(This is from a page titled "Biology" on my morel mushroom web site.)

There are two types of evolution. Minor changes occur in a slow and continuous manner with all species. Major changes are made in large leaps which occur rapidly. Both are caused by changes in environmental conditions.

Morel mushrooms are undergoing rapid change, as a yeast growing on trees adapted to the soil. The end point must be a cup fungus for long term survival, because ascospores require that strategy. A cup-like indent is appearing on some variants of the morel, but only a related genus, Helvella, is apt to complete the process.

1. Yeast Origins

Yeasts and higher fungi evolved in opposite directions creating major differences in many of their important characteristics. Yeasts evolved toward high speed growth through simplicity, while higher fungi sacrificed time for complexity. The basis for the difference is that yeasts must battle more directly against competitors, while higher fungi battle against the elements growing on surfaces in ways which competitors cannot.

Yeasts began their evolution about 50 million years ago, as indicated by fossil evidence* and circumstances. Modern plants began producing sugary substances at about that time, which is the primary factor causing yeasts to evolve. Sugar is a dehydrating agent, so it must be in the form of dilute liquid to be utilized by microbes. Since competitors including bacteria and molds can grow in sugary solutions, high speed growth was needed by yeasts.

To maximize growth rate, yeasts discarded all possible complexity. Besides simplifying morphology, they also minimized the number of enzymes they maintained. Discarded were most extracellular enzymes for breaking down large molecules plus enzymes for using unusual nutrients. Of the remaining enzymes, about ninety percent were repressible being synthesized only when needed. The result for typical yeasts was a mass doubling time of about 90 minutes at room temperature compared to 45 minutes for bacteria.

High growth rate was not the only adaptation to increase competitiveness of yeasts in sugary solutions. Also important was the excretion of acid and alcohol. Yeasts normally break down sugars into two-carbon compounds through glycolysis, while repression of TCA enzymes prevents further reduction. Some of the carbon is used for synthesis, but a limited amount is excreted as acid and alcohol to inhibit growth of competitors. The excreted carbon also causes sugar to be depleted more rapidly making it less available to competitors. When the sugar is depleted, TCA enzymes are synthesized, and the excreted carbon is remetabolized.

Acid and alcohol excretions by yeasts are under intrinsic control, and the quantities depend upon adaptations. While alcohol must be excreted for anaerobic glycolysis, the extent to which such fermentation is used varies with adaptations.

This procedure is effective, because yeasts can develop a high tolerance for acid and alcohol. They usually tolerate pH 3.0 well. Bacteria in general do not tolerate acid as well as yeasts, because their environment is not usually acidic, and also because their prokaryotic characteristics do not meet the demands as well as does the complexity of eukaryotic cells.

Molds have a low tolerance for acid in organic form, at least when it is absorbed. Circumstantially, the reason appears to be that protons must be pumped out of the cell, while molds must avoid excreting substances which concentrate on exposed mycelium and kill it.

The evolution of filamentous fungi traces back in fossil records to the beginning of terrestrial life. New forms of fungi continued to originate from different starting points for several hundred million years. Since yeasts appeared much later, they could only have evolved from higher fungi. The diversity of yeasts indicates that they evolved from several different lines of higher fungi. If this assumption is true, there should be more phylogenetic difference between some yeasts than between some yeasts and some higher fungi.

There is a window of opportunity for all evolution; and it closes, because basics cannot change much after they are depended upon. Early in evolution, while basics were still alterable, higher fungi acquired specialized characteristics. Yeasts gave up some of those characteristics and cannot regain them, because that type of evolution no longer occurs.

The demanding evolution which higher fungi underwent created the following characteristics:

• external spores
• extracellular enzymes for breaking down solids
• the ability to tolerate dehydration on exposed surfaces
• and high efficiency metabolism for preventing the excretion of toxic substances which would be harmful to exposed mycelium.

Yeasts not only excrete in a controlled manner but also excrete a variety of metabolites in a manner which appears to be inadvertent. The uncontrolled excretion indicates a lower efficiency of metabolism than higher fungi have. Influencing metabolic efficiency is the spatial arrangement of enzymes which are attached to membranes, and this characteristic would be resistant to change.

There is no evidence of filamentous yeasts tolerating dehydration. For example, when insects make holes in trees causing sap to be exuded, the filamentous yeasts grow in the bore holes, where there is protection from dehydration, while unitary yeasts grow in the exudate on the bark.

(The original page continues on the characteristics of the morel mushroom.)