| BEN |
| BOTANICAL ELECTRONIC NEWS |
|---|
| ISSN 1188-603X |
| No. 179 November 28, 1997 | aceska@victoria.tc.ca Victoria, B.C. |
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[Kendrick, B. 1994. Evolution in action: from mushrooms to truffles. II McIlvainea 11 (2): 39-47.] [Continued from BEN 177 and BEN 178]
I have not mentioned all the sequestrate genera connected with the families listed in Part 2: many of them are rare, or are known only from the southern hemisphere. But I have given you enough information to realize that the evolution of sequestrate forms is a widespread phenomenon. And from what I have said about the Russulaceae and the Boletaceae, it will be obvious that more than one evolutionary pathway may evolve within a single family, and perhaps even within a single genus.
One or two interesting questions arise from my survey. Why have sequestrate forms evolved? The generally accepted explanation is that during dry periods of the Earth's recent history some mushrooms mutated in such a way as to remain closed, and lose their spore-shooting mechanism. This gave these lines a selective advantage over those which exposed their gills to the hot, dry air. It is easier to maintain an appropriate level of humidity for spore development inside a closed fruit body. The next step, of remaining underground, is another way of escaping drought. Of course, once the spores are retained inside the fruit body, or kept underground, the problem of dispersal arises. In many cases, this has been solved by involving small mammals as vectors. That means evolving mechanisms for attracting these mammals and getting them to dig up or eat the fruit bodies. So one kind of adaptive change is complicated by the need for other adaptations. But that is what evolution is all about, and any organism that survives and propagates itself has obviously hit on a successful, or at least a functional, combination.
It is less easy to explain the geographic distribution of these sequestrate and hypogeous forms, since they appear to be concentrated in such areas as western North America, parts of South America, New Zealand and Australia, and to be relatively few in number in other areas such as eastern North America and northern Europe.
No sequestrate fungi have yet been connected with two agaric families, the Hygrophoraceae and the Pluteaceae. Do such fungi exist, and have we simply not seen or recognized them? And although the Tricholomataceae is a very large and diverse family of agarics, a sequestrate derivative (Hydnangium) is known only for Laccaria. Why have none of the other more than 30 widely recognized and often very common genera in this family produced sequestrate offshoots? Or have we simply not yet found them, or recognized them for what they are?
In most cases, the sequestrate forms are much less common than their spore-shooting ancestors (though this is not true of Rhizopogon). Is this scarcity more apparent than real because they are more difficult to find, since many of them grow below-ground? Does it indicate that most of these fungi are no more than rather unsuccessful evolutionary experiments, on their way to extinction? Or have they arisen so recently that they have not yet had time to spread very far?
How long ago did the oldest, and the youngest, of these fungi arise? This question, at least, we may attempt to solve by means of our newly acquired molecular techniques, which can measure the amount, and the rate, of change in the genetic material. Could sequestrate forms be appearing regularly, even now? Are the changes taking place gradually, as the necessary mutations slowly accumulate in mushrooms. Or do they appear suddenly and sporadically as a result of what is called "punctuated" evolution, involving larger jumps during periods of great environmental stress?
Why has all this happened? Is it the new trend among mushrooms? Will all mushrooms eventually become sequestrate? Will our descendants have to dig if they want to see the fall flush of fleshy fungi, or fill their cooking pots with boletes and other fine edibles? Only, I suspect, if the greenhouse effect goes all the way and our climate becomes much drier and hotter than it is now. But we'll have to wait and see.
We are not yet in a position to answer all of those questions, but at least we know know that there is a wide range of such fungi out there. There is a message here for the amateur: Don't just throw away those aberrant closed or distorted or partly hypogeous agarics. Cut them open to see if their gills are normal vertical plates, and check them to see whether they can be persuaded to yield a spore print. If the answer to both of the above is no, then you may very well have a sequestrate fungus on your hands. One of the professional agaricologists in your area should be able to check this. If it is indeed one of these most recently evolved taxa, you may congratulate yourself on your sharp eyes, and science may thank you for one more piece of the evidence we need to unravel this great jigsaw puzzle.
Acknowledgments: I would like to acknowledge stimulating discussions with Drs. Jim Trappe, Michael Castellano, Neale Bougher and Harry Thiers.
Readers who wish to explore the "sequestrate" agarics further should consult the publications listed below.
Submissions, subscriptions, etc.: aceska@victoria.tc.ca. BEN is archived at http://www.ou.edu/cas/botany-micro/ben/