A five-step experimental cycle based on haploid models of sexual selection (cf. Barton & Turelli 1991). One mating type, MATα, was designated the signaller, and the other, MATa, the receiver. At the start of each cycle, strong and weak signallers were mixed with receivers at a predetermined ratio and allowed to mate. The exact ratio of signallers to receivers determines the level of sexual selection. Diploids arising from these matings were forced to sporulate, producing tetrads of new haploid spores (diamonds). Any remaining unsporulated diploids and unmated haploids (circles) were destroyed by heating and the spores were allowed to germinate and grow asexually on selective media, producing separate pools of each mating type. Mating types were then mixed to start the next cycle (see §2c for details). This design introduces three measures to lift the restrictions on sexual selection imposed by the natural yeast mating system. First, we prevented self-fertilization by selectively germinating MATα or MATa spores. Second, we blocked autodiploidization using heterothallic mutants incapable of switching mating types, allowing cultures of pure MATα and MATa gametes to be propagated. Third, by manipulating the ratio of signallers to receivers away from the natural 1 : 1 ratio, we altered the amount of competition for mates, resulting in higher (or lower) levels of sexual selection.

Variation in sexual signal strength in ancestral strains. α-Pheromone production was measured using the signal size assay (see §2e). Weak signallers (A, YDG1093) produced a much smaller halo than did strong signallers (B, YDG1074). The signalling locus is silent in MATa cells and therefore no halo was observed around receivers carrying the weak signalling allele (C, YDG1096) or the strong signalling allele (D, YDG1075).

Evolution at the signalling locus under different levels of sexual selection. Change in frequency of the initially rare (1%) strong signalling allele in six replicate populations exposed to high sexual selection (solid lines) and six replicate populations exposed to low sexual selection (dotted lines). Each replicate of each treatment is indicated by a different symbol. High sexual selection: filled up triangles, A; filled down triangles, B; filled diamonds, C; filled circles, D; pluses E; filled squares, F. Low sexual selection: open squares, A; open diamonds, B; down triangles, C; crosses, D; open circles, E; up triangles, F.

Evolution of sexual signal strength in populations under high sexual selection treatment. Signal strength (α-pheromone production) is shown by the size of the dark ‘halo’ surrounding each round patch of signalling cells. The halo is caused by the inhibition of growth of an overlaid lawn of MATa tester cells that are hypersensitive to α-pheromone and cannot grow when it is at a high concentration. Thus, a larger halo indicates greater α-pheromone production. Replicates are ordered clockwise from A to F starting at the top of each plate. Results are shown only for the initial population (t0) and experimental cycles 3, 5, 7, 10 and 13.