Zygomycota
Zygomycota, or zygote fungi, is a former division or phylum of the kingdom Fungi. The members are now part of two phyla: the Mucoromycota and Zoopagomycota. 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. Zygomycota is no longer recognised as it was not believed to be truly monophyletic.
Etymology
The name Zygomycota refers to the zygosporangia characteristically formed by the members of this clade, in which resistant spherical spores are formed during sexual reproduction. Zygos is Greek for "joining" or "a yoke", referring to the fusion of two hyphal strands which produces these spores, and -mycota is a suffix referring to a division of fungi.Spores
The term "spore" is used to describe a structure related to propagation and dispersal. Zygomycete spores can be formed through both sexual and asexual means. Before germination the spore is in a dormant state. During this period, the metabolic rate is very low and it may last from a few hours to many years. There are two types of dormancy. The exogenous dormancy is controlled by environmental factors such as temperature or nutrient availability. The endogenous or constitutive dormancy depends on characteristics of the spore itself; for example, metabolic features. In this type of dormancy, germination may be prevented even if the environmental conditions favor growth.Mitospores
In zygomycetes, mitospores are formed asexually. They are formed in specialized structures, the mitosporangia that contain few to several thousand of spores, depending on the species. Mitosporangia are carried by specialized hyphae, the mitosporangiophores. These specialized hyphae usually show negative gravitropism and positive phototropism allowing good spore dispersal. The sporangia wall is thin and is easily destroyed by mechanical stimuli, leading to the dispersal of the ripe mitospores. The walls of these spores contain sporopollenin in some species. Sporopollenin is formed out of β-carotene and is very resistant to biological and chemical degradation.Zygomycete spores may also be classified in respect to their persistence:
Chlamydospores
are asexual spores different from sporangiospores. The primary function of chlamydospores is the persistence of the mycelium and they are released when the mycelium degrades. Chlamydospores have no mechanism for dispersal. In zygomycetes the formation of chlamydospores is usually intercalar. However, it may also be terminal. In accordance with their function chlamydospores have a thick cell wall and are pigmented.Zygophores
Zygophores are chemotropic aerial hyphae that are the sex organs of zygomycota, except for Phycomyces in which they are not aerial but found in the substratum. They have two different mating types and. The opposite mating types grow towards each other due to volatile pheromones given off by the opposite strand, mainly trisporic acid and its precursors. Once two opposite mating types have made initial contact, they give rise to a zygospore through multiple steps.Once contact between the zygophores has been made, their walls adhere to each other, flatten and then the contact site is referred to as the fusion septum. The tips of the zygophore become distended and form what is called the progametangia. A septum develops by gradual inward extension until it separates the terminal gametangia from the progametangial base. At this point the zygophore is then called the suspensor. Vesicles accumulate at the fusion septum at which time it begins to dissolve. A little before the fusion septum completely dissolves, the primary outer wall begins to thicken. This can be seen as dark patches on the primary wall as the fusion septum dissolves. These dark patches on the wall will eventually develop into warty structures that make up the thickness of the zygospore wall. As the zygospore enlarges, so do the warty structures until there are contiguous around the entire cell. At this point, electron microscopy can no longer penetrate the wall. Eventually the warts push through the primary wall and darken which is likely caused by melanin.
Meiosis usually occurs before zygospore germination and there are a few main types of distinguishable nuclear behavior. Type 1 is when the nuclei fuse quickly, within a few days, resulting in mature zygospore having haploid nuclei. Type 2 is when some nuclei do not pair and degenerate instead, meiosis is delayed until germination. Type 3 is when haploid nuclei continue to divide mitotically and then some associate into groups and some do not. This results in diploid and haploid nuclei being found in the germ sporangium.
Cell wall
Zygomycetes exhibit a special structure of cell wall. Most fungi have chitin as structural polysaccharide, while zygomycetes synthesize chitosan, the deacetylated homopolymer of chitin. Chitin is built of β-1,4 bonded N-acetyl glucosamine. Fungal hyphae grow at the tip. Therefore, specialized vesicles, the chitosomes, bring precursors of chitin and its synthesizing enzyme, chitin synthetase, to the outside of the membrane by exocytosis. The enzyme on the membrane catalyzes glycosidic bond formations from the nucleotide sugar substrate, uridine diphospho-N-acetyl-D-glucosamine. The nascent polysaccharide chain is then cleaved by the enzyme chitin deacetylase. The enzyme catalyzes the hydrolytic cleavage of the N-acetamido group in chitin. After this the chitosan polymer chain forms micro fibrils. These fibers are embedded in an amorphous matrix consisting of proteins, glucans, mannoproteins, lipids and other compounds.Trisporic acid
Trisporic acid is a C-18 terpenoid compound that is synthesized via β-carotene and retinol pathways in the zygomycetes. It is a pheromone compound responsible for sexual differentiation in those fungal species.History
Trisporic acid was discovered in 1964 as a metabolite that caused enhanced carotene production in Blakeslea trispora. It was later shown to be the hormone that brought about zygophore production in Mucor mucedo. The American mycologist and geneticist Albert Francis Blakeslee discovered that some species of Mucorales were self-sterile, in which interactions of two strains, designated and, are necessary for the initiation of sexual activity. This interaction was found by Hans Burgeff of the University of Goettingen to be due to the exchange of low molecular weight substances that diffused through the substratum and atmosphere. This work constituted the first demonstration of sex hormone activity in any fungus. The elucidation of the hormonal control of sexual interaction in the Mucorales extends over 60 years and involved mycologists and biochemists from Germany, Italy, the Netherlands, the UK and the USA.Functions of trisporic acid in Mucorales
Recognition of compatible sexual partners in zygomycota is based on a cooperative biosynthesis pathway of trisporic acid. Early trisporoid derivatives and trisporic acid induce swelling of two potential hyphae, hence called zygophores, and a chemical gradient of these inducer molecules results in a growth towards each other. These progametangia come in contact with each other and build a strong connection. In the next stage, septae are established to limit the developing zygospore from the vegetative mycelium and in this way the zygophores become suspensor hyphae and gametangia are formed. After dissolving of the fusion wall, cytoplasm and a high number of nuclei from both gametangia are mixed. A selectional process results in a reduction of nuclei and meiosis takes place. Several cell wall modifications, as well as incorporation of sporopollenin take place resulting in a mature zygospore.Trisporic acid, as the endpoint of this recognition pathway, can solely be produced in presence of both compatible partners, which enzymatically produce trisporoid precursors to be further utilized by the potential sexual partner. Species specificity of these reactions is among others obtained by spatial segregation, physicochemical features of derivatives, chemical modifications of trisporoids and transcriptional/posttranscriptional regulation.
Parasexualism
Trisporoids are also used in the mediation of the recognition between parasite and host. An example is the host-parasite interaction of a parasexual nature observed between Parasitella parasitica, a facultative mycoparasite of zygomycetes, and Absidia glauca. This interaction is an example for biotrophic fusion parasitism, because genetic information is transferred into the host. Many morphological similarities in comparison to zygospore formation are seen, but the mature spore is called a sikyospore and is parasitic. During this process, gall-like structures are produced by the host Absidia glauca.This, coupled with further evidence, has led to the assumption that trisporoids are not strictly species-specific, but that they might represent the general principle of mating recognition in Mucorales.
Phototropism
Light regulation has been investigated in the zygomycetes Phycomyces blakesleeanus, Mucor circinelloides and Pilobolus crystallinus. For example, in Pilobolus crystallinus light is responsible for the dispersal mechanism and the sporangiophores of Phycomyces blakesleeanus grow towards light. When light, particularly blue light, is involved in the regulation of fungal development, it directs the growth of fungal structures and activates metabolic pathways. For instance, the zygomycota use light as signal to promote vegetative reproduction and growth of aerial hyphae to facilitate spore dispersal.Fungal phototropism has been investigated in detail using the fruiting body, sporangiophore, of Phycomyces as a model. Phycomyces has a complex photoreceptor system. It is able to react to different light intensities and different wavelengths. In contrast to the positive reaction to blue light, there is also a negative reaction to UV light. Reactions to red light were also observed.