Nonmotile sexual and asexual spores—microscopic in size—are the common means of reproduction and the primary agents of fungal dispersal. They are readily carried in air or attached to the bodies of insects and other animals and are not resistant structures like bacterial endospores. Although they can withstand desiccation, they are killed by heat. Sexual spores often require a period of dormancy after they are formed, but asexual spores usually germinate and produce new hyphae whenever and wherever moisture is available.
Asexual spores are produced in special hyphae called sporangia in the zygomycetes and conidia in the ascomycetes and basidiomycetes. Unlike many organisms that produce embryos, the fungal spores form hyphae directly with no immature or embryonic stage between spore and adult.
Among fungi, there are no female and male individuals, and no eggs and sperm. Physiological differences among the hyphae do exist, however, and result in different mating types; only compatible strains fuse. In the zygomycetes the strains are designated simply as (+) and (−). Haploid ( n) gametes are produced by mitotic division from haploid ( n) parent nuclei in specialized hyphae called gametangia.
In the ascomycetes and basidiomycetes, sexual reproduction starts with hyphae from two mating strains fusing, but the nuclei remain independent within the merged cytoplasm. The name for this process is plasmogamy, and the cells with the two genetically distinct haploid nuclei are called dikaryons. In genetic shorthand dikaryotic cells are n + n rather than the 2 n of diploid cells. At some point, the nuclei combine, mixing the DNA from the two separate mating types. This type of fertilization is called karyogamy, the union of nuclei following plasmogamy. Mycologists frequently use the term syngamy for the process of fertilization; both syngamy and fertilization, however, mean the same thing: the union of two haploid gametes to form a diploid zygote. In most fungi, karyogamy is followed almost immediately by a reduction division (meiosis) that restores the haploid chromosome number to the resultant spores and the new hyphae that are produced when the spores germinate.
Both meiosis and mitosis in the fungi are different from nuclear division in most other organisms. Before the nuclear material is divided in plants and vertebrates, the nuclear membranes (nuclear envelopes) disintegrate, and the DNA condenses into discrete chromosomes that divide and move into new cells. In the fungi, the nuclear membranes do not disintegrate, and in many taxa no discrete chromosomes appear nor do centrioles develop. In some fungi, spindles form outside of the nucleus and move into it, while in others a spindle apparatus forms within the nucleus. Lacking the elaborate apparatus of more advanced organisms, the nuclei of fungi simply elongate, constrict near the middle, and pinch or tear apart into two daughter nuclei.
The predominant phase in the life cycle of fungi is haploid, the zygote is the only diploid cell in the entire cycle. This is called a zygotic life cycle and is the type prevalent in algae and some protists, in addition to the fungi.