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The genetics of selective breeding in western hemlock Tsuga heterophylla Wellman, Hugh Francis

Abstract

Rates of genetic diversity were inferred from allozyme allelic variation and used to compare differing seed crops that had originated from seed orchards under various seed production conditions with natural stands of western hemlock (Tsuga heterophylla (Raf.) Sarg). The supplemental mass pollination (SMP) technique was found to maximize both Ht and H0 when compared with unimproved wind pollinated seed crops and controlled full-sib crosses. This was identified as being the result of minimized self-fertilization and an improved parental balance. It also can be noted that 8 private alleles were lost in the seed orchard populations when compared with natural populations. Results of an inbreeding analysis have suggested that when compared with natural populations, inbreeding decreased in the wind pollinated unimproved and controlled fullsib seed crops of first generation seed orchard. The SMP treatment also showed lower values of inbreeding and when compared with controlled crosses had the highest genetic diversity. These finding suggest that the SMP technique best maintains genetic diversity and minimizes inbreeding while retaining the required selection intensity for genetic gains. Fifteen microsatellite markers were isolated from western hemlock genomic DNA and six of these markers were optimized for use in mountain hemlock [Tsuga mertensiana (Bong) Carr.]. The developed SSR's were then implemented in a test of the Heterozygosity-Fitness Correlation (HFC) in western hemlock elite families. In general no significant relationship was identified between the genetic distance/relatedness of top western hemlock families and the fitness of their progeny based on phenotypic indicators. One exception were the MKL and STV progeny trials which were found to be significantly correlated at 9 5 % . They both showed negative slope indicating that as genetic distance increase, height decreases which is indicative of outbreeding depression. Reasons that may have lent to the null result include limited parental lines, concluding in too little distance variation, a low degree of relatedness resulting in very little inbreeding depression and very little evidence for linkage disequilibrium, a cause of associative overdominance. The lack of structure identified in Western hemlock populations may not have allowed for strong heterosis therefore resulting in the poor predictive power of genetic distance.

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