Chapter 9: Archiascomycetes

The chapter surveys four major representatives that illustrate the remarkable morphological and ecological variation within this clade. The Taphrinales comprise obligate biotrophic plant parasites that cause significant agricultural diseases through the generation of host tissue distortion and hypertrophy. Taphrina deformans, responsible for peach leaf curl, develops intercellular mycelial networks and subcuticular asci that trigger dramatic leaf deformation through fungal-derived growth regulators, particularly indoleacetic acid and cytokinin compounds, which promote excessive cell proliferation and tissue swelling. Related Taphrina species generate witches' brooms on deciduous trees and pocket plum deformations in stone fruits, while the yeast form called Lalaria enables overwinter survival and saprotrophic persistence on plant surfaces, with disease management relying on dormant-season fungicide applications. The Schizosaccharomycetales encompass fission yeasts, particularly Schizosaccharomyces pombe, which has become indispensable in molecular and cell biological research. Unlike typical budding yeasts, fission yeasts reproduce through binary division, alternating between haploid somatic cells and diploid zygotes that undergo meiosis to produce asci containing four or eight spores. This organism revealed fundamental mechanisms of eukaryotic cell cycle control through identification of the cdc2 gene and its encoded cyclin-dependent kinase, discoveries that established the universal conservation of cell cycle regulation across eukaryotes and earned recognition with the Nobel Prize. Research utilizing S. pombe has also advanced understanding of cytoskeletal organization, polarized growth mechanisms, and septate wall formation. Pneumocystis represents an unusual pathogenic member formerly classified as protistan but now definitively recognized as fungal through molecular phylogenetic analysis. This lung-dwelling pathogen causes opportunistic pneumonia in immunocompromised patients, particularly those with acquired immunodeficiency syndrome, through a life cycle involving both ascus-derived spore forms and trophic cells undergoing binary fission, with unusual biochemical requirements including reliance on exogenous cholesterol rather than ergosterol synthesis, explaining its characteristic resistance patterns to standard antifungal therapeutics.