Ascospore

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.

Ascospore biology also matters outside microscopy. Airborne showers initiate apple scab epidemics and other plant diseases, heat-resistant spores of Talaromyces and Paecilomyces spoil shelf-stable fruit products, and geneticists dissect ordered tetrads of Saccharomyces to map genes and breed new brewing strains. Industry banks hardy spores of Aspergillus and Penicillium to seed cheese-ripening and enzyme production, and melanin-rich ascospores are also tracked in the night-time lower atmosphere, where they can act as nuclei for cloud droplets and even ice at −5 °C (23 °F). Because of their combined functions in evolution, ecology, agriculture, biotechnology and atmospheric processes, ascospores are a key means by which many fungi persist and spread.