Introduction to Fungi, Third Edition

DICTYOSTELIOMYCETES: DICTYOSTELID SLIME MOULDS
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Fig 2.2 Life cycle of Dictyostelium discoideum.The central feature is the haploid amoeba which is free-living in the soil. It divides
mitotically to produce two daughter amoebae or, under unfavourable conditions, may form a microcyst. If two amoebae of
compatible mating type meet, a diploid macrocyst may be formed which can remain dormant for some time and eventually
germinates by meiosis and then mitosis to release numerous haploid amoebae.Under certain circumstances, starvation may lead to
aggregation of amoebae to form a slug and a sorocarp in which individual amoebae become converted into spores.These are purely
asexual, and meiosis is not involved in their formation or germination.Open and closed circles represent haploid nuclei of opposite
mating type; diploid nuclei are larger and half-filled. Key events in the life cycle are plasmogamy (P), karyogamy (K) and meiosis (M).
amoebae from an area of 1 cm 2 of soil can be
involved. Aggregating amoebae adhere to each
other and secrete a common slime sheath
(Figs. 2.3c,d). Eventually they pile up to form a
compact bullet-shaped slug which flops over
onto the substratum. In D. discoideum and some
other species, the slug undergoes a period of
migration towards the light (Figs. 2.3e g). The
individuality of amoebae is retained within the
slug. As the slug moves along, it leaves behind a
slime trail. Within the slug, the amoebae are
divided into two functionally different populations,
i.e. an anterior group of large, highly
vacuolated cells (pre-stalk cells) and a posterior
group of smaller ones, the pre-spore cells
(Fig. 2.4). It is the pre-stalk group of cells which
co-ordinates slug migration by secreting cAMP.
Various environmental stimuli can direct movement.
For instance, the anterior end of the slug
follows an oxygen gradient but is repelled by
ammonia. Temperature as well as light can also
act as triggers of directed movement. The end of
the migration phase is marked by the roundingoff
and erection of the pseudoplasmodium to
form a flat-based, somewhat conical structure,
which undergoes further development by differentiating
into a multicellular stalk composed of
the large anterior cells, and the sorus which rises
up on the outside of the stalk (Figs. 2.3h j, 2.4).
This final stage of development is called culmination.
About 80% of the amoebae become
converted into spores, with the remainder
being sacrificed for the formation of the fruit
body structure.