GENOTYPE-ENVIRONMENT INTERACTIONS AND THE ESTIMATION OF THE GENOMIC MUTATION-RATE IN DROSOPHILA-MELANOGASTER

We have studied the relative fitnesses of three genotypes of Drosophil a melanogaster in 50 environments. Two genotypes, the MA lines, had ac cumulated mutations in the absence of natural selection over 62 genera tions. The third was a related strain where selection had continued to act. The environments differed in three factors: parental density, di lution of the medium, and the temperature regime and medium compositio n. Our measure of fitness assessed fecundity and viability relative to a reference genotype. Both MA lines always had lower fitnesses than t he selected line, but the difference increased dramatically with dilut ion of the medium and, especially, crowding. Under the most severe con ditions, the performance of the MA lines approached 0. This increased difference in harsh conditions may be caused both by a uniform increas e in the magnitude of deleterious effects of all mutations and by the exposure of mutations which are essentially neutral under benign condi tions. If the second cause is important, previous experiments are like ly to have underestimated the genomic deleterious mutation rate in Dro sophila melanogaster more than previously thought.