The Mucorales are the largest and best studied of the order zygomycete fungi . Members of this sequence are sometimes called pin molds . The term mucormycosis is now preferred for infections caused by molds belonging to the order Mucorales.
The order includes: 11 families , 56 genera , and about 300 species . Mucorelian classification has traditionally been based on morphological, developmental and ecological characteristics. Recently, molecular data has shown that some aspects of traditional classification are quite artificial. For example, Mucoraceae are believed to be polyphyletic , as are Thamnidiaceae , Chaetocladiaceae and Radiomycetaceae . Some genera, ( including Mucor , Absidia and Bacchusella ) are polyphyleticseem to be. Today, the traditional system is still largely in use, as further studies are needed to reconcile the morphological and molecular concepts of families and species.
The order includes the following families:
Mucoralean fungi are generally fast-growing, and their elaborate hyphae ( long, filamentous structures ) lack septa (multi-perforated septa are present only in sporangiophores and gametangia ). Hyphae mostly grow within the substrate . Sporangiophores are upright (simple or branched) hyphae, supporting sac-like sporangia filled with asexual sporangiospores. Other structures include merospores, odia and sporangiola.
Many are known for the damage done to stored food like bread. Others can cause mucormycosis , usually in immunosuppressed patients, or in patients already infected with other diseases.
Sporangiospores are asexual , formed through mitospores ( mitosis ), producing sporangia ( thousands of spores ) or sporangioles ( single or few spores ). They are released as a whole sporangiole when mature, or separated from the sporangiophore, by the dissolution of the sporangium wall.
The sporangiospores germinate to form the haploid hyphae of a new mycelium . Asexual reproduction is often frequent.
In heterothallic species, sexual reproduction occurs when opposite mating types (designated + and -) come into close proximity, inducing the formation of specialized hyphae called gametangia . The gametes move towards each other, then fuse, forming a diploid zygote at the point of fusion. The zygote develops a resistant cell wall, leading to the formation of a single-celled zygote , the characteristic that gives this group of fungi its name. Meiosis occurs within the zygospore (see article Phycomyces). Upon germination, a new haploid mycelium or sporangium is formed. Some species are homothallic.
The original report of sex in fungi, dated two centuries ago, was based on observations of the fungus Syzygites megalocarpus ( Mucoromycotina ) (reviewed by Idnurm  ). This species was later used to represent self-fertile species in 1904, when the concept of two major mating strategies for fungi was developed. These strategies are homothallism for self-fertile fungi and heterothallism for self-incompatible, outcrossing fungi.
Most mucorelian 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. Some species cause the human and animal disease zygomycosis, as well as allergies.