Mating system and multilocus associations in a natural population of Pseudotsuga menziesii (Mirb.) Franco

Summary

Arrays of open-pollinated seeds were assayed for allozyme polymorphisms at ten loci (Aat2, Est1, G6pd, Idh, Mdh2, Mdh3, Pgm, Sod, 6Pgd1, 6Pgd2) to obtain estimates of the outcrossing rate and assess multilocus association in a natural population of coastal Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco. The allele frequencies in the samples of adult trees and pollen-gamete pool were similar. Maximum-likelihood estimators of the outcrossing rate for individual loci and two multilocus models were derived using counting methods. The single-locus maximum likelihood estimates (MLEs) of the outcrossing rate were significantly heterogeneous; they varied over a more than two-fold range from 0.404 to 0.935, with an average MLE of 0.741. Both multilocus MLEs of the outcrossing rate were 0.887. The sample of trees was in random mating equilibrium when assessed on a pairwise-locus basis using Burrows' composite measure of gametic disequilibrium, with one exception (Mdh2 Sod) that was attributable to a rare “gametic” class. In the sample of pollen gametes, 5 of the 45 pairwise-locus associations were nominally significant at the 0.05 level: Idh Est1, Mdh2 Sod, Aat2 Est1, Aat2 Mdh3, and Est1 Mdh3. These apparent associations were attributable in most cases to the relative excess of uncommon or rare paternal gametes of discernibly outcrossed embryos. An additional two-locus association was identified for Mdh2 Pgm which was marginally significant for the major partition of the contingency table that excluded paternal gametes with the rare allele Mdh2 ².