by Michael Kuo
Other "Taxonomy in Transition" pages at this site have focused on the often astounding relationships between mushrooms discovered in recent DNA studies, representing the final species groupings ("clades") in such studies. But looking at DNA discoveries in this way ignores the evolutionary stories that are told when the branches of phylograms are not conflated. So here, for a change of pace, I will attempt to represent one such story--one about the genera Russula and Lactarius (among others), told by Steven Miller and his co-authors in a 2001 paper on the Russulales (citation below).
But I have already misrepresented things. Miller and his colleagues did not tell this story at all; they simply read it. The DNA molecules of about 40 sampled mushrooms were the ones doing the story telling. Stop and think about this for a moment. Scientists can read accurate evolutionary stories by decoding the sequences in DNA. The evidence is thorough and reliable even in the absence of a fossil record--which is something you might want to mention publicly the next time some moron on your child's school board starts talking about how evolution is "just a theory," on par with the nonsense that is now labeled "intelligent design theory," and how the "gaps in the fossil record" prove his point.
As most mushroomers know, Russula and Lactarius are very similar. The latter genus, however, produces a latex or "milk" when injured. Some microscopic features have also been used (with varying degrees of success) to separate the two genera. And experienced mushroomers can testify that there is a certain physical je ne sais quois that makes the two genera distinguishable to the naked eye, most of the time, without requiring close inspection. But how did these genera evolve over the millennia? Did one evolve from the other? Or did they both evolve from some common ancestor that we might call a "Russultarius?" Here is the partial answer to these questions provided by the DNA analysis of Miller and his colleagues:
I have left off some branches, and over half of the mushrooms included by Miller and his colleagues, in an effort to simplify the picture somewhat--but even this very simplified version raises many questions. For now, let's put off an answer to "What on earth are Macowanites, Zelleromyces, Gymnomyces, and Arcangeliella?" and focus on Russula and Lactarius.
I'll start with an explanation for the gray lines, since this explanation is necessary in order to understand the rest of the picture, and since it reveals something important and wonderful about scientists. Some of the lines are gray because they represent results that Miller and his colleagues are less certain of. In Molecular Biologese, the results in gray have "low bootstrap support," which means that the software was unwilling to certify the gray results as very reliable. Any number of factors could have produced the "gray area"--but let's save a discussion of such things for another day. It is worth mentioning, however, that any decent theory has a good sense of its restrictions and potential fallibility; try asking a proponent of "intelligent design" to discuss the limits of his or her theory.
But in the case at hand the picture is clear enough even with the gray areas that will eventually be brought into focus with further research. In short, the big story is that Lactarius appears to have evolved from Russula. Look at point A on the diagram, which could be said to represent a common ancestor of all the mushrooms that follow. From this common ancestor, to quote Robert Frost, "two roads diverged in a wood" (quite literally in the woods, since the mushrooms are mycorrhizal). The study's results are clear on this divergence; hence the black arrows. In the word of Miller and his colleagues, there are two groups of russulas: "one group arising separately from the main body of the tree and one group basal to Lactarius" (344).
If you follow the bottom branch from point A, you will see that, assuming the branch between points A and B is accurate, point B represents the dawning of the age of Lactarius, where a common ancestor gives rise to the milky caps as well as several species of Russula. The picture is not very clear after this point, but the authors note that Russula and Lactarius in this area of the diagram are less clearly distinct, and that "[t]he potential for persistence of a mixed Russula-Lactarius clade remains a strong possibility which will require further testing" (350).
And now to the funky names on the diagram (Macowanites, and so on). They represent "gasteroid" species, the sort of thing that mushroomers in most areas never encounter--in part because many of them develop underground, and in part because they tend to occur in arid areas of western North America, where fewer mushroom collectors are scrounging around. The mushrooms in these genera look like deformed versions of gilled mushrooms in which the gills never really developed and the cap wound up approximating something more like the ball on a puffball. Many of these "gasteroid agarics" have been described over the years, and mycologists have noted that the various species tend to align themselves with "normal" genera of gilled mushrooms, sharing many of their features. Macowanites and Gymnomyces species look like aborted species of Russula, while Arcangeliella and Zelleromyces share affinities with Lactarius (including the production of latex). I'm afraid I can offer you no photos (or links to photos) of these mushrooms, but to give you an idea of the concept, MykoWeb's California Fungi has nice photos--by Hugh Smith, who is also a contributor to this site--and a description of Longula texensis, which is a gasteroid species related to the genus Agaricus (be sure to click the links to additional photos at the bottom of the page to see what Longula texensis looks like when sliced open.)
Mycologists have always struggled with an explanation for the gasteroid agarics and boletes. One of the prevailing theories in recent years has been that gasteroid forms evolved from the normal forms as natural selection pressures caused them to adapt to low-moisture environments, but this idea is fairly hotly debated. What is clear from the diagram above, however, is that the gasteroid, Russula-like and Lactarius-like genera are indeed closely related to their counterparts--so closely related that they should probably be treated, genetically at least, as species of Russula and Lactarius. DNA studies of other gasteroid genera have made similar findings (for example, that species of Gastrosuillus should be treated as species of Suillus).
In the diagram above I have left out the various "pleurotoid" and "annulate" species studied by Miller and his colleagues. The former lack stems, and the latter have rings. Believe it or not, they belong to Russula and Lactarius, though most of us have never heard of them. They are species from the tropics (Guyana, Tanzania, and so on), and they are not likely to be in your field guide to North American mushrooms. But the study by Miller and his colleagues makes it clear that these species truly are species of Russula and Lactarius, since the mushrooms are embedded among the others throughout the phylogram.
Miller and his colleagues make it clear that further research into the Russulaceae is required before grand proclamations about the family can be made. But their preliminary study indicates the strong likelihood that Lactarius evolved from Russula, that the two genera may be very closely related in the Russula brevipes-Lactarius piperatus area, and that previous suggestions that the associated gasteroid genera belong in the family are correct.
Miller, S. L. et al. (2001). A molecular phylogeny of the Russulales including agaricoid, gasteroid and pleurotoid taxa. Mycologia 93: 344-354.
Cite this page as:
Kuo, M. (2005, January). The Russulaceae. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/russulaceae.html