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Morchella: A Mushroom Evolved from a Yeast

I. An Anomaly of Morchella as a Tool for the Study of Differentiation

Gary Novak

Abstract

Mycelium of Morchella esculenta and Morchella angusticeps differentiated on agar media creating an anomaly, when suitable nitrogen sources were used. The anomaly was a disc of high density (felt-like) mycelium containing ascocarp pigment and having a rubbery, brittle tissue, similar to that of the ascocarp, below it. The formation of the anomaly demonstrated rudimentary control of morphology and indicated a reversion to a primitive evolutionary form resulting from recent evolution from a yeast.

Methods, Results and Discussion

The study of differentiation of mushroom mycelium is usually encumbered by a bulky medium in which the mushrooms form. The mycelium of morel mushrooms (Morchella spp.) is readily established in pure culture from ascospores (Hervey, Bistis & Leong, 1978; Gessner, Romano & Schultz, 1987) or stipe tissue (Ower, 1982; Brock, 1985), but ascocarps have only been produced in a soil-like medium (Ower, Mills & Malachowski, 1986). In This study, Morchella mycelium differentiated into an anomaly on agar media providing an unusual opportunity to study differentiation.

1. Cultural Conditions

2. Differentiation

In agar slants (20x150 mm), a high density mycelium formed at the top after normal growth was completed, which was about 10-15 d. The thin agar below the high density mycelium would normally support little mycelial growth demonstrating that the differentiation was endotrophic. In other words, some of the nutrients were derived from mycelium extending down the slant.

With petri plate cultures and suitable progression of differentiation, ascocarp pigment appeared within the high density mycelium. Morchella esculenta produced gray pigment, but only when a cutout was made in the high density mycelium, as resulted from subculturing (Fig. 1).

figure 1

The need for a cutout indicated that increased aeration was stimulatory to pigment production or the progression of differentiation. With that mycelial type, an orange pigment sometimes formed below the gray pigment (Fig. 2).

figure 2

The high density mycelium always formed around the inoculum and extended out from it. It could be distinguished from surrounding mycelium in about 14 d, whereupon a boundary developed around it. The progression is shown with Morchella angusticeps in Figs 3-6.

4 plates

On day 11, the white disc had a diameter of 82 mm. On day 16, a high density brown disc had a diameter of 65 mm and a boundary around it, while the outer mycelium was thin. The shrinkage of disc diameter, development of a boundary and thinness of extended mycelium are evidence of cell material migrating from the outer mycelium to the high density mycelium in a endotrophic manner. A similar translocation of cell material has been found to occur during sclerotia formation with motion in the direction of greatest nutrient utilization (Amir et al., 1993).

3. Cell Structure

The cell structure of the high density mycelium was filamentous on the upper surface, with cell density increasing toward the agar. On the agar surface below the high density mycelium, a tissue formed having a rubbery, brittle texture similar to that of the ascocarp. The cells extending farthest above the high density mycelium were long and straight with few septa. The septa became more closely spaced at lower depths breaking ultimately into separate polymorphic cells which aggregated into tissue. The agar provided a support base for a layered structure indicating an adaptation to differentiation on a surface. High aeration at the surface along with rich nutrient availability appeared to create the conditions which promoted differentiation.

The onset of differentiation could not be determined. The most objective criterion for differentiation was boundary formation. The differentiated mycelium maintained viability for 4-6 wk from inoculation reforming exotrophic mycelium when subcultured. Mycelium exposed on an agar surface deteriorated due to dehydration and toxic effects requiring frequent reisolation.

4. Reversion Anomaly

The formation of a tissue, as found on agar, would not be advantageous during normal growth, which demonstrates that the differentiated mycelium was an anomaly. Considering the structured nature of the growth, including zoned ascocarp pigment and a tissue, the high density mycelium appeared to be a rudimentary formation of a mushroom as it existed earlier in evolution. The primitive morphology including a lack of strict controls indicates evolution from a yeast. The ability to revert to a primitive form indicates that the evolution was very recent. Such a formation could have been found at the base of trees during the cold conditions of an ice age (Erickson, 1990), which would reduce dehydration. The base of trees could provide soluble nutrients as exudate and an interface between above and below ground growth for the transitional state of evolution.

5. Yeast Ancestor

Two likely genera from which Morchella evolved are Nadsonia and Schizosaccharomyces--the first based on physiology, and the second based on morphology. Nadsonia includes species found in tree exudate (Miller & Phaff, 1984). Those species have adapted to survival on tree bark through endotrophic sporulation (Novak, 1981), which allows survival after being washed over the surface by rain.

Nadsonia has life cycle characteristics similar to those of Morchella, both being haploid (Gessner et al., 1987; Yoon, Gessner & Romano, 1990; Miller & Phaff, 1984) with karyogamy and meiosis occurring in succession during ascospore formation (Volk & Leonard, 1990, Miller & Phaff, 1984; Phaff, Miller & Mrak, 1966).

If, however, morphology is weighed more heavily than physiology, the ancestor may be in the genus Schizosaccharomyces, which contains filamentous yeasts and sometimes shows 8 spores per ascus (Yarrow, 1984), as does Morchella (Volk & Leonard, 1990). The life-cycle characteristics of Schizosaccharomyces are similar to those described above, except that somatic conjugation of vegetative cells usually precedes ascus formation (Yarrow, 1984; Phaff et al., 1966).

6. Nitrogen Depletion

In this test, the boundary, being an objective criterion for differentiation, formed only where the agar depth was moderately thin, which meant around the inoculum when placed on the upper half of the slant but not when placed lower down. The agar depth determined the amount of nitrogen available and its rate of depletion indicating that depletion of nitrogen was stimulatory to differentiation. A similar effect was observed with slanted petri plates. The boundary was sharp where agar was thin, but it became invisible where agar was thick.

The partial influence of nitrogen depletion indicates that the direct inducer of differentiation was an energy peak which was promoted by inhibited synthesis due to nitrogen deficiency. This mechanism was found to control the sporulation of yeasts, both exotrophic (Croes, 1967) and endotrophic (Novak, 1981), where the physiology has been studied in some detail. Evidence indicates, as described below, that endotrophic differentiation controlled by the energy level is the method by which soil mushrooms in general form.

7. Induction Physiology

With yeasts, the test of endotrophism is sporulation in distilled water (Novak, 1981). In this study, the test was differentiation on thin agar, where nutrients were depleted. The general mechanism of endotrophism for mushroom formation would be the accumulation of cell mass within the mycelium before it is transferred to the sporocarp as nutrients, preformed molecules and cell components.

8. References

Amir, R., Levanon, D., Hadar, Y., & Chet, I. (1993). Morphology and physiology of Morchella esculenta during sclerotia formation. Mycological Research 97, 683-689.

Brock, T. D. (1951). Studies on the nutrition of Morchella esculenta Fries. Mycologia 43, 402-422.

Croes, A. F. (1967). Induction of meiosis in yeast. II. Metabolic factors leading to meiosis. Planta (Berl.) 76, 227-237.

Erickson, J. (1990). Ice Ages. Tab Books: Blue Ridge Summit, Penn.

Gessner, R. V., Romano, M. A. & Schultz, W. R. (1987). Allelic variation and segregation in Morchella deliciosa and M. esculenta. Mycologia 79, 683-687.

Hervey, A., Bistis, G. & Leong, I. (1978). Cultural studies of single ascospore isolates of Morchella esculenta. Mycologia 70, 1269-1274.

Magasanik, B. (1992). Regulation of nitrogen utilization. In: The Molecular and Cellular Biology of the Yeast Saccharomyces, Vol. 2. Gene Expression (ed. E. W. Jones, J. R. Pringle & J. R. Broach), pp. 283-318. Cold Spring Harbor Laboratory Press: Cold Spring Harbor, N.Y.

Miller, M. W. & Phaff, H. J. (1984). Genus 17. Nadsonia Sydow. In: The Yeasts. (ed. N. J. W. kreger-van Rij), pp. 279-284. Elsevier Science Publishers B. V.: Amsterdam.
  
Novak, G. E. (1981). Endotrophic sporulation by the yeast Nadsonia fulvescens. Canadian Journal of Microbiology 27, 967-970.

Ower, R. (1982). Notes on the development of the morel ascocarp: Morchella esculenta. Mycologia 74, 142-144.

Ower, R. D., Millls, G. L. & Malachowski, J. A. (1986). Cultivation of Morchella. U.S. Patent No. 4,594,809.

Phaff, H. J., Miller, M. W. & Mrak, E. M. (1966). The Life of Yeasts. Harvard University Press: Cambridge, Mass.

Stamets, P. & Chilton, J. S. (1983). The Mushroom Cultivator. Agarikon Press: Olympia, Wa.

Volk, T. J. & Leonard, T. J. (1990). Cytology of the life-cycle of Morchella. Mycological Research 94, 399-406.

Wickerham, L. J. (1951). Taxonomy of yeasts. U.S. Department of Agriculture, Washington, D. C. Technical bulletin 1029.

Yarrow, D. (1984). Genus 25. Schizosaccharomyces Lindner. In: The Yeasts (ed. N. J. W. Kreger-van Rij), pp. 414-422. Elsevier Science Publishers B. V.: Amsterdam.
 
Yoon, C., Gessner, R. V. & Romano, M. A. (1990). Population genetics and systematics of the Morchella esculenta complex. Mycologia 82, 227-235.


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