Identifying Morels with Morphology

by Michael Kuo

Analyzing a mushroom's physical features has traditionally been the way to identify it, and the morel species have been separated this way for over 200 years, ever since Linnaeus described "Phallus esculentus" (which later became Morchella esculenta) as "a phallus with an egg-shaped head." The arrangement of pits and ridges on a morel's cap, the attachment of the cap to the stem, the size of the stem, the shape of the cap, and the colors of both cap and stem . . . all of these physical features have been used to define morel species over the years. Mycological arguments about how many morel species there are have amounted to disagreements over the extent to which a difference in physical features justifies a separate "species" or not.

Mycologists who studied morels were spared the taxonomic headaches produced by the glorification of microscopic analysis in the 20th Century since morels, for the most part, fail to demonstrate substantial microscopic differences. Thus the genus Morchella did not burgeon with new species defined on the basis of micro-morphological differences, and the total number of morel species worldwide ranged somewhere between a handful and 30, depending on which mycologist one asked. However, though they stayed under the radar for the Microscopic Revolution of the last century, the morels can no longer hide from this century's DNA Revolution, and the morel portrait being painted by molecular biologists is very different.

But before viewing the new portrait and comparing it to the old one, it is worth asking why we ever expected the old picture to be accurate. When we define "species" on the basis of physical differences we can perceive, we are merely projecting our perceptive abilities (and limitations) onto the natural world if we do not accompany our decisions with some sort of working theory about why organisms look different. If one morel has ridges that darken with maturity and another morel does not, we are not necessarily justified in calling them separate "species" just because of a physical difference seen by the human eye, since there is no scientific theory behind the decision other than "these organisms conveniently evolved so that I can see their differences."

Though mycology has not ignored evolutionary theory altogether, it has unfortunately not paid much attention, preferring to play "One of These Things Is Not Like the Others," erecting an enormous fungal taxonomy based on morphological differences without thinking about what the differences mean. Why does one morel have darkening ridges while another morel does not? How about this: because the morel with darkening ridges has evolved from an ancestor that was mycorrhizal with conifers, while the morel with pale ridges has evolved from a hardwood loving ancestor. Morels appear in the spring, when there is plenty of sunlight on the forest floor in hardwood forests, but not much sunlight under conifers, which have retained their evergreen needles over the winter. Perhaps the sterile ridges on a morel's cap absorb light and heat, warming the asci in the fertile pits until, at an optimum temperature, spores are released. Darker ridges, under conifers, would absorb light and heat more effectively than reflective, paler ridges.

Yes, I pulled that one out of thin air (though I kind of like it, now that I've typed it). The point, however, is not the theory itself, but the fact that having such a theory allows us to be scientific about the physical differences in the mushrooms; we can collect data that will support, or not support, the theory. But when our theory is merely "They look different to me," it is in essence a theory about ourselves and the mushrooms, rather than the mushrooms alone--and while we can collect data that will support or fail to support our theory, it will only be a happy coincidence if we stumble upon observable physical differences that correlate to significant genetic distinctions. In this sense, defining species based on physical differences is not necessarily a scientific process at all. Think of all the species of gilled mushrooms that have been defined on the basis of cystidia--and now reflect that no one knows what cystidia are, or what function they serve. If we discover that the "species" we have thus defined are not reflective of significant genetic differences, should we be surprised?

Molecular biologists, much to the frustration of previous generations of morphology-based mycologists, carry a "Get Out of Jail Free" card when it comes to the theory question. What they are studying is DNA, and their theory goes like this: Organisms evolve through genetic changes; DNA expresses these changes. You will not find a responsible scientist on the planet who will disagree with this theory. Problems do arise from the fact that science does not completely understand how DNA works, from the fact that molecular biologists are still figuring out the best ways to look at the DNA of organisms, and (more importantly) from the fact that DNA studies of fungi are unavoidably and inextricably immsersed in the "Looks different to me" milieu simply because molecular biologists study mushrooms that have been collected and identified on the basis of morphological differences. Even so, a morel "species" defined on the basis of DNA differences carries with it an evolutionary theory that generations of scientists have been working on, adapting, and correcting.

DNA Morels Versus Morphological Morels

The ongoing study of hundreds of North American morels sent in to the Morel Data Collection Project has led, at the time of this writing, to the preliminary identification of 16 genetically different morels. See MDCP Morel Taxa for descriptions and illustrations of these mushrooms. Based on the present data, it appears that only four of the 16 DNA-defined morels can be easily separated on the basis of physical features (in combination with consideration of distribution and ecological factors), while four "species groups" can also be determined with morphology. Fortunately, the two or three morels that are difficult to identify to one of the species groups are among the four morels that are easily identified individually. It should be noted that I am using the term "species group" rather uncritically, since the groups merely consist of morels that cannot be separated by examination of physical features; the mushrooms in the groups are not necessarily related in any scientific sense, despite their apparent similarities.

Below is a morphology-based key demonstrating the extent to which physical differences can be used to separate DNA-defined morels in North America. Further data may suggest ways to separate some of the morels that are currently grouped together, of course, but at the moment it is simply not possible to identify any North American morel merely by looking at it closely. Several "species," defined by both mycologists and morel hunters on the basis of physical features, have fallen by the wayside as a result of DNA studies; these are discussed below the key.

DNA testing of MDCP collections supports the idea that thick-stemmed yellow morels are not genetically distinct and do not merit species status. Specimens with crassipes-like, inflated stems have not grouped together, DNA-wise, and have spread themselves among morels in the esculenta-like group (the Classic Yellow, Taxon 12, Taxon 13, and Taxon 15).

Here are two specimens that certainly deserve the label Morchella crassipes, if it is valid in the sense of North American authors:

Under the scrutiny of DNA study, the specimen on the right grouped with many other specimens that lacked swollen stems, and was identified by separate methods (RFLP analysis and sequencing) as the morel I have called the Classic Yellow Morel. The specimen on the left was identified by RFLP analysis as belonging to Taxon 15 and by sequencing as a Classic Yellow. However, although the two DNA analysis methods see things differently for this specimen, neither result supports the idea that thick-stemmed morels are a genetically distinct species, since thin-stemmed specimens can be found in both the Classic Yellow group and the Taxon 15 group.

All of that said, it is possible, in my opinion, that North American authors (with a few exceptions) have consistently misinterpreted the original European descriptions of Morchella crassipes, focusing on the stem dimensions rather than details about the cap shape and arrangement of pits and ridges--and that the "true" Morchella crassipes may be a better-defined morphological species than simply a yellow morel with a big stem. Whether or not this more clearly distinct morel (on the basis of physical features) is genetically distinct, however, has not been determined with any certainty. The name Morchella crassipes has been applied to a genetically distinct morel in several European DNA studies (Wipf et al., 1999; Kellner et al., 2005), but not because its DNA matches a physically distinct species. The authors used the name because someone "deposited" a DNA sequence in "GenBank" (an international repository for DNA information) for a specimen labeled "Morchella crassipes" (for whatever reason)--and they found that several of their morels, which they were unable to separate from others on the basis of physical features, matched the GenBank sequence.

In general, North American mycologists have never suspected that what midwestern and eastern collectors often call "gray morels" actually constitute a separate species of morel, but amateur morel hunters can be pretty adamant about the idea. Though usage of the term varies, "gray morel" usually applies, east of the Rocky Mountains, to an esculenta-like morel with dark (sometimes nearly black) pits and pale ridges (in western North America the term is used by collectors for Morchella tomentosa).

DNA analysis of MDCP collections confirms what mycologists have long suspected: that "gray" morels are not genetically distinct, as a group. The many specimens labeled "grays" by collectors have not grouped together, DNA-wise, to the exclusion of other morels that do not have dark pits and pale ridges. In short, DNA evidence does not support the idea that "gray morels," in the midwestern and eastern sense, are a separate species.

That said, gray morels are certainly distinct in a non-scientific sense. In my opinion they are better for the table, and have better texture and taste than yellow morels. Although I have not examined every specimen in the MDCP that has been labeled a "gray" by the collector or that, in photographs, matches the description of a gray morel, most of the "grays" I have examined with a microscope have been immature (maturity in morels is easily assessed with a microscope, since immature asci do not contain spores). I suspect that "gray morels" are often just immature forms of yellow morels. My field observations also support this idea; many "grays" I have watched over a period of several days have developed into yellow morels that do not match the description of a "gray morel."

However, it is true that some "gray morels" are clearly mature--and it is even the case, in my experience, that some populations of morels are consistently "gray" from youth to maturity, year to year. The explanation for these consistently gray "grays" is not entirely clear (it may have something to do with weather conditions or other ecological factors)--but it is not because the morels in question constitute a separate species, according to the available scientific evidence. As one of many examples in the MDCP, here are two collections, both from Ontario, that DNA study has indicated represent the same morel species or taxon:

But the "pickles" in the MDCP have DNA that places them with various black morels (including the Classic Black, Taxon 06, and Taxon 07), suggesting that the greenish colors result not from genetic difference but from ecological conditions (exposure to sunlight, soil type, associated trees, and so on).

The same explanation appears to apply to "conicas," which western collectors label on the basis of their pointy caps. Burn-site morels labeled "conicas" by collectors have primarily belonged to Taxon 07--but Taxon 7 also includes morels bearing the labels "pickle" and "gray." Collections of conical-capped, western burn morels I have studied in the fresh state, like 08070407, have belonged to Taxon 05, the Classic Black Morel.

The "gray morel" referred to by western collectors is Morchella tomentosa. When young, this species typically features a dark gray or nearly black cap and stem, both of which are densely fuzzy. It is this form of Morchella tomentosa that is fairly consistently labeled a "gray" by western collectors. However, the species is sometimes brownish or even pale yellowish, especially when exposed to prolonged sunlight, and in these forms it is often given a different label by collectors, who then call it a "blond" or a "brown" (see 06150409 and 08070404 for examples). In other words, the western label "gray morel" is fairly consistently used to indicate a genetically distinct morel, Morchella tomentosa--but this species is also given other labels.

Cite This Page As:

Kuo, M. (2008, November). Identifying morels with morphology. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/mdcp/kuo_08.html