By Gary Novak
The morel mushroom evolved from a single-celled yeast into a multi-celled mushroom over the past 70 thousand years. This appears to be one of the few examples of a single-celled organism evolving into a multi-celled organism in the past 100 million years, beyond some molds which could be doing something similar on a smaller scale.
Rapid change is still occurring with the morel. It must evolve into a cup fungus to survive through the next ice age. It won't get there, but a related species, Helvela crispa (H.c.) probably will. H.c. is like a potato chip on a stalk. The stalk can shrink and touch the ground, while the potato chip curves upward into a cup shape.
There are two closely related cup fungi which evolved as the morel did during previous ice age cycles. They have the same surface pattern on their spores as the morel, which means they evolved from the same yeast, which is probably Schizosaccharomyces japonicus.
There have probably been dozens of cup fungi evolved in the past, but they die out, because the cup shape is a failing attempt to cope with ascospores, which result from evolving from a yeast.
Ascospores form within cells. They do not disperse easily. To get them out, the tissue must dry and shrink to create a propelling force. But if the tissue dries too soon, the spores fail to form. So there must be rainy conditions for three days, and then dry conditions to propel the spores out.
The cup shape copes with the problem by creating slow-drying tissue near the ground and fast drying tissue extending into the air. Someplace in between, the tissue is supposed to dry at just the right rate to get spores out. It seems to work for a couple of ice age cycles before die-off of the species occurs. Ice ages have been cycling at precisely 100 thousand-year intervals for the past ten cycles.
The age of the morel can be pinned down to 20 thousand years as a free-growing mushroom based on the length of an ice age. About 30 thousand years would be required to form an ice sheet. About 50 thousand years would be required for a yeast growing at the base of trees to evolve multi-cellular characteristics. And the remaining 20 thousand years allowed the mushroom to evolve crude morphology and variant strains.
Something about the front edge of a glacial ice sheet allows a yeast to evolve into the morel mushroom. It is also where human agriculture began at the same time. As the ice sheet forms, it draws moisture out of the air due to the cold temperatures above it. The resulting dry air creates desert-like conditions in front of the ice sheet. Between the ice and the desert would be every imaginable ecosystem in a small amount of space.
The reason why the yeast cannot evolve into the morel at this time is because the tree sap gets overrun by algae, which is better adapted to growing on trees than the yeast is. Something about the front of an ice sheet prevents algae from overgrowing the tree sap that the yeast needs to grow on.
The morel only grows in sandy soil or something similar such as mountain humus, because sand never dries out due to its lack of capillary action. Yeasts do not tolerate dehydration, because they evolved in sugary solutions, as modern biology created sugary solutions about 50 million years ago.
The morel is like a yeast in every respect except its macro-morphology. Macro-morphology evolves easily, while micro-morphology and physiology do not. Physiology is dependent upon the location of enzyme systems, which are attached to membranes. These cannot be re-arranged easily. Changes in micro-morphology would re-arrange the enzyme systems, so it to does not change easily.
So the morel is still physiologically a yeast with no micro-structures other than what it carried from the yeast. The morel has an ascus with eight spores lined up in a row, as Schizosaccharomyces japonicus does.
For the yeast to evolve multi-cellular characteristics, it had to acquire signal molecules for cells to communicate with each other. Once these existed, crude morphology could take form. The morel does not have stable morphology because of its ancestor lacking multi-cellular morphology. When the morel is grown using an elaborate procedure developed by Ronald Ower, it will lack normal ridges on the surface and look like smooth-surface balloons, when large strains are used, though small morels will have normal appearance. No other species can produce such odd morphological variations.
Another morphological oddity of the morel is that it will form a tissue when growing on a surface such as liquids or gels. The tissue will include pigments characteristic of the strain along with complex cell structures. This structure indicates that the ancestor spent much time adapting to surface growth at the base of trees.
The morel cannot use its ascospores for seasonal survival, because they cannot be created and emitted reliably. It therefore transforms its underground cells into a resistant mass of cells called sclerotia. Sclerotia consists of blocky cells with characteristics similar to the ascospores. Sclerotial cells store energy, which allows them to transform back and forth into mycelial cells without much loss of mass. Under wet conditions, sclerotial cells grow into mycelium, and they revert back into sclerotia as conditions get drier. Sclerotia also carries the organism through freezing conditions during the winters.
In this way, morels do not have to form every year. In fact, it takes about three years for a mycelial mass to get adequately developed underground, before morels emerge. When morels emerge, the mycelial mass is 2-3 meters across.
The size of the mycelial mass can be determined, because each spore outgrowth produces different appearing morels. These differences are an attempt to cope with environmental variations. All species do this to some extent, but the phenomenon was mysterious and unrecognized in science. With the morel, it is so extreme that it can easily be studied.
Scientifically, this phenomenon is "phenotypic variation as an adaptation mechanism." It means variants are produced which have the same genetic make-up but different outward characteristics in order to cope with rapidly changing conditions such as seasonal variations. The test for it is variations which are randomly distributed through a population rather than distributed in a Mendelian manner from parents to offspring.
Most species produce only a few phenotypic variants which are designed to cope with particular needs. The morel produces unlimited variations, because it has not had time to sort them out.
Numerous enzymes have been studied for the morel and shown to have 3-5 variant types. Scientists have seen such variants and their genes, called alleles, for a long time and have wondered why they exist. The morel shows why more clearly than usual.
There would statistically never have been any two morel phenotypes the same, because there are such a huge number of possible variations. Since these variations have never been whittled down to practical purposes, most of them are nonfunctional. I find that only about 20% of morels actually form spores, but this varies from area to area depending upon the stability of the environment. In some areas where environments are extreme, most morel forms are so odd that they are nonfunctional. By contrast, other species only form a small number of phenotypic variants, and they are all functional and quite practical. It takes evolutionary time to sort out the variants and select for the most practical ones. The morel has not had time to do that.
The morel has not had time to evolve the ability to grow vertically. Everything which emerges from the soil can measure gravity for vertical growth except the morel. Whatever direction the morel starts growing, it keeps growing. Morels will grow horizontally, if they start in that direction. It shows that the morel has been a mushroom for a very short amount of time.
Gary Novak
Mushroom Physiologist
Evolution Biologist