Let’s take a very bad organism, such as Colletotrichum gloeosporioides. I say bad, but it is neither bad nor good, it just is. I mean bad, because it is a label for some species that go their way screwing crop species. Plants we cultivate for food. As a result, we’re a bit upset by this fungus life-form. It is threatening. Not that it is scary, just it’s screwing all around, causing anthracnose diseases in a whole bunch of plant species.
I’m not going to speak about plant disease, though. I’m going to tell you about trends in taxonomy, related to plant-diseasing things. I’m not going to tell you about fungus taxonomy, though. It’s… Let’s put it straight, messy! Or say it’s been until we got into molecular taxonomy. But then, even now that species identification may be replaced by a few polymerase-chain-reactions well adjusted for your model thingie, it is not always that easy, or it still needs some workaround. (Look here how it looks — not much different from other related organisms). I’m going to tell you about trends in taxonomy.
See, the wiki page for Colletotrichum gloeosporioides is named Glomerella cingulata. For a reason: the latter is the sexual stage of the earlier. Both were morphologically enough distinct that early taxonomists considered them different species. Notwithstanding that most relevant information is dispersed into so morphs vs. so other morphs in litterature, it doesn’t help much more. Entities and identities are mixing up and down at multiple scales. It makes things a bit complex. Because with different stages, not everything is going to reconnect to phases or stages at the same pace and space. The entity is a messy flow in genetic information, lineages variously purposed in their specialised ecology, that may fit their niche but not our ability to make them match our own biological concepts. But we give a try.
In the past, once basic features of this fungal thingy special were worked out, along with a bit more precise morphological approach, scientists working on plant disease (aka phytopathologist, if you stick to words), and specially those working with diseases caused by Colletotrichum-Glomerella sweeties, considered things they were collecting on their favourite plant model were specific and specialised to that plant. So if it looked like a Colletotrichum, and was found, say, on a strawberry, then it was described as ‘the species living on Fragaria‘. And so forth, grounding information to an ever growing listing of over 750 species that were once thought to exist in nature. No matter what, I would add. No matter if it was already known that pathogens may strive on a bunch of different hosts, which would blow an approach based merely on assigning a species epithet on each host a strain was isolated from.
Eventually, a scientist decided to reconsider available taxonomy, on the legitimate ground that morphological features were beginning to demonstrate discrepancies, and that science was in need for new descriptions and expert rearrangement of things. Well, this scientist’s name was von Arx and he worked out a grand new Colletotrichum systematic in 1957*. Von Arx decided morphology to prevail over host specificity, given that boundaries were not known well enough, or that it was fairly limited at best (and indeed it was, since the fungis were apparently able to develop on a variety of plant species). Which resulted in a dramatic decrease of identified species: from 750 and more to about a dozen.
That was way simpler. I am grasping of this understanding of species, considering it to be easy and broadly defined, and only merely refined. Except that species are not always conveniently labeled with morphological approaches only. Or if it does, it doesn’t stay like that for long. Under things that look alike first, are things that can’t interbreed at all, things that may interbreed under special circumstances, and other things that have sex a lot. There may well be different species under the same gross category. As a result, with growing evidence that we shall not name different things after a too stereotypical and confusing label, refining species boundaries have had another start. And we know currently of 100 species in the Colletotrichum species complex. And there’s probably more, awaiting for identification. As a result, we may be a bit upset by this fungus life-form.
This is how reality shows. Bad in prospect. This pathogen is going to stand our expectations for simplicity’s sake. We’ll have to manage the unavoidable mess.
* If you’re interested in knowing more about this group and its evolving taxinomy, you may have a look to:
Cannon PF, Damm U, Johnston PR, Weir BS (2012). Colletotrichum – current status and future directions. Studies in Mycology 73(1):181-213.