Allelic Exchange of Pheromones and Their Receptors Reprograms Sexual Identity in Cryptococcus neoformans

Abstract

Cell type specification is a fundamental process that all cells must carry out to ensure appropriate behaviors in response to environmental stimuli. In fungi, cell identity is critical for defining “sexes” known as mating types and is controlled by components of mating type (MAT) loci. MAT–encoded genes function to define sexes via two distinct paradigms: 1) by controlling transcription of components common to both sexes, or 2) by expressing specially encoded factors (pheromones and their receptors) that differ between mating types. The human fungal pathogen Cryptococcus neoformans has two mating types (a and α) that are specified by an extremely unusual MAT locus. The complex architecture of this locus makes it impossible to predict which paradigm governs mating type. To identify the mechanism by which the C. neoformans sexes are determined, we created strains in which the pheromone and pheromone receptor from one mating type (a) replaced the pheromone and pheromone receptor of the other (α). We discovered that these “αa” cells effectively adopt a new mating type (that of a cells); they sense and respond to α factor, they elicit a mating response from α cells, and they fuse with α cells. In addition, αa cells lose the α cell type-specific response to pheromone and do not form germ tubes, instead remaining spherical like a cells. Finally, we discovered that exogenous expression of the diploid/dikaryon-specific transcription factor Sxi2a could then promote complete sexual development in crosses between α and αa strains. These data reveal that cell identity in C. neoformans is controlled fully by three kinds of MAT–encoded proteins: pheromones, pheromone receptors, and homeodomain proteins. Our findings establish the mechanisms for maintenance of distinct cell types and subsequent developmental behaviors in this unusual human fungal pathogen.

Cryptococcus neoformans and Cryptococcus gattii, the Etiologic Agents of Cryptococcosis

“…The haploid cells propagate asexually by budding until the strains of opposite mating type come into contact on substrates conducive for mating. Cells of the MATa strains secrete MFa pheromone and MATa cells secrete MFa pheromone; in response to the mating partner's pheromone, the MATa cells undergo isotropic growth, whereas the MATa cells form conjugation tubes (McClelland et al 2004;Stanton et al 2010;Gyawali and Lin 2013) to enable fusion and produce dikaryotic hyphae with fused clamp connections (Fig. 2E).…”

The cryptic sexual strategies of human fungal pathogens

“…Unlike most basidiomycetes where mating is regulated by a tetrapolar mating locus 86 , the MAT locus of C. neoformans is bipolar and encodes homeodomain proteins, pheromones, and pheromone receptors, which all contribute to cell identity 89–91 . Mating in C. neoformans and C. gattii was reported almost four decades ago and involves fusion of a and α cells to produce dikaryotic filaments (FIG.…”

Sex in Fungi

“…Basidiomycetes have only a factor–like pheromones and pheromone receptors, which are encoded by genes in the MAT locus. In the human pathogenic basidiomycete Cryptococcus neoformans , pheromones and their receptors contribute to both opposite- and same-sex mating (85, 131, 137). We can therefore conclude that pheromone-receptor systems are integral for both heterothallism and homothallism.…”