Black bread mold is the common name for a species of fungus called Rhizopus stolonifer. Despite its name, this fungus also attacks substrates other than bread, including cheese and a variety of fruits and vegetables. Rhizopus grows by extending filaments, called hyphae, along the surface of a substrate and by penetrating the substrate with rootlike structures (also made up of hyphae) called rhizoids. Like all fungi, Rhizopus digests its food outside its body and then transports the digested nutrients inside.
Rhizopus is classified in the group Zygomycota, the zygospore fungi . The name for this group comes from the only diploid structure—called the zygosporangium—that exists in the entire life cycles of these organisms. In the accompanying animation we depict the life cycle of a zygospore fungus, which includes both sexual and asexual reproduction.
Fungi have complex life cycles consisting of asexual and sexual reproductive phases. Here we examine the life cycle of the black bread mold, Rhizopus stolonifer. The black speckles that Rhizopus produces on bread are capsules, called sporangia, that the fungus produces through asexual reproduction.
The fungus consists of a network of filaments, called hyphae, that penetrate the bread and absorb nutrients. During asexual reproduction, spore-filled sporangia develop at the tops of stalklike hyphae. The sporangia and hyphae are haploid. Sporangia break open, and their haploid spores disseminate.
Spores travel by air to other locations, and may land on suitable food sources, such as a flaky croissant. A spore germinates and sends an elongated hypha into the croissant to absorb nutrients. As it develops, the new haploid fungus can produce more sporangia, initiating additional cycles of asexual reproduction.
In addition to asexual reproduction, the fungus may initiate sexual reproduction. Although Rhizopus does not have two distinct sexes, it does have two distinct mating types, designated "+" and "-". They reproduce sexually when adjacent hyphae of the two types release pheromones.
The pheromones trigger the hyphae to grow side branches. Note that Rhizopus generally lacks cross walls between its nuclei, and most nuclei share a common cytoplasm. As the side branches grow, nuclei stream in. A cross wall forms near the end of each branch, isolating several haploid nuclei at the ends.
The nuclei are now isolated in sexual structures called gametangia. These gametangia fuse, and the nuclei inside pair up. The pairs of nuclei eventually fuse, as well, and the entire multinucleate structure becomes a zygosporangium. The zygosporangium develops a thick, tough wall, and can remain dormant for months.
The diploid nuclei within the zygosporangium then undergo meiosis and a sporangium sprouts. The sporangium contains the products of meiosis: haploid nuclei that are incorporated into spores. When the sporangium opens, these spores will disperse and germinate to form the next generation of haploid hyphae.
A close inspection of bread being attacked by black bread mold will reveal a cottony mass of filaments spotted with black dots. The cottony mass consists of thousands of hyphal strands, and the black speckles are the sporangia that are borne atop hyphal stalks. From their elevated positions, these sporangia can crack open and disseminate the spores to new substrates at distant locations.
If two fungi of opposite mating types meet, these organisms can reproduce sexually by fusing their nuclei. In this process, a tough zygosporangium forms. The zygosporangium can wait out the worst of environmental conditions for months, and then revive when environment conditions are again favorable. When they revive, the zygosporangia produce sporangia elevated on hyphal stalks. When the sporangia crack open, their spores disseminate, and, if the spores land on appropriate substrates, they will germinate and continue the life cycle of the fungus.