II. Fungal life cycles.

Fungi have diverse life cycles, ranging from very simple to very complex. Mating and sexuality in fungi also take diverse forms. The following seven examples illustrate some of the diversity in life cycles in fungi, beginning with relatively simple life cycles. Please realize that each of the major groups of fungi has a diversity of life cycles beyond those listed here.

1. Unicellular asexual fungi. Example: Candida albicans (ascomycete) is an asexual animal pathogen. Reproduction is by budding of yeast cells.

2. Unicellular sexual fungi. Example: Chytriomyces hyalinus (chytridiomycete) is an aquatic fungus that grows on chitin such as the exoskeletons of aquatic insects. It produces a single diploid cell that immediately undergoes meiosis. Single celled zoospores perpetuate the haploid phase.

3. Filamentous asexual fungi. Example: Fusarium oxysporum (ascomycete) and related taxa include many of the most serious plant pathogens. Reproduction and dispersal is via conidia.

4. Filamentous fungi with sexual and asexual reproduction, but no multicellular fruiting bodies. Example: Rhizopus stolonifer (zygomycete) is a common, fast-growing black mold of bread, strawberries and other foods, which has non-septate hyphae. Asexual spores are produced on stalked sporangia. Sexual reproduction involves the fusion of haploid hyphae, and production of a single diploid cell, the zygospore, which undergoes meiosis on germination to recycle the haploid phase.

5. Filamentous fungi with sexual reproduction and multicellular fruiting bodies. Example: Agaricus bisporus (basidiomycete) is the common button mushroom. Sexually produced spores give rise to primary (haploid) mycelia, which fuse to form the secondary mycelium. Karyogamy (nuclear fusion) is delayed, so the secondary mycelium is said to be dikaryotic, or simply a dikaryon. The dikaryon produces diagnostic clamp connections at the septa. When conditions permit, the dikaryon produces a multicellular fruiting body. The club-shaped meiosporangia are called basidia. Karyogamy occurs in the basidia and is immediately followed by meiosis and spore production. As in the other fungi discussed, there is only a single diploid cell in the life cycle.

6. Filamentous fungi with sexual and asexual reproduction. Example : Peziza vesiculosa (ascomycete) produces cup-shaped multicellular fruiting bodies, where sexual reproduction occurs. The meiosporangia are sac-shaped cells called asci. As in basidiomycetes, karyogamy and meiosis occur in the asci. Haploid ascospores germinate to form the primary mycelia, which can produce microscopic asexual reproductive structures. The asexual form has been given its own name, Oedocephalum. Conidia produced by the Oedocephalum stage can recycle the haploid phase. Fusion of primary mycelia produces a dikaryon, which gives rise to the fruiting body, as in basidiomycetes. In fungi with sexual and asexual phases, the sexual phase is called the teleomorph and the asexual phase is called the anamorph. These are also called meiosporic and mitosporic phases, respectively.

7. Plant pathogens with sexual and asexual reproduction on multiple hosts. Example: Puccinia graminis (barberry-wheat rust; basidiomycete). Sexually produced basidiospores infect barberries. Spermogonia are reproductive structures that are produced on the upper surface of the barberry leaves. Spermogonia produce single celled spermatia and receptive hyphae. Spermatia contact receptive hyphae and fuse to form the dikaryon. The dikaryon produces an asexual reproductive structure, the aecium, on the underside of the barberry leaf. Dikaryotic aeciospores infect wheat. The dikaryotic mycelium on wheat produces reproductive structures called uredinia, which produce asexual, dikaryotic urediniospores that can reinfect wheat. Eventually, the dikaryon on wheat produces reproductive structures called telia. Teliospores are produced asexually and are dikaryotic overwintering structures. In spring, the nuclei in teliospores fuse (karyogamy), produce a stalked basidium, undergo meiosis, and produce basidiospores that infect barberry. In total there are two hosts, four kinds of spore-producing structures, and one diploid cell in the life cycle!

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