Reproductive structures of the Mucorales. Reproductive structures of the Mucorales. (A) Sporangium. Development of asexual sporangiospore in sacks called sporangia is characteristic of the families Mucoraceae andSaksenaea. The aerial mycelium of these organisms terminate in swollen structures that develop into sporangial sacks and columellae. Sporangiospores develop asexually by free-cell cleavage within the sporangial membrane. At maturity, the sporangium of theMucorales becomes deliquescent, releasing the sporangiospores. A related mechanism where sporangiospores are released from the sporangiospore by dissolution of a gelatinous plug is seen forSaksenaea vasiformis. The fungus pictured here demonstrates the sporangium produced by Mucor spp. Bar, 20 μm. (B) Merosporangia. Cylindrical or finger-like projections surround a swollen vesicle in this isolate of Syncephalastrum racemosum. A single row of sporangiospores form inside these tubular merosporangia (arrowhead). Spores may be released as entire merosporangial units or singly as the merosporangial membrane dissolves. This form of reproduction is characteristic of S. racemosum. Bar, 20 μm. (C and D) Sporangiola. Sporangiola or conidia develop singly around a swollen vesicle on stalks called sterigmata. Single celled sporangiola (arrowhead), typically produced by members of the genus Cunninghamella (C), are often echinate and form on short sterigmata. Those produced byCokeromyces recurvatus (D) are multicelled (arrowhead) and occur on long recurving stalks. Bar, 20 μm. (E) Yeast. Several members of the order Mucorales are dimorphic. Yeast forms have been identified in vivo for some Mucor spp. as well asCokeromyces recurvatus (pictured here). Yeast production in vitro often requires increased temperature of incubation, high carbon dioxide tension, or anaerobic culture conditions. Bar, 20 μm. (F and G) Chlamydospores and gemmae. These asexual reproductive structures are derived from the vegetative hyphae of certain species of theMucorales. Chlamydospores may be formed intercalated with the mycelium (endogenous formation) (arrowhead in panel F), while gemmae are separated from the mycelium and often demonstrate yeast-like budding (exogenous formation) (G). Chlamydospores and gemmae are often considered together, since they may be difficult to differentiate and are similarly derived. These are produced by some but not all members of the Mucorales. Their morphology may vary substantially but is not particularly useful for species determination (355). Bar, 40 μm. (H) Zygospores. This is the only form of sexual reproduction employed by the zygomycetes. Zygospores may form within a single isolate without mating (homothallic reproduction) or may require mating with an appropriately oriented mating strain (heterothallic reproduction). Zygospore morphology is often characteristic for an organism when color, size, shape and surface decoration are taken into account. Mating of two isolates to produce mature zygospores provides definitive taxonomic identification of an unknown isolate. The zygospores pictured here are fromCokeromyces recurvatus. Bar, 40 μm. Julie A. Ribes et al. Clin. Microbiol. Rev. 2000; doi:10.1128/CMR.13.2.236