of eastern white cedar, Thuja occidentalis (Cupressaceae)

PREMISE OF THE STUDY

Geographically peripheral (marginal) populations are expected to have lower genetic diversity and higher genetic differentiation than geographically core (central) populations as a result of supposedly lower effective population size (N(e)) and higher genetic drift, founder effect, fragmentation, and isolation in peripheral than in core populations. Here we address this issue for a long-lived plant species, eastern white cedar (Thuja occidentalis).

METHODS

Genetic diversity and population structure of 13 natural populations of eastern white cedar from its Canadian eastern peripheral and core natural ranges in New Brunswick, Nova Scotia, and Prince Edward Island were studied using six nuclear microsatellite DNA markers.

KEY RESULTS

The core populations of eastern white cedar had significantly higher allelic diversity (mean A = 8.83, A(r) = 8.13, A(e) = 4.03) and N(e) (428) than the peripheral populations (A = 6.64, A(r) = 6.15, A(e) = 3.12, N(e) = 198). However, expected heterozygosity was similar in the core (H(e) = 0.64) and peripheral (H(e) = 0.60) populations. Genetic differentiation was significantly higher among the peripheral (F(ST) = 0.089) than among the core (F(ST) = 0.032) populations. No genetic differentiation (F(ST)/Φ(RT) = 0.000) was detected between core and peripheral regions.

CONCLUSIONS

Peripheral populations have significantly lower N(e) and genetic diversity in terms of allelic diversity (richness) and significantly higher genetic differentiation than the core populations of eastern white cedar in its Canadian eastern range. However, core and peripheral populations have similar levels of expected heterozygosity. Implications for conservation of eastern white cedar genetic resources are discussed.

Random amplified polymorphic DNA diversity of marginal and central populations in Pinus contorta subsp. latifolia

Fifteen populations of lodgepole pine (Pinus contorta subsp. latifolia) were surveyed for diversity across 52 random amplified polymorphic DNAs (RAPDs). The objective was to compare single-locus and multilocus structures in four marginal, three intermediate, and eight central populations. Single-locus estimates indicated average observed and expected heterozygosity to be 0.19 and 0.17, respectively. When these estimates were split into population categories, a clear trend of increasing diversity was detected in the direction of marginal to central populations. F-statistics indicated an excess of heterozygotes, with FIS ranging from -0.08 for marginal populations to -0.15 for central populations and averaging -0.12 over 15 populations. The estimates of FST decreased towards the margins of the species range, indicating increased population differentiation. Forty-nine of 52 RAPDs tested neutral in the Ewens-Watterson analysis. Multilocus analysis showed significant two-locus and high-order gametic disequilibria in all 15 populations. The most prominent components of the two-locus analysis were the variance of disequilibrium (VD, 46.2%) and the multilocus Wahlund effect (31.9%). This high value for VD indicated that founder effects could explain much of the observed multilocus associations. When analyzed by population categories, the VD showed a decreasing trend indicating that variation due to founder effects was more prominent in marginal populations. The two-locus Wahlund effect (WC) that is characteristic of strong population subdivision was highest in the central populations. This indicated significant levels of gene flow between populations with different allelic combinations.Key words: multilocus genetic structure, central and marginal populations, RAPD, Pinus contorta subsp. latifolia.