Dr. Ron Petersen
I try to keep two kinds of studies cooking simultaneously. In one type, I want to know the mating systems of many species of mushrooms (and their relatives) for the following reasons: 1) the mating system of a mushroom (bipolar or tetrapolar or amphithallic) is just as good a taxonomic character as, say, clamp connections or pigment deposition; 2) only by assessing several species of a genus can the consistency of mating systems be analysed. For example, it was always assumed that Marasmius was a coherent genus, but when we assessed mating systems within the genus, we found that certain SECTIONS (groups WITHIN a genus) were tetrapolar, others bipolar, and that one SUBSECTION seemed consistently amphithallic. This sent up a red flag of concern: was Marasmius really coherent, or were there groups which exhibited similar taxonomic characters, but had their origins in several separate places? Now, some years later, DNA sequences are showing a heterogeneous evolution, telegraphed previously by mating studies.
The second type of research deals with deeper, more exploratory work with a small group of species. The latest completed research has been with Omphalotus, the "jack o'lantern mushrooms." Five "species" were represented, and each was mated with all the rest, so that almost 100 matings were done in each experiment. The result was that the California taxon (O. livascens), the Florida taxon (O. subilludens) and the southern European taxon (O. olearius) were clearly one species, nearly 100% compatible in all combinations. Amazingly, the Appalachian taxon (O. illudens) was sexually very separate, an unexpected result considering its location so near Florida, and even more surprising, the Australian species (O. nidiformis) was able to mate at low frequencies with all of those mentioned above. Conclusion: While allopatric speciation has clearly occured between O. nidiformis and other Omphalotus species, the formation of reproductive barriers has been incomplete. In this case, evolution of reproductive barriers and morphological evolution are disconnected. In fact, NONE of the "species" was reproductively totally isolated, regardless of geographic distribution, geographic isolation, morphological dissimilarities, or presumed time-length of separation from one another (i.e. the Australian species differs considerably in color and form, and may have been separated from the others by a minimum of 175,000,000 years!).
Our latest big project concerns the "fuzzy-foot mushroom" (Flammulina velutipes and relatives). We have attacked the poor, defenseless genus from the following flanks: 1) Scott Redhead (Ottawa, Canada) has examined fruitbodies, and has concluded that there are several additional undescribed taxa within Flammulina; 2) Petersen finds the same general conclusion using compatibility experiments; 3) Karen Hughes and Andy Methven have produced phylogenies based on DNA RFLPs and DNA sequences, and they show the RELATIONSHIPS between these taxa, not just their similarities; and 4) Nadya Psurtseva (St. Petersburg, Russia) has grown the cultures on agar media for physiological characterization.
Two things are clear: 1) lots of the physiological literature (i.e. how to fruit the mushroom; the effects of light, temperature, humidity, etc.) have NOT dealt with the mushroom named in their titles; and 2) most of the taxa are NOT clearcut, but interbreed or hybridize at low levels, presumably in nature. This makes our man-made taxonomy somewhat muddy.
I hope this gives you a taste of my pursuits. Take a peek at the interests of Karen Hughes and our students for a more complete picture.
Dr. Ron Petersen
The University of Tennessee
437 Hesler Biology Building
Knoxville, TN 37996-1100
Phone: (423) 974-6217