The evolution of spore size in Agarics:
do big mushrooms have big spores?
P. MEERTS
Laboratoire de GeÂneÂtique et Ecologie veÂgeÂtales, Universite Libre de Bruxelles, ChausseÂe de Wavre 1850, B-1160 Bruxelles, Belgium (e-mail: [email protected])
Keywords:
Abstract
Basidiocarp;
Basidiomycotina;
fungal development;
fungi;
offspring size;
PIC;
reproductive strategy;
scaling.
As a ®rst attempt to investigate evolutionary patterns of spore size in Agarics, I
tested whether this trait was correlated to the size of the fruit-body
(basidiocarp). Based on phylogenetically independent contrasts, it was shown
that big mushroom species had on average 9% longer, 9% wider and 33%
more voluminous spores (all with P < 0.05, one-tailed tests) than small
congeneric species (a three-fold difference in cap diameter was used to
discriminate big and small mushrooms). It is argued that larger spore size does
not consistently confer higher ®tness in fungi, owing to aerodynamic
constraints. Surprisingly, the cap±spore correlation was strongly lineagespeci®c. Thus, spore volume correlated signi®cantly with cap diameter in ®ve
of 16 large genera (four positive and one negative correlation). Positive cap±
spore correlations are interpreted in terms of developmental constraints,
mediated by hyphal swelling during cap expansion. The possible mechanisms
which can account for the breakdown of this constraint in the majority of
genera investigated are discussed.
Introduction
Offspring size is subject to optimization by natural
selection because larger offspring tend to have a higher
®tness but are more costly to produce (Lloyd, 1987;
Morris, 1987; Silvertown, 1989). Trade-offs between
offspring size and number are often masked by variation
in parental size because larger organisms, having higher
absolute allocation to reproduction, can increase both
offspring size and offspring number at the same time
(Stearns, 1992; Begon et al., 1996). Thus, interspeci®c
scaling is a pervasive source of variation in offspring size
in plants and animals (Stearns, 1992). In ¯owering
plants, for instance, seed size is often correlated positively
with fruit size and vegetative size (Primack, 1987;
Thompson & Rabinowitz, 1989; Niklas, 1994).
Compared to animals and plants, the evolutionary
ecology of offspring size in the third kingdom of multicellular eucaryotes, i.e. fungi, has received much less
attention (but see Ingold, 1971; Kreisel, 1984; Parmasto
& Parmasto, 1987). As a structure whose function is to
Correspondence: P. Meerts, Laboratoire de GeÂneÂtique et Ecologie veÂgeÂtales,
Universite Libre de Bruxelles, ChausseÂe de Wavre 1850, B-1160
Bruxelles, Belgium.
E-mail: [email protected]
J. EVOL. BIOL. 12 (1999) 161±165 Ó 1999 BLACKWELL SCIENCE LTD
produce and liberate spores as ef®ciently as possible
(Webster, 1980; PoÈder, 1983), there is little doubt that
fruit-body (basidiocarp) morphology has been subjected
to optimizing selection, particularly in Homobasidiomycetes (McNight & Roundy, 1991; PoÈder, 1992; PoÈder &
Kirchmair, 1995).
Basidiospores are unicellular, haploid propagules dispersed by air. This mode of dispersion, in addition to
narrow habitat requirements, must result in high rates of
density-independent mortality, a selective regime known
to favour small offspring size (Stearns, 1992; Begon et al.,
1996). In Agarics, i.e. Homobasidiomycetes with a stipe, a
pileus (cap) and a hymenophore consisting of lamellae
(gills), spore volume varies by about three orders of
magnitude among species (Pegler & Young, 1971; Singer,
1975; Webster, 1980). In this paper, I test the simple
hypothesis that spore size is correlated with basidiocarp
size. I anticipate a positive correlation, on the grounds that
basidiocarp growth occurs mostly through cellular swelling and spores are produced by unicellular sporocysts
(basidia) (Webster, 1980; Oberwinkler, 1982) (Fig. 1).
Materials and methods
The taxonomic groups considered are the Agaricales and
the Russulales. Spore dimensions [length (L) and width
161
162
P . ME E R T S
Fig. 1 Schematic diagram of a typical Agaric
fruit-body. PM: primary mycelium (haploid
hyphae), SM: secondary mycelium (dicaryotic
hyphae); S: stipe; C: cap; TS: transverse section
of a gill; H: hymenium; T: trama; B: basidium;
B1: caryogamy; B2: meiosis; B3: spore formation.
(w)] and cap diameter are from the standard ¯ora of
Moser (1983). Therein, spore and cap dimensions are
reported as the two extremes of the species' normal
variation range; typically, the range of values is about 10±
25% of the average value for spore dimensions (e.g. spore
length: 10±12 lm) and 50±100% for cap diameter (e.g. 2±
4 cm). Cap size indeed shows large phenotypic plasticity
depending on age and growth conditions (Webster,
1980). The average value of the two extremes of the
normal variation range was used for subsequent analysis.
Species for which only a single value was available (e.g.
cap diameter `up to 3 cm') were not considered. Cap
diameter was preferred over other basidiocarp size measurements because it is positively correlated with total
hymenophore area and thus, with total spore number
(PoÈder, 1983). Spore volume was calculated as that of a
revolution ellipsoid: 4p/3 (L/2)*(w/2)2 (Gross, 1972). Two
different approaches were then conducted.
Phylogenetically independent contrasts (PICs)
Phylogenetically independent contrasts (PICs) (Harvey
& Pagel, 1991) were constructed by the following
procedure. A three-fold difference in average cap
diameter was chosen as the size difference criterion
for discriminating between big and small mushrooms.
This criterion was a compromise between maximizing
the number of PICs included in the analysis and
minimizing overlaps in the size range of small and big
species. One big and one small species (as de®ned
above) were selected randomly from each of 54 genera
showing suf®ciently large variation in cap size among
species, from a total of about 95 genera comprising two
species or more. The genera included were (total
number of species in parentheses, round ®gures):
Agaricus (67), Agrocybe (18), Amanita (36), Bolbitius
(6), Calocybe (13), Camarophyllus (12), Clitocybe (94),
Clitopilus (9), Collybia (33), Conocybe (30), Coprinus (92),
Cortinarius (530), Crepidotus (25), Cystoderma (13),
Cystolepiota (8), Entoloma (150), Flammulaster (19),
Galerina (57), Gerronema (8), Gymnopilus (13), Hebeloma
(53), Hohenbuehelia (12), Hygrocybe (57), Hygrophorus
(46), Hypholoma (16), Inocybe (170), Lactarius (89),
Lentinellus (9), Lepiota (52), Leucoagaricus (13), Leucocoprinus (15), Leucopaxillus (14), Lyophyllum (19),
Macrolepiota (11), Marasmius (32), Melanoleuca (31),
Micromphale (6), Mycena (130), Omphalina (31), Panellus,
Pholiota (34), Pluteus (47), Psathyrella (100), Pseudobaeospora (4), Psilocybe (17), Rhodocybe (11), Ripartites
(6), Russula (160), Squamanita (7), Stropharia (16),
Tephrocybe (24), Tricholoma (67), Tubaria (11), Volvariella
(16). Spore length, width and volume were then
compared between big and small mushrooms by means
of two-sample paired t-tests.
Spore±cap correlations within genera
Those 16 genera of Agaricales and Russulales comprising
more than 50 species were used to investigate correlations between cap size and spore size at the within-genus
J. EVOL. BIOL. 12 (1999) 161±165 Ó 1999 BLACKWELL SCIENCE LTD
Spore size in mushrooms
163
level. For the largest three genera (Cortinarius, Entoloma
and Inocybe), a subsample of about one-third of all species
were randomly selected; for the other 13 genera (Agaricus, Clitocybe, Coprinus, Galerina, Hygrocybe, Hygrophorus,
Lactarius, Lepiota, Mycena, Pluteus, Psathyrella, Russula,
Tricholoma), all species for which the relevant data were
available were considered. For Coprinus, in which some
species have dorsi-ventrally compressed spores, two
measures of spore width were included where appropriate (w1 and w2) and spore volume was calculated
accordingly. All data were transformed to natural logarithms before analysis. Associations between cap diameter, spore length, spore width and spore volume were
computed as Pearson correlation coef®cients.
Results
Phylogenetically independent contrasts
On average, big mushrooms had 9% longer, 9% wider
and 33% more voluminous spores than small mushrooms; these differences were, however, signi®cant only
at a one-tailed error rate (mean values ‹ standard errors
over 54 species, length: big 8.37 ‹ 0.41 lm, small
7.71 ‹ 0.35 lm, t ˆ 1.73 P ˆ 0.045; width: big 5.19 ‹
0.28 lm, small 4.75 ‹ 0.21 lm, t ˆ 1.93 P ˆ 0.029;
volume: big 169 ‹ 31 lm3, small 120 ‹ 19 lm3,
t ˆ 1.85 P ˆ 0.035) (Fig. 2). Compared to their 54
small counterparts, the spores of the large species were
longer in 29 cases, wider in 35 cases and more voluminous in 34 cases.
Correlations within individual genera
The correlation patterns with cap diameter were very
similar for all spore size measurements so that detailed
results are presented only for spore volume (Table 1). At
a tablewide error rate, spore volume was signi®cantly
correlated with cap diameter in four of 16 genera
(positive correlation in Agaricus, Coprinus and Cortinarius,
negative correlation in Psathyrella).
Discussion
Why are evolutionary changes in cap size only weakly
correlated with changes in spore size in Agarics? A ®rst
likely explanation might lie in the fact that larger spores
do not consistently have a higher ®tness in fungi, owing
to aerodynamic constraints. Theoretical aspects of particle transfer by air indicate that spore size is negatively
correlated with dispersability and positively correlated
with impaction ef®ciency (Whitehead, 1969; Dix &
Webster, 1995). Thus, small spores are less prone to
being trapped by obstacles before reaching a suitable
substrate because they tend to follow the airstream
around a potential collecting object (Whitehead, 1969;
Ingold, 1971). The relationship between offspring size
J. EVOL. BIOL. 12 (1999) 161±165 Ó 1999 BLACKWELL SCIENCE LTD
Fig. 2 Phylogenetically independent contrasts of spore volume
between big and small mushroom species (i.e. at least a threefold difference in cap diameter) in each of 54 genera of Agarics.
Each point represents one pair of species. The diagonal line
represents equal spore size for big and small mushrooms.
and ®tness might therefore be quite different than in
animals and plants and the optimal spore size might
conceivably vary depending on way of life. For instance,
mushrooms which colonize twigs and living leaves tend
to have relatively larger spores because these are more
easily captured by the host (Ingold, 1971); conversely,
low wind speed and high vegetation density might favour
smaller spores in forest-¯oor species, but this does not
Table 1 Pearson correlation coef®cients (r) between cap diameter
and spore volume in the 16 largest genera of Agarics in Europe.
n = number of species considered. Coef®cients in bold type remain
signi®cant when a tablewide error rate is applied. ***P < 0.001,
**P < 0.01, *P < 0.05, ns not signi®cant.
n
Agaricus
Clitocybe
Coprinus
Cortinarius
Entoloma
Galerina
Hygrocybe
Hygrophorus
Inocybe
Lactarius
Lepiota
Mycena
Pluteus
Psathyrella
Russula
Tricholoma
57
80
74
142
73
50
47
43
40
46
43
119
44
89
96
57
r
0.688***
0.169 ns
0.292**
0.310***
)0.063 ns
)0.172 ns
0.363*
)0.220 ns
0.269 ns
)0.254 ns
0.269 ns
0.090 ns
0.042 ns
)0.324**
0.141 ns
0.081 ns
164
P . ME E R T S
seem to have ever been tested. The positive correlation
between spore size of small and large species across
genera (Fig. 2) does support the idea that spore size tends
to be canalized within lineages.
Secondly, unlike plant fruits and animal body cavities,
basidiocarps are open, not closed, structures in which
total spore mass represents only a limited proportion of
the energy devoted to reproduction (Webster, 1980).
These peculiarities might loosen the constraint imposed
by basidiocarp size on spore size. Clearly, fungal spores
are not homologous either structurally or functionally of
plant seeds and animal eggs.
Surprisingly, however, the pattern of correlation of
spore size with basidiocarp size is strongly lineagespeci®c. What might be the mechanism underlying the
positive correlation found in four genera? At the intraspeci®c level, spore±cap correlations have been circumstantially reported on several occasions (e.g. Hanna,
1926; Watling, 1975; CleÂmencËon, 1979). Buller (1922,
1924) already noted that hairs on the pileus and spores of
abnormally small fruit-bodies of Coprinus lagopus were
smaller than in normal fruit-bodies. Recent progress in
the study of morphogenesis of the Agaric fruit-body
(reviewed in Wells & Wells, 1982; Chiu & Moore, 1996)
might offer a mechanistic explanation to these observations. It is now generally accepted that Agarics primarily
depend on cell in¯ation (hyphal swelling) for fruit-body
expansion (Moore et al., 1979; Reijnders & Moore, 1985;
Moore, 1996). On the other hand, basidiospores are
produced by unicellular meiosporocysts (basidia). A tight
correlation between basidium volume and total spore
volume per basidium does exist in Basidiomycetes (Corner, 1948; PoÈder, 1986), indicating that spore size is
strongly constrained by basidium size. The size of hyphal
articles therefore appears as a possible mediator of the
spore±cap correlation.
Two conditions must be ful®lled for cell size to generate a
positive correlation between spore and basidiocarp size: (i)
interspeci®c differences in cap size must be due, to a
signi®cant extent, to differences in the magnitude of
hyphal swelling during basidiocarp growth and (ii) hyphal
swelling must be coordinated throughout the basidiocarp.
Based on the results of this study, it would appear that only
a few Agaric genera do ful®l both aforementioned conditions. Oberwinkler (1982) recognized two fundamental
types of basidia, namely `non in¯ating' basidia and basidia
that strongly expand apically during hymenophore development. He cites three genera as examples of the second
category (Agaricus, Coprinus, Russula). He also comments
that `very often, those species forming basidia that expand
apically form basidiocarps in which the hyphae of the
trama, subhymenium and hymenium also show similar
secondary expansion. Possibly this feature is associated
with the capacity of basidiocarps to undergo rapid expansion under optimum environmental conditions.' It is
striking that two of the three genera cited as having
in¯ating basidia showed a positive cap±spore correlation
(Agaricus, Coprinus). In Russula, basidium size is probably
uncoupled from cap size because specialized cells
(`sphaerocysts') account for most of the expansion process
of the basidiocarp (Reijnders, 1963). The timing of the
hyphal in¯ation process, relative to basidium formation
(see Hammad et al., 1993), might also be of importance in
the evolution of cap±spore correlations. Speci®cally, if the
charging of the basidium with protoplast, which sets the
upper limit of total spore volume (Corner, 1948), is
achieved before the onset of the expansion process, there
would be limited opportunity for a correlation to evolve.
The morphogenetic basis of the negative cap±spore correlation in Psathyrella is, however, less clear and would
deserve further investigation.
In conclusion, the results reveal unsuspected differences between genera of Agarics in the pattern of covariation of spore and basidiocarp size, the ecological and
evolutionary signi®cance of which remains to be elucidated. On the whole, the factors which govern spore size
evolution in fungi need further investigation.
Acknowledgments
J. Rammeloo and A. De Kesel commented on an earlier
draft of the manuscript. This paper is dedicated to the
memory of P. Heinemann.
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Received 9 December 1997; accepted 8 January 1998