FREQUENCY-DEPENDENT SELECTION IN DROSOPHILA: ESTIMATION OF NET FITNESS IN PSEUDOHAPLOID POPULATIONS

There is much evidence that the viability of Drosophila larvae depends on the genotypes of other larvae with which they develop. There is, however, little evidence concerning frequency dependence of net fitness. This report documents variation in net fitness among male D. melanogaster from 17 lines studied in 49 discrete-generation populations. The experimental design, which uses attached-X females, eliminates effects of dominance, heterosis, recombination, genetic background, and selection in females. Selection operates only on patroclinously inherited X-chromosomes in males, causing frequency dynamics to mimic those of haploid populations. Analysis of one-generation transitions shows a general pattern of frequency dependence of net fitnesses. Analysis of multigeneration trajectories suggests frequency dependence in 12 populations, but trajectories tend to be heterogeneous among replicates. For a deterministic model in which fitness w(p) is a function of allelic frequency (p), oscillatory dynamics can occur where |dw(p)/dp| > w(p̂) (Curtsinger, 1984a). The derivative measures the strength of frequency-dependent selection, and divides models into two categories: dynamically "well-behaved" and "complex." For the pooled data, this criterion is violated in about half of the frequency space, though several factors probably cause strength to be overestimated. The strength criterion is a useful predictor of evolutionary dynamics: observed oscillations tend to occur most often where frequency dependence is strong, and less often where it is weak.