Zygomycota, or zygote fungi, is a phylum of fungi. The name comes from zygosporangia, where resistant spherical spores are formed during sexual reproduction. Approximately 1060 species 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. Zygomycete hyphae may be coenocytic, forming septa only where gametes are formed or to wall off dead hyphae.
The common example of a zygomycete is black bread mold (Rhizopus stolonifer), a member of the Mucorales. It spreads over the surface of bread and other food sources, sending hyphae inward to absorb nutrients. In its asexual phase it develops bulbous black sporangia at the tips of upright hyphae, each containing hundreds of haploid spores. As in most zygomycetes, asexual reproduction is the most common form of reproduction. Sexual reproduction in Rhizopus stolonifera, as in other zygomycetes, occurs when haploid hyphae of different mating types are in close proximity to each other. Growth of the gametangia commences after gametangia come in contact, and plasmogamy, or the fusion of the cytoplasm, occurs. Karyogamy, which is the fusion of the nuclei, follows closely after. The zygosporangia are then diploid. Zygosporangia are typically thick-walled, highly resilient to environmental hardships, and metabolically inert. When conditions improve, however, they germinate to produce a sporangium or vegetative hyphae. Meiosis occurs during germination of the zygosporagium so the resulting spores or hyphae are haploid. Grows in warm and damp conditions.
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 2 m, 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.
The Zygomycota are generally placed near the base of the fungal phylogenetic tree, having diverged from other fungi after chytrids.
Molecular phylogenetics reveals that they form a polyphyletic group and could see a split into several new phyla.
The order Glomales was removed in 2001 and elevated to Division (or phylum) Glomeromycota due to their lack of zygospore formation, mycorrhizal habit, and lack of DNA sequence homology.
Evolution of 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. 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.
^ . 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.
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