Final answer:
Haploid yeast strains undergo meiosis when cells of different mating strains fuse with each other through chemical communication. This fusion produces diploid cells, which later undergo meiosis to form haploid spores.
Step-by-step explanation:
Haploid yeast strains undergo meiosis when haploid cells of different mating strains encounter each other. Chemical communication through pheromones triggers the production of extensions ('Shmoo's') that allow cells to fuse with each other (plasmogamy). This fusion eventually leads to the creation of diploid cells, which will then undergo meiosis to produce haploid spores that can germinate.