Zygomycota
| Zygomycota | |
|---|---|
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| Sporangium of a Phycomyces sp. | |
| Scientific classification | |
| Kingdom: | |
| Division: | Zygomycota Moreau 1954 (technically invalid)
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| Class: | mold
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| Orders | |
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Mucoromycotina:
Endogonales | |
Zygomycota, or zygote fungi, are a phylum of fungi. The name of the phylum comes from zygosporangia, where resistant spherical spores are formed during sexual reproduction. Approximately 1060 species of zygomycetes are known. They are mostly terrestrial in habitat, living in soil or on decaying plant or animal material. Some are parasites of plants, insects, and small animals, while others form symbiotic relationships with plants[1]. Zygomycete hyphae may be coenocytic, forming septa only where gametes are formed or to wall off dead hyphae.
Reproduction
In Zygomycetes, they reproduce asexually during favorable conditons. It starts with a walled haploid spore landing on old food. The spore germinates into a coenocytic mycellium. Septal walls form to wall off dead hyphae or gametangia. After feeding for a while, some hyphae grow upward. The end of this hypha bulges and becomes covered with microscopic appendages. The haploid nuclei inside divide by mitosis, using an internal spindle in the nuclear membrane. The result is hundreds of clone haploid nuclei. Each falls into an appendage and is covered with a protective coat. this often happens in multiple hypae, which form sporangia. The spores fly off in a short air current. Sexual reproduction occurs when food runs out. On two hyphae, three nuclei become walled off with septa. This forms two gametangia. They fuse by two stages of sygnamy. the first is plasmogamy; where the cytoplasm of the gametangia fuses. This forms a dikaryon with haploid nuclei in pairs. The Zygosporangium secretes a resistant wall. The dikaryotic zygosporangium can withstand harsh conditions for 2 years. When conditons improve, karyogamy occurs and forms a diploid zygosporangium. It then undergoes meiosis to make 12 haploid nuclei. these divide by mitosis to make many genetically diverse haploid nuclei. A single haploid hypha bore through the wall to become a sporangium. The haploid nuclei become spores. They are dispersed and may land on old food, these new mycelia will reproduce asexually until harsh conditions resume
Some zygomycetes disperse their spores in a more precise manner than simply allowing them to drift aimlessly on air currents. Pilobolus, a fungus which grows on animal dung, bends its sporangiophores towards light with the help of a light sensitive pigment and then "fires" them with an explosive squirt of high-pressure cytoplasm. Sporangia can be launched as far as 2m, placing them far away from the dung and hopefully on vegetation which will be eaten by an herbivore, eventually to be deposited with dung elsewhere. Different mechanisms for forcible spore discharge have evolved among members of the zygomycete order Entomophthorales. Meiosis occurs during gerination of the zygosporagium so the resulting spores or hyphae are haploid.
Phylogeny
The Zygomycota are generally placed near the base of the fungal phylogenetic tree, having diverged from other fungi after chytrids.
Molecular phylogenetics reveal that they form a polyphyletic group and could see a split into several new phyla.[2]
The order Glomales was removed in 2001 and elevated to Division (or phylum) Glomeromycota[3][4] due their lack of zygospore formation, their mycorrhizal habit, and lack of DNA sequence homology.
Evolution of the Conidia
The evolution of sporangiospores typical of zygomycetes to conidia similar to those found in ascomycetes can be modeled by a series of forms seen in zygomycetes. The evolution of the conidia from the sporangiospore is the main defining difference between Zygomycetes and Ascomycetes [5]. Many zygomycetes produce multiple sporangiospores inside a single sporangium. Some have evolved multiple small sporangiola that contain few sporangiospores. In some cases, there may be a few as three spores in each sporangiolum, and a few species have sporangiola which contain just a single spore. ‘‘Choanephora’’, a zygomycete, has a sporangiolum that contains one spore with a sporangium wall that is visible at the base of the sporangium. This structure is similar to a conidium, which has two, fused cell walls, an inner spore wall and an outer sporangium wall.
References
- ^ . Raven, P. H., Evert, R. F., & Eichhorn, S. E. (2005). Fungi. In Biology of plants (pp/ 268-269). New York: W.H. Freeman and Company.
- ^ Hibbett DS, Binder M, Bischoff JF; et al. (2007). "A higher-level phylogenetic classification of the Fungi". Mycol. Res. 111 (Pt 5): 509–47. doi:10.1016/j.mycres.2007.03.004. PMID 17572334.
{{cite journal}}: Explicit use of et al. in:|author=(help); Unknown parameter|month=ignored (help)CS1 maint: multiple names: authors list (link) - ^ "Zygomycota". Retrieved 2009-03-07.
- ^ Schüßler, A., D. Schwarzott and C. Walker. 2001. A new fungal phylum, the Glycomycota: phylogeny and evolution. Mycol. Res. 105: 1413-1421.
- ^ Cain, R. F. (1972, January/February). Evolution of the Fungi. Mycologia, 64(1), 1-14.
External links
- Zygomycota at the Tree of Life Web Project
- Zygomycetes.org
- List of all Zygomycetes species from Zygomycetes database by PM Kirk in Catalogue of Life 2008
- Mucorales at the U.S. National Library of Medicine Medical Subject Headings (MeSH)