Zygomycota

Zygomycota, often called zygote fungi, was once considered a single division of the kingdom Fungi. Scientists now classify these organisms into two separate phyla: Mucoromycota and Zoopagomycota. There are approximately 1,060 known species within these groups. Most of these fungi are terrestrial, meaning they live on land. They are commonly found in soil or growing on decaying plant and animal matter.

Some species act as parasites on insects or small animals. Others form symbiotic relationships, which are helpful connections, with plants.

These fungi reproduce using spores, which are structures used for dispersal and growth. They can produce spores through both asexual and sexual means. Asexual reproduction involves mitospores, also called sporangiospores. These are produced in specialized containers called mitosporangia. These containers can hold anywhere from a few to several thousand spores. The mitosporangia are held up by stalks called mitosporangiophores. These stalks often show negative gravitropism, growing away from gravity, and positive phototropism, growing toward light.

This helps the spores spread effectively. In some species, the spore walls contain sporopollenin, a substance made from beta-carotene that resists chemical and biological breakdown.

Another type of asexual spore is the chlamydospore. Unlike sporangiospores, chlamydospores are used for the persistence of the mycelium, which is the body of the fungus. They are released when the mycelium degrades. These spores have thick, pigmented cell walls and do not have a mechanism for dispersal. Zygomycetes can also enter a dormant state before germination. This dormancy can last from a few hours to many years. It can be exogenous, controlled by external factors like temperature, or endogenous, which depends on the internal metabolic features of the spore itself.

Sexual reproduction in zygomycetes is a complex process involving zygophores. These are aerial hyphae that act as sex organs. They come in two mating types, designated as (+) and (-). The opposite types grow toward each other using volatile pheromones, specifically trisporic acid.

When the zygophores make contact, their walls adhere and flatten at a site called the fusion septum. The tips of the zygophores swell into structures called progametangia. A septum then develops to separate these gametangia from the base. The zygophore then becomes a suspensor. As the fusion septum dissolves, the cell wall thickens and develops warty structures. These warts eventually cover the entire cell to form a mature, resistant zygospore.

The study of these fungi has a rich history. The American mycologist Albert Francis Blakeslee discovered that some Mucorales species are heterothallic. This means they are self-sterile and require two different strains, (+) and (-), to initiate sexual activity. Hans Burgeff of the University of Goettingen later showed that this interaction happens because of low molecular weight substances diffusing through the environment. This was the first demonstration of sex hormone activity in any fungus. In 1964, trisporic acid was discovered as a metabolite in Blakeslea trispora. It was later identified as the hormone that triggers zygophore production in Mucor mucedo.

The cell walls of zygomycetes have a unique chemical structure. While most fungi use chitin, zygomycetes synthesize chitosan. Chitosan is a deacetylated homopolymer of chitin. To build these walls, specialized vesicles called chitosomes transport precursors and the enzyme chitin synthetase to the cell membrane. Through a process called exocytosis, the enzyme catalyzes bond formations using a sugar substrate. An enzyme called chitin deacetylase then cleaves the group to form chitosan polymer chains. These chains become microfibrils embedded in a matrix of proteins, lipids, and glucans.

Zygomycetes also interact deeply with light and gravity. Many species use phototropism to direct their growth. For example, the sporangiophores of Phycomyces grow toward light to help disperse spores. Some species, like Pilobolus crystallinus, use light to trigger their dispersal mechanisms. Light can also regulate metabolic pathways, such as the synthesis of beta-carotene. In contrast, light can sometimes inhibit sexual development in certain species like Phycomyces. Additionally, these fungi exhibit gravitropism, which is a growth movement in response to gravity.

This allows them to orient themselves correctly in their environment.