Introduction to Fungi, Third Edition

2
Protozoa: Myxomycota (slime moulds)
2.1 Introduction
When the first slime moulds were described
by Johann H. F. Link in 1833, they were given
the term myxomycetes (Gr. myxa ¼ slime). Link
used the suffix -mycetes because of the superficial
similarity of the fructifications of slime moulds
with the fruit bodies of certain fungi, notably
Gasteromycetes (see Chapter 20). Although it
has been appreciated for some time that they
lack any true relationship with the Eumycota
(de Bary, 1887; Whittaker, 1969), slime moulds
have none the less been studied mainly by
mycologists rather than protozoologists, probably
because they occur in the same habitats
as fungi and are routinely encountered during
fungus forays. Since slime moulds are only
rarely covered by zoology courses even today,
they are briefly described in this chapter,
referring to more specialized literature as
appropriate.
Slime moulds differ substantially from
the Eumycota not only in phylogenetic terms,
but also regarding their physiology and ecology.
Their vegetative state is that of individual
amoebae in the cellular slime moulds, or of
a multinuclear (coenocytic) plasmodium in the
plasmodial slime moulds. Motile stages bearing
usually two anterior whiplash-type flagella may
be present in the plasmodial slime moulds
(Sections 2.4, 2.5) and in the Plasmodiophoromycota
(Chapter 3). Amoebae or plasmodia feed
by the ingestion (phagocytosis) of bacteria,
yeast cells or other amoebae. This is followed by
intracellular digestion in vacuoles. The mode of
nutrition in slime moulds is therefore fundamentally
different from extracellular degradation
and absorption as shown by Eumycota.
Numerous phylogenetic analyses of DNA
sequences encoding rRNA molecules and various
structural proteins or enzymes have been carried
out, but the results obtained are difficult to
interpret because the comparison of different
genes have led to rather variable phylogenetic
schemes. Of the four groups treated in this chapter,
it seems that the Dictyosteliomycetes,
Protosteliomycetes and Myxomycetes are related
to each other whereas the Acrasiomycetes have
a different evolutionary origin (Baldauf, 1999;
Baldauf et al., 2000). The general evolutionary
background is, however, still rather diffuse in
these lower eukaryotes.
2.2 Acrasiomycetes: acrasid cellular
slime moulds
The Acrasiomycetes, or Acrasea as they are called
in zoological classification schemes, are a small
group currently comprising 12 species in six
genera (Kirk et al., 2001). Although appearing
somewhat removed from the bulk of the slime
moulds, they still clearly belong to the Protozoa
(Roger et al., 1996). The trophic stage consists
of amoebae which are morphologically distinct
from those of the dictyostelid cellular slime
moulds (Section 2.3) in having a cylindrical,
rather than flattened, body bearing a single