The taxonomy of trichomycetes is replete with the same kinds of classificatory and nomenclatural problems found in many other large, variable, and understudied groups of organisms. The problems with trichomycetes, however, are compounded by the habitation of the fungi within arthropod guts. This presents some peculiar situations not found in other taxa of fungi. Part of what follows is paraphrased or quoted (with permission) from Lichtwardt, 1978a.
Many species of trichomycetes have well-defined characters with narrow ranges of variation and therefore are readily described or identified. At the other extreme are some species and even genera that have such a variable morphology that their description or identification is difficult unless sufficient material is available in the right stages. Unlike many fungi whose developmental stages can be induced in culture or where visual selection of particular structures can be made directly from their natural substrates, the trichomycetes, with relatively few exceptions, are not currently culturable and are not visible until the host is dissected. The stage of fungal development one finds may depend on the season of the year and the phase of the host at the time of degutting (see Chapter 7). Dissection kills the arthropod and inhibits further development of the fungus in most cases.
It is recognized that species of some genera of Eccrinales (Alacrinella, Astreptonema, Enteromyces, Ramacrinella) are dimorphic. That is, the thalli fall into two size categories. However, in other genera one finds some species whose many individual thalli exhibit extensive morphological variation within one gut. In fact, using taxonomic criteria applied to many other species of trichomycetes, we arrive at the initially disturbing conclusion that, in comparing some Eccrinales, intraspecific morphological variation can be greater than interspecific differences. This is not a taxonomic absurdity if one recalls that we are considering the expression of only a small part of the total genotype. Somewhat analogous morphological situations occur in other groups of fungi. For example, single species of Fusarium may produce conidia ranging from small, oval, and unicellular to large, lunate, and multiseptate. At the same time, considerably more subtle spore differences are used to separate species in other related genera, and even to make generic distinctions. We simply accept the fact that variability of spore morphology in Fusarium species is a characteristic of that genus but not others.
In general, the larger and more complex the gut, the more variable is the trichomycete species within it. A large gut such as that of some millipedes, crustaceans, and beetles may contain a range of microhabitats. Certainly the physiological conditions near the anus would be different from those near the Malpighian tubes. One can measure differences in pH values and dryness, and one can observe that other organisms present in the gut frequently are more or less compartmentalized. Species of Eccrinales that are restricted to particular regions of the gut often show less variability than those which grow throughout the length of the gut. If the variability is very great or disjunct, it may be difficult to determine if the morphologically different thalli represent one species.
A case in point is Enterobryus borariae, one of the variable species of Eccrinales studied by the Lichtwardt (1958). The hindgut of its millipede host, Boraria carolina, is relatively long and has several distinct structural regions often containing bacteria, nematodes, and protozoans with more or less restricted localization within the hindgut. The fungal thalli can be attached to the gut cuticle from the anterior to the posterior end, or even to the surface of nematodes in the gut. The mature anterior thalli may be as narrow as 3.5 µm, whereas farther back they may be up to 50 µm wide. Various kinds and sizes of spores have been observed (see Chapter 11), but because thallial forms more or less intergrade, it seems preferable to regard these as an assembly of one species that is subjected to various environmental conditions differentially influencing thallial development and morphology. Not all morphological variants are to be found in any single gut. Obviously, identification of this species, as well as other species of Eccrinales with variable morphology, may be difficult if based on examination of only one or two guts. It is difficult to write diagnoses for such variable species according to conventional standards, not to mention designing practical identification keys. See also the description of Enterobryus tuzetae and the note under E. isoporostrepti in Chapter 11 that relate to the problem of identifying species of Enterobryus.
Other examples of extreme variation within species of Eccrinales have been reported. Enteropogon sexuale has four thallus types with different spore morphologies, according to Hibbits (1978). The prevalence of particular morphological forms within the long hindgut of any given specimen of its marine crustacean host (Upogebia pugettensis) can depend on the season when it was collected. To complicate matters further, the same host may be infested with Enteromyces callianassae in the cardiac and pyloric stomach (foregut). Enteromyces callianassae, in turn, has a much wider host range including both anomurans and decapods. As another example, passalid beetles are hosts to Leidyomyces attenuatus, Passalomyces compressus, and an unnamed (but good) species of Enterobryus described by Heymous and Heymous (1934). All three have been found by the authors (rarely) in single passalid specimens, and are distinguished by their location in the long and complex beetle hindgut and by their distinct morphologies (Lichtwardt et al., 1999).
Most hosts of Eccrinales, however, seem to contain but one trichomycete species. This generalization is not true for some larval aquatic insects, which commonly contain two or more species of Harpellales and Amoebidiales (see Chapter 6). While the trichomycetes in larval insects are usually quite distinct and do not show the range of variation exhibited by some Eccrinales, the proliferation of growth and mixture of species may make it difficult at times to associate zygospores with their related trichospores.
Experimental methods using axenic cultures and host inoculations can aid in the resolution of taxonomic problems, but often they are not practicable or possible. Furthermore, culture conditions do not replicate conditions in the gut, and even if cultures of some of the more variable species were available, they would not likely produce the full range of variation found in natural development. In the absence of such experimental methods, several criteria may be used to help in taxonomic determinations of trichomycetes:
(1) Host taxa alone should not be used to distinguish trichomycete species; notwithstanding, host identification can be a useful primary consideration in restricting the options for identifying a given trichomycete on hand.
(2) Studies should include reasonably large samples of hosts so as to comprehend the fungal variations possible within species.
(3) Collections should preferably be made at different seasons of the year and at different. stages of host development (immature, adult, or molting individuals).
(4) If any given trichomycete species appears to have a wide latitude of morphological features within one host species, one can suspect that, if it does occur in other hosts, it may be variable there also, and this should be taken into consideration when making taxonomic assessments.