A yeast is a stage in the life-cycle of a fungus where it lives in a single-celled state. A fungus in the yeast state reproduces by budding, not by forming spores, although it may undergo sexual conjugation with other yeasts from time to time.
Perhaps it is a misnomer to call the yeast growth habit a stage, as many of these fungi never grow in any other way in the wild. Some of these permanent yeasts can be induced to form a mycelium in the laboratory, but some cannot - - they are truly permanent yeasts. This is very annoying for scientists who want to classify them, as the first thing you know is whether a fungus bears its spores in asci or basidia. But if they never form a mycelium and bear spores, you can never find this out! Most of the yeasts that we've been able to get to fruit have turned out to be ascomycetes, but there are some basidiomycetous yeasts also.
Back to top