Trembling aspen (Populus tremuloides Michx.), a clonal angiosperm, is the most geographically widespread tree in North America. It is generally thought that most extant populations in the western interior of Canada and the United States became established shortly after glacial retreat, but sexual recruitment then effictively ceased owing to inimical climatic conditions. Six populations of trembling aspen were studied in the prairie and montane environments of Waterton Lakes National Park, Alberta. Vegetative tissues were analyzed for electrophoretically detectable variation in 13 enzymes encoded by 14 polymorphic loci and three monomorphic loci. All populations maintained high levels of inter‐ and intrapopulation diversity (P = 0.891; H = 0.319; A = 2.4). The mean fixation index, F, was ‐0.102, indicating some deviation from Hardy‐Weinberg expectations. Genetic differentiation (FST = 3.0) was apparent in this ecologically diverse, but geographically small‐scale, spatial setting. It is suggested that the maintenance of diversity in the absence of frequent modern‐day recruitment, and resistance to further geographic differentiation in this spatially heterogeneous environment reflect occasional seedling establishment through “windows of opportunity” and more importantly, the species' clonal morphology. The phalanx growth form and concomitant physiological integration between ramets combine to spread the risk of death and buffer the effects of selection over time and space.
Effects of asexual reproduction as a primary reproductive strategy on population structure and levels of variability were investigated electrophoretically in natural populations of a woody plant species, trembling aspen (Populus tremuloides Michx.), from Alberta. As expected, levels of genic diversity, 42%, and proportion of polymorphic loci, 92%, averaged over all clones are considerably greater than those reported for comparable samples of sexually reproducing plant and animal species. These measures of genic variability of a primarily asexual plant species are similar to those reported for asexual species of insects, fish and bacteria. In addition, each of the 222 clones was electrophoretically unique. Since neutral theory would predict each individual clone to be heterozygous for a unique mutation at each gene locus at equilibrium, these results can be interpreted in a number of ways: (i) insufficient time to reach equilibrium, (ii) inability of electrophoresis to detect all variation at a locus, (iii) periodic establishment of sexually derived propagules in the population, and (iv) selection for similar genotypes at each location or against mutations at particular gene loci. Re-invasion of Pleistocene-glaciated areas by trembling aspen likely was by sexual means, with subsequent reproduction being primarily asexual.
Electrophoretic techniques which can be used to aid the identification of clones are described. An example using members (putative ramets) from 10 morphologically distinct putative clones of trembling aspen is presented. In this example, it is demonstrated that phenotypically uniform groups of trees (putative clones) can actually be composed of several electrophoretically distinct genotypes.
Inheritance of allozymes of 12 polymorphic loci was demonstrated using haploid megagametophytic tissues of viable seed from a total of 47 white spruce trees in one stand. In general, allozymes segregated as expected in a 1:1 ratio. However, one allozyme of phosphoglucose isomerase (Pgi-2) did not segregate as expected. Significant deficiency in two different heterozygous combinations was observed. This result could be indicative of selection against the allozyme. The level of genetic variability in this stand of white spruce was similar to average levels of variation observed with many conifer species. Overall, an excess of heterozygotes was observed in the mature population. However, after 9 years of storage in a seedbank, no more than random selfing could be detected in the viable filial generation. Significant heterogeneity in allele frequency distribution of several loci was observed in the pollen pool in this stand. The calculated ratio of genetically effective males to females in this population was only 0.4. These results were interpreted as evidence for changes in the spatial genetic structure of the population, likely marking demes of related individuals, and restricted effective transmission distances of male gametes. Some implications for tree improvement programs are discussed.
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