Echinodontium ballouii – from eyeballs to DNA
by Leon Shernoff
Fungal Classification: In the Beginning
Way back in 1751, the Swedish botanist Carol Linnaeus had great success with a new system for classifying plants – the system that is the basis of the one that we use today. Much of Linnaeus’ success is often credited to his invention of binomial nomenclature – the idea that each plant should be referred to be a combination of a genus and species name. However, he also got a lot of mileage by categorizing plants according to how they ran their sex lives – not just flower structure, as you might expect, but whether each flower had both male and female parts or whether they were separated, and whether plants with separate male and female flowers would bear them on the same plant or whether there were separate male and female plants. He actually described these criteria in very provocative terms: as whether the male and female parts were in the same bed or the same marriage chamber or not, and he was roundly attacked by the fundamentalists of his time. He usually responded by naming small weeds after them.
Any successful system demands expansion, of course, and Linnaeus was soon extending his system from the green plants that he was originally interested in to fungi, lichens and even animals. Linnaeus really knew his plants, and his focus on sexual systems worked really well for them. It worked much less well for fungi, but he was pretty clueless about them (his first classification of fungi included only about a hundred altogether) so it didn’t bother him that much. By that time, people were pretty sure that mushrooms reproduced by means of spores, so Linnaeus made a fungus’ spore-bearing surface the basis of its classification and gave this surface the typically provocative name of ‘hymenium.’ All mushrooms that bore their spores on gills went into the genus Agaricus. All mushrooms with a honeycombed hymenium on a stalk went into the genus Phallus (another typical Linnaeus name), which sounds familiar until you realize that the morels went in there, too. So the first name for the morel under Linnaeus’ system was Phallus esculenta, the edible phallus; no wonder the fundamentalists were disturbed.
Luckily, help was on the way. In the 1790s, Christian Hendrik Persoon started shaking things up a bit, and in 1821 Elias Magnus Fries redid the classification of fungi in a major way. He separated the morels from the stinkhorns and liberated the chanterelles from the strange spot that Linnaeus had stuck them. Linnaeus had created the genus Merulius for fungi with an irregularly wrinkled hymenium – when many of you learned your mushrooms, you learned Merulius as a group of wrinkly jelly-like things that lie flat (resupinate) on wood. Because chanterelles also have a wrinkly hymenium, Linnaeus decided that they were somehow closely related and it fell to Fries, later, to separate them. Fries also separated the polypores from the boletes – Linnaeus had put all the “pored fungi” together – and Fries later supported the division of polypores into further categories like Fomes (for the hard, woody, perennial conks), using “polypores” as a larger-scale category that included all these new genera. Fries brought a lot of common sense to the classification of fungi. But he was still working within the basic outlines of Linnaeus’ system, just with further refinements.
So a hundred years ago, when Echinodontium ballouii was discovered, fungi were still primarily categorized according to the structure of their hymenium – whether it was composed of gills, pores, spines, or something harder to categorize. Within these broad categories, mushrooms were then broken down into genera on the basis of whether they had a cap, a stem, or whether they were just resupinate (lying flat on what they were growing on).
When Ballou sent Howard James Banker at the New York Botanical Garden his first specimens of Echinodontium ballouii, Banker placed it in the genus Steccherinum. Steccherinum was a sort of catch-all genus for fungi with a spiny hymenium and a cap but no stem. Other people disagreed with Banker and placed Ballou’s find in other genera, and it didn’t end up in Echinodontium until 1964, when Henry Louis Gross took a comprehensive look at the genus Echinodontium and pointed out how closely it fit in. The genus is based on E. tinctorium, which has some unique features – a thick cluster of spore-bearing spines and brightly-colored flesh – so it took the work of Henry Louis Gross to point out that the other Echinodontiums – most of them less spiny and more dully colored – were so similar to it microscopically.
Morphology, then and now
What features merit inclusion in a genus? The mycological consensus on this has changed drastically in the last fifty years. Linnaeus started us off with an approach based on morphology: in broad terms, simply how the mushroom is shaped. His approach was very much based on the naked eye – and, I might add, what the naked eye can taken in quickly. My favorite example is an example from his first classification of the fungi: Agaricus (Russula) rubra, where Russula is a subgenus that was later raised to genus level. Yes, rubra means red, and his treatment amounts to giving all red-capped Russulas, everywhere, the same name. Currently there are about two hundred red-capped Russulas currently named in North America alone, and probably a dozen just in Linnaeus’ native Sweden. But to Linnaeus, they were all just red Russulas, Russula rubra
Persoon and Fries brought [ahem] more sophistication to their classification systems. and during their lifetime more and more microscopic features were included in descriptions of fungi – extending the concept of morphology to the microscopic level. Fries also tried to use developmental features as one of the criteria for a genus. He didn’t do a very good job of it, but by the mid-20th century, some mycologists advocated taking microscopic and developmental features into account in fungal classification.
Back to Echinotontium
When Gross monographed the genus Echinodontium in 1964, he did something that was at that time quite “modern” and daring: he included the two species of the white crusty genus Laurilia. Why? There are a variety of reasons. The illustrations below shows a microscopic view of the hymenium of Echinodontium tinctorium and Laurilia sulcata, the species upon which both genera are based. These illustrations are taken from Gross’ paper. Both fungi have thick-walled cystidia that are encrusted with crystals. Cystidia are cells that protrude from the main body of the fungus out into the hymenium. Since they seem to evolve quite rapidly, such a strong similarity in cystidia has been considered a marker of evolutionary relatedness. The hyphae with the thickened walls, by the way, are what stiffen the fruiting body and give it its woody (E. tinctorium) or tough (L. sulcata) texture.
Gross also emphasized a developmental feature: that these fungi are perennial, and form each year’s new hymenium by growing it directly on top of last year’s. If you cross-section each of these mushrooms, you’ll see a set of growth layers outlined in the flesh, like the growth rings on a tree, each parallel to the one before, outlining the same set of teeth, or (in the case of Laurilia) the same set of random bumps and other irregularities. Gross noted that the spore-bearing spines of these fungi thus get thicker each year, and in the case of E. tinctorium the spines eventually grow into one another, starting from the base outward. The spines in the specimen of E. tinctorium pictured here seem to have grown together quite a bit. But, Gross said, if you cut the conk down the middle, you can still see the outlines of where each previous year’s growth of each spine occurred, embedded in the now-solid flesh.
All of this sounds like what many other conks do: don’t the big Fomes and Phellinus species also form new layers of pores each time they sporulate? Well, they do, and they also exhibit “growth rings” in cross section, but with them, the next layer of pores comes from hyphae growing up through the previous layer of pores from the main body of the fungus. Gross emphasized that after each hymenium of a polypore sporulates, it basically dies, so the next wave of pore formation comes from hyphae from the main body of the fungus growing down through the existing pore openings and forming a new pore layer on top of them. In the Echinodontiums, the older hymenium stays alive and grows a new one directly on top of itself. He also emphasized that in polypores, the hymenium will correct for previous errors and try to present the pore openings on a smooth horizontal plane. For instance, if the conk has to grow around a branch at some point, this may distort the pore layers that enclose the branch, but once it has grown past the branch, the pore layers will become regular again. Not so in Echinodontium and Laurilia – each irregularity of the hymenium is perpetuated in subsequent layers.
There were also more subtle similarities, difficult to put into words. For instance, when the Japanese mycologist Rokuya Imazeki was describing his new find, Echinodontium tsugicola in 1935, he mentioned that its white, tough and irregularly bumpy upper surface looked exactly like that of Laurilia sulcata. It’s hard to formalize a similarity like that, and make it part of a system, but it impressed Imazeki enough that he felt obliged to mention it, and Gross of course felt that this was significant also.
All of this adds up to a pretty hefty set of similarities; why were the Echinodontium and Laurilia species ever put in different genera? Because the two Laurilia species have a “smooth” spore-bearing surface, without regular structures of the types (such as gills, teeth, pores) that were then officially recognized. Linnaean/Friesian mycology, the first systematic approach to the classification of the fungi, emphasized the shape of the hymenium and made its major divisions of fungal classification according to its shape – gills versus pores versus spines and so on. Laurilia sulcata thus started out its scientific career in 1901 as Stereum sulcatum, where Stereum is the old general-purpose genus for fungi with a protruding cap whose hymenium is “smooth.” Now, in fact, its hymenium is anything but smooth – it is covered with all sorts of irregular bumps, protrusions and folds. But Edwin Angus Burt, who named it, felt that this should get recognition at the species level, not the genus level: sulcata means “folded.”
Does Morphology Include Chemistry?
At the time that Gross wrote his paper, chemical features were being added to microscopic and developmental ones in determining which fungi were related to which. For instance, around the same time as Gross’s paper, Rolf Singer placed the genus of huge polypores, Bondarzewia, in with the Russulas and Lactarius in his classification system, because all three genera possess a unique type of spore: one having amyloid ridges or flanges. “Amyloid” is the fungal-speak term for mushroom parts (usually microscopic) that have a starchy structural component, so they turn black when exposed to iodine. It’s not particularly rare for fungi to have an amyloid spore wall, but it is quite unusual for a fungus to have an inamyloid spore wall with amyloid ornamentation, as these do. Amyloidity, as it has turned out, is a taxonomically important feature in the group of mushrooms that includes Echinodontium.
And at the same time, the Dutch mycologist Marinus Donk was pondering whether to include the delicately-spined genus Hericium in the same group with Lactarius, Russula and Bondarzewia, because some of the hyphae in Hericium contain a latex similar to that in Lactarius (although the Hericium species never contain enough to exude “milk”, the way the Lactarius species do). In 1971, Donk would be the first to propose that all the fungi now known as the Russulales be grouped together because of these latex-containing hyphae.
But this sort of thing was slow to catch on. In 1959, the Czech mycologist Zdenek Pouzar had decided to create the genus Laurilia, because he felt that the fungi with encrusted cystidia and such an irregular spore-bearing surface deserved to be separated from the more ‘ordinary’ Stereums. But he didn’t take the next step that Gross took and look to see if there was already another place to put them – one where they fit according to microscopic and developmental features, but not according to spore-bearing surface. So most mycologists have historically not gone along with Gross’ transfer of Pouzar’s Laurilia species into Echinodontium.
An Attractive but Incorrect Picture of Evolution
If one accepts Laurilia and Echinodontium as being in the same evolutionary group of fungi, one can map out a very smooth and convincing transition from “primitive” to “advanced” spore-bearing surfaces, as you can see from the sequence of pictures above. From Laurilia, which is bumpy and irregular but doesn’t have anything on it that you could call an organized structure, one moves to E. ryvardenii, which seems to have developed some of Laurilia’s little bumps into real stalactites. Then it’s on to E. tinctorium, which has a whole bunch of stalactites, and finally the fine-toothed Hericium, which would represent admirable modern efficiency.
However, now we also have DNA evidence, which has turned everything on its head.
We are lucky in that Echinodontium ryvardenii was discovered just in time to be included in a huge DNA sequencing project, Assembling the Fungal Tree of Life (AFTOL). In fact, it arrived in time to be included in a 2003 paper by Ellen Larsson and Karl-Henrik Larsson on the Russulales, the large-scale taxonomic group that also includes Russula and Lactarius. A later publication in the official unveiling of the AFTOL results three years later was very much the same, and the monstrosity on page 33 is a schematic view of their results, incorporating features from both papers. For those of you who wish to look at the original charts and original papers, the citations are provided at the end of this article. You should be able to find Mycologia (where most of these articles were published) in any college library; or, for those of you with an internet connection, most of these papers (as well as many additional ones) can be downloaded from David Hibbett’s website: http://www.clarku.edu/faculty/dhibbett/publications.html. He was an author on almost all of them.
From this, you can see the great diversity of hymenial surface and fruiting body form in the Russulales. In this and the interweaving of different hymenium and fruiting body types within the same derived group, we find more confirmation that the older view of evolutionary relationships – that they are indicated simply by hymenium type – is incorrect. For example, not only are Laurilia sulcata and Echinodontium tinctorium as close as can be, but the two other fungi in the same group with them are Heterobasidion annosum, a hard, extremely fine-pored (i.e. sophisticated-looking) “polypore” that from looking at it to be just a typical conk, and Bondarzewia berkeleyii, a huge floppy thing that is also pored but grows on the ground, is fleshy rather than hard, and has pores that break apart and become maze-like in age. Three different types of hymenium, and a great difference in features between the two poroid species.
Likewise, nutritional lifestyle doesn’t seem to be a good mid-scale indicator of relatedness – the fungi just named are all wood-rotters, on dead or living trees, but a neighboring group includes the mycorrhizal genus Albatrellus.
The placement of names requires a little explanation. As one moves to the left in the chart, one is moving backwards in time. The vertical black line running down the left-hand side of the chart is called the backbone. It connects to the presumed single ancestor (the black line leading off to the left) of the group as a whole, and represents the initial radiating of this single ancestor into different species. These newer species are said to be derived from it, and the newer species that have evolved from them are even more derived.
Most of the branches coming off the backbone lead to groups of varying size. For instance, near the top, beside the picture of Echinodontium ryvardenii, is a node labeled “Gloeocystidiellum 5”. This means that the original chart has five species of Gloeocystidiellum in this position. Larger groups usually contain more than one genus of fungi; in this case, each genus is listed in turn, separated by a plus sign. Where a species had no discernible relatives among the other fungi sampled for this project, it is connected directly to the backbone. In cases where a genus appears in separate locations within a group, it is listed only once but with slashes between the number of species at each appearance – for instance “Aleurodiscus 2/1” means that there are two Aleurodiscus species present at one point in the group, and another one by itself later on, with species from other genera in between.
Thus, another thing shown by the chart is how many of the more poorly understood genera are really a random collection of unrelated species. Scytinostromella and Wrightoporia seem to be the champions in that regard. These are resupinate genera – Scytinostromella has a smooth hymenium and Wrightoporia is pored – that are defined by having dextrinoid thickened structural hyphae and crystalline bumps on their regular hyphae. This combination of features seems to be absolutely meaningless as a way to indicate evolutionary relationships, as you can see by finding various species of these genera all over the chart.
The champion for being scattered among different groups, however, is Gloeocystidiellum, which appears in four different groups, and within one of them in four different places. Its criteria of prominent latex-containing cystidia is not meaningless, however, as the species fulfilling this criteria are often grouped in clumps. What it does show is that prominent latex-containing cystidia seem to have evolved independently several times.
Horizontal position in these charts usually indicates when something evolved – the farther to the left a species is, the longer ago it evolved. In this chart, that dimension is almost meaningless, since the relation of each group to the backbone – and to each other – is so poorly defined. The chart depicts the larger groups as starting more to the left, but that’s something that’s forced by the fact that within each group something had to come first, so in a larger group that first something gets shoved further back in line. The smaller groups could each be very recent or very ancient – there’s no way to tell from the data in the study.
Presumably vertical position in the chart means something, but it’s not clear what, and the authors do not discuss this. So it’s not clear, for example, among the species that are connected directly to the backbone, why there’s a cluster of four of them at the top of the chart, and one at the bottom. There is only one by itself in the middle in the original chart – Pseudoxenasma verrucisporum. Scytinostromella cerina is mentioned in the original text as connecting directly to the backbone, but the authors didn’t include it in their chart. I stuck it in at this arbitrary location because its presence supports a couple of observations I want to make.
This use of “basal” and “derived” gets us away from the older style of talking about more “primitive” and “advanced” fungi, which is preferable for two reasons: first of all, this isn’t obvious from my chart, which has horizontally compressed things a bit, but in the original pretty much everything in it is about equally derived – in fact very much so – or of unknown placement. One way you can see this on the chart is that there’s no group of fungi with another group coming out of it. So there isn’t really a spread of taxa over enough evolutionary time to make the older labels – “primitive” and “advanced” – meaningful.
Second, you can see that the flat, resupinate hymenium that was formerly thought to be a “primitive” state (shown in magenta on the chart) is present all over the place, in groups alongside fungi with a more structured one. So that old notion goes down the tubes also.
The first thing that strikes one is the poor quality – or more precisely, the non-usefulness – of some of the data. There are seven isolated species that are connected directly to the backbone, which means we have no idea whatsoever of their relatedness to anything else in the chart. If we add to these the two species of Aleurocystidiellum towards the bottom, which the authors especially single out as being unusually unrelated to all the other groups, and the strange duo of a Scytinostromella and a Wrightoporia species in the next-to-last position, we have eleven species that are essentially floating freely within the chart, with no indication of their relations to any other taxa. This is just under 10% of the total number of species sampled.
This is not a criticism of the quality of work that these scientists did – it is perfectly possible that the 127 species that make up the original tree are less than 1% of all the currently existing Russulales; so it should not be surprising to see that some of the taxa sampled have not had any near neighbors included. This work is preliminary, and even the follow-up work will remain preliminary for some time.
A more troubling problem is the poor resolution at the middle level of detail – between these small groups and the backbone. We have good resolution of the smaller groups that I have shown as single nodes in my schematic, and we are quite confident that all the fungi shown belong in the Russulales. However, we have hardly any information at all about how each group relates to one the other ones.
This is a consequence of the two DNA areas that were sequenced to provide the data. These were two locations sequenced in the fungi’s ribosomal DNA: the first is the nuclear large subunit rDNA, which is thought to evolve fairly slowly, and so is used to broadly position a species in the fungal tree. The second is the so-called ITS region, which consists of two non-coding spacer regions and the 5.8S rDNA gene. Since spacer regions don’t code for anything (they just separate coding genes), mutations are thought to accumulate more rapidly so this locus is used to compare closely related species. Unfortunately, the effect of using these two locations is to give us good small-scale and whole-group resolution, but to leave the middle-ground almost blank. This is a shame, because the middle-ground should have been the most valuable part of such a survey.
There are those who will say, “No, no! It’s the small-scale detail which is valuable,” but really, if we want to do small-scale work on Russula (for instance), we’ll sample more than five species, and undoubtedly at more than these two DNA locations.
These are the same two locations that were used for the whole large-scale AFTOL project: a decision was made to limit the sequencing to these two, in order to sequence the fungi uniformly across the whole project. In addition to this desire for compatibility between the results of all the branches of the project, there was also a practical consideration. First of all, these were two of the first locations discovered to be useful in sequencing fungal DNA, so they have been used more than any others which means that they’ve been obtained from more fungi. Work that was done a while ago means less work to do now.
Secondly, using only two locations made the project more manageable, and even so it was five years late (and I think we still don’t have the results from the Polyporales). But it’s time to get working on more locations, and get the middle-ground of the Russulales resolved. As it is, the most valuable results of the survey are to show that many of these resupinate genera are badly defined, and the function of including things like those five Russula species is in effect to calibrate the system – to show that the procedure does in fact produce the results that we were dead certain of already, so the surprising ones are probably correct also.
Well, enough of this analysis of the study. Let’s see what help it gives us with the fungi we’re most interested in at the moment. First of all, it takes that tidily linked evolutionary chain that I outlined in photos a while ago and blows it out of the water (the photos are included in the chart). Two of the fungi that we’ve been discussing as being part of that sequence, Laurilia sulcata and Echinodontium tinctorium (towards the bottom), turn out to be sister taxa, at exactly the same level of evolutionary derivedness.
Of the other fungi in our model evolutionary sequence, Hericium actually looks like it’s slightly more basal, but this is just an artifact of it being part of a larger group. Echinodontium ryvardenii, on the other hand, is way up at the top of the chart, and is connected directly to the backbone, which means that nothing directly related to it was included in the survey. Larsson and Larsson report that “despite having [a whole bunch of anatomical similarities that we haven’t even discussed here,] it is not related to the other Echinodontium species. Its true affinities are unclear. [That’s ‘species’ in the singular, by the way. They are speaking of the relationship of E. ryvardenii to E. tinctorium, the only other Echinodontium in the survey.]” It could be extremely basal, with no descendants; it could be extremely derived, with all its close relatives extinct; or it could be in pretty much the same position as the other fungi in the chart, but no related fungi were included in the sampling. We just don’t know.
Whither the Russulales?
So is there any physical similarity across the whole Russulales? Well, there turn out to be two. Unfortunately for you and me, they turn out to be (usually) microscopic. The one absolutely diagnostic feature – all the Russulales have it, and no fungus outside the Russulales does – is hyphae that contain a latex. Only three groups of Russulales (to my knowledge) produce enough latex to exude it when cut – the genera Lactarius is of course named for this ability; some Stereums also exude latex; and Bondarzewia will do so when young.10 However, whether or not they have enough latex to exude, all the Russulales turn out to have hyphae that are filled with these compounds. There has been little formal research into their function, but the AFTOL paper on the Russulales mentions the hypothesis that “based on the chemistry of the fluid contained in hyphae from Lactarius velutinus, Camazine and Lupo (1984) suggested that the laticiferous [I did not make up this word] hyphae functioned as a storage depot for precursors of pungent dialdehyde compounds. These compounds are largely unstable and change rapidly from nontoxic to toxic form, leading to the hypothesis that they are chemical defensive agents protecting the spore-producing structures from mycophagy.” To which I would add that because most of these would-be mushroom grazers are tiny bugs, the fungi don’t need to produce amounts of these chemicals that are visible to you and me to be effective as a deterrent.
The other diagnostic feature, somewhat less reliable, is amyloidity, the reaction where a starch compound reacts with iodine to turn blue-black. Readers of field guides know that many gilled mushrooms have amyloid spores (for instance, two of the more toxic sections of the genus Amanita), so simple amyloidity is not the exclusive privilege of the Russulales. However, a layer of amyloid ornamentation on an otherwise inamyloid spore does seem to be, and having cystidia or some other bodily feature that are amyloid is vastly more common in the Russulales than outside them.
If you look at the chart, you’ll see that almost all the names that are totally unfamiliar to you start with amylo- or gloeo-, indicating that this is a resupinate fungus with some part (usually microscopic) that was either amyloid or latex-bearing. Those names serve as an indication of the completeness with which those two features cover the Russulales. It could mean that such features were present in the presumed single ancestor of this large group of fungi, or that they’ve simply evolved independently multiple times. Or both – they could have been lost in some fungi and then re-evolved. We simply don’t know at this point.
This isn’t to say that closely related species don’t share these sorts of features. We’ve already mentioned how Laurilia sulcata and Echinodontium tinctorium both have thick-walled, crystal-encrusted cystidia. But the other two genera in the same little clade, Heterobasidion and Bondarzewia, have nothing of the sort.
Meanwhile, the next genus up from them is Aleurocystidiellum, which has minutely spiny, amyloid spores and thickened hyphae that terminate in the hymenium as encrusted cystidia. In other words, its hymenium, spores and hyphal system are exactly the same as the L. sulcata and E. tinctorium; apparently the only reason that no one seems to have ever thought about them being related is that Aleurocystidiellum is a cup fungus – it produces disc-like flat fruiting bodies. Unfortunately, this little group of Aleurocystidiellums connects directly to the backbone, so its relationship to any of the other groups is undefined. We’ve seen this already with Echinodontium ryvardenii, where the presence of encrusted, thick-walled cystidia turned out not to be an indicator of relatedness with E. tinctorium; instead, it likewise connects directly to the backbone.
But What About Echinodontium ballouii?
Well, Manfred Binder, one of the mycologists who did a lot of the sequencing work on the AFTOL project, also sequenced a bit of Bill and Larry’s E. ballouii, using the same two genes as the rest of the AFTOL project, and it came up extremely close to E. ryvardenii; so first of all, everything we’ve been saying about the evolutionary position of E. ryvardenii also applies to E. ballouii. Second, the rediscovery (and sequencing) of E. ballouii puts another data point into this widely isolated area of the chart. True, it now makes for two linked, very close points that are collectively isolated from everything else, but it’s a start. We need more data – more sampling of the other species currently placed in Laurilia and Echinodontium.
What does all this do for Echinodontium‘s status as a living fossil fungus? Well, as far as E. tinctorium is concerned, it blows it out of the water – it is clearly a recently-derived fungus. However, as far as E. ryvardenii and E. ballouii are concerned, the jury’s still out. Connection straight to the backbone of the larger group like this has been used as an indicator of basalness in other large-scale groups, but here the backbone is still very poorly defined, so we really can’t use the DNA evidence make any inferences about the large-scale relationships between any of the species clusters in the Russulales.
So Where Does This Leave Us?
With both DNA evidence and morphological evidence ruled out, this leaves us only with ecology. The world polypore expert Leif Ryvarden notes that these fungi all occur on pre-pine gymnosperms – apparently pines, with their big, woody cones and large, nut-like seeds are the most recently-evolved gymnosperms. He concludes, “Thus the genus must be a very old one.” His case is actually strengthened by the separation of E. ryvardenii and E. ballouii – perhaps we can call them the paleo-Echinodontiums – from the more modern group represented by Echinodontium tinctorium and Laurilia sulcata. The modern pair occur among a wide variety of old-growth conifers, while the paleo-Echinodontiums are each limited to a single species of host in the Cupressaceae, the basal family for the Pinales, the largest group of conifers.
Their situation reminds me of an unusual but recurring scenario in the evolution of other kingdoms, where an early family of organisms is displaced by a more specialized one, but retains a hold on its most inhospitable niches, which the newcomer for one reason or another is unable to annex. For instance, the aardvark, armadillo, and South America’s giant anteater are all members of a group of mammals called paenungulates, which used to be primarily herbivores (think elephants, which are also paenungulates) and were a very populous and diverse group in the southern hemisphere. However, they were outcompeted in their primary niche by more specialized grazers, and these three animals ended up getting more and more of their nutrition from insects, which had played a minor role in their diet up until that point… until all three ended up getting more and more specialized as nocturnal raiders of insect nests. As for the newcomers, the very specialization that allowed them to take over the local grazer niche also prevented them from competing with these surviving paenungulates for the role of insect-eater.
Likewise, the unique plant Welwitschia grows in the Namibian desert, the hottest and driest climate on the planet. Presumably at one point there were lots of related plants, from which this one species evolved to live in the desert. The ones that lived in more hospitable places were outcompeted by newcomers, but none of the more recent plants made the jump into the desert. My idea here is that the Chamaecyparis wood, which is so tremendously resistant to rot, may here serve in the role of the inhospitable environment that the older fungus managed to adapt to, and the younger ones have not caught up with it yet.
Greg Mueller, the head mycologist at the Field Museum, strongly objects to these speculations. He emphasizes that there’s no actual evidence placing these paleo-Echinodontiums at an earlier point in history than any other fungus, and that furthermore what little backbone evidence there is for the larger-scale basidiomycete group hints that the Polyporales/Thelephorales group is a little older than the Russulales/Agaricales/Boletales group, so odds are that there were “mainstream” polypores around prior to these paleo-Echinodontiums. He may be right. The emergence of these particular fungi as the dominant attackers of this ancient group of trees may just be one of those random things.
As Gross presciently noted, “the family may prove to be phylogenetically important because it is a small taxonomic group containing characters proposed to be both primitive and advanced.” Distinguishing between characters that are “primitive” and those that are “advanced” – or, as we prefer to say nowadays, characters that are basal and those that are derived – is an issue that continues to bedevil us today.
For the Future
Much basic work remains to be done – for instance, we still don’t really know whether the Laurilia/Echinodontium taxodii from southeastern North America is really the same thing as the L./E. taxodii of east Asia. Let’s get all these things sequenced – and at more than two DNA locations! And let’s see some more attention to their continued survival in the wild. Gross, for instance, notes that only a few collections of E. japonicum have ever been made. There are cultures of it listed in the stock of various culture banks, but it’s not clear if it’s present in the wild anymore. Perhaps a program of inoculating seedlings with the rarer of these fungi can be undertaken. If nothing else, it will at least demonstrate whether these cultures are viable or whether something like E. japonicum is gone for good. And if the fungus does fruit on the saplings, it will likewise demonstrate if the culture is of the correct mushroom! And it will hopefully somewhat increase the vigor of the cultures, which (according to the CBS notes on them) seem to be fading away on their refrigerated Petri dishes.
There are all sorts of other connections waiting to be made – for instance, the orange-red flesh of E. tinctorium is a striking feature, and definitely rare – and it is also present in E. tsugicola. Gross also notes that it fruits at branch stubs. In hindsight, this all seems significant – and it may also be significant in another genus.
The other genus of fungi I’d like to see attacked is Pyrofomes. Gross notes that Mildred Nobles, the great American expert on fungal cultures, felt that there was a great similarity between E. tinctorium and Fomes juniperinus. F. juniperinus turns out to have been renamed Pyrofomes demidoffii; it is a woody, perennial conk with bright orange-red flesh that grows only on juniper. Sound familiar? Furthermore, this pigment, which seems to be one of the defining features of the genus Pyrofomes, seems to be the same in both Echinodontium and Pyrofomes: “An orange pigment named echinotinctone was isolated from the wood-rotting polypores Echinodontium tinctorium and Pyrofomes albomarginatus.” (“Isolation of a fluorone pigment from the Indian paint fungus Echinodontium tinctorium and Pyrofomes albomarginatus,” Yang Yea, Ingrid Jostena, Norbert Arnolda, Bert Steffana and Wolfgang Steglich, Tetrahedron 52:16, 1996, pp. 5793-5798.) And if that weren’t enough, the pigment is fairly unique in the entire biosphere: not only is echinotinctone “the first simple fluorone pigment from fungi,” it is also “the first naturally occurring pigment [anywhere!] with a simple fluorone chromophore.” I have no idea what any of the chemistry means, but there’s clearly some explaining that needs to be done about why this unique pigment is found only in these two (groups of?) fungi – and now that we know what to look for on a chemical basis and not just by color, perhaps related but less colorful compounds will be found in closely related species. Pyrofomes is not closely related to Echinodontium – it is in the Polyporales rather than the Russulales – so maybe this pigment is a metabolic by-product of the specialized chemistry needed to feed on the wood of these tough-to-decay ancient conifers. The story should be interesting. As for the rest of these fungi!
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David Hibbett, “A phylogenetic overview of the Agaricomycotina,” Mycologia 98(6), 2006, pp. 917–925.
Ellen Larsson and Karl-Henrik Larsson, “Phylogenetic relationships of russuloid basidiomycetes with emphasis on aphyllophoralean taxa,” Mycologia 95(6), 2003, pp. 1037-1065.
Steve Miller, Ellen Larsson, Karl-Henrik Larsson, Annemieke Verbeken and Jorinde Nuytinck, “Perspectives in the new Russulales,” Mycologia 98(6), 2006, pp. 960–969.
Yang Yea, Ingrid Jostena, Norbert Arnolda, Bert Steffana and Wolfgang Steglich, “Isolation of a fluorone pigment from the Indian paint fungus Echinodontium tinctorium and Pyrofomes albomarginatus,” Tetrahedron 52:16, 1996, pp. 5793-5798.