Further Experiments on Competition Between Strains of House Flies

Abstract

Experiments on competition in house flies reported here extend two previous studies by Sullivan and Sokal (1963) and Sokal and Sullivan (1963). The wild typed OL strain, the autosomal recessive bwb, and the reciprocal crosses bwb x OL (called A for short) and OL x bwb (called B) were reared at seven densities ranging from 40 to 2,560 eggs per 36 g of CSMA larval medium, both in pure and mixed cultures. Three possible mixtures, OL: bwb, A: bwb, and B: bwb, were set up in three proportions, 10:90, 50:50, and 90:10 parts of wild phenotype to mutants, respectively. The nine possible combinations were replicated minimally five times per density. The numbers, dry weights, and times of emergence of adults were recorded by genotype. The responses of the hybrids resembled those established previously for the wild type flies. The reciprocal heterozygotes were not markedly different from each other. In mixed culture the emergence of bwb flies is affected more severely by an increase in density than that of their competitors. Hence the relative fitness of the wild phenotype increases markedly with density. At the highest densities hybrids weigh more in competition than they do in pure culture. Crowding increases the mean and variance of duration of adult emergence. Mutants emerged fastest in pure culture, whereas the wild phenotypes are fastest in mixed culture. Competition reduces the variance of duration of emergence in the wild type, though increasing it in the mutant. When the wild phenotype is present in low proportions in dense mixed culture its survival, weight, and duration of emergence are better than when it is frequent. These findings are Reviewed and discussed from the point of view of genetic factors in competition, genetic facilitation, and gene frequency dependent changes in components of fitness. A brief comparison is made with a related study of the ge strain in the house fly.

COMPETITION AMONG GENOTYPES IN THE HOUSEFLY AT VARYING DENSITIES AND PROPORTIONS (THEGREENSTRAIN)

“…Thus while the wild phenotypes are mildly slowed down in dense competition, the ge flies are considerably retarded when mixed with any of the other three genotypes. There appear to be differences in duration of emergence of ge depending on the genotype of the competitor (A retards ge the most) and the proportions of the mixture, although the latter do not give the clear-cut effects found for the bwb strain by Sullivan and Sokal (1964). The present data also differ from the findings on bwb, in that duration of emergence of the wild phenotype was accelerated by competition with bwb while the opposite is true in this study.…”

“…4a shows the effect of increasing densities on the mean weights of ge, OL, and their reciprocal crosses in pure cultures. The pattern of response of OL is in general agreement with that observed by Sullivan and Sokal (1963), except that these authors noted the first significant drop in weight at density 320, while OL, B, and ge do not show a significant decrease until density 640. The absolute values of mean weights in Sullivan and Sokal's data are much higher for OL, ranging from 4.15 mg to 0.69 mg per fly, while the range in the present series is from 2.92 mg to 0.96 mg.…”

“…As density increases, both weight and emergence decrease, but the former decreases before the latter in what Sullivan and Sokal ( 1963) have termed a type 2 response. By maintaining a high emergence at the expense of body weight under unfavorable food conditions, populations are able to continue to disperse into many more habitats than if crowding would immediately reduce their number, producing a few but well-fed individuals.…”

“…The medium is prepared from 600 g of dry CSMA medium, 500 cc of water, 20 g of baker's yeast, 15 g of malt, and 400 cc of "Pet" brand evaporated milk. This involves minor changes in formulation from the earlier medium prepared by Sullivan and Sokal (1963) and Sokal and Sullivan (1963) to obtain a drier and richer medium. The cultures were kept in a constant temperature room at 27 ± 2°C.…”

Selection for changes in genetic facilitation: negative results in Tribolium and Musca.

“…Lewontin (1955), Lewontin and Matsuo (1963), Sokal and Huber (1963), Sokal and Karten (1965), Sokal and Sullivan (1963), Sullivan and Sokal (1965), and Bhalla and Sokal (1964) provide evidence for Drosophila, Tribolium and Musca. Decreased overlap in environmental requirements may be achieved by preferential utilisation of fine differences in the environment by different genotypes, or indirectly by the life processes of one competitor beneficially affecting the other, e.g.…”