3.6.1: Fungal Spores

Figure 1 Although many fungi reproduce without sex, the sexual organ, if present, is the most visible structure of the organism and, until the advent of molecular techniques, it was the structure used to identify fungi. In many fungi the sexual organ is too small to be seen but in some, like this pale chanterelle, the 'fruiting body' is not only large, its also edible!

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 fungibecause 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 thesegroups (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.

Fungi have the following features associated with sex and reproduction that are different from most familiar sexual organisms:

• R eproduction often involves both sexual aspects and asexual aspects. 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 wouldconsider '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 themfrom the normal hyphae.
• Syngamyoften 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, wherea cell hastwo 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 NOTperpetuated; they undergo meiosis to form haploid nucleiwhich develop walls to become haploid cells. Thus, the diploid state is very brief in extent(i.e., size and number of cells) and often brief in time as well—the diploid cells neverdivide mitotically to form more diploid cells, they only divide meiotically to form haploid cells. These haploid cells are, or soon become, spores and aredispersed from the parent fungus to a new location where they germinate and form haploid hyphae.
• 'Mating strains' are a common mechanism thatensures that fungi do not mate with themselves. Within a particular species there are two to many mating types. A particular mating type needs to find a different mating type in order to interact sexually (i.e., fuse hyphae). The simplest situation has two mating types: “+” and “-” and they need to find each other to mate, but there may bemore than just two types, e.g., types A, B, C, D, E, and A could mate with any of the othersbut not with another A. Generally, the interaction, or lack of it, involves chemical signals (pheromones)that are emitted by one mating type and sensed only by fungi of a different mating type. In response to the pheromone, hyphae grow and find each other, bringing about plasmogamy.

TOPICS

• • Zygomycetes (bread molds)
  • Ascomycetes (cup fungi)
  • Basidiomycetes (club fungi)