Epistasis in measured genotypes: Drosophila P-element insertions.

Transposon tagging provides an opportunity to construct large numbers of strains of organisms that differ by single insertional mutations. By scoring the phenotypes of these "measured genotypes," powerful tests of effects of mutations on phenotypic expression have been performed. Here we extend this approach by constructing with simple crosses all possible two-locus genotypes for each of eight pairs of P-element insertions. Analysis of metabolic phenotypes (fat and glycogen contents, enzyme activities, total protein, and body weight) of the resulting nine genotypes provides direct estimates of additive, dominance, and epistatic effects of the mutations. Nested two-way analysis of variance identified significant epistatic effects in 27% of the tests (35/128 of the trait x P-element combinations). Posterior contrasts were performed to partition the epistatic variance into the four orthogonal components of COCKERHAM, and the data exhibit a tendency toward additive x dominance and dominance x dominance epistasis. Mutations in this study have epistatic effects on metabolic traits that are on the same order of magnitude as main (additive and dominance) effects. Measured genotypes have been used in other contexts to quantify epistatic effects on phenotypic expression, and these results are also briefly reviewed.