Asci (s. ascus) are the spore-containing structures of a fungus found in the hymenium. In lichens, the mycobiont produces spores through meiosis, which are housed in an ascus. A single ascus represents a single meiotic event followed by a mitotic event. Therefore each spore within an ascus results from the same mother cell after it undergoes meiosis, making ascospores useful in the study of fungal genetics.