Wednesday, April 27, 2005

463: Sexual reproduction between partners of the same mating type in Cryptococcus neoformans : Nature

Sexual reproduction between partners of the same mating type in Cryptococcus neoformans : Nature:

Although this laboratory-defined sexual cycle of C. neoformans has been known for three decades, the environmental and clinical predominance of alpha strains has posed a conundrum3. If most of the population is limited to one mating type, how would a sexual cycle occur in nature? Haploid C. neoformans alpha strains can also undergo a developmental transition involving filamentation and sporulation, known as haploid or monokaryotic fruiting4. This process resembles mating but was assumed to be mitotic and asexual. We show here that fruiting in fact represents a form of sexual reproduction between strains of the same mating type.

Fruiting and mating are both stimulated by similar environmental conditions (nitrogen starvation, desiccation, darkness and pheromones4, 5, 7), and both involve hyphal growth and the production of basidia and spores. However, the two pathways do have distinguishing features; hyphal cells produced during fruiting are mononucleate, with unfused clamp connections, whereas mating hyphal cells are dikaryotic, linked by fused clamps.

Compared with mating, fruiting is inefficient, requires prolonged incubation, and occurs in a stochastic manner at isolated points at the periphery of a growth patch, suggesting that a rate-limiting step might restrict entry into this developmental pathway. The observation that diploid alpha/alpha strains undergo more rapid and robust fruiting compared with isogenic haploid alpha strains suggested that diploidization might normally occur during fruiting. Because isolating hyphal cells to determine ploidy is technically challenging, we made use of the fact that hyphae bud to produce vegetative yeast cells (termed blastospores)4, 5, which are readily isolated by micromanipulation and grow as budding yeast cells (Fig. 1). Blastospores are uninucleate, and their DNA content presumably reflects the nuclear content of the hyphae from which they are derived (by mitosis).
Evolutionary reasoning lead to this research, because the α mating type is too common, and fruiting would impose an evolutionary cost.