Basidiospores are the sexual spores that characterize a large group of fungi, the Basidiomycota, or basidiomycetes. In comparison with the morphological diversity of ascospores, basidiospores are more uniform.
They also show a smaller size range, from about 3 to 20 μm, which is possibly related to their unique method of discharge. They are normally found in groups of four attached by tapering sterigmata to the cell which bears them, the basidium.
At the time of their discharge, all basidiospores are unicellular, but they may become septate after release in some members of the Heterobasidiomycetes.
In shape, basidiospores are asymmetric and vary from sub-globose, sausage-shaped, fusoid, to almond-shaped (i.e. flattened), and the wall may be smooth or ornamented with spines, ridges, or folds.
The color of basidiospores is important for identification. They may be colorless, white, cream, yellowish, brown, pink, purple, or black. The spore color may be due to pigments in the spore cytoplasm or in the spore wall.
The appearance of pigments in the wall occurs relatively late in spore development. This explains the change of color of the gill of a domestic mushroom (Agaricus) from pink, due to cytoplasmic spore pigments, to dark purplish-brown when mature, due to wall pigments.
Most basidiospores have a flatter adaxial face and a more curved abaxial face. The point of attachment of the spore to the sterigma is the hilum, which persists as a scar at the base of a discharged spore. Close to the hilum is a small projection, the hilar appendix.
This is involved in the unique mechanism of basidiospore discharge, in which a drop of liquid perched on the hilar appendix coalesces with a second blob of liquid on the spore surface, creating a momentum that leads to acceleration of the spore.
The spore is projected for a short distance (usually less than 2mm) from the basidium. Violently projected spores are termed ballistospores (Lat. ballista = a military engine for throwing large stones), but whilst most basidiospores are ballistospores, some are not.
For example, in the Gasteromycetes, which include puffballs, stinkhorns, and their allies, violent spore projection has been lost in the course of evolution from ancestors that possessed it. Likewise, the basidiospores of smut fungi (Ustilaginales,) are not violently discharged. The term statismospore (Lat. Statio = standing still) is sometimes used for a spore that is not forcibly discharged.
The cytoplasm of basidiospores usually contains a single haploid nucleus resulting from meiotic division in the basidium; sometimes a post-meiotic division gives rise to two genetically identical nuclei. The structure of the wall is complex. In Agrocybe acericola there are two layers, a thicker, dark-pigmented, electron-dense outer layer or epispore, and a thinner, electron-transparent inner layer, the endospore.
The cultivated mushroom, Agaricus bisporus, has a three-layered wall making up some 35% of the dry weight of the spore, whereas the wall of the Coprinus cinereus basidiospore comprises six distinct layers.
A histochemical feature of the walls of some basidiospores is that they are amyloid, i.e. they include starch-like material which stains bluish-purple with iodine-containing stains such as Melzer’s reagent. This reaction is used as a taxonomic character.
The amyloid reaction is due to the presence of unbranched, short-chain amylose molecules. It has been suggested that this ‘fungal starch’ may aid dormancy by creating a permeability barrier to oxygen in dry spores. When the amyloid material is dissolved as water becomes available, dormancy is lost and spore germination can proceed.
In some basidiospores, e.g. those of Coprinus cinereus and Agrocybe acericola, the basidiospore has a distinct germ pore at the end opposite to the hilum. In other basidiomycetes, e.g. Oudemansiella mucida, Schizophyllum commune, and Flammulina velutipes, the basidiospores have no specialized pore.
The reserve contents of the spore may vary. In some species, the lipid is the major storage product, and there is an apparent lack of insoluble polysaccharides such as glycogen. In other spores, glycogen predominates.
Where lipid is present, germination may be fuelled by its breakdown and utilization, but where it is absent spores are dependent on external nutrient supplies before germination and further development is possible.
In addition to the usual organelle complement, microbodies are also prominent in basidiospores. These are single membrane-bound organelles often associated with mitochondria and lipid globules; they may function as glyoxisomes containing enzymes involved in the oxidation of lipids.