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of a mature plasmodium. K, Beginning of
sporulation. L, Mature fruiting body with spores still enclosed. (Adapted from Stephenson and Stempen
54 W. Foissner et al. (eds)
Liceales, Physarales, Stemonitales, and Trichiales). However, members of the Ceratiomyxales
are distinctly diVerent from members of the other orders, and many modern workers have
removed these organisms from the myxomycetes and reassigned them to the protostelids
(Olive 1970, 1975; Olive and Stoianovitch 1979). The exact evolutionary aYnities of the
myxomycetes are still debated, but these organisms constitute a well-deWned and homoge-
nous group. Evidence from DNA sequence analysis (Baldauf and Doolittle 1997) suggests
that even what appear to be closely related taxa on the basis of morphological similarity
may have diverged from each other a long time ago (Clark 2000). In the Wrst phylogenetic
study based on molecular data, Fiore-Donno et al. (2005) suggested that the Echinosteli-
ales, which produce fruiting bodies with a simple structure, represented the most basal
clade of myxomycetes, with two more advanced groups, the Wrst with light-colored spores
and consisting of the Trichiales and the (presumably not monophyletic) Liceales, and the
second having dark spores and made up of the Physarales and Stemonitales. Because of
their small size and the limited array of morphological characters upon which their taxon-
omy is based, determination of what constitutes a natural biological species, in the same
sense that the concept is used for many of the more familiar groups of organisms (Mayr
1970), is sometimes rather problematic. It is now known that a number of the more
common and widespread morphospecies actually consist of complexes of geographically
restricted apomictic clonal lines (El Hage et al. 2000; Clark 2000; Clark and Stephenson
2000; Irawan et al. 2000). These genetically isolated lines are capable of independent
evolution, which can lead to the accumulation of minor morphological diVerences that
reXect speciWc adaptations to the particular set of environmental conditions in which they
occur. For example, some of the forms found in special microhabitats (e.g., the inXores-
cences of tropical herbs) diVer in some respects (e.g., color and size of the fruiting bodies)
from specimens of the same species collected from more typical habitats. These almost
certainly represent biotypes that are adapted to the microhabitat in question. Approximately
50% of all described species of myxomycetes are known only from the type locality or
fewer than Wve localities worldwide. It seems likely that many of these “species” are no
more than morphologically distinct biotypes present in particular habitats or conWned to a
certain regions of the world. If so, then the criteria that need to be applied before describing
a taxon as new should be reconsidered to account for this phenomenon (Schnittler and
Mitchell 2000). As shown by these authors, the annual number of species described as new
to science is steadily increasing, although the morphological species concept as currently
applied hardly considers reproductively isolated lines within a morphological species, as
seen in culture experiments.
Limiting factors for myxomycete occurrence
Temperature and moisture are thought to be the main factors limiting the occurrence of
myxomycetes in nature (Alexopoulos 1963), and species richness tends to increase with
increasing diversity and biomass of the vascular plants providing the resources (various
types of detritus) that support the bacteria and other microorganisms upon which the two
trophic stages in the myxomycete life cycle feed (Madelin 1984; Stephenson 1989). The
pH of the substrates potentially available to myxomycetes in a particular habitat also repre-
sents an important factor inXuencing the distribution of these organisms (Härkönen 1977;
Stephenson 1989; Wrigley de Basanta 2000; Mosquera et al. 2000). Although many myxo-
mycetes appear to have a relatively wide pH tolerance, this is not the case for all species.
Härkönen (1977), who measured the pH of substrates upon which fruitings occurred in a
Protist Diversity and Geographical Distribution 55
study of the distribution patterns of myxomycetes associated with the bark of living trees in
southern Finland, concluded that species of myxomycetes have diVerent pH optima and
amplitudes. In her study, some species seemed to prefer an acidic substrate, whereas others
never developed under low pH conditions. Stephenson (1989) found the same to be true for
both bark and forest Xoor litter in a study carried out in the eastern United States. In
general, members of the Stemonitales developed under more acidic conditions than did
members of the Physarales and the Trichiales.
Temperate forests
Much of what is known about the distribution and ecology of myxomycetes in terrestrial
ecosystems has been derived from studies carried out in temperate forests of the Northern
Hemisphere. In such forests, myxomycetes are associated with a number of diVerent micro-
habitats. (As used herein, the term “microhabitat” simply denotes a speciWc portion of the
total forest habitat represented by a homogenous substrate and similar microclimatic condi-
tions (Stephenson 1989)). These include coarse woody debris on the forest Xoor, the bark
surface of living trees, forest Xoor litter, the dung of herbivorous animals, and aerial por-
tions of dead but still standing herbaceous plants. Each of these microhabitats tends to be
characterized by a distinct assemblage of species (Stephenson 1988, 1989; Stephenson and
Stempen 1994).
Lignicolous myxomycetes associated with coarse woody debris are the best known,
since the species typically occurring in this microhabitat tend to be among those character-
istically producing fruiting bodies of suYcient size to be detected in the Weld (Martin and
Alexopoulos 1969). Many of the more common and widely known myxomycete taxa,
including various species of Arcyria, Lycogala, Stemonitis and Trichia, are predominantly
lignicolous. Much less is known about the myxomycetes associated with the microhabitats
represented by the bark surface of living trees and forest Xoor litter. The primary reason for
this is that many of the species involved are rather inconspicuous or sporadic in their occur-
rence and thus diYcult to detect in the Weld. However, the moist chamber culture technique
as it applies to myxomycetes (Gilbert and Martin 1933) provides a convenient and often
very productive method of supplementing Weld collections when studying such microhabi-
tats as bark and litter. Since its introduction, the technique has been used with considerable
success by many researchers (e.g., Keller and Brooks 1976; Härkönen 1981; Blackwell and
Gilbertson 1980; Stephenson et al. 1999). More than 100 species of “corticolous” myxo-
mycetes have been reported from the bark microhabitat as Weld and/or moist chamber col-
lections (Mitchell 1980). Many of these are also known to occur in other microhabitats, but
at least some species seem restricted to bark of living trees. Prominent examples include
various species of Echinostelium, Licea, and Macbrideola (Alexopoulos 1964; Mitchell
1980). The litter microhabitat of temperate forests is more heterogeneous than the bark
microhabitat, since it usually consists of a mixture of leaves from diVerent tree species
along with other types of plant debris (e.g., pieces of bark, fragments of wood, fruits, seeds,
inXorescences, and small twigs). Some of the species associated with litter also occur on
bark, but others (e.g., various species of Diderma, Didymium, and Physarum) are found
predominantly or even exclusively on litter. The assemblage of species associated with lit-
ter derived from coniferous trees tends to be distinctly diVerent from that associated with
litter from broadleaf trees (Härkönen 1981; Stephenson 1989).
Another microhabitat potentially available for myxomycetes is represented by the dung
of herbivorous animals. A few species