Rhizopus

Class - Zygomycetes; Order - Mucorales; Family - Mucoraceae; Genus - Rhizopus. Rhizopus, popularly called bread mould, is a saprophyte that grows on substrates such as bread, jelly, leather and decaying vegetables and fruit; Rhizopus stolonifer specifically causes leak and soft rot of vegetables. Its vegetative structure is a profusely branched, coenocytic (aseptate, multinucleate) mycelium, made of horizontally-growing aerial hyphae called stolons; the stolons put down branched rhizoids that penetrate the substrate to absorb water and nutrients, while sporangiophores are produced exactly opposite the rhizoids. The cell wall is chitin and chitosan, sitting outside a plasma membrane, with a granular, multinucleate protoplast containing mitochondria, ribosomes, endoplasmic reticulum, and inclusions such as glycogen and oil droplets. Asexual reproduction: under favourable conditions, erect, unicellular, unbranched, multinucleate sporangiophores rise opposite the rhizoids and each bears a single sporangium; the sporangium has a sterile central columella around which spores are produced, and when the sporangial wall breaks, the columella collapses and the spores disperse, germinating on a suitable substrate into new mycelium. Sexual reproduction proceeds by gametangial copulation. Most species are heterothallic (needing two physiologically distinct, though visually identical, strains, called '+' and '-'), while Rhizopus sexualis is unusual in being homothallic. Mycelia of opposite strain each put out a special hypha called a zygophore; the tips of two opposite zygophores swell into progametangia, a septum then cuts off a terminal gametangium plus a suspensor cell on each, and the two gametangia fuse - first plasmogamy, then karyogamy - with many nuclei of opposite strain pairing and fusing to give many diploid nuclei inside a single diploid zygosp …