Messages from STEM and Health Science Scholars
Sexual and Asexual Reproductive Stages of Fungi
Article Excerpt with Reflection
Charles Adair
The traditional name for the sexual spore stage of fungi in the field of Mycology had originally been the “perfect stage,” but this has been replaced by the term teleomorph (“end form”). Parallel with this terminology, the traditional name for the asexual spore stage had been the “imperfect stage,” and this has been replaced by the term anamorph which describes all structures (including hyphae) that are not involved in the sexual stage of the organism. Finally, the term holomorph describes the complete picture of both the sexual and asexual stages of a fungus, so it is a combination of the teleomorph and the anamorph.
Sexual spore stage
The sexual stage of a fungus life cycle consists of the events leading up to the fusion of two compatible haploid nuclei to form a diploid zygote and its subsequent division by meiosis to produce haploid nuclei typically packaged within spores. The fusion of the compatible haploid nuclei is more accurately known as karyogamy [“nuclear marriage”] than “fertilization,” since there are no equivalents to male or female gametes among fungi. Some fungi employ elaborate methods for bringing together the two compatible haploid nuclei in this process, often having mating types that prevent karyogamy of nuclei within the same colony or between closely related strains. A fusion of fungal cells is necessary to bring these two compatible haploid nuclei together, whether these are two hyphae, a hypha and a specialized spore, two spores, or other specialized structures, and this cell fusion is known as plasmogamy [“cell marriage”]. Following meiosis of the diploid zygote (or comparable cell), the resulting haploid nuclei, or their mitotic products, are immediately or eventually packaged into characteristic spores that are associated with the sexual stage of the fungus. The events of plasmogamy, karyogamy and meiosis are so unique to each phylum of Mycota, and to each of the various taxonomic subgroups within each of those phyla, that they are a dependable basis on which to identify and classify these organisms.
Asexual spore stage
Those spores that are produced by mitosis from various hyphal structures are known as zoospores, sporangiospores, or conidia depending on their properties and the structure in which or on which they are formed. There is a tremendous variety of asexual spore morphology, and also in the modes of asexual spore formation and dispersal, all of which are utilized in recognizing fungal species. However, the asexual spore stage is much less reliable than the sexual spore stage for establishing the identity and taxonomic placement of an unknown fungus because of the wide variability in spore morphology and in the mode of production of these spores that can occur within a given taxon. Because their mitotic origin results in cloning, the reproductive function of zoospores, sporangiospores and conidia is asexual.
Practical challenges caused by the teleomorph/anamorph duality of many fungi
A recurring theme in the field of Mycology has been the difficulty in making the connection between the teleomorph and the anamorph of a large number fungi in several of the phyla of the Mycota, especially those in the Ascomycota. One of the reasons for this is that the sexual and asexual spore stages are typically quite dissimilar, and often produced at different times during the development of a fungus in its natural habitat. Also, the successful initiation and outcome of the sexual spore stage in many fungi requires the interaction of two compatible mating types which may not always be present in the same location at the same time, so the teleomorph may occur rarely or never. The mitotically-produced spores associated with the anamorph of most fungi function effectively as a means of asexual reproduction, and they can also function as an effective means of initiating host infections by necrotrophic and biotrophic fungi of clinical or agricultural importance. Because of this characteristic of many fungi that have an impact on human affairs, these fungi have been historically recognized by their anamorph stage, have been given Latin binomials based on that stage, and are described in the scientific literature by those binomials.
At the same time, the teleomorphs of many of these same fungi have been identified as unique species and given Latin binomials, but the connection with the corresponding anamorph may or may not have been recognized at the time they were first described. As a result, there are many fungi that have two Latin binomials, one for the anamorph and one for the teleomorph. Advances in mycological techniques, including both the availability of genomic data and methods of ultrastructural analysis, have resulted in the publication of increasing numbers of teleomorph binomials that are linked to their corresponding anamorph binomial.
While the publication of two different binomials for the same fungal species would appear to be cause for confusion, most scientists investigating these organisms in the context of agricultural, clinical, and industrial applications have recognized the basis for these binomials and have cited them appropriately in the scientific literature. However, there is a debate arising from the principle of “one fungus one name” that is consistent with the current interpretation of the International Code of Botanical Nomenclature, the recognized authority on scientific naming of fungi. While the use of both the teleomorph binomial and anamorph binomial was originally considered to be acceptable, the current standard that is being advocated is to utilize only the teleomorph binomial, even in cases when the anamorph binomial was published first. In practical terms, much of the scientific literature regarding clinical, agricultural, and industrial investigations of fungi has identified these organisms by the anamorph binomial, since that is the stage that is relevant to these fields. The exclusion of the anamorph binomial from future publications in these fields would appear to be a more significant source of confusion than following the earlier convention of identifying the organism with both binomials when appropriate.
Reflection
My intention in offering this piece is to provide a relatively accessible example of my writing in my field of Mycology. This selection is part of a rather long chapter on fungi that I was invited to write for the Practical Handbook of Microbiology, 4th ed., CRC Press 2021, edited by Lorrence H. Green and Emanuel Goldman, and I hope it serves as a relatively accessible window into this field of specialization and some of the issues that are encountered within it.
In writing this chapter I was conscious of the fact that my audience would be quite varied in their backgrounds but also very pragmatic regarding their use of the information that I was presenting. The nature of the audience for this Handbook is so effectively described on the Routledge/CRC Press website that I would like to quote it here: “Useful to anyone interested in microbes, the book is intended to especially benefit four groups: trained microbiologists working within one specific area of microbiology; people with training in other disciplines, and use microorganisms as a tool or “chemical reagent”; business people evaluating investments in microbiology focused companies; and an emerging group, people in occupations and trades that might have limited training in microbiology, but who require specific practical information.”
Given this audience, I tried my best to define the technical terms used to describe the organisms and their structures and processes without letting those definitions intrude on the flow of the language and the concepts that I wanted to present. I felt that the only way I could accomplish this was to take the approach that I would take in introducing a fairly unfamiliar Biological topic to a classroom full of fairly advanced undergraduates who had some previous background in Biology. I certainly didn’t want to appear to be talking down to the audience of professionals described in the quotation above, but I didn’t want to lose them by using terms that might be unfamiliar to them.
There was an extensive introductory section of this chapter in which I took this stepwise approach in describing the way fungi grow and obtain their nutrition, but in which I referred to fungal spores in very general terms. The section of the chapter that I have presented here deals with the Sexual and Asexual Stages of Fungi, but the fairly straightforward definitions and comparisons of these stages lead to a discussion of the very inconsistent ways in which these processes occur in these complex organisms. I have tried to guide the reader through the rather involved nature of the sexual reproductive process itself, the importance of the sexual stage in the classification of fungi, the challenges posed by those fungi whose sexual stage is unknown or rarely seen, and the practical necessity to identify and name many fungi by their asexual stage even though it is seen as “less reliable” as a basis for naming these organisms.
I had to assume that my audience was familiar with terms describing relative chromosome numbers (haploid or diploid), the modes of cell division (mitosis or meiosis), the characteristic numbers of daughter cells produced, and the genetic implications of sexual vs. asexual reproduction. At the same time, I made a point of defining certain seemingly obscure terms (such as plasmogamy and karyogamy) for which there are no common synonyms, since they become essential in later sections of the chapter as the only way to explain certain events.
Another objective that I felt was important to achieve in this section of the chapter was to “re-educate” those members of my audience who may have learned Mycological terminology at an earlier period of time when less precise definitions were applied. A specific case is the naming of the sexual and asexual stages of fungi as the “Perfect” and “Imperfect” stages, which was so prevalent for so long that one of the standard reference books still on my shelf is the Manual of Imperfect Fungi (with beautiful line drawings of the asexual spores used for identification of species). Practitioners in a field like Plant Pathology tend to be very attached to familiar names of organisms that they deal with on a regular basis, so I wanted to gently prepare them for the current terminology that is employed in this field.
Finally, I felt I had to address the controversy regarding “correct” names of fungi, given the fact that many of them have two entirely independent names—one for the sexual stage and one for the asexual stage—yet the purists insist that the name for the “Imperfect” stage should be discarded entirely once the named sexual stage of the organism is known. Naturally for those practitioners who know (and often look for ways to control) the asexual stage in their work, this approach is impractical and confusing. I try to hint strongly that it is better to keep both names in circulation and just refer to the other whenever either one is used in a publication.
Aside from the issues of audience and my method of communicating with them, I realize this reflection should also address my writing method. I suspect that it is fairly typical, in that I am not very disciplined and just write as much as I can when I am mentally focused on the topic at hand. I’m often most productive in the evening, but this can result in working too late at night.
The first draft is very transitory, since I tend to edit and re-edit constantly as I write (something much harder to do years ago when everything was on a typewriter). I always edit further when looking at my work the next day or next week, and I often have to revise an earlier section if I discover that a prior understanding of an additional point is needed in order to effectively present a concept in a later section.
My teaching experience definitely influences my narrative style, since I tend to present concepts in a stepwise approach, building on earlier definitions when presenting new topics. I don’t like long paragraphs or lengthy chapter sections, and I like to employ a hierarchy of headings and subheadings to provide clear guidance to the reader as to where they are. And I hope my readers have a good memory, since terms defined early will then show up later without a definition.
