Fly husbandry and collection
In this experiment, we used laboratory adapted Drosophila melanogaster wild-type (wt ) Dahomey flies. We also usedDrosophila melanogaster sparkling poliert (spa ) mutants flies in order to discriminate the paternity-share of focal wtmales. The spa allele being recessive, individuals homozygous for this locus display the spa eye phenotype, whereas heterozygouswt /spa individuals display the wt phenotype. We kept stock populations at 24°C on a 12h light/12h dark cycle, with overlapping generations, and fed them with standard food weekly (solidified aqueous mix containing 60g.L-1 corn flour, 50g.L-1 white sugar, 40g.L-1 fresh baker’s yeast, 10g.L-1 soy flour, 10g.L-1 industrial agar, 3g.L-1Methyl 4-hydroxybenzoate, 10mL.L-1 96 % EtOH, 5mL.L-1 99% propionic acid). We collected eggs directly from stock populations using yeasted grape juice agar plates (FlyStuff grape agar premix, Genesee Scientific). We ensured a controlled density of ca. 200 larvae per 250mL bottle filled with ca. 75mL of standard food. Using ice anaesthesia, we isolated flies by sex 6 hours upon emergence in order to ensure virginity. We kept females by groups of 15 per vial and males by groups of 20 per vial. All vials used in this experiment contained a large amount of the same food the populations were fed with, both for adult feeding purposes and to provide an adequate egg-laying substrate to females.