White “a” cells promote same-sex mating of opaque cells (α×α) and opposite-sex mating between “a” opaque cells of the mfa1Δ/Δ mutant and “α” opaque cells (a-mfa1Δ/Δ×α).

The “α-op×α-op” cross: 9.6×107 white cells of “helper” strains (60%) were mixed with 3.2×107 opaque cells of SZ306α (Δ/α, ura3Δ/Δ, 20%) and 3.2×107 opaque cells of GH1349 (α/α, arg4Δ/Δ, 20%). The “a-op(mfa1Δ/Δ)×α-op” cross: 9.6×107 white cells of “helper” strains (60%) were mixed with 3.2×107 opaque a cells of GH1609 (a/a, mfa1Δ/Δ, ura3Δ/Δ 20%) and 3.2×107 opaque cells of GH1349 (α/α, arg4Δ/Δ, 20%). The mating mixtures were spotted onto Lee's glucose medium plates and cultured at 25°C for 5 days. Then, mixed cells were replated onto SD medium for prototrophic selection. SZ306 (a/Δ, ura3Δ/Δ wor1Δ/Δ) is locked in the white phase since it is a wor1Δ/Δ mutant. In the mating mixture where SZ306a served as the “helper” strain, white cells of SZ306a were able to mate with opaque α cells at extremely low efficiencies. The progeny of this mating cross were able to grow on selectable plates because SZ306a only carries a ura3-auxotropic marker. To exclude these mating products from our calculations, all mating progeny colonies were subject to PCR verification, and only correctly validated colonies were used in the calculation. “White “a” cells promote same-sex mating of opaque cells (α×α) and opposite-sex mating between “a” opaque cells of the mfa1Δ/Δ mutant and “α” opaque cells (a-mfa1Δ/Δ×α).