DISCUSSION OF THE ASEXUAL DIKARYA

Most fungi some ability to reproduce asexually. Although reproduction without sex does not promote genetic diversity it does allow a population to increase in numbers rapidly. Fungal life histories often involve establishing a small population from sexually produced spores at the beginning of the growing season and then a rapid increase through asexual means. This strategy is pronounced in the Dikarya, especially the Ascomycota.

The relative infrequency of sexual reproduction and the staggeringly large amount of asexual reproduction in the Dikarya has resulted in many species being known only in their asexual state. Most fungal life histories are still poorly understood and their once-a-year sexual reproduction may never have been seen. Added to this deficiency in our knowledge is the fact that some fungi may have evolved means of genetic recombination that are not according to the usual methods that we recognize. Some fungi (probably not many) may not have any means of recombination.

The fact that many fungi are well-known to us in their asexual state and rarely or never seen producing sexual structures leads us to a dilemma. The systems of fungal classification used by all mycologists are based on the nature of sexual structures. These structures reflect taxonomic relationships better than any other means short of nucleic acid sequences. And, in fact, modern genetic studies have so far strengthened our confidence in the predictive value of sexual structures. In some cases these studies have changed the way we interpret the importance of these structions but not the importance of the structures themselves.

So, how do we deal with fungi we find only in their asexual state? With some difficulty. Since the 1850's, when mycologists first understood the sexual cycles of fungi, it was recognized that many fungi were seen only in an incomplete or "imperfect" state. Such fungi came to be known as "Fungi Imperfecti" and, although recognized as incomplete portions of life cycles, were classified as though they were a separate group. For example, the picture at left shows a species of the ascomycete genus Hypocrea with all stages of its reproductive cycle present. The sexual structures are embedded within the orange cushions while the asexual structures are present in the green areas. If you were to try to grow this fungus in a petri dish you would get the green asexual state but not the orange sexual one. If you were to investigate a pile of poorly dried firewood or the bottom of an old mushroom you would also be likely to find the asexual state but not the sexual one. Since mycologists have only very slowly discovered the connections between the sexual and asexual states of fungi, or have known only the asexual states, it has become accepted practice to apply names to asexual fungi, even when the "complete" fungi are already known and named. Thus the fungus at left is clearly a species of Hypocrea bearing sexual and asexual structures, yet when the asexual structures are encountered alone they are called Trichoderma. Many species of Trichoderma have been named, some with known connections to Hypocrea species and others without.

Classification of asexual Dikarya

Making a workable classification for these fragments of life cycles has not been easy. Ever after 150 years of trying mycologists are still hotly debating the issue. Major discussions at the beginning of the 21st century have centered on the problem of integrating a classification based on form, one that does not require evidence of genetic relatedness, with what we now can easily determine using the tools of molecular genetics. It certainly makes sense to integrate these incomplete fungi into the more satisfying system used for complete ones except that often what one can see under a microscope is not very helpful in bringing this about. Of course the genetic tools will do this fairly easily, but these are mainly available to well-financed labs having highly-trained personnel. So for the present most mycologists continue on with the traditional methodology. See the pages on Moulds for a practical application of these systems.

Some terminology

A great deal of terminology has been applied to the asexual Dikarya. While much of this terminology is superfluous it is necessary to master some of it in order to understand what has been written in this area.

Holomorph, teleomorph, anamorph

These terms came into use in the mid-1970's and were accepted by most mycologists soon after. They refer to structures produced during the life cycle of a fungus. A holomorph is defined as the whole fungus in all of its parts, sexual, asexual and non-reproductive. A teleomorph refers to the sexually reproducing structures of a fungus as distinct from other structures. An anamorph is any asexually reproducing part of a fungus. The asexual Dikarya discussed on this page can be referred to as anamorphs. A holomorph can be said to be composed of a teleomorph, one or more anamorphs and of course various structures not involved in reproduction

Mitospore, meiospore

Specific, genetically-based terms referring to the origins of fungal spores. Meiospores are produced by a fungus following meiosis and are thus the result of sexual reproduction. Mitospores are produced by mitosis and do not involve sexual processes. Since the precise nuclear events leading to spore-formation in most fungi have not been studied these terms are applied in much the same way as the previous set to denote sexual and asexual parts of life cycles. Current mycologists are divided in their preferences, with some using anamorph and teleomorph and others using the terms mitosporic and meiosporic.

Conidium, conidiogenesis

A conidium (plural, conidia) is an asexually produced fungal spore. The term is equivalent to mitospore but is of much earlier origin and is the term most commonly used to describe the spores produced by asexual Dikarya. Conidiogenesis, a term dating back to the 1950's, refers to the way that conidia are produced. In the second half of the 20th century conidiogenesis became the most widely used means of classifying the asexual Dikarya.

Thallic, blastic

Fundamental types of conidiogenesis. Thallic conidia are produced by the transformation of existing cells into spores. Blastic conidia begin their existance as specialized cells that develop as conidia. The usual rule-of-thumb is that blastic conidia can be recognized as such before they are cut off by a septum while thallic conidia are defined by septa and then later become recognizable as conidia. In practice the distinction is not difficult. There are several terms denoting various types of thallic and blastic conidia but these are beyond our purpose here. The pages on Moulds treat the subject of conidiogenesis in much greater detail

Mononematous, synnematous

Terms referring to the simple and compound structures bearing conidia. Mononematous species are those having their spores borne on structures arising from a single hypha. Synnematous species produce their conidium-bearing structures on relatively large compound fructifications made up of bundles of hyphae.

Hyphomycete, coelomycete

Hyphomycetes bear their spores on structures borne directly from the substrate and never enclosed in any kind of protective tissues. Coelomycetes have their conidium-bearing structures arising from a "rind" or wall, often completely enclosing them with only a small pore for their escape