ascospore

thumb|right|upright=1.2|alt=Microscope view of a transparent sac (ascus) holding four oval, golden-brown spores. The spores are covered with a raised, net-like pattern that looks like a mosaic of tiny scales.|Four ornamented ascospores of the Oregon white truffle (Tuber oregonense) inside a single ascus. Each [[ellipsoid spore shows the typical honey-gold colour and ornamentation of polygonal pits bordered by low ridges.]] In fungi, an ascospore is the sexual spore formed inside an ascus, a sac-like cell. Asci define the division Ascomycota, the largest and most diverse division of fungi. After two parental nuclei fuse, the ascus undergoes meiosis (which halves genetic material) followed by a mitosis (a cell division). This typically produces eight genetically distinct haploid ascospores. Many yeasts produce four instead, whereas some moulds produce dozens by adding extra divisions. Many asci build internal pressure and shoot spores past the thin still-air layer that clings to the fruit body. By contrast, subterranean truffles usually rely on animals to spread spores.

Development shapes both the form of ascospores and how well they survive stress. In many fungi, a hook-shaped crozier helps organize the paired nuclei. The ascus then partitions its contents with a double-membrane system, and the young spores build layered walls of β-glucan and chitosan, plus additional protective layers that vary among groups. Mature ascospore walls may be smooth, ridged, spiny, or gelatinous, and range in colour from hyaline to jet-black. In some species these walls allow spores to survive pasteurization, deep-freezing, desiccation and ultraviolet radiation. Dormant spores can lie inert for years until heat shock, seasonal wetting or other cues trigger germ tube emergence. Such structural and developmental traits are mainstays of fungal taxonomy and phylogenetic inference.