Minority Advantage of Certain Eye Color Mutants of Drosophila melanogaster. II. A Behavioral Basis

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Abstract
To establish a behavioral basis for the rare-male mating advantage in D. melanogaster, studies were done with a pair of alleles at the brown locus (bw, bw75) combined with scarlet (st/st) of D. melanogaster. Some details of male courtship and female acceptance were described for 2 genotypes, bw75/bw75, bw75/bw. Preliminary multiple choice tests of homozygous red (R) and heterozygous orange (O) produced a uniformly graded series from greatest advantage at extreme minority ratios (2:18) to near equality in mating success (R males 8% more successful than O) when at equal frequency (10:10). R females were 3-7% less receptive than O throughout all tests. Single females were tested with 6 males and details of courtship and acceptance observed with timing by a metronome and recording on tape of the courtship elements. At the control ratio (3:3) there was no significant difference between numbers of courtships for R and O males, and females were equally receptive to each eye-color type. At both minority ratios (5:1) the rarer male of either genotype was accepted at an absolute rate of 70%, > 4 .times. the expected rate (16.7%). The female received more courtships (5-6) from majority males when they were accepted than when they were rejected (.ltoreq. 4), i.e., when a minority male was accepted, while the minority male courted 2-3 .times. on the average whether he was successful or not. On examination of type male 1st to court and mating success, > 70% of 1st males to court were avoided by the female in the mating, consistently at all 3 ratios. With the 2nd male to court, matings were close to random with just 52% avoided. It is not the development in females of a sensory habituation against majority males'' cues but a simple avoidance of first courting males'' cues that accounts for the minority advantage. At unequal ratios, chance alone determines first courtship by majority males, and thus by avoiding that cue the female accepts the type of rarer male. How a female may discriminate between courtship of 2 eye-color type males was evidenced by analysis of tape recordings made during the 30 min tests. While the order of courtship elements was about the same for both types, they differed significantly in duration of wing vibration, courtship sequence and vibration as a percentage of the sequence (orientation and jumping being the remaining important elements). The R males in all ratios had shorter wing vibration bouts (1-2 s) than O (3-4 s), longer sequence (15-20 s) than O (10-12 s) and vibration as a smaller percentage of courtship (17%-20%) than O (40%-60%). These facts are discussed as relevant to O''Donald''s (1977) alternative model for rare-male mating advantage in which females were presumed to have had a constant preference for each male type. No constant preference on the part of females was observed through the constant avoidance tendency toward the 1st courting male type could be interpreted as a constant attitude on the part of the female. Females showed no significant bias toward either male type, and labeling one type of male as more vigorous than another in order to predict an outcome is to be repudiated.