Isozyme Studies of Bark Beetle Population Genetics and Systematics1

Electrophoretic analysis was used extensively in the 1970's to estimate genetic relationships among populations, species, and closely related genera of many different plants and animals, including many insect groups. Some of the first electrophoretic work on bark beetles was done by Anderson and others (1979), Anderson and others (1983), Florence and Kulhavy (1981), Florence and others (1982), and Namkoong and others (1979). These studies demonstrated that bark beetles are highly polymorphic and amenable to population studies using this technique. The first isozyme study of the mountain pine beetle (MPB) Dendroctonus ponderosae (Stock and Guenther 1979) showed differentiation among geographically separated populations. A subsequent study of MPB from four locations (Stock and Amman 1980) suggested that there might also be genetic differences at individual gene loci between MPB from lodgepole and ponderosa pine, and greater overall heterozygosity in beetles from ponderosa pine. Some preliminary data also showed differences between beetles emerging from trees with thin and thick phloem and between earlyand late-emerging beetles from the same tree. Inferences that could be made from these observations of differences related to host species were, however, confounded by geographic distance among sites. Sturgeon (1980) and Sturgeon and Mitton (1986) studied sympatric groups of MPB attacking different hosts in mixed pine stands and found significant differences related to host tree species. Further evidence of differentiation of MPB by host was found in a comparison of isozyme frequencies of MPB collected from lodgepole and ponderosa pine at a single site in Utah (Stock and Amman 1985). Deviations in genotype frequencies from Hardy-Weinberg (random mating) expectations occurred when isozyme data from all beetles were pooled (Wahlund effect); frequencies were much closer to random mating expectations when separate analyses were conducted for beetles from each of the host species. Once again, greater heterozygosity was observed in beetles from ponderosa pine. Overall, beetles from thin-phloem lodgepole were more heterozygous than those from thick phloem, though these differences were not detected in beetles from ponderosa pine.