Introduction to Fungi, Third Edition

DICTYOSTELIOMYCETES: DICTYOSTELID SLIME MOULDS
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Fig 2.1 Amoebae of cellular slime moulds.The arrows indicate the direction of movement at the time when the photomicrographs
were taken. (a) Limax-type amoeba of Acrasisrosea, an acrasid cellular slime mould.Note the absence of granular contents from the
lobose pseudopodium at the tip of the amoeba. (b) Amoeba of Protostelium mycophaga with filose pseudopodia. Reproduced from
Zuppinger and Roos (1997), with permission from Elsevier; original prints kindly supplied by C. Zuppinger.
large-lobed (lobose) anterior pseudopodium. The
granular cellular contents trail behind the pseudopodium,
which appears clear. The posterior
end is knob-shaped and is called the uroid
(Fig. 2.1a). Such amoebae are of the limax type
because their movement resembles that of slugs
of the genus Limax. Good accounts of the acrasids
have been given by Olive (1975) and Blanton
(1990).
Acrasid slime moulds are common on decaying
plant matter, in soil, on dung and on rotting
mushrooms, but they are rarely recorded
because of their small size, which necessitates
observations with a dissecting microscope. The
most readily recognized species is Acrasis rosea,
which has orange- or pink-coloured amoebae
due to the presence of carotenoid pigments,
including torulene (Fuller & Rakatansky, 1966).
Acrasis rosea can be observed if dead twigs,
leaves or fruits are incubated on weak nutrient
agar for a few days. Spore-bearing structures
called sorocarps (Gr. sorus ¼ heap, karpos ¼ fruit)
will develop, and spores can be transferred to
fresh agar with yeast cells as a food source
(Blanton, 1990). The uninucleate amoebae feed
on yeast cells, bacteria or fungal spores and can
encyst under unfavourable conditions, especially
drought, to form microcysts. Each microcycst
germinates again to release a single amoeba.
Eventually amoebae aggregate to form a pseudoplasmodium,
in which the individual amoebae
retain their identity but are surrounded by a
common sheath. The chemical signal for aggregation
is unknown but it is not cyclic AMP (cAMP)
as in the dictyostelid slime moulds (see below).
The pseudoplasmodium develops into a
branched sorocarp in which the amoebae align
themselves in single rows and then round off,
each forming a walled spore. Each spore germinates
to release a single amoeba. The cells
making up the stalk of the sorocarp also encyst
and are capable of germination (Olive, 1975).
Sexual reproduction in the acrasid slime
moulds is unknown.
2.3 Dictyosteliomycetes:
dictyostelid slime moulds
The Dictyosteliomycetes (zool.: Dictyostelia) are
a group of cellular slime moulds comprising