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Department of Botany, The Field Museum, 1400 S Lakeshore Dr, Chicago, Illinois 60605, USA
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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A laboratory experiment was performed which examined tradeoffs between production of mycelium and reproduction (using stipe dry weight as an estimator of spore production) in the coprophilous mushroom species Coprinus cinereus. Isolates of the fungus taken from a single dikaryotic mycelium were grown in Petri plates containing yeast extract agar. Plates varied in diameter and resource density, but the total volume of agar was kept constant. Isolates grown in 100 mm and 150 mm diameter plates produced significantly less mycelium compared to isolates grown in 60 mm diameter plates. Within 60 mm plates there was no correlation between the efficiency of mycelium production and fruit body production, but in larger plates there was a significant negative correlation between the two. These results indicate that isolates grown on larger plates were less efficient at using resources than isolates grown on small plates, and that mycelium production is curtailed on larger plates to maintain spore production.
Key words: Coprinus cinereus, fungi, mycelium, resource allocation, tradeoffs, unit-restricted decomposer
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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). Unit-restricted individuals decompose the resources present on their patch. However, since the patch is composed of dead or waste material, there will be no renewal of at least some of the resources on the patch. As a result, "resource density"—the total availability of resources—on the patch will decrease until all of the resources on the patch are decomposed, and the unit-restricted individuals inhabiting the patch die. Propagule dispersal, usually by spores in the case of fungi, is the sole means by which new individuals can become established on new patches.
In recent years, researchers have focused on the ecology of species that inhabit patchy resources (Rhodes and Odum 1996
, Hanski and Simberloff 1997
, Tilman et al 1997
). In general, a species is considered successful if its population can persist despite the extinction of sub-populations on individual patches. Similarly, a species is considered a successful competitor if it can persist despite the extinction of sub-populations due to competition pressure (Holt 1997
). In the context of unit-restricted decomposers, the long-term success of a population depends on the ability of individuals to colonize new patches, as established individuals will inevitably die when the resources on their patches are depleted. Propagule production is the contribution of an individual to the persistence of a population, as it is only through propagules that new patches will be colonized. Therefore, propagule production is a useful measure of the competitive abilities of an individual unit-restricted decomposer (Schmit 1999
).
Clearly, propagule production is crucial to the survival of unit-restricted species and it is expected that individuals will maximize their propagule production. However, individual fungi also have to allocate resources to growth in order to utilize the resources of the patch they inhabit. Each individual will have access to limited amounts of resources that must be carefully allocated between growth and reproduction in order to maximize total reproductive output. Relatively little research has explicitly examined the relative resource allocation to mycelium and reproduction in unit-restricted fungal decomposers.
To date, research that has focused on mycelium and fruit body production has examined the effects of patch size and nutrient quality. Madelin (1956a)
, working with the basidiomycete Coprinus lagopus Fr., demonstrated that isolates given more substrate produce a greater dry weight of fruit bodies. In a laboratory experiment on wood decay fungi, Holmer and Stenlid (1993)
demonstrated that the territory size of an individual fungus has a significant effect on its ability to invade the territory of other species. Plunkett (1953)
, working with Collybia velutipes (Curt.) Fr. and Madelin (1956a)
, working with C. lagopus, demonstrated that the ratio between the dry weight of the fruit bodies and the dry weight of the mycelium is not constant.
In this paper, a simple framework is provided to examine the relative allocation made by unit-restricted fungi to mycelium and fruit body production in order to examine the tradeoff between resource acquisition and spore production. An experiment, which examined the effect of resource density and patch size on resource allocation to mycelium and fruit bodies, is then described.
Theory –
The tradeoffs faced by unit-restricted fungi growing on variable patches can be examined by comparing the resource allocation a fungus makes to different structures. For the purposes of this paper, the resources that are captured by an individual can be allocated in two ways:
 | (1) |
 | (2) |
The amount of resource allocated to reproduction is
 | (3) |
 | (4) |
, 1.1 g/L yeast extract agar—Schmit 1999
).
Clearly, this framework only considers a single resource and is only applicable to resources that are not renewed at the scale of individual patches. Resources that fall under this framework include carbon resources, such as lignin, cellulose and hemicellulose, in substrates such as dung or dead wood (Schmit 1999
). These resources are not renewed within patches and will ultimately be limiting for the entire community.
Equations (1)–(4)
indicate that the amount of resource allocated to growth and to reproduction depends on the total amount of resource available to the fungus, which in turn is determined by the initial resource density of the patch. This coincides with studies (Plunkett 1953
, Madelin 1956a
, Schmit 1999
) that have shown that the amount of resource allocated to reproduction is strongly influenced by the amount of resource initially present.
This does not imply, however, that there can be no tradeoffs between mycelium production and fruit body production. While more mycelium may be produced at high resource densities than low resource densities, it may be that percentage of captured resources allocated to mycelium differs for isolates grown at different initial resource densities. Similarly, isolates grown at different initial resource densities may differ in their relative allocation of resources to reproduction.
It is possible to determine if the proportion of resources allocated to these structures changes with initial resource density by estimating u and v over a range of resource densities. This could be a positive relationship if increases in mycelium biomass improve the efficiency of the fungus in capturing resources, or it could be a negative relationship if allocating additional resources to mycelium results in fewer resources available for reproduction.
An experiment was performed to examine the tradeoffs made by unit-restricted fungi between mycelium production and reproduction. In particular, two questions were asked: (i) What is the relationship between the resource density of the substrate and the allocation made by the fungus to mycelium and spore production? (ii) What is the relationship between resource allocation to mycelium production and resource allocation to spore production? The study consisted of isolates of Coprinus cinereus (Schaeff : Fr.) S.F. Gray grown in three different diameter Petri plates on yeast extract agar with six different initial resource densities. The different initial resource density treatments were used to determine the relationship between resource density and mycelium and spore production. It was assumed that isolates growing on the different diameter plates would produce different amounts of mycelium. The differences in mycelium production were then used to characterize the relationship between mycelium and spore production.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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In order to provide sufficient time to measure the growth rate of the isolate in each treatment, the plugs of mycelium used as inoculum were placed on the edge of the plate. There were eight replicates of each combination of plate diameter and resource density, giving a total of 144 replicates, although due to lack of fruiting or other problems, not all measurements were made for all replicates. All replicates were grown together in a controlled temperature chamber at 27 C, 
69% humidity, and a 12 h light/dark cycle until one week had passed after all isolates had finished fruiting. All Petri plates were sealed with a single layer of Parafilm, which permits gas exchange.
Growth rate – Linear growth rate was estimated for each replicate by marking daily growth rings on the lid of the Petri plates. All replicates had a lag time of approximately two days from the time they were initiated until growth began. There was no evidence for a slowing of growth rate before the entire plate was covered in mycelium. Linear growth was then calculated as the slope of a linear regression between cumulative growth and total time elapsed since the beginning of growth (regression performed through the origin).
Resource density – Rinit was the initial resource density of the agar in g/L from Table I . Rmin was estimated as zero, as there was mycelial growth in all replicates.
Allocation to reproduction –
Erepro was estimated from the combined stipe dry weight of the fruiting bodies of each replicate. Stipes were harvested 1 wk after all fruiting had completed. Previous studies have shown that the stipe dry weight of a C. cinereus fruit body is positively correlated with the number of spores it produces (Schmit 1999
). The relative allocation of resources to reproduction, u, was measured as the total stipe dry weight of a plate divided by the initial resource density from Table I
.
Allocation to mycelium – Emyc was estimated from the mycelium dry weight of each replicate. After the stipes were harvested, mycelium dry weight was measured by boiling the agar and mycelium from each replicate in water. Once the agar had been liquefied the mycelium and agar were passed through filter paper to collect the mycelium. The mycelium was then dried overnight on a food dehydrator and weighed. This technique allows the mycelium full access to the substrate, and allows the isolate to remain undisturbed until the end of the experiment. A drawback of this technique is that some of the hyphal contents will be lost during the boiling process. Because of this loss, the analyses in this paper only compare mycelium dry weights of a treatment against mycelium dry weights of other treatments; no attempt is made to directly compare the mycelium dry weights with the stipe dry weight. In order to determine the relative allocation of resource to mycelium, v, the mycelium dry weight of each replicate was divided by its initial resource density.
Analyses –
General Linear Model ANOVAs were used to examine the effects of initial resource density, plate diameter and the interaction between the two on growth rates, mycelium dry weight, stipe dry weight, v values and u values. Correlation was used to examine the relationship between stipe dry weight and mycelium dry weight and between v and u values. Separate analyses were carried out for the data set as a whole and for each plate diameter. All analyses were performed using the Minitab statistical package (Minitab 1999
), except for power analyses, which were performed using the online Power Calculator (Statistics UCLA 2001
).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Allocation to reproduction – Both plate diameter and resource density influenced fruit body production. At the two lowest nutrient densities, 1.1 and 2.75 g/L, isolates growing on the 100 and 150 mm plates frequently failed to fruit, while almost all isolates growing on plates with a resource density of 5.5 g/L and higher successfully fruited (Fig. 4 ).
Plate diameter and resource density had a significant impact on both u (Table II , Fig. 5 ) and stipe dry weight (Table II , Fig. 6 ) of the isolates. There was also a significant plate diameter x resource density interaction term in both cases. Stipe dry weight and u were higher on plates with a high initial resource density. Smaller diameter plates had higher stipe dry weights and u values, and this difference was particularly pronounced on plates with a low resource density.
Tradeoffs between mycelium and fruit body production – When all treatments are considered, there is a strong, positive, correlation between mycelium dry weight and stipe dry weight (Fig. 7 ). While there is a negative relationship between relative resource allocation to mycelium (v) and allocation to fruiting (u), it is not significant (Fig. 8 ). A power analysis of this correlation test indicates that it had a power above 0.80 to detect significant correlation coefficients with an absolute value of 0.26 and greater.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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, Ballou and Holton 1985
) When the data from each plate diameter are analyzed separately, there is evidence for a tradeoff between mycelium and fruit body production. Isolates demonstrated a positive correlation between mycelium dry weight and stipe dry weight regardless of the diameter of the plate on which they were grown. When v and u values are examined, however, isolates from 100 and 150 mm plates show a negative correlation, indicating that increases in resource allocation to mycelium came at the expense of allocation to reproduction and vice versa. This is due to changing resource allocation over the gradient of initial resource densities. When grown at low initial resource densities, isolates made a relatively high resource allocation to mycelium compared to those grown at high initial resource densities (Fig. 3
). Conversely, the resource allocation to reproduction increased with increasing initial resource density (Fig. 5
). One possible explanation for the emphasis on mycelium production at low resource densities is that, in nature, more mycelium is produced at low resource densities to enhance foraging ability. By emphasizing mycelium production, fungi may be able to discover resource rich territory. Under this hypothesis, fungi that have resource rich territories would be free to allocate a greater proportion of their resources to reproduction. Thus, the transition from an emphasis on mycelium production to an emphasis on reproduction may be a foraging strategy. This hypothesis assumes that unit-restricted fungi often grow on patches that are heterogeneous and that an increased allocation to mycelium production would allow the fungus to encounter a portion of the patch with a high resource density.
Effects of plate size – When fungi grown on plates with different diameters are compared, those on larger plates produce less mycelium (Fig. 2 ) at all, but particularly at high, resource densities, and produce fewer, lighter fruitbodies, particularly on plates with low initial resource densities (Figs. 4, 6 ). The negative relationship between Petri plate diameter and mycelium dry weight is surprising, as is it was initially expected that the fungus would have to produce more mycelium to cover the larger plates. The fungi growing on the larger plates instead allocated fewer resources to mycelium production, with the result that they were able to allocate almost as much to fruit body production as were the fungi growing in the small sized plates.
This implies that fungi growing on the larger Petri plates were less efficient in making use of the resources, perhaps due to some cost involved in transporting resources across the plates. While this may be relatively unimportant at high initial resource densities, fungi on larger plates were less likely to fruit at low initial resource densities (Fig. 4
). In a natural setting, many substrates, particularly wood, occur in a variety of shapes and sizes. While many physical characteristics of substrates are known to influence fungal growth and reproduction (Cooke and Rayner 1984
, Rayner and Boddy 1988
), the shape of a patch has not been considered among them. This experiment demonstrates that on marginal patches the shape of an individual's territory may determine if it can successfully reproduce.
The differences in mycelium production between plates raises the question as to why the isolate, when grown on the 60 mm plates, produced so much "extra" mycelium instead of allocating more resources to spore production. The data from this experiment cannot answer that question. We do know, however, that the mycelium plays a variety of roles beyond nutrient absorption, including defense, aggression, and exploration for new resources (Cooke and Rayner 1984
). It could be that while the isolate seemed to over-allocate resources to mycelium production in this experiment, in a natural system these resources would be used in territorial defense and/or acquisition, and saving resources for this purpose would enhance the fitness of the individual.
Territory size of unit restricted fungi –
Previous ecological studies on decomposer fungi have indicated that territory size can have important implications for reproductive success and competitive ability. Individuals with larger territories are more likely to successfully defend that territory and are more likely to invade the territory of their competitors (Holmer and Stenlid 1993
, Holmer et al 1997
). Territory size has also been shown to determine the reproductive output in experiments on unit-restricted individuals showing deadlock interactions while competing for agar both inter-specifically (Schmit 1999
) and intra-specifically (Schmit 2001
).
The current study, however, indicates that it is not territory size per se which is important to the fungi; it is the amount of resources captured. When the amount of resources captured is held constant, larger territories can be detrimental. In previous studies, individuals with a larger territory were more successful as competitors only when compared to individuals grown on an identical substrate and when an increase in territory size led to an increase in resources captured. This could be an important distinction when investigating fungi in nature. When predicting the competitive ability of a fungus, the quality of the territory may be as important as its actual size. It is well known that many species of decomposers show little host specificity (e.g., see Gilbertson and Ryvarden 1986, 1987
for the range of tree species decomposed by individual polypore species). When a single decomposer inhabits a variety of host species, the resources of the host species will inevitably differ in ways that are important to the decomposer. Therefore when comparing individual fungi living on different substrates, size of territory alone may not explain differences in competitive ability. Similarly, if a fungus is growing on a heterogeneous substrate, quality of its territory may have a bigger influence on its competitive ability than the size of its territory.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Accepted for publication May 25, 2001.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Cooke RC, Rayner ADM., 1984 Ecology of saprotrophic fungi New York: Longman Inc
Gilbertson RL, Ryvarden L., 1986 North American polypores. Vol. 1 Oslo, Norway: Fungiflora. 433 p
Gilbertson RL, Ryvarden L., 1987 North American polypores. Vol. 2 Oslo, Norway: Fungiflora. 885 p
Hanski I, Simberloff D., 1997 The metapopulation approach, its history, conceptual domain, and application to conservation In: Hanski IA, Gilpin ME, eds. Metapopulation biology: ecology, genetics and evolution. San Diego, California: Academic Press. p 5–26
Holmer L, Renvall P, Stenlid J., 1997 Selective replacement between species of wood-rotting basidiomycetes, a laboratory study Mycol Res 101:714-720
Holmer L, Stenlid J., 1993 Competitive hierarchies of wood decomposing basidiomycetes in artificial systems based on variable inoculum sizes Oikos 79:77-84
Holt RD., 1997 From metapopulation dynamics to community structure: some consequences of spatial heterogeneity In: Hanski IA, Gilpin ME, eds. Metapopulation biology: ecology, genetics and evolution. San Diego, California: Academic Press. p 149–165
Madelin MF., 1956a Studies on the nutrition of Coprinus lagopus Fr., especially as affecting fruiting Ann Bot (London) 20:307-330
Madelin MF., 1956b The influence of light and temperature on fruiting on Coprinus lagopus Fr. in pure culture Ann Bot (London) 20:467-481
Minitab Inc. 1999 Minitab for Windows, release 12.23
Plunkett BE., 1953 Nutritional and other aspects of fruit-body production in pure cultures of Collybia velutipes (Curt.) Fr Ann Bot (London) 17:193-217
Rayner ADM, Boddy L., 1988 Fungal decomposition of wood: its biology and ecology New York: John Wiley and Sons. 587 p
Rhodes OE, Odum EP., 1996 Spatiotemporal approaches in ecology and genetics: the road less traveled In: Rhodes OE, Chesser RK, Smith MH, eds. Population dynamics in ecological space and time. Chicago, Illinois: University of Chicago Press. p 1–7
Schmit JP., 1999 Resource consumption and competition by unit-restricted fungal decomposers of patchy substrates Oikos 87:509-519
Schmit JP., 2001 Intraspecific competition in two unit-restricted fungal decomposers, Coprinus cinereus and C. congregatus Mycol Res 105:112-118
Statistics UCLA. 2001 Power Calculator, based on STPLAN by Barry Brown et al, online at http://ebook.stat.ucla.edu/calculators/powercalc/
Tilman D, Lehman CL, Kareiva P., 1997 Population dynamics in spatial habitats In: Tilman D, Kareiva P, eds. Spatial ecology: the role of space in population dynamics and interspecific interactions. Princeton, New Jersey: Princeton University Press. p 3–20
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