A range of long established inbred lines derived from the TEXAS population of Drosophila melanogaster have been used to elucidate the nature of the competitive interactions which occur in genetically heterogeneous mixtures. A prerequisite for this type of investigation is the ability to distinguish the genotypes which compete in mixed culture. Specific marker alleles are generally used to achieve this distinction although in the past little attention has been given to the possibility of competitive bias introduced by the marker alleles themselves. For the experiments reported in this paper two specific marker alleles (y~ and w ~) have been introduced independently into the TEXAS inbred lines. In this way the original wild type inbred lines could be compared with similar series of genotypes marked with either ye or w a and the effects of the marker alleles determined.
The results indicated that the body colour mutation (v 2) was neutral in its effect on the competitive interaction of recipient strains. The introduction of the white apricot eye colour mutation (w ~) however, had a pronounced and deleterious effect on competitive ability. This effect was to render genotypes less able to compete effectively in mixed culture by depressing inter-genotypic competitive ability. These effects were found to be consistent over a range of genotypes and for each of two characters measuring competitive success.