Does kin selection moderate sexual conflict in Drosophila?

Abstract

Two recent studies provide provocative experimental findings about the potential influence of kin recognition and cooperation on the level of sexual conflict in Drosophila melanogaster. In both studies, male fruit flies apparently curbed their mate-harming behaviours in the presence of a few familiar or related males, suggesting some form of cooperation mediated by kin selection. In one study, the reduction in agonistic behaviour by brothers apparently rendered them vulnerable to dramatic loss of paternity share when competing with an unrelated male. If these results are robust and generalizable, fruit flies could be a major new focus for the experimental study of kin selection and social evolution. In our opinion, however, the restrictive conditions required for male cooperation to be adaptive in this species make it unlikely to evolve. We investigated these phenomena in two different populations of D. melanogaster using protocols very similar to those in the two previous studies. Our experiments show no evidence for a reduction in mate harm based upon either relatedness or familiarity between males, and no reduction in male reproductive success when two brothers are in the presence of an unfamiliar, unrelated, 'foreign' male. Thus, the reduction of sexual conflict owing to male cooperation does not appear to be a general feature of the species, at least under domestication, and these contrasting results call for further investigation: in new populations, in the field and in the laboratory populations in which these phenomena have been reported.

Keywords: Drosophila; cooperation; familiarity; kin selection; mate harm; sexual conflict.

Figure 1.

Mate harm experiment. Tukey box plots compare treatments designed to evaluate the effects of three brothers (AAA; n = 43 vials) versus three unrelated males (ABC; n = 52) on a female's (a) longevity, (b) period of offspring production and (c) lifetime reproductive success (LRS; total number of progeny). Graph (d) shows the average number of progeny (±95% CL) produced per day by females in the presence of AAA and ABC male threesomes; symbols for each day (shown on the x-axis) are jittered for clarity. (Online version in colour.)

Figure 2.

Foreign male experiment. Tukey box plots compare the proportion of progeny sired by the ‘foreign’ male in treatments where the female was also housed with two brothers raised together (n = 43) two brothers raised apart (n = 39), two unrelated males raised together (n = 47), or two unrelated males raised apart (n = 47). The dot and solid line inside the box are the mean ± 95% CL of the proportions for each treatment, and the horizontal dashed line is at 0.33, the expected value if the males achieve equal reproductive success. (Online version in colour.)