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The selective value of modifiers of the eyeless locus in Drosophila melanogaster Wills, Christopher James

Abstract

The reactions of a spectrum of eyeless phenotypes of Drosophila melanogaster, ranging from complete eyelessness to overlapping of the wild type, were examined with respect to the action of modifiers of the ey locus. Flies with less than 1/3 of a compound eye were found to show no response to light. Modifiers of eyeless could be selected for and against under constant environmental conditions. The continuous range of variation exhibited by the locus indicates that a large number of modifiers are involved. Under laboratory conditions, the effect of light alone was shown to have no influence on the rate of accumulation of modifiers in large populations, in spite of the fact that the absence of eyes was demonstrated to exert a great effect on the photokinetic response of the flies. This indicates that under the conditions found in population cages the absence or reduction of compound eyes and optic lobes is not detectably disadvantageous to the flies. Three sampling techniques were used to determine the phenotypic distribution of the flies in the cages. Mass culturing of egg samples produced great variability in the phenotypic means of the adult flies, due to crowding and the variable number of flies which emerged. Suction samples of flies taken directly from the cages were biased towards those flies which were most normal and vigorous and therefore most likely to be trapped by the suction device. The method which introduced the least variability involved isolating a small known number of eggs from the cages and culturing them under uncrowded conditions.

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