Experimental constraints on mate preferences in Drosophila pseudoobscura decrease offspring viability and fitness of mated pairs

Abstract

Using Drosophila pseudoobscura, we tested the hypothesis that social constraints on the free expression of mate preferences, by both females and males, decrease offspring viability and reproductive success of mating pairs. Mate preference arenas eliminated intrasexual combat and intersexual coercion. The time female and male choosers spent in arena tests near either of two opposite-sex individuals measured the preferences of choosers. We placed choosers in breeding trials with their preferred or nonpreferred discriminatee when they met the minimum criteria for showing the same preference in two consecutive tests. There was no statistically significant difference in the frequency of female and male choosers meeting minimal preference criteria. There was a significant difference between female and male choosers for offspring viability, with female choice having the greater effect, but there was not a significant difference in the overall reproductive success of male and female choosers. There were significant differences in fitness between matings to preferred and nonpreferred partners. Female and male choosers paired with their nonpreferred discriminatees had offspring of significantly lower viability, as predicted by the constraints hypothesis. Reproductive success, our measure of overall fitness, was greater when males or females mated with the partner they preferred rather than the one they did not prefer.

Fig. 1.

Experimental mate preference arenas consisted of tygon tubing (0.79-mm internal diameter and 7.6 mm in length), fine plastic mesh, and two Eppendorf tubes. The single chooser was able to move throughout the length of the long corridor. The Eppendorf tubes were the cells (A and B) containing discriminatees of the opposite sex. Opaque tape on one side of each tube blocked visual contact between the discriminatees, one per cell during preference testing. The side of each Eppendorf cell connected to a longer corridor of tubing in which we placed focal individual choosers. Very fine plastic mesh prevented the discriminatees from moving into the corridor and the chooser from entering either of the cells. We divided each corridor into three regions (A and B in front of each Eppendorf cell, and C designating the space between them) designated by dark lines drawn on the surface of the tubing.

Fig. 2.

Time course for female longevity, offspring viability, fecundity, and reproductive success (RS) of mating pairs in female and male choice tests, for choosers paired with their nonpreferred (filled circles) or preferred partners (open squares). Differences in longevity for females in preferred or nonpreferred matings were not statistically significant by using Kaplan–Meier statistics (for female choice tests: Wilcoxon χ² = 2.02, df = 1, P > 0.15; for male choice tests: Wilcoxon χ² = 0.1140, df = 1, P > 0.73).

Fig. 3.

The difference scores for fecundity between preferred and nonpreferred matings in female (A) and male (B) choice studies. The distribution of difference scores was significantly different in both female choice tests (Wilcoxon signed rank t = −81.5, df = 24, P > |t| = 0.013) and male choice tests (Wilcoxon signed rank t = 59.5, df = 24, P = |t| = 0.055). NP, nonpreferred; P, preferred.

Fig. 4.

The difference scores for offspring viability between preferred and nonpreferred matings in female (A) and male (B) choice studies. The distribution of difference scores was significantly different both for female choice tests (Wilcoxon signed rank test = 97.5, P > |t| = 0.006) and for male choice tests (Wilcoxon signed rank test = 134.5, P > |t| < 0.0001). NP, nonpreferred; P, preferred.