Ascospore

In fungi, an ascospore is the sexual spore formed inside an ascus—the sac-like cell that defines the division Ascomycota, the largest and most diverse division of fungi. After two parental nuclei fuse, the ascus undergoes meiosis (halving of genetic material) followed by a mitosis (cell division), ordinarily producing eight genetically distinct haploid spores; most yeasts stop at four ascospores, whereas some moulds carry out extra post-meiotic divisions to yield dozens. Many asci build internal pressure and shoot their spores clear of the calm thin layer of still air enveloping the fruit body, whereas subterranean truffles depend on animals for dispersal.

Development shapes both form and endurance of ascospores. A hook-shaped crozier aligns the paired nuclei; a double-membrane system then parcels each daughter nucleus, and successive wall layers of β-glucan, chitosan and lineage-specific armour envelop the incipient spores. The finished walls—smooth, ridged, spiny or gelatinous, and coloured from hyaline to jet-black—let certain ascospores survive pasteurisation, 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 resonates far beyond the microscope slide. 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, while aerosol scientists trace melanin-laden ascospores in the nocturnal boundary layer, where they seed 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.