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Chapter 7 : Sexual Reproduction of Cryptococcus
Category: Clinical Microbiology; Fungi and Fungal Pathogenesis
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This chapter reviews the discovery of both opposite-sex and unisexual reproduction and illustrates how these pathways are molecularly controlled and the central cell biology questions that remain to be addressed. Fungal sexual reproduction is genetically regulated by the mating-type locus (MAT), a specialized region of the genome that is idiomorphic or allelic between different sexes. Similar to the model fission yeast Schizosaccharomyces pombe, mating and meiosis of Cryptococcus neoformans occur sequentially in response to limitation or specific nutrient cues. Other environmental cues such as light and temperature also affect mating of C. neoformans. Those factors that have been connected to the sexual reproduction of C. neoformans are summarized in the chapter. Furthermore, several Cryptococcus species can be isolated from trees and fermenting fruits, suggesting that there might be potential plant-fungus interactions that contribute to sexual reproduction of C. neoformans and Cryptococcus gattii in nature. Apart from nutritional cues, other environmental signals also influence sexual reproduction. However, the predominance of the α mating type (>99%) in the Cryptococcus population represents a paradox as to how sexual reproduction might occur in this essentially unisexual population. The discovery of same-sex mating in C. neoformans resulted in a paradigm shift in considering how genetic diversity is generated in a unisexual population and the evolutionary role of a unisexual reproductive mode. Moreover, the transitions that occur in the fungal kingdom between heterothallic outbreeding and homothallic inbreeding modes promise to reveal general features by which sexual reproduction enhances fitness and enables evolutionary success throughout biology.
Sexual cycle of C. neoformans. (A) a-α opposite-sex mating. a and α haploid yeast cells secrete peptide pheromones that trigger a-α cell-cell fusion under nutrient-limiting conditions. After fusion, cells switch to filamentous growth, and the two nuclei congress but do not fuse in the resulting dikaryotic hyphae. Clamp cells are formed to ensure that two nuclei are faithfully segregated during hyphal growth. Blastospores (yeast-like cells) can bud from the hyphae and divide mitotically. Some hyphal cells can enlarge and form chlamydospores ( 94 ). In the basidium, two nuclei (a and α) fuse and undergo meiosis to produce four chains of basidiospores. In cases where cell fusion is immediately followed by nuclear fusion between a and α haploid yeast cells, heterozygous diploid a/α yeast cells are created. These diploid cells produce monokaryotic hyphae with unfused clamp cells. Eventually meiosis occurs in the basidium, and basidiospores with a and α mating types are generated. (B) α-α same-sex mating (fruiting). During same-sex mating, cells of one mating type become diploid either by endoduplication or by cell-cell and nuclear fusion. The diploid monokaryotic hyphae form rudimentary clamp connections that do not fuse to the preceding cell. Blastospores and chlamydospores are also produced during fruiting. In the basidium, meiosis occurs and haploid basidiospores are produced in four chains. In an alternative model, haploid α cells produce monokaryotic hyphae with haploid nuclei, and diploidization occurs in the basidium, followed by meiosis to produce spores with only the α mating type.
Factors that affect mating in C. neoformans. Plant materials, copper ions, pigeon guano, nitrogen starvation, V8 juice, IAA, and myo-inositol are compounds and conditions that promote sexual development of C. neoformans. Light, high temperature (37°C), high CO2, and water, on the other hand, are known to inhibit mating.
Signaling cascades that contribute to sexual development of C. neoformans. The pheromone response pathway, cAMP-PKA pathway, Ca2+-calcineurin pathway, and the ammonium sensor Amt2 all positively regulate sexual development, while the light- and stress-sensing pathways negatively regulate this process (see text for details).