Fungi

1. Fungi • Fungi are non-motile (they don’t move) heterotrophs (they get food and energy from other organisms). • The defining characteristic of fungi is their mode of getting food: “extracellular digestion and absorption”. They secrete digestive enzymes into the environment, then absorb the nutrients released by those enzymes. • Most fungi are decomposers: they live on decaying organisms. • Some fungi are parasites: they extract food from living organisms.

2. Other Fungal Characteristics • Fungi are haploid for most of their life cycle, becoming diploid only for the purpose of meiosis. • Fungi are non-vascular: they have no internal pipes to distribute nutrients • Fungi have a cell wall, like plants do, but it is composed of chitin, the same material that covers insects. • Fungi reproduce by means of spores, which can be sexual (the products of meiosis) or asexual (the products of mitosis). Each group of fungi has a unique set of spores. Asexual reproduction is more common than sexual. • Classically, the fungi were classified into the yeasts (which are unicellular) and the molds (which have a mat of fibers called hyphae as the main body of the organism). • Recent DNA-based studies show that fungi are more similar to animals than to plants.

3. Major Groups of Fungi • Within the past few years, several groups that were considered fungi have been re-classified into the protists. A few were put in with the Eubacteria—they were prokaryotes that superficially resembled simple fungi. • The basic groups: zygomycetes, ascomycetes (sac fungi), and basidomycetes (club fungi). “mycete” means fungus. There is also a group of miscellaneous types called “imperfect fungi”.

4. Basidomycetes: Club Fungi • Mushrooms are the most common club fungi. Others include rusts and smuts that harm crop plants. • The visible mushroom is merely a fruiting body. The bulk of the organism is underground, a mat of hyphae (strands) called a mycellium that can be quite large. One example in Oregon covers 2200 acres (3 ½ square miles), to a depth of 3 feet, and it is at least 2400 years old.

5. Basidomycete Life Cycle • The hyphae in the mycellium are haploid. If the hyphae of two different organisms that have different mating types meet, the cells fuse into a dikaryon: hyphae with cells having 2 different nuclei in each cell. The nuclei don’t fuse: the new dikaryon hyphae multiply into a large mycelium. • When the dikaryotic mycelium finds proper conditions, it gives rise to a fruiting body: the mushroom. In the gills of the mushroom, the two different nuclei fuse together in the spore-forming cells. The cells are now diploid, and they undergo meiosis. • After meiosis, the haploid meiotic products develop into spores (basidospores) growing on small stalks, which get dispersed by the wind. • The spores land on soil, then germinate into haploid hyphae once again. • Mating types: fungi often have multiple “sexes’, which are mating types that are compatible with each other, that can form a dikaryon together. The current record is about 28,000 mating types.

6. Mushroom Life Cycle Haploid hyphae meet and form a dikaryon: hyphal cells with 2 nuclei. After growth, the dikaryon sends up mushrooms as fruiting bodies. The 2 nuclei fuse in some of the mushroom cells, creating a diploid nucleus. The diploid nuclei undergo meiosis, resulting in new haploid cells. The haploid meiotic products turn into spores, which blow away and germinate into new hyphae.

7. Zygomycetes • Zygomycetes form a diploid spore (think “zygote”) that undergoes meiosis after germination. Zygomycetes also have asexual, haploid spores. • Bread mold is a common zygomycete. • Another important zygomycete group is the mycchorrhyzae, fungi that infect the roots of most plants. These fungi have a symbiotic relationship with the plants: the fungi gather nutrients form the soil as an extension of the roots, and the plant supplies nutrients from photosynthesis.

8. Ascomycetes • Ascomycetes are sac fungi: they produce spores in sacs. • Truffles and morels are good examples of ascomycetes: they taste good! • Penicillium, the mold that gave penicillin, the first antibiotic, is an ascomycete. Pennicillium also gives flavor to certain cheeses. • Sac fungi also include some important single celled yeasts. Saccharomyces cerevesiae is used to make bread rise and also to ferment beer and wine. Candida albicans produces the common human yeast infections.