STUDIES ON THE POPULATION-STRUCTURE OF THE 2-SPOTTED SPIDER-MITE, TETRANYCHUS-URTICAE KOCH, BY ALLOZYME VARIABILITY ANALYSIS

Electrophoretically detectable variation in phosphoglucoisomerase (PGI ) was found in a Tetranychus urticae population on vinyl-house strawbe rries. Three types of band patterns were present in this population, i ndicating that two alleles exist at the PGI locus. Frequencies of the genotypes were determined at five different levels of sub-population ( leaflet, leaf consisting of 3 leaflets, patch of injured plants, sub-d ivided section and the whole house). The proportion of sub-populations which deviated from HARDY-WEINBERG equilibrium was smallest at the le aflet level, and increased with the size of the sub-population. In con trast, the proportion of sub-populations that were monomorphic was gre atest at the leaflet level. Further, WRIGHT's F-statistics were calcul ated at each of the levels. At the leaflet level, the inbreeding coeff icient (F(IS)) was smallest, whereas the fixation index (F(ST)) was gr eatest. The F(IS) for the whole population was about 0.5. These result s suggest that the mites tend to form breeding colonies on individual leaflets. However, because random genetic drift easily occurs within e ach colony when population size is small, the whole population is thou ght to maintain a large degree of genetic diversity.