Chapter 12: Fungal sex and fungal groups
The ‘vegetative’ (i.e., not associated with sex and reproduction) structure of most fungi are generally very consistent: they are composed of branched filaments that explore the volume of material that they feed on (be it living or dead) and this structure provides for acquisition of materials that they need to grow and survive. The consistency of vegetative form makes it difficult to classify fungi because they mostly look alike, although some vary on the basis of whether or not the filaments on coenocytic or septate (with cross walls and therefore cellular). However, fungi do show variation, and can be grouped, based upon structures that are associated with sexual reproduction. The exceptions to this are the two groups where sex is either completely absent (Glomeromycota — the endomycorhizal forming fungi) or rare (Chytridomycota — the chytrids). The remaining three groups (bread molds = Zygomycota, cup fungi = Ascomycota, and club fungi = Basidiomycota usually exhibit specific features associated with sexual reproduction that are used define these groups (generally considered phyla). Fungi that show no sexual features used to be put into a group called the ‘Fungi Imperfecti’ (Deuteromycota), but molecular techniques now allow these fungi to be placed in one of the groups mentioned above.
Cells called spores — defined as cells with both dispersal capabilities (i.e., mobile cells) and reproductive capabilities (i.e., cells that can grow into new fungal organisms) are often important in both asexual and sexual reproduction.
- Gametes (cells that can fuse with other gametes) are quite different from what most would consider ‘typical’; they are usually hyphal cells (part of the fungal filaments) that have the ability to fuse with other hyphae. Depending upon the group, these special hyphae may or may not have structural features that would distinguish them from the normal hyphae.
- Syngamy often involves two steps separated in time and often in space. This is a consequence of the fact that the fusion of the hyphae serving as gametes is generally not followed immediately by the fusion of nuclei. Cellular fusion is called plasmogamy and nuclear fusion is called karyogamy. Plasmogamy followed by a delayed karyogamy allows fungi to have a novel condition, the dikaryon state, where a cell has two nuclei (‘dikaryon’ means ‘two nuclei’), one from each parent. This condition is often perpetuated: the dikaryon cell divides while both nuclei divide, thereby forming a new cell that is also dikaryon. This process can continue, producing multiple dikaryon cells and dikaryon hyphae. Note that although a dikaryon cell has two copies of each chromosome, it is not considered diploid because each nucleus is haploid, with only one copy of each chromosome.
- At some point, some of the dikaryon cells become diploid as a result of the fusion of the two nuclei (karyogamy). The diploid cells are NOT perpetuated; they undergo meios