Fungi

Species in this phylum reproduce sexually by forming a zygospore, a thick-walled, diploid cell which contains thousands of nuclei. There are about 600 species in this phylum. Most species are terrestrial and feed on organic matter, although there are a few parasitic species. The conjugating fungi are coenocytic, in that they have a continuous mycelium, containing hundreds or thousands of haploid nuclei, with no divisions between them. However, the Zygomycota do have septa (cross walls) between their reproductive structures and the rest of their mycelium.

The conjugating fungi have a life cycle that includes a sexual phase and an asexual phase. In the asexual phase, thousands of spores develop inside a sporangium, a small spherical structure. The sporangium grows on the tip of a sporangiophore, a specialized aerial hypha, typically about as thin as a hair.

In the sexual phase of their life cycle, these fungi form specialized hyphae, called gametangia, which are of two different strains (sexes), plus and minus. The plus and minus strains are very similar morphologically, but differ physiologically and biochemically. Plus and minus gametangia conjugate with one another and form a structure with hundreds or thousands of nuclei from each strain.

Then, a thick-walled structure, called the zygospore, develops from the conjugated gametangia. Inside the zygospore, the many thousands of nuclei from the plus and minus strains pair off and fuse together to form thousands of diploid nuclei. The zygospore is typically spherical in shape and has a thick, dark outer wall. It usually remains dormant for several months or more before development continues.

As the zygospore germinates, it produces germsporangia which are born on germsporangiophores, structures morphologically similar to the asexual sporangium and sporangiophore (see above). The germsporangium contains thousands of haploid germspores which arose from the diploid nuclei of the zygospore by meiosis. Each germspore is liberated, germinates, and gives rise to a new haploid mycelium.

One of the best known of the conjugating fungi is Phycomyces blakesleeanus, a species which grows on animal feces in nature. The sporangiophores of Phycomyces respond to a variety of sensory stimuli. For example, they bend in response to light (phototropism), gravity (gravitropism), wind (anemotropism), and nearby objects (avoidance response). Physiologists and biophysicists have intensively studied the response to light. One important finding is that the light sensitivity of the sporangiophore is about the same as the eyes of humans. Furthermore, like humans, the sporangiophore can adapt to a one-billion-fold change in ambient light intensity. One of the pigments involved in the extraordinary light responses of Phycomyces is a flavin (vitamin B2) bound to a special protein. This pigment is commonly called the blue light photoreceptor, since it is most sensitive to blue light.