Mucoromycotina is a subphylum of uncertain placement in Fungi. It was considered part of the phylum Zygomycota, but recent phylogenetic studies have shown that it was polyphyletic and thus split into several groups, it is now thought to be a paraphyletic grouping. Mucoromycotina is currently composed of 3 orders, 61 genera, and 325 species. Some common characteristics seen throughout the species include: development of coenocytic mycelium, saprotrophic lifestyles, and filamentous. With the treatment of Tedersoo et al. 2018, Mucoromycotina is the only subphylum under Mucoromycota. It includes a diverse group of various molds, including the common bread molds Mucor and Rhizopus. The other treatment of Mucoromycota is equivalent to current Mucoromyceta.
History Zygomycete fungi were originally only ascribed to the phylum Zygomycota. Such classifications were based on physiological characteristics with little genetic support. A genetic study of Zygomycete fungi performed in 2016 showed that further classification of the group was possible, thus splitting it into Zoopagomycota, Entomophthoromycota, Kickxellomycotina, and Mucoromycotina. The study put these groups as being sister to Dikarya, but without further research, their exact locations in Fungi remain unknown. Many of the questions regarding these groups stem from the difficulty of collecting and growing them in culture, so the current groupings are based on the few that have been successfully collected and which could undergo genomic testing with a certain level of accuracy.
Taxonomy The exact placement of Mucoromycotina is currently unknown. It currently resides in the subphyla incertae sedis, alongside Zoopagomycota, Entomophthoromycota, and Kickxellomycotina, whose’ placements are also currently unknown. These groups originally comprised Zygomycota alongside others that were assigned to Glomeromycota, which was elevated to phylum in 2001. These groups are sister to Dikarya, which contains Ascomycota and Basidiomycota. Studies have currently divided Mucoromycotina into 3 orders: Endognales, Mucorales, and Mortierellales. All three orders contain species that are saprotrophic, with others forming relationships with other organisms. There are still many questions regarding Mucoromycotina and the organisms that compose it, owing to limited collected samples.
Orders
Endogonales
This order currently contains 2 families, (Endogonaceae and Densosporaceae) 7 genera, and 40 species. Not much is known about this order, other than readily noticeable characteristics. They produce subterranean sporocarps, which are ingested by small mammals attracted by the fetid odor they produce. Cultured specimens have shown that they produce coenocytic mycelium, and can be saprotrophic or mycorrhizal. This order was first described in 1931 by Jacz. & P.A.Jacz., after being monographed in 1922 by Thaxter.
Mucorales
Often referred to as pin molds, members of this order produce sporangia held up on hyphae, called sporangiophores. There are currently 13 families in this order, divided into 56 genera, and approximately 300 species. They can be parasitic or saprotrophic in nature and reproduce asexually. Much is known about this order since some of the species cause damage to stored food, with several others causing mycosis in immune compromised individuals. The order was proposed in 1878 by van Tieghem, as the examined samples did not fit in with what was Entomophthorales at the time.
Ecology The species described in this subphylum have evolved 3 main lifestyles: saprotrophic, mycorrhizal, or parasitic. Saprotrophic species are involved in decomposition of organic matter, mycorrhizal species form symbiotic relationships with plants, and parasitic species form harmful symbiotic relationships with other organisms.
Saprotrophs Saprotrophs breakdown decomposing matter into different components: proteins into amino acids, lipids into fatty acids and glycerol, and starches into disaccharides. The species responsible usually require excess water, oxygen, pH less than 7, and low temperatures.
Parasitism Parasitic species seen in Mucorales cause infections in crops and immune compromised animals. A common infection of plants by some species in Mucorales is referred to as crown rot or stem rot, common symptoms are: rotting near the soil line, rotting on one side or on lateral branches. Treatment is difficult if not caught in its early stages, and usually results in the death of the plant. Crown rot is seen in cereal plants (wheat, barley), with experiments from 2015 showing crop losses at 0.01 t/ha per unit increase in crown rot index or more. In addition to cereal plants, crown rot is seen in strawberries and other such low growing plants.
Mycorrhizal Mycorrhizal, literally “fungus-root”, interactions are symbioses between fungi and plants. Such interactions are based on nutrient acquisition and sharing, the fungi increases the range over which nutrients are gathered and the plant provides materials that the fungi cannot produce. There are two main types of interactions: arbuscular endomycorrhizal, and ectomycorrhizal. Arbuscular endomycorrhizal interactions are when the fungi is allowed to enter the plant, and inhabit special cells. The fungi produce structures that look like trees, called “arbuscules,” inside these cells. Mucoromycotina within the Planticonsortiaceae form arbuscular mycorrhizae (M-AMF) which have similar ecology and morphology to mycorrhizae formed by Glomeromycota. Ectomycorrhizal interactions are similar symbioses, however the fungi are not allowed into any plant cells, though they may grow between them.
Plant-microbe interactions
Endogonales A new genus proposed in 2017, Jimgerdemannia, contains species with an ectomycorrhizal trophic mode. Further research is needed to understand these species. Several studies have observed fossils of some potential members forming mycorrhizal interactions with ancient plants. The genus Endogone is important in nutrient-deficient soils, such as sand dunes. The presence of species in this genus stabilizes the soil and provides some assistance to dune plants. Planticonsortiacae form ‘fine root endophyte’ (FRE) arbuscular mycorrhizae with higher plants. These are often termed Mucoromycotina arbuscular mycorrhizal fungi (M-AMF) since other fungi from the Mucoromycotina may have the potential to form arbuscular mycorrhizae within plant roots.
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