White “a” cells facilitate opposite-sex mating of opaque cells.

Briefly, 4.8×106 of “helper” white cells of different genetic backgrounds (as indicated) were added to the mating mixture (1×104 opaque a cells plus 3.2×106 opaque α cells). The mating mixtures were spotted onto Lee's glucose medium plates and cultured at 25°C for 48 hours. The mating mixtures were replated onto SD-histidine-uridine, SD-arginine-uridine and SD-uridine-arginine-histidine media for prototrophic selection. All the “helper” strains are ura3Δ/Δ mutants and are unable to grow on media without uridine. Mating tester strains: a-op (as GH1013, but his1Δ/Δ URA3+ARG4+) and α-op (GH1349, arg4Δ/Δ). Three groups of white “helper” strains of different genetic backgrounds (including BWP17, SZ306, SN152 and their derivatives) were used.White “a” cells facilitate opposite-sex mating of opaque cells.