General fungus structure
Fungi are a kingdom of eukaryotes, typically either single cellular (yeasts) or made up of multicellular filaments (hyphae). Their cell walls are reinforced not with cellulose as in plants, but chitin, a strong but flexible molecule also found in arthropod exoskeletons. In multicellular fungi, the hyphae are interwoven to create a mass called a mycelium. Fungi are heterotrophic, and feed by decomposing and absorbing dead material, or by living parasitically or mutualistically with other organisms (some fungi have hyphae specialized enough to extract nutrients from plant cells, or to capture, penetrate, and digest the inner tissues of animals such as nematodes[1, p638]). Being made of so many thin bands, myceliums are structures with massive amounts of surface area in a small volume to maximize surface area for absorption. Fungi expend their resources on extending their hyphae rapidly, radiating into new food sources and increasing surface area further[1, p637].
Reproduction
Depending on the species, fungi are able to produce spores sexually or asexually[1, p640]. Since hyphae are usually haploid,[2] in asexual reproduction spores are simply formed from a hypha by mitosis[2]. In particular species there may be specialized structures to bear the spores in asexual reproduction, such as sporangia in zygomycetes or conidiophores in ascomycetes[1, p643, p645]. In sexual reproduction, two hypha will release pheremones and determine if they are a match (different mating types). If they are, they'll grow towards the source of the pheremones and meet, and the two touching cells can fuse cytoplasms (plasmogamy). This results in a dikaryotic hypha growing, which may remain with unfused nuclei for extended periods of time[1, p639][2]. Eventually the two haploid nuclei fuse (karyogamy), forming a zygote and then immediately undergoing meiosis and producing spores. The haploid spores then grow into new masses of hyphae, restarting the life cycle.
|
Diveristy
Fungi are classified into 5 groups:
Chytrids are the earliest diverging group, and are the only group of fungi to retain the flagellal element of an ancestral opisthokont. Both spherical single-cellular and hyphae-forming multicellular species exist. They possibly do not form a clade.
Zygomycetes can grow into foods (bread molds). Others live symbiotically with living organisms.
Glomeromycetes form symbiotic relationships with approximately 90% of all plants roots, gaining carbohydrates from the plants photosynthesis, and lending their higher absorpative ability for water and minerals[1, p644].
Ascomycetes are the most diverse division, encompassing 65,000 species. They bear sexual spores from sac-like structures in dikaryotic fruiting bodies, called ascocarps[1, p644-645], and release spores asexually from conidiophores (pictured).
Basidiomycota are also a diverse group, with about 30,000 species. They are familiarly seen by their unusually long-lived dikaryotic fruiting bodies: mushrooms. They are important decomposers of wood and other dead plant material[1, p646].
|
Origin & Evolution
There is evidence fungi, along with animals, diverge from an aquatic single-celled posteriorly-flagellated ancestor, leading to the clade containing all of them to be called opisthokonta (rear + pole)[1]. Chytrids, the earliest diverging division of fungi (see first entry) retain flagellated spores[1, p641], however terrestrial fungi have lost all traces of flagellation[3].
References
[1]Campbell & Reece, Biology 8th Editio
[2]http://www.rci.rutgers.edu/~uzwiak/GBSummer13/GB101Summer13Lect/GB101Summer_Lect3.htm
[3]Sekimoto, S., Rochon, D., Long, J., Dee, J., & Berbee, M. (2011). A multigene phylogeny of Olpidium and its implications for early fungal evolution. BMC Evolutionary Biology, 11331. doi:10.1186/1471-2148-11-331
[2]http://www.rci.rutgers.edu/~uzwiak/GBSummer13/GB101Summer13Lect/GB101Summer_Lect3.htm
[3]Sekimoto, S., Rochon, D., Long, J., Dee, J., & Berbee, M. (2011). A multigene phylogeny of Olpidium and its implications for early fungal evolution. BMC Evolutionary Biology, 11331. doi:10.1186/1471-2148-11-331