Population genetics of Pinus contorta on coastal and pygmy forest sites in Mendocino County, California

It was established that no phenological crossing barrier exists between the two subspecies of Pinus contorta. An isozyme study was conducted to identify patterns of genetic variation within and between the two subspecies, and to investigate the evolutionary relationship between the subspecies. Seeds were collected from 11 pygmy-forest and 6 coastal populations. Isozyme analyses of gametophytes for 25 loci showed ssp. bolanderi to have significantly less isozyme variation than ssp. contorta. The two subspecies did not exhibit the phylogenetic dichotomy expected for subspecific classification. Within ssp. bolanderi, genetic distances exhibited a pattern that correlated better with edaphic differences between sites, rather than with geographic distance.; The population genetic structures of mature trees and their progeny were compared using 21 isozyme loci for the same 11 pygmy-forest and 6 coastal populations.; The deviation from Hardy-Weinberg equilibrium was greater for the embryonic cohort than for the mature-tree genotypes in both subspecies. Fixation indices were higher on average for embryos than for mature trees. Single-locus outcrossing rates were estimated, averaging 0.79 for coastal populations and 0.82 for pygmy forest populations, based on the paired genotypes of each embryo and its seed parent.; Coastal populations exhibited higher levels of allozymic variation than did pygmy-forest populations for both mature trees and their progeny; and, in both subspecies mature trees had higher levels of both observed and expected heterozygosity than did embryos.; The six coastal populations exhibited similar levels of population differentiation for both embryos and mature trees in terms of both F{dollar}sb{lcub}rm st{rcub}{dollar} and genetic-distance estimate. Pygmy-forest populations showed more population differentiation for mature trees than did the coastal populations. The pygmy-forest embryo samples indicated higher levels of gene flow than did either embryo samples of the coastal populations or mature-tree samples of both the pygmy-forest and coastal populations.; It is concluded that ssp. bolanderi is a relatively recently evolved edaphic derivative of ssp. contorta, and that isozyme genetic differentiation on severe sites may be limited either due to recent colonization of these sites by a small number of individuals or due to hitchhiking of neutral isozyme genes with adaptation-related genes. (Abstract shortened with permission of author.)...