Fig. 1.

Sexual cycle of Cryptococcus neoformans. During the opposite-sex mating, a and α haploid yeast cells secrete peptide pheromones that stimulate cell–cell fusion. The resulting zygote develops first as hyphae, which are dikaryotic. After the first clamp cell is formed, a narrow ‘pioneer’ hypha is present at the apex. During hyphal growth, a basidium develops from the apical cell. Nuclear fusion and meiosis take place most likely concomitant with the formation of the basidium. Postmeiotic nuclei undergo rounds of mitotic divisions and four chains of spores are formed by subsequent budding from the surface of the basidium. Alternatively, a and α haploid yeast cells can form an a/α diploid, which grows as yeast at 37 °C in a rich medium and forms hyphae at 24 °C on mating-inducing medium. These diploid-derived hyphae are monokaryotic, have unfused clamp cells, and produce a and α spores. During same-sex mating, two yeast cells of the same mating type (α is depicted) undergo fusion and form monokaryotic hyphae with haploid nuclei. Concomitant with the formation of the basidium, nuclear fusion and meiosis occur. The resulting recombinant spores are of a single mating type. A single yeast cell can undergo autopolyploidization (i.e. endoreplication), resulting in a diploid, which develops into monokaryotic hyphae with diploid nuclei and unfused clamp cells. Autopolyploidization can also occur in the hyphae. Both opposite- and same-sex mating hyphae can develop chlamydospores and blastospores (not depicted) (see the text for a more detailed description).