3.3: Zygospore-forming Fungi

These organisms were formerly classified in a group called the Zygomycota because they sexually reproduce by forming a structure called a zygospore. However, they have since been broken into several different lineages.

Mitosporangium (Asexual Reproduction) and Coenocytic Hyphae

Figure \(\PageIndex{1}\): Rhizopus stolonifer asexually reproducing. The globose structure is a mitosporangium. The peridium (outer covering of the sporangium) has split and haploid spores are being released. At the center of the sporangium is a swollen structure called the columella. Photo by Maria Morrow, CC BY.

Figure \(\PageIndex{2}\): These branching structures called rhizoids are formed at the base of each mitosporangium. Photo by Maria Morrow, CC BY.

Zygospore Formation

When the mycelium of one fungus encounters another fungus of the same species and a complementary mating type, it can start to produce compounds to interact with the new fungus. Through a series of chemical exchanges, the two fungi each begin to extend toward each other. When they touch, they wall off an area of that extension by creating a septum. This area, filled with haploid nuclei, is called a gametangium (see Figure \(\PageIndex{3}\)). The walls between the gametangia of each fungus dissolve and the two fungi combine cytoplasm (plasmogamy) and then fuse the nuclei together (karyogamy) to form many diploid nuclei. As this happens, a thick, orange, ornamented wall forms around the nuclei. This is the zygosporangium.

Figure \(\PageIndex{3}\): Gametangia of Phycomyces blakesleeanus. Two complementary mycelia have formed gametangia (indicated by white arrows) by forming a cell wall to make a distinct compartment at the end of each hyphal filament. These structures are filled with haploid nuclei that will fuse together when the cell walls between the two gametangia dissolve. This new joined structure is the zygosporangium. User:Xerantheum, CC BY-SA, via Wikimedia Commons.

Figure \(\PageIndex{4}\): Phycomyces blakesleeanus forming zygosporangia. Many pairs of hyphae have begun conjugation. In (1), gametangia have formed, but plasmogamy has not yet occurred. In (2), plasmogamy has occurred as the walls separating the gametangia from each other have dissolved. In (3), the developing zygosporangium and suspensor hyphae begin to darken and ornamentations begin to form. In (4), the zygosporangium is nearing maturity with black branching ornamentations. Photo by Xerantheum, CC BY-SA, via Wikimedia Commons with labels added by Maria Morrow.

Figure \(\PageIndex{5}\): Rhizopus stolonifer sexually reproducing. The large, warty zygosporangium (left) is being held by two suspensors, one from each mycelial parent. On the right side of the image, suspensors hold gametangia that would soon fuse to form a zygosporangium. Photo by Maria Morrow, CC BY.

Rhizopus stolonifer Life Cycle

The life cycle below shows both sexual and asexual reproduction in Rhizopus stolonifer. Both sexual and asexual reproduction result in the production of haploid spores that can germinate and grow into a haploid mycelium. However, the spores produced by the mitosporangia will all be genetically identical, while the spores produced by the sporangia emerging from the zygosporangium will be genetically distinct.

Figure \(\PageIndex{6}\): In sexual reproduction, two compatible haploid mycelia identify each other. Where the two mycelia meet, gametangia are formed, each containing haploid nuclei. The wall between the gametangia dissolves and plasmogamy occurs. The nuclei fuse to form diploid zygospores within the zygosporangium. A sporangium germinates from the zygosporanigum and produces haploid spores via meiosis. These haploid spores can germinate and grow into new mycelia. In asexual reproduction, mitosporangia produce haploid spores by mitosis. Mitosporangia do not emerge from a zygosporangium. Instead, they have a network of hyphae (rhizoids) that look like roots and lateral connections (stolons) to clusters of other mitosporangia. Artwork by Nikki Harris, CC BY with labels added by Maria Morrow.

Watch the video below to see the microscopic structures involved in the asexual portion of the life cycle of Rhizopus stolonifer.

• 0:50 The video starts, showing coenocytic hyphae growing
• 1:25 Mitosporangium formation (asexual reproduction)
• 2:00 Mitospores, haploid spores produced via mitosis, are shown
• 2:30 Video ends

Video \(\PageIndex{1}\): Asexual structures of Rhizopus stolonifer. Sourced from YouTube.

Examples of Zygospore-forming Fungal Lineages

These groups of fungi can be found in your daily life, if you know what to look for. Molds on fruits and bread are often (but certainly not always) from the Mucorales, as well as molds that form on dog poop (specifically, a genus called Phycomyces). You can also find a diverse assortment of former "Zygomycota" members parasitizing other fungi and insects.

Figure \(\PageIndex{7}\): A fly killed by Entomophthora, a parasitic fungus whose name means "insect destroyer". The powdery white spores can be seen on the wings and forming in bands on the abdomen. Photo by Maria Morrow, CC BY.

Figure \(\PageIndex{8}\): Three Mycena at different stages of parasitization by the mycoparasitic fungus Spinellus fusiger. Photo by Maria Morrow, CC BY.

Figure \(\PageIndex{9}\): Fruiting bodies of Pilobilus fruiting on deer scat. The swollen region acts as a lens, focusing light to heat the liquid and create turgor pressure. This causes the black spore packet to rocket off onto an adjacent grass patch. Photos by Maria Morrow, CC BY.