Rarer need not be better if commoner is worse: Frequency-dependent selection for developmental time at the alcohol dehydrogenase locus of the olive fruit fly, Bactrocera oleae

Whereas the importance of frequency-dependent selection in Life-history tra its, behavioral characters and source allocation patterns is widely accepte d, its role in governing biochemical and molecular polymorphisms remains po orly understood. Here we demonstrate a case of allozyme frequency-dependent selection. When olive fruit flies (Bactrocera oleae) are reared on an arti ficial larval medium, an allele at the alcohol dehydrogenase locus that is present in very low frequency in natural populations increases to about one -third in less than five generations. We show here that the time from the h atching of the egg to the eclosion of the adult is affected by the genotype composition of the larval population that grows in the same cup of food. C ultures consisting of one genotype only have the longest developmental time , and two-allele cultures in which the two homozygotes and the heterozygote occur in a 1:1:2 ratio show the shortest developmental time. Cultures with intermediate genotypic compositions show intermediate levels of developmen tal time. The results can be explained by assuming that the developmental t ime of a genotype depends an the frequency array of all genotypes in the la rval population and is not merely a function of its own frequency. It is ev en possible that the developmental time of a genotype becomes longer as the genotype becomes rarer, yet the genotype will be favored because the devel opmental times of the competing genotypes become even longer owing to the a ssociated increase of their frequencies. Given that developmental time is i nversely related to fitness, this generates a frequency-dependent selection , with developmental times changing progressively until the population arri ves at an equilibrium. One optimum population composition that provides a s atisfactory fit to allele frequency changes in our experimental populations is when the two alleles occur in equal frequencies and genotypes are in Ha rdy-Weinberg proportions. We argue that this type of selection is consisten t with the role of alcohol dehydrogenase as a detoxifying enzyme in a mediu m that undergoes continuous chemical changes during its use by the feeding larvae.