The fungi covered so far in this chapter have all been in the subdivision Agaricomycotina. These fungi follow some version of the life cycle in Figure \(\PageIndex{1}\) below.
Figure \(\PageIndex{1}\): Generalized life cycle of the mushroom-forming basidiomycetes (Agaricomycotina). On the right side of the diagram, haploid basidiospores of two different mating types produced (dark blue and red). The life cycle follows one of each of these basidiospores as they each germinate and grow by mitosis into a monokaryon (a haploid thallus). Unlike ascomycetes, basidiomycetes do not spend long in the monokaryotic state. The two monokaryons fuse (P!=plasmogamy) via a conjugation bridge and a dikaryon is formed. This new dikaryotic (n+n) thallus has clamp connections due to the way nuclei divide and are kept in the dikaryotic state (one dark blue nucleus and one red nucleus between each septum). Sexual reproduction occurs with the formation of a basidium. Within the basidium, the two different nuclei fuse (K!=karyogamy), forming a diploid nucleus. This nucleus instantly divides by meiosis (M!), forming 4 haploid nuclei (two dark blue, two red). Small projections called sterigmata emerge from the basidium and each nucleus is moved through a sterigma, extending the cell wall around each to form basidiospores. M. Piepenbring, CC BY-SA 3.0, via Wikimedia Commons.
Pucciniomycotina
Rust fungi are in the Pucciniomycotina. These fungi have complex life cycles that can have a variety of different spore stages that require different plant hosts. A macrocyclic life cycle (containing all of the five spore stages) of Puccinia gramminis is represented in Figure \(\PageIndex{2}\).
Figure \(\PageIndex{3}\): Generalized life cycle of the rusts (Pucciniomycotina). This life cycle is macrocyclic (contains all five stages) and heteroecious (involves two different host plants). In the upper left corner of the diagram, haploid basidiospores of two different mating types produced (dark blue and red). The life cycle follows one of these basidiospores (dark blue) as it lands on a new host plant that has a vase-like structure called a spermatogonium. The spermatogonium has receptive hyphae with red nuclei. The basidiospore fuses with the receptive hyphae, forming a dikaryon (n+n). The dikaryotic hyphae then grow to the underside of the leaf and form an aecidium where dikaryotic aecidiospores are produced. These aecidiospores are released and infect a second, different species of host plant where the fungus forms a pustule in the plant tissue called a uredinium. Within the uredinium, dikaryotic urediniospores are produced and can cause new infections in the same host plant species. Later in the season (end of summer, in this diagram), nuclei fuse (K!=karyogamy) and a telium is formed. Diploid teliospores (resting spores) are released. The teliospores germinate to produce basidia, meiosis (M!) occurs, and haploid basidiospores are formed. M. Piepenbring, CC BY-SA 3.0, via Wikimedia Commons.
Ustilaginomycotina
Smut fungi are in the Ustilaginomycotina. These fungi often have a saptrotrophic phase where they form yeasts and a biotrophic phase during which they act as parasites. The life cycle of the corn smut fungus Ustilago maydis is shown in Figure \(\PageIndex{3}\).
Figure \(\PageIndex{3}\): Generalized life cycle of the smuts (Ustilaginomycotina). In the upper left corner of the diagram, haploid basidiospores of two different mating types produced (dark blue and red). These basidiospores begin a saprotrophic yeast phase (sporidia), dividing by mitosis. When the fungi switch to a biotrophic (parasitic) feeding strategy, two different mating types of sporidia will begin to grow conjugation tubes, fusing cytoplasm (P!=plasmogamy) and growing together as a dikaryon. The fungus forms an infection sorus within its host, producing sporogenous hyphae. These fragment into dikaryotic cells within a gelatinous matrix. The two nuclei with each cell fuse (K!=karyogamy) forming a diploid nucleus inside the now teliospore. Teliospores are released from the host plant as a resting stage. The teliospores germinate to produce basidia, meiosis (M!) occurs, and haploid basidiospores are formed. M. Piepenbring, CC BY-SA 3.0, via Wikimedia Commons.
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