Zygomycota: The Conjugation Fungi
The Zygomycota, or conjugation fungi, include molds, such as those that invade breads and other food products. The identifying characteristics of the Zygomycota are the formation of a zygospore during sexual reproduction and the lack of hyphal cell walls except in reproductive structures. Many (~100 species) are known plant root symbionts.
The mycelia of Zygomycota are divided into three types of hyphae. The rhizoids reach below the surface and function in food absorbtion. Above the surface, sporangiophores bear the spore-producing sporangia. Groups of rhizoids and sporangiophores are connected above the surface by stolons. Cell walls separating individual cells are absent in all but reproductive structures, allowing cytoplasm and even nuclei to move between cells.
Like all fungi, Basidiomycota can undergo both asexual and sexual reproduction. Asexual reproduction in Zygomycota is similar to that in other types of fungi, while sexual reproduction bears some similarity to that in Ascomycota.
Asexual reproduction in Zygomycota varies greatly among orders and species. Spores may be formed by the separation and thickening of hyphal cells. They may also be produced in specialized organs, whose structure is also widely varied.
Like Ascomycota, some Zygomycota have two mating types, though individual species may only have one mating type. When hyphae from opposite mating types meet, they produce structures called progametangia that are dense and multinucleate. Cell walls form to separate the tips of the progametangia into gametangia, which continue to be attached to the mating hyphae by the remaining suspensors. Plasmogamy then occurs between the two gametangia to form a zygote. Next, karyogamy takes place within the zygote. The cell walls of the zygote are thin at first, but later thicken into a zygospore. Germination begins when the diploid nucleus undergoes meiosis and a sporangium develops at the end of a germ tube. Spores are produced within the sporangium.