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Isozyme patterns of a selected Pseudotsuga menziesii (mirb.) franco population El-Kassaby, Yousry Aly


Open pollinated seed samples were collected from 42 Douglas-fir trees in September 1978. The individuality of cone lots and subsequent seed lots had been retained in these trees. The trees are located in the University of British Columbia Research Forest, Haney, B.C. and were studied previously for phenology, growth, and flower and cone production by Griffith (1968). Isozyme variations were studied (using the gel electrophoresis technique) for both the haploid megagametophyte and the diploid embryo tissues at 27 loci, coding for 18 different enzymes for each tree separately. The objectives of this study were: 1. to study the mode of inheritance for these loci; 2. to determine the linkage relationships among these loci; 3. to estimate the outcrossing rate (t) for this population; 4. to determine the most effective sample size for estimating allelic frequencies; 5. to study the amount and organization of isozyme variation in this population and compare the results with the variation for some quantitative traits, and 6. to study the association between isozyme genotypes and quantitative traits. The results from the inheritance analyses for the 14 heterozygous enzyme systems showed that these electrophoretic variants segregated in a co-dominant fashion with distinct simple Mendelian expression. The linkage study yielded two tightly linked pairs [(AAT-2:PGI-2) and(AAT-3: SOD)] with recombination frequencies of 1.5 and 22.4 percent respectively. In addition, seven loosely linked pairs were detected with recombination frequencies varying between 32.7 and 41.9 percent. It was not possible to study three-point linkage due to the lack of appropriate combinations. Conditional probabilities were used to estimate the outcrossing rate (t) in the population using four enzyme systems. The estimated outcrossing rate was 0.9 with a standard deviation of 0.11, giving an inbreeding rate of 10 percent. By the minimum sampling variance criterion it was estimated that sample sizes between 42 and 60 trees from the base population are optimal to obtain reliable estimates for the allelic frequencies. Striking agreement on the appointment of genetic variation was found between results obtained from the gene diversity analysis for the electrophoretic data and the analysis of variance for seven different quantitative traits. These two independent sets of information confirmed that the majority of the variation existed within and not between populations. Finally, the association between the mother trees' genotypes and the performance of their half-sib families showed that the mother trees appeared to exert minimal influence on those characteristics analysed. The pollen parent contribution to the genetic constitution of the progenies should be investigated and it is recommended that isozyme studies should be extended to full-sib progenies.

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