Ecological partitioning by drosophila populations

Abstract

Pure and mixed cultures of two populations of the fruit fly (Drosophila melanogaster) were grown together for up to 15 generations. Interbreeding and reproductively isolated mixtures of Bar and wild type genotypes were maintained in two experiments. Isolated mixtures only were maintained in a third, although the populations had been interbreeding previously for 30 generations. Bar populations were as productive as wild type in pure cultures but less so in mixed cultures. Mixed cultures outyielded pure cultures in the third experiment but not in the first two. Flies from mixed cultures emerged earlier than those from pure cultures towards the end of Experiment 2. The competitive ability of Bar flies from the reproductively isolated mixtures was inferior to that of Bar flies from pure cultures, while that of wild type flies from these cultures was only sometimes worse than wild type flies from pure cultures. However, competitive ability of Bar and wild type flies from interbreeding cultures was superior to that of flies from pure cultures. This can be explained by the former suffering inbreeding depression, while the latter were becoming more outbred. It was also suggested that Bar, as it became a much worse competitor than wild type, was having strong selection pressures imposed upon it by the wild type population. By favouring only a part of the population, this would also reduce its genetic variability. This is a factor which has not been considered before in experiments of this type. Replacement series tests, measuring performance of the two genotypes grown together at different frequencies, showed that frequency dependent selection was operating in the reproductively isolated mixtures, and in the pure cultures in the third experiment. Both Bar and wild type yielded proportionately higher when they were the minority component. This implied ecological differences, and probably competitive avoidance between the two populations. This is possibly supported by extinction rate tests, where some of the replicates of selected Bar (from reproductively isolated mixtures) appeared to reach stability. This was not true for unselected Bar (from pure cultures). In the second part of this thesis, a single population was subjected to two unpleasant types of medium, one containing hydrochloric acid and the other sodium hydroxide. Populations of flies grown on acid or base medium for 15 generations gave higher yields on their own type of medium than on the alternative medium and even higher than on normal medium. Flies grown on a choice of acid or base medium over this time also grew best on the medium from which they emerged in the final generation. Thus populations subjected to either directional or disruptive selection will produce flies adapted to either unpleasant types of medium. There was no indication of assortative mating having evolved between acid and base flies from the mixed cultures, so that although divergence had taken place, there was no incipient speciation.