#009: Overview of the Fungal Life Cycle

1. Filamentous fungi begin their lives as sexual spores. Each spore has a haploid (only one copy of each chromosome) nucleus, which is designated “n.” When the spore lands in a favorable environment, it germinates and produces a mycelium.
2. There are two ways that this mycelium can grow vegetatively: through growth of the mycelium or through asexual reproduction. The fungus can produce asexual mitospores (n) which are dispersed and grow a new mycelium that is genetically identical to the parent.
3. When this mycelium encounters another fungus with a compatible mating type they fuse together (plasmogamy). However, their nuclei remain separate and the cells become dikaryotic (meaning “two nuclei,” designated “n+n”). This gives rise to a dikaryotic mycelium. In ascomycetes and some other fungi, the dikaryotic mycelium only results in the production of fruiting bodies. In basidiomycetes, the dikaryotic mycelium grows vegetatively and produces fruiting bodies. Some basidiomycetes form clamp connections to make sure that each hyphal cell gets exactly one nucleus from each parent.
4. Before fungal cells undergo meiosis, the nuclei fuse (karyogamy) and create a diploid (two copies of each chromosome) nucleus (designated “2n”). Nuclear fusion is necessary in order for crossing over to occur during meiosis.
5. Fungal cells then undergo meiosis and produce sexual spores (n), which begin the life cycle over again.

Fungi modify this general life cycle in a number of ways. For example, the time spent in each step can be varied. There are also a variety of mechanisms to achieve sexual reproduction. Additionally, some fungi are only known to reproduce asexually (they only carry out the cycle described in step two). Yeast also use this general life cycle, but have adapted it to unicellular growth.

See Further:

http://www.clt.astate.edu/mhuss/intro_to_the_fungi1.htm (see the picture under part II)

http://www.fungionline.org.uk/7sexual/5dikaryon.html (explore the “sexual reproduction” and “asexual reproduction” sections for a more complete picture)