SELECTIVE MATING AND EYE PIGMENTATION: AN ANALYSIS OF THE VISUAL COMPONENT IN THE COURTSHIP BEHAVIOR OF DROSOPHILA MELANOGASTER

A number of investigations have shown that in competitive mating tests mutant Drosopkila are at a disadvantage when competing with wild type flies. Over long time intervals the mutant genes are either eliminated or maintained at a very low frequency in the population (Reed and Reed, 1948; Merrell and Underhill, 1956). This disadvantage has usually been attributed to a reduction in fitness on the part of the mutant when competing for the available food supply. Behavior is also a component of fitness, and there is evidence indicating that the disadvantage exhibited by many mutant genotypes is due to some impairment of their mating behavior. Reed and Reed (1950) found that males from the sex linked white eye mutation (w) of Drosophila melanogaster were at a considerable disadvantage when in competition with wild type males. Bastock (1956) demonstrated that the yellow mutant (y) of D. melanogaster gives rise to males which show a quantitative change in the wing vibration element of the courtship display. The slight reduction in mean vibration bout lengths of the mutants results in their being at a disadvantage. Petit (1959) found that wild type males had an advantage over wkite eye and Bar eyed males in a competitive situation, this she attributed to differences in the wing vibration component of the courtship display. Geer and Green (1962) were unable to confirm this effect on wing vibration in studies with vestigial mutant males. Geer and Green (loc. cit.) working with a series of eye pigment mutants obtained evidence of a relationship between the amount of eye pigmentation and mating success; the greater the pigment density the greater the mating success. This is a particularly interesting finding since it has been argued that vision plays only a minor role in the courtship of Drosophila melanogaster (Spieth and Hsu, 1950), although it has an important influence on the courtship of certain other members of the melanogaster species group (Grossfield, 1968). Geer and Green (loc. cit.) showed that the disadvantage suffered by the eye pigment mutants was no longer present when matings were allowed to take place in total darkness, indicating that vision may also be important in the courtship of Drosophila melanogaster. In none of the investigations where eye mutants have been used, has the behavior of the animals been observed, mating success being measured by progeny analysis or by the dissection of females to determine the presence or absence of sperm. Ewing and Manning (1967) in their review of behavior genetics in insects single out a difficulty in the interpretation of Geer and Green's results. They argue that although Geer and Green demonstrate a relationship between pigment density and mating success it is not possible to attribute the disadvantage shown by the eye pigment mutants solely to that sensory defect. An alternative hypothesis is that the eye pigment mutations produce some pleiotropic effect in the animal's central nervous system which affects some aspect of the male's sexual behavior. Since the behavior of the mutants has not been examined in detail this remains a plausible hypothesis. These two alternative explanations may be examined by using the phenocopying technique described by Burnet et al. (1968).