The Wonderful World of Fungi
Part XIV – Division Basidiomycota – Unique Somatic Structures
By: H. Wayne Shew, Ph.D.
NAB Certified Pollen Counter
AAAC Collection Station—Birmingham, Alabama
In addition to having unique reproductive bodies called basidia which produce basidiospores, basidiomycetes are also unique somatically. Basidiospores germinate to produce thread-like or filamentous structures called hyphae. (Some species produce basidiospores that germinate and give rise to yeast cells instead of hyphae.) The hyphae are typically haploid and cells of the hyphae are uninucleate. The hyphae of basidiomycetes are septate and most species of basidiomycetes have septa that when viewed with an electron microscope show a distinctive morphology. These septa are called dolipore septa. Unlike the septa seen in most fungi, these septa have a barrel-shaped swelling that surrounds the central pore. (Figure 1) The structure has specialized membranes at each end that resemble parentheses and are hence called parenthesomes. These parenthesomes cap the ends of the septal pore and allow migration of cytoplasm and some organelles but not nuclei in dikaryotic hyphae. (Figure 2) These dolipore septa are quite different in appearance from any other septa found in other fungal groups. (Note: the basidiomycete fungi known as rusts and smuts have not been observed to have dolipore septa.)
The “typical” reproduction in Basidiomycota requires the fusion of haploid cells followed at some later time with the fusion of haploid nuclei inside of the basidium to form a zygote. The zygote undergoes meiosis in the basidium to form haploid nuclei that become incorporated into the basidiospores. However, the dominant phase of the basidiomycete life cycle is neither the haploid somatic hyphae nor the diploid basidium but rather the dikaryon stage. This stage is characterized by hyphae having cells that have the two nuclei brought together in mating in a side-by-side arrangement, so-called n + n.
The successful formation of these dikaryotic cells results from unique structures in basidiomycete fungi called clamp connections. (Figures 3 & 4) One of the nuclei divides mitotically in the main part of the hypha, the other divides mitotically with one nucleus migrating into the clamp while one remains in the main part of the hypha. Septa then form across the mitotic spindles. The monokaryotic clamp cell fuses with the subapical cell to reestablish the dikaryotic condition. If clamp connections are present on hyphae, then the fungus is a basidiomycete. Of course, not all Basidiomycota produce clamp connections.