Competitive interactions in Drosophila melanogaster.

A competition experiment with Drosophila melanogaster is reported which comprised a replacement series with two genotypes, together with pure cultures of both genotypes at the same densities as they appeared in the mixtures. The results for total numbers and weights make clear the need to separate the intergenotypic from the intragenotypic interactions if an unambiguous interpretation of the outcome of such mixtures is to be obtained. Further experiments were carried out to investigate the behaviour of pure cultures in order to gain insight into the relative simplicity or complexity of the intragenotypic interactions that needed to be taken into account when investigating mixtures. It was found that transformation of the measurements into percent survival and average weight per fly led to relations with density which were more nearly linear than the originals and, hence, were more readily amenable to analysis. Two main points emerged from the experiments. First, the relationship between the two transformed characters did not appear straightforward. Secondly, the intragenotypic interactions, particularly for percent survival, were complex and thus provided not support for assuming them to be simple when present in mixtures of genotypes.