Fig. 3.
Reproductive isolation of novel mating-type pairs from the WT population as revealed by a sensitive genetic recombination assay. Results of the genetic recombination assay. The same numbers of P and M cells were mixed and cultured on conjugation medium (malt extract medium) for 2 d. Cells were spread onto YEA plates containing 100 µg/mL appropriate drugs: YEA + Nat, YEA + Hyg, and YEA + Nat + Hyg. After 3 d of incubation, colony numbers were counted. The number of Nat/Hyg double-resistant colonies was normalized by the colony numbers on plates containing single Nat or Hyg. High recombination frequencies were observed in crosses between WT maters as well as between the mutant pair mfm1-V5H and map3-F214H. Note that the mutant M factors were completely rejected by the WT Map3 (recombinant frequency, <10−7); similarly, the mutant receptor Map3(F214H) barely accepted WT M factor (recombination frequency, 4.5 × 10−5).