Abstract
Electrophoretically detectable variation in phosphoglucoisomerase (PGI) was found in a Tetranychus urticae population on vinyl-house strawberries. Three types of band patterns were present in this population, indicating 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-divided section and the whole house). The proportion of sub-populations which deviated from HARDY-WEINBERG equilibrium was smallest at the leaflet level, and increased with the size of the sub-population. In contrast, the proportion of sub-populations that were monomorphic was greatest at the leaflet level. Further, WRIGHT'S F-statistics were calculated at each of the levels. At the leaflet level, the inbreeding coefficient (FIS) was smallest, whereas the fixation index (FST) was greatest. The FIS for the whole population was about 0.5. These results suggest that the mites tend to form breeding colonies on individual leaflets. However, because random genetic drift easily occurs within each colony when population size is small, the whole population is thought to maintain a large degree of genetic diversity.
