GENETICS AND THE ENVIRONMENT IN INTERSPECIFIC COMPETITION - A STUDY USING THE SIBLING SPECIES DROSOPHILA-MELANOGASTER AND DROSOPHILA-SIMULANS

The outcome of interspecific competition of two closely related specie s may depend upon genetic variation in the two species and the environ ment in which the experiment is carried out. Interspecific competition in the two sibling species, Drosophila melanognster and D. simulans, is usually investigated using long-term laboratory stocks that often h ave mutant markers that distinguish them. To examine competition in fl ies that genetically more closely resemble flies in nature, we utilize d freshly caught wildtype isofemale lines of the two species collected at the same site in San Carlos, Mexico. Under ordinary laboratory con ditions, D. melanogaster always won in competition. However, in hotter and drier conditions, D. simulans competed much more effectively. In these environmental conditions, there were genetic differences in comp etitive ability among lines with the outcome of competition primarily dependent upon the line of D. melanogaster used but in some cases also influenced by the line of D. simulans used. Differences in the measur es of productivity and developmental time did not explain the differen ces in competitive ability among lines. This suggests that the outcome of competition was not due to differences in major fitness components among the isofemale lines but to some other attribute(s) that influen ced competitive ability. When lines of flies were combined, the outcom e of competition was generally consistent with competitive outcomes be tween pairs of lines. In several cases, the combination of lines perfo rmed better than the best of the constituent lines, suggesting that co mpetitive ability was combined heterotically and that the total amount of genetic variation was important in the outcome of interspecific co mpetition.