Mucorales

The Mucorales is the largest and best studied order of Zygomycete fungi.

Systematics
The order includes 12-13 families (see the taxbox->), 56 genera and approximately 300 species. Mucoralean classification has traditionally been based on morphological, developmental, and ecological characters. Recently, molecular data have revealed that some aspects of traditional classification are quite artificial. For example, the Mucoraceae is believed to be polyphyletic, as are the Thamnidiaceae, Chaetocladiaceae and Radiomycetaceae. Some of the genera, (including Mucor, Absidia and Backusella) appear to be polyphyletic. Today, the traditional system is still largely in use, as further studies are needed to reconcile morphological and molecular concepts of families and genera.

Characteristics
Mucoralean fungi are typically fast-growing, and their wide hyphae lack septa (multiperforate septa are present only in sporangiophores and gametangia). The hyphae grow mostly within the substrate. Sporangiophores are upright (simple or ramified) hyphae that support sac-like sporangia filled with asexual sporangiospores.

Life cycle
The sporangiospores are asexual mitospores (formed via mitosis) that are produced inside sporangia (thousands of spores) or sporangioles (single or few spores). They are released when mature by the disintegration of the sporangium wall, or as a whole sporangiole that separates from the sporangiophore.

The sporangiospores germinate to form the haploid hyphae of a new mycelium. Asexual reproduction often occurs continuously.

Sexual reproduction occurs when opposite mating types (designated + and -) come into close proximity, inducing the formation of specialized hyphae called gametangia. The gametangia grow toward each other, then fuse, forming a diploid zygote at the point of fusion. The zygote develops a resistant cell wall, forming a single-celled zygospore, the characteristic that gives its name to this group of fungi. Meiosis occurs within the zygospore, and one of the resulting recombinant nuclei survives. Upon germination, a new haploid mycelium or sporangium is formed. Some species are homothallic.

Ecology
Most Mucoralean species are saprotrophic, and grow on organic substrates (such as fruit, soil, and dung). Some species are parasites or pathogens of animals, plants and fungi. A few species cause human and animal disease zygomycosis, as well as allergic reactions.