DIFFERENCES IN FITNESS COMPONENTS AMONG ALCOHOL-DEHYDROGENASE GENOTYPES OF THE OLIVE FRUIT-FLY (DIPTERA, TEPHRITIDAE) UNDER ARTIFICIAL REARING

Previous studies have shown that natural populations of the olive frui t fly, Bactrocera oleae (Gmelin), segregate for 3 electrophoretically distinguishable alleles (allozymes S, F, and I) at the alcohol dehydro genase (ADH) locus. Allele I is rare, but in colonies maintained on an artificial larval medium its frequency rapidly rises to approximate t o 35%. Analysis of allelic frequency trajectories have suggested stron g selection in favor of heterozygotes. Here we report on the results o f an extensive study designed to estimate fitness components of the va rious genotypes at the ADH locus and to identify the stage in the life cycle of the insect at which selection pressure is most intense. Sign ificant differences among genotypes were observed for egg hatchability , larva-to-pupa viability, pupa-to-adult viability, and for egg-to-pup ation time. When combined into an overall fitness value, these differe nces produced clear evidence for heterozygote advantage. In addition, our results reinforce previous claims that selection acts directly on the ADH locus and narrows the time of selection to the early larval st age. The ADH system of B. oleae serves as a model case of a predictabl e and drastic genetic change that occurs in an insect colony as it ada pts to an artificial environment.