Reference Review: The Trials of Anamorphic Taxa

Skovgaard, K., S. Rosendahl, K. O’Donnell & H. I. Nirenberg. 2003. Fusarium commune is a new species identified by morphological and molecular phylogenetic data. Mycologia 95(4): 630-636.

Fusarium is a genus of filamentous soil fungi (shown above in a diagram from here) that is best known as a cause of a selection of nasty diseases of crop plants. It is an anamorphic genus - that is, it includes taxa that reproduce asexually. Fungal taxonomy maintains a complicated system of classifying asexual anamorphs separately from sexual teleomorphs, at least at the generic level (for instance, Fusarium anamorphs are associated with various teleomorphs of the family Nectriaceae - Rossman et al., 1999). In the past, there were separate families and higher for anamorphic taxa, but these have largely been abandoned. This system remains in place despite the fact that some "individual" hyphal masses (inasmuch as one can recognise an individual in fungi) may reproduce both asexually and sexually. In a previous post, I commented that the double taxonomy system was due to a "combination of history, theory and a certain degree of pragmatism". Anamorphs are usually completely different in appearance to teleomorphs, and there is generally no way to tell easily whether a given teleomorph corresponds to a given anamorph (usually, the only way to make a connection is to luck out and find one of the double-dipping hyphae I refered to a moment ago). Even when a connection is made, there is not necessarily a one-to-one relationship between anamorph and teleomorph - one anamorph may correspond to more than one teleomorph. There are even cases known where an anamorphic taxon is found worldwide, but its apparent teleomorph is only known from a very restricted location. A theoretical component can be invoked, too - species concepts are supposed to reflect gene flow, and gene flow is generally not occurring between anamorphic and teleomorphic lines. There are issues with the double taxonomy system, of course - perhaps most significantly, anamorphic taxa seem to be something of the poor cousins of mycology. Despite their being far more abundant in the environment, anamorphs seem to receive only a fraction of the attention given to their more glamorous teleomorphic counterparts.

I think it's worth noting that almost all anamorphic taxa are treated as essentially artificial form-taxa. Thus, while Fusarium seem to all fall within the Nectriaceae, there is no assumed guarantee that taxa with a Fusarium anamorph necessarily form a monophyletic unit. One teleomorphic genus may include members with a number of different anamorphic forms, that each may be shared with members of other teleomorphic genera. Attempts to try to restrict anamorphic genera phylogenetically, such as Sampaio et al. (2003), are relatively few and far between.

With that background explanation dealt with, on to the description of Fusarium commune Skovgaard et al., 2003. One of the big problems with taxonomy of anamorphic is that, well, there's often not that much to work with. All the flashy characters, the colourful mushrooms, the pungent truffles, the wierd-shaped fruiting bodies, are sexually-reproducing structures of teleomorphs. When a fungus is not actively fruiting, one collection of hyphae looks much like another. And conidia, the structures that give off asexually-produced spores in anamorphs, are often not much more than budding extensions of hyphae. As a result, useful morphological characters of anamorphs are few and often somewhat vaguely distinguished.

It should therefore come as no surprise at all that when molecular data was applied to anamorphs, it seemed that the amount of diversity present had been significantly underestimated. Convergence in anamorphs is rampant, and two morphologically near-identical samples may easily turn out to be very distant phylogenetically. So when morphological taxonomy has proven insufficient, in steps the substitute of molecular taxonomy. And that, I'm afraid, is where my hackles start to raise themselves just a little.

The use of molecular data in taxonomy is a much-abused field. Generally speaking, molecular data cannot resolve species. Any analysis of molecular data results in a branching tree, but species identifications are supposed to be about identifying gene flow in networks. There is no magic figure for "x% genetic divergence = different species". A single species with a large, widespread population (say, a wide-ranging bird species) may feature a large amount of genetic divergence without barriers to gene flow. In contrast, a cluster of short-range endemic species (e.g. snails that don't move about much at all) may have very little genetic variation within or even between populations without gene flow occurring between them. So any use of molecular taxonomy should be approached with extreme caution.

I'm glad to say that Skovgaard et al. seem to get it mostly right as far as I can tell. They use 15 different isolates of the new molecular species - a very important step in fending off the spectre of sample contamination. And they also identify some morphological traits supporting the new species. Fusarium commune differs from the closely related F. oxysporum in producing polyphialides and long, slender monophialides when grown in the dark*, while F. oxysporum produces short monophialides only (phialides are the hyphal branches that produce conidia - if I interpret correctly, polyphialides produce spores in multiple axes, while monophialides only have one axis). I am a little mystified as to why there are no samples of F. blasticola, referred to in the article text as very similar to F. commune, included in the molecular analysis. However, Skovgaard et al. do demonstrate the distinction of F. commune from F. blasticola through a pathogenicity test. Fusarium blasticola is a pathogen of Picea (spruces) and Pinus (pines). Despite specimens of these two hosts being grown for five months in soil inoculated with cultures of F. commune, no sign of infection was noticed. Fusarium commune has since been shown to be able to cause infection in Pseudotsuga (the Douglas fir), another commercial conifer (Stewart et al., 2006).

REFERENCES

Rossman, A. Y., G. J. Samuels, C. T. Rogerson & R. Lowen. 1999. Genera of Bionectriaceae, Hypocreaceae and Nectriaceae (Hypocreales, Ascomycetes). Studies in Mycology 42: 1-248.

Sampaio, J. P., M. Gadanho, R. Bauer & M. Weiss. 2003. Taxonomic studies in the Microbotryomycetidae: Leucosporidium golubevii sp. nov., Leucosporidiella gen. nov. and the new orders Leucosporidiales and Sporidiobolales. Mycological Progress 2(1): 53-68.

Stewart, J. E., M.-S. Kim, R. L. James, J. R. Kasten Dumroese & N. B. Klopfenstein. 2006. Molecular characterization of Fusarium oxysporum and Fusarium commune isolates from a conifer nursery. Phytopathology 96 (10): 1124-1133.