Fig. 1.

Schematic of the experimental design. From each of four isofemale strains (a, b, c, and d) of Drosophila melanogaster, flies were collected and placed into vials, each with two males and two females (A; controlled density vials). After 4 d, these flies were transferred to a single population cage (one cage per strain) to lay eggs (B). Eggs were collected and placed into vials at a density of 50 per vial (standard density vials). Most flies emerging from the controlled density vials were used in mating trials (C). We performed competitive (C⁺) and noncompetitive mating trials (C^-), where females were either virgin (V⁺) or non-virgin (V^-). Non-virgin females were first mated to males from an isogenic, brown-eyed strain. In competitive trials, one male from each strain was present with their abdomens dyed different colors, and females from all four strains were presented (singly) to these groups of males. All noncompetitive trials had one male and one female, with males from all four isofemale strains presented to females from all four isofemale strains. The remining flies emerging from the controlled density vials were crossed within and between strains in a full factorial design (D; strain crosses). For fitness, these crosses involved three replicate vials of two males and three females, and the offspring emerging from these crosses were used to measure male and female fitness, where they were competed against a brown-eyed fly of the same sex for access to a brown-eyed fly of the opposite sex. Fitness was measured (the log ratio of the numbers of wild-type [WT] and brown-eyed [BE] flies produced) 10 times for each sex and cross combination. For egg-to-adult viability, each cross combination was replicated 10 times, each with one male and one female, and the numbers of eggs and adults they produced after being held together for 22 h were recorded. Finally, flies emerging from standard density vials were used to measure wing shape and size and sex comb tooth number (E). In this schematic, smaller flies with dark abdomens are male, larger flies with striped abdomens are female, and flies with dark-brown eyes are from an isogenic brown-eyed strain. This design was replicated five times, with four different isofemale strains represented in each randomized block.