Introduction to Fungi, Third Edition

MYXOMYCETES: TRUE (PLASMODIAL) SLIME MOULDS
49
Fig 2.9 Life cycle of the myxomycete Physarum polycephalum. Spores released from the sporangium are haploid and can germinate
by releasing either a single myxamoeba or a swarmer cell.These two cell types are interconvertible.The myxamoeba can divide
mitotically. In P. polycephalum, plasmogamy (P) usually takes place between swarmers which must belong to different mating types.
Karyogamy (K) follows, and the diploid zygote establishes a phaneroplasmodium.When nutrients become limiting, a sporophore is
formed and differentiates sporangia in which meiosis (M) occurs.Unfavourable conditions can be overcome at the haploid stage
when the myxamoeba forms a microcyst, or at the diploid stage when the plasmodium forms sclerotia.Open and closed circles
represent haploid nuclei of opposite mating type; diploid nuclei are larger and half-filled.
by wind or insects and, depending on environmental
conditions such as moisture, germinate
by releasing either amoebae or zoospores
(swarmers) with usually two anterior whiplash
flagella, of which one is shorter than the other
and is thus often invisible (Fig. 2.10). The amoebae
are called myxamoebae, in order to distinguish
them from the amoebae of cellular
slime moulds which have a different function
in the life cycle. Myxamoebae are capable of
asexual reproduction by division. Swarmers
cannot divide, but can readily and reversibly
convert into myxamoebae. Under adverse conditions,
myxamoebae secrete a wall to form
microcysts. Both swarmers and myxamoebae
form filose pseudopodia with which they engulf
their prey. Sexual reproduction is initiated when
two haploid myxamoebae or swarmers of compatible
mating type fuse to form a zygote from
which the diploid plasmodium develops. The
plasmodium can survive adverse conditions by
turning into a resistant sclerotium in which
numerous walled compartments (spherules),
each containing several nuclei, are formed.
Upon resumption of growth, the protoplasts
emerge from their spherules and fuse to re-establish
the plasmodium. When sexual reproduction
ensues, the entire content of a plasmodium is
converted into one or more sporangia in which
meiosis takes place. Beneath the developing sporangia,
the plasmodium deposits a specialized
layer, the hypothallus, which is very variable in