3.9 The species concept in fungi

Most of what we’ve discussed so far depends absolutely on the ability to identify each organism so that you can state confidently the exact name of the species you are dealing with. That, of course, applies to everything you might want to do with a fungus; whether you want to paint it or sequence its genome, you have to give it an accurate name. The identification side of this depends on accurate descriptions, and the knowledge and skill of the person making the identification. Accurate descriptions should be assured by the international rules of taxonomy and nomenclature; knowledge and skill can be learned. What is worrying is that we’re not very clear about the ‘unit’ you might need to identify.

We know you need to identify a particular species; but what is ‘a species’? The debate over how species should be defined (the ‘species concept’) has been going on for many years but we don’t have a universal definition of ‘species’ that meets with widespread agreement. As recently as 1997 a paper appeared that discussed 22 different species concepts. The most commonly applied species concepts in fungal taxonomy are:

  • The morphological species concept. Species are based on morphological similarity, but most biologists and mycologists in particular, believe that the morphological species concept is the least satisfactory. It is practical for identification, but it obscures the phylogenetic origins of many features.
  • The biological species concept. Species is defined as an interbreeding population that is somehow reproductively isolated from other populations. This cannot be applied to all fungi because within the group there are many that do not reproduce sexually. Overall, there are too many severe restrictions on application of the biological species concept widely in fungi for it to be a universal definition.
  • Ecological and physiological species concepts. Differentiates species by their ecological niche and the constraints on their evolution that determine their maintenance and reproduction in that niche. The niche may be a particular species of host plant, or even a specific cultivar of a host species. Application is severely limited in practice. Can be ideal for pathogens butt there is a vast range of other fungi for which this approach simply fails. It is another concept that cannot offer the universal definition.
  • Evolutionary/phylogenetic species concept. Envisages a species as being a monophyletic group of organisms sharing molecular characters that derive from a common ancestor. The concept does not have any obvious built-in exclusions or limitations. The analysis can be applied to asexual organisms; anamorphic and teleomorphic stages can be covered by a single species concept. No shortage of characters – fungal genomes are big enough to provide more than enough sequences for this to be a definition of universal applicability.

The use of different methods of defining species inevitably results in recognition of different things; and remember the practice of assigning different taxonomic names to the imperfect (non-sexual) and perfect (sexual) stages of fungi. The indications are that application of the phylogenetic species concept will lead to a 2-4 times increase in the number of species recognised. Implying that biodiversity and species richness of ecosystems are 2-4 times greater than we currently imagine.

Resources Box

The fungal species: a matter of definition

CLICK HERE to visit a page providing further discussion of this.

Updated July, 2020