Ascospores are the characteristic spores of the largest group of fungi, the Ascomycota or ascomycetes. They are meiospores and are formed in the developing ascus as a result of nuclear fusion immediately followed by meiosis.
The four haploid daughter nuclei then divide mitotically to give eight haploid nuclei around which the ascospores are cut out. In most ascomycetes, the eight ascospores are contained within a cylindrical ascus, from which they are squirted out together with the ascus sap when the tip of the turgid ascus breaks down and the elastic ascus walls contract.
The distance of discharge maybe 1 cm or more. In some cases, for example, the Plectomycetes and in ascomycetes with subterranean fruit bodies, such as the false truffles (Elaphomyces spp.) and truffles (Tuber spp. and their allies), ascospore release is non-violent and their asci are not cylindrical but globose.
Ascospores vary greatly in size, shape, and color. In size, the range is from about 4-5 x 1 μm in small-spored forms such as the minute cup fungus Dasyscyphus, to 130 x 45 μm in the lichen Pertusaria pertusa.
The shape of ascospores varies from globose to oval, elliptical, lemon-shaped, sausage-shaped, cylindrical, or needle-shaped. Ascospores are often asymmetric in a form with a wider, blunter, anterior part and a narrower, more tapering posterior.
This shape increases their acceleration as they are squeezed out through the opening of the ascus. Ascospores may be uninucleate or multinucleate, unicellular, or multicellular, divided up by transverse or by transverse and longitudinal septa.
In some genera, e.g. Hypocrea or Cordyceps, the multicellular ascospores may break up into part-spores within the ascus prior to discharge.
The ascospore wall may be thin or thick, hyaline or colored, smooth or rough, sometimes cast into reticulate folds or ornamented by ridges, and it may have a mucilaginous outer layer which is sometimes extended to form simple or branched appendages, especially in marine ascomycetes where they aid buoyancy and attachment.
In many cases, ascospores are resting structures that survive adverse conditions. They may have extensive food reserves in the form of lipids and sugars such as trehalose. Because the formation of ascospores involves meiosis, they are important not only as a means of dispersal and survival but also in genetic recombination. It is obvious that there is no such thing as a typical ascospore.
Neurospora tetrasperma will serve as an example of an ascospore whose structure has been extensively studied. This fungus is somewhat unusual in that it has four-spored asci and the ascospores are binucleate.
The spores are black, thick-walled, and shaped rather like a rugby football, but with flattened ends. The name Neurospora refers to the ribbed spores because the dark outer wall is made up of longitudinal raised ribs, separated by interrupted grooves.
Within the cytoplasm of the spore are the two nuclei, fragments of endoplasmic reticulum, swollen mitochondria, and vacuoles, bounded by single-unit membranes. The wall surrounding the protoplast is composed of several layers.
The innermost layer is the endospore, outside of which is the epispore. The ribbed layer is termed the perispore. Between the ribs are lighter intercostal veins containing material that is chemically distinct from the ribs.
This material is continuous over the whole surface of the spore, giving it a relatively smooth surface. The spore germinates by the extrusion of germ tubes from a preexisting germ pore, a thin area in the epispore at either end of the spore.
In many ascomycetes a trigger is required for germination, e.g. heat shock in Neurospora or a chemical stimulus, for example in ascomycetes which grow and fruit on the dung of herbivorous mammals and whose spores are subjected to digestive treatment.