Mixed evidence for intralocus sexual conflict from male-limited selection in <em>Drosophila melanogaster</em>

Sexual conflict over shared traits – intralocus sexual conflict (IaSC) – may be common and consequential, but experimental tests of its relative magnitude are challenging and limited in number. We use a sex-limited selection experiment, designed to subject haplotypes of Drosophila melanogaster to selection for male fitness without opposing selection acting on female fitness. Importantly, we use three novel base populations to compare results with those from the LHM population, the sole population investigated using this technique. In contrast with previous studies, we find that the male fitness of haplotypes subject to male-limited selection (ML populations) is not consistently better than their matched (MC) controls when tested in the “wildtype” state. Males from ML lines did not outperform controls in competitive fitness assays, mate choice trials, fecundity induction, or sperm offense tests. As predicted, genetic variation for male fitness was reduced, with low fitness haplotypes apparently removed by selection, but this was only surveyed in one replicate population pair and included a potential artifact in the protocol. Female fitness was markedly reduced by carriage of ML haplotypes, as predicted by sexual antagonism. Hence, our results are only partially consistent with the IaSC hypothesis, raising questions about the relative contribution of sexual conflict to the standing genetic variation in these populations and the potential role of artifacts in the protocol that may have obscured our ability to detect IaSC. Methods Full methodological details are provided in the associated publication. Breeding designs used to produce animals from various treatments can be found in the online supplement. Competitive reproductive fitness (CRF) was assayed for both males and females against competitive recessive (Cr) marker stocks in equal numbers. Test females were collected as virgins and test males at 10–11 days post-oviposition. On day 12, groups of 30 MC or ML flies were combined with 30 Cr of the same sex and 50 Cr of the opposite sex in ventilated “mini-cages” with yeast for 2 days. Six competition arenas (“contests”) were set up per population, and offspring proportions from six egg vials per arena were counted to estimate CRF. Fitness assays for hemiclonal analysis followed a similar design with reduced competition size to accommodate 76 lines. For males, 4 test flies were combined with 4 Cr males and 6 Cr females. For females, 4 virgins were combined with 3 Cr males, and productivity (pupal count) was used as a proxy for fitness. Up to 5 contests were run per line, and complete hemiclonal data were obtained for 76 of 80 lines. Male mating success: Single test males were combined with a Cr male and a Cr virgin female. When one pair initiated copulation, the rival male was removed without disturbance, and the mating event was monitored until separation. Mating latency, mating duration, and the offspring produced (sex ratio, fecundity induction, and eye-color marker) were recorded to identify the successful sire. Fifty binary choice vials were set up per treatment within each replicate line. Sperm competition (P2): Groups of 12 virgin Cr females were combined with Cr males, and matings were closely observed. After mating, Cr males were removed. On day 13, 10 “target” males (MC or ML) were added to each vial for a second mating, observed for ~3 hours to prevent multiple matings. Ten females per vial were then isolated for 24 hours to lay eggs. Offspring were scored by eye color to determine the proportion sired by the second male. Each treatment included 30 replicate vials.