Amoebae that form slime moulds

Most amoebae live as single-celled organisms in the water and soil, but
some amoebae can also form multicellular slime structures. Amoebae are
not bacteria, rather they are micro-organisms belonging to a group called
the Protoctista. Amoebal cells are typically 10-100 times the diameter of
bacterial cells and have the structure typical of animal cells, but they are
not animals because animals always form complex multicellular bodies.
Some amoebae (called myxamoebae), such as
Dictyostelium, will live in
the soil as single cells that feed and reproduce for many generations, but
if these cells start to run out of food in their neighbourhood, then they
send chemical signals to one another and the amoebae respond by
streaming in long conveys to a common rendezvous. When they arrive,
these amoebae do something very strange, they form a
multicellular
mound
or aggregate that piles on new coming cells, getting taller and
taller. Eventually the cells at the tip of the mound form a nipple-like
protuberance and this takes charge as it is designed to become the
'head' of our new organism. All this happens on a small scale, these
mounds are only a few millimetres in diameter.
Above a mound of assembling Dictyostelium
amoebae on a glass slide, left, and a later
stage with a tip, right.
Eventually, this tipped mound falls over and starts crawling around like a slug, with the tip raised up like a snout behind which the
rest of the body follows, leaving a trail of slime behind it as it does so. So our single celled creatures have all come together to
form a temporary multicellular body! The reason is, that this way they are bigger and so can move faster and further. The job of
the snout is to find a suitable place high up in the light and air, from which to release spores into the wind or running rain-water.
This slug-like creature is called a
grex (or pseudoplasmodium or slug, not to be confused with garden slugs!), and is one to a
few millimetres long, which is not bad for something that started out as amoebae one hundredth of a millimetre in diameter! Each
grex contains about 100 000 separate amoebae, all encased in slime and working together as a single unit! The grex of
Dictyostelium discoideum is white and translucent, but experimenters frequently add colouring agents to make the different cells in
the grex apparent.
Above: a slime mould grex (rendered with Pov Ray) crawling across a glass
slide, leaving a trail of slime behind it.

Left: when a grex finally finds a suitable place (or runs out of time) it will stop
moving, then form a mound which elongates into a relatively long stalk (several
millimetres long) with a rounded structure at the tip (the colour and form varies
tremendously depending upon species). This structure, called a
sporangium
(spore capsule)
will dry and break open, releasing amoebae in the form of
spores, into the wind or rain water to be carried off to new habitats.
This lead
me to consider the issue that the sporangia are not really tall enough to break
through a typical boundary layer of still air, to reach the turbulent layer for
optimum wind dispersal: when we think of toadstools they are typically several
cm in height in order to achieve this. However, John Bonner (2009, The social
Amoebae: The Biology of Cellular slime Molds, Princeton University Press)
notes how the spores remain glued together and apparently do not dry out and
separate well for efficient wind dispersal, but they may however be dispersed
by rain splash or perhaps by flowing rainwater. (If my calculations are correct,
they should be high enough to pass a typical boundary layer in water to reach
the mainstream for efficient spore dispersal). Bonner also notes that often they
are dispersed by insects; indeed the typical height of 2 mm for a
Dictyostelium
stalk would be ideally placed to brush against passing insects and other
arthropods to which a packet of sticky spores can readily attach.

The spores are dormant cells with tough walls to resist drying out. Hopefully
some of the spores will find a suitable place and germinate into single-celled
amoebae and live and reproduce happily, until they run out of food that is ...
then the cycle will start all over again!


It should be noted that the fruiting bodies of different species of cellular slime
molds can be strikingly different (there are about 100 known species of cellular
slime mold). Some have branches, which may occur as whorls along the main
axis, with each branch ending in a small sporangium. They are very beautiful
structures!


A more detailed account of
Dictyostelium discoideum is given below.
mound
grex
In Dictyostelium discoideum about 100 to 100 000 free-living amoeboid cells come together, in times of harsh
conditions like starvation, to form one of the most simple animal-like multicellular organisms.

Cell differentiation and cell types

In Dictyostelium discoideum the cells in the mound, grex and fruiting body, can be distinguished into different
types on the basis of morphology and biochemical markers – different cell types develop with different
development pathways and end fates. This is differentiation and division of labour, characteristics of
multicellular organisms.

Two principal classes of cells develop from the aggregate cells –
prespore cells (psp) destined to become
spores and
prestalk cells (pst) destined to form the stalk. About 80% of the cells in the aggregate will
eventually become spores, the remaining 20% become the stalk and attachment/basal disc, sacrificing their
lineage in an act of sacrifice to increase the odds of survival of their kin, an example of kin selection, since
the amoebae will be more-or-less related to one-another.
The prestalk cells can be divided into several distinct cell types:

1) pstA cells

These occur at the tip of the mound and grex, occupying the anterior-most 1/3 of the prestalk region (accounting for 5-10% of the
total cells). They will develop into pstAB cells during culmination. PstA cells have active promoters for the ecmA gene, ecm for
extracellular matrix, since this gene encodes an extracellular matrix protein. These cells originally form in random positions in the
very early mound but move to the tip during cell-sorting. PstA cells are capable of transdifferentiating into pstO cells.

2) pstO cells

These occur in the anterior of the mound and grex behind the pstA cells, accounting for 5-15% of the cells. They also form originally
in random positions within the mound. They are capable of undergoing transdifferentiation into either pstA cells or ALC cells.

3) pstB cells

These initially form scattered around the mound but sort to the base and form an anteroventral band in the grex and form the inner
basal disc. They express the ecmB extracellular matrix gene. The pstB cells of the mound will form the basal disc if the gex stage is
skipped, otherwise they are removed from the grex and deposited in the slime trail to be replaced by cells from the anterior prespore
region which will form the inner basal disc.

4) pstAB cells

These develop from pstA cells when these cells enter the stalk tube and form a core inside the tip of the mound and grex. These
cells express both ecmA and ecmB extracellular matrix genes and are destined to become stalk cells.

Non-prestalk cell types:

5) psp cells (prespore cells)

These are destined to become spores, though they may transdifferentiate into ALC cells.

6) ALC (anterior-like cells)

These are intermingled, as small groups, with psp cells and cluster to form the rearguard cells in the grex and culminant. One
subtype of ALC cells are the pstO/ALC cells (ALC/pstO cells) which have active pstO-specific promoters and interchange with pstO
cells in the prestalk. Other cell types include ALC/pstA, ALC/pstAB, ALC/pstB cells, according to the cells they most closely resemble
biochemically. ALC cells also go on to form the lower cup in the sorus of the fruiting body, situated below the spore mass, and the
upper cup above the spore mass. ALC cells also form the outer part of the basal disc. There is lack of clarity or uncertainty in the
literature about how readily different cell types can interchange. It appears that cell type is determined by position within the mound
or grex and that cells crossing region boundaries may transform into the same type as their neighbours.

Other cell types:

7) Tip-organiser cells

These are pstA cells at the very tip. They possibly act as pacemaker cells by setting the frequency and direction of the cAMP waves
that direct the other cells in the mound and grex.

8) Sentinel cells (S cells)

These are cells that seem to specialise in phagocytosis and so may have an immune or nutritive function. Most cells stop feeding in
the aggregate and show signs of starvation in the grex, including the formation of autophagic vacuoles as they start to break-down
and digest their own non-essential organelle systems.

9) Peripheral layer cells (PLC)

These form a flattened proto-epithelium covering the grex. They don’t seem to be biochemically specialised or genetically
differentiated, but rather any cell type finding itself on the surface of the grex will change morphology and form part of the epithelium,
perhaps as a result of mechanical forces. These cells are closely joined to one-another by what appear to be junctional contacts,
especially in their outermost apical regions where there are no intercellular spaces. The posterior prespore region is covered in
flattened squamous cells, elongated with anterior-posterior polarity. The anterior prestalk region is covered by less flattened cells
with apical-basal polarity (they have some sort of tight junctions in their apical regions and wider intercellular spaces of 10 nm width
basally) and which put-out fine pseudopods (filopodia) that interdigitate between interior cells. It is possible that these filopodia are
the sites of material exchange, perhaps taking up materials released by interior cells to fuel secretion of the slime coat, though this is
my speculation. The proto-epithelium on the anterior and ventrum are formed from pst cells, those on the dorsum from psp cells.

10) Stalk cells

These develop from prestalk cells and form the stalk and part of the basal disc. They are about 8 micrometres in diameter and
develop single large autophagic vacuoles and a single layer of thin cell wall material and then die on terminal differentiation. They
are polygonal in outline (probably due to close packing, though they do have thin walls). The walls contain randomly arranged
cellulose microfibrils and ecmA/B glycoproteins. Those that are outermost secrete the stalk tube – a cylinder that traverses the sorus
at its anterior end and embeds in the basal disc posteriorly. This sheath consists of cellulose microfibrils, with the outermost fibrils
being arranged parallel to the long axis of the stalk. Outside the stalk sheath is generally a thin layer of ALC-derived cells which
extend to the basal disc to form the outer region of the disc.
Stages in Development - The Dictyostelium life-cycle

Free amoebae

The amoebae live free in the soil, eating primarily bacteria by phagocytosis, which they locate by chemotaxis. The bacteria are
phagocytosed into endosomal vesicles and processed in the endosome vesicle system and the waste exocytosed, with the whole
cycle taking about 90 minutes. They have a contractile vacuole network (CVN) of cisternae (membranous sacs) and interconnecting
ducts and large cisternae acting as bladders that periodically expel excess water to the outside by contracting and discharging
water through a pore in the plasma membrane. The CVN appears to be a distinctly separate membrane system that does not
exchange membrane material with either the plasma membrane or the endoplasmic reticulum or endosomal vesicle system. The
CVN functions in osmoregulation, expelling excess water to prevent the cell bursting by osmosis, and also helps regulate calcium ion
levels inside the cell. The amoebae of
Acytostelium leptosomum feeds on yeast as well as bacteria.

The amoebae, which are usually haploid, divide by mitosis, undergoing asexual binary fission. When food begins to run out, the
amoebae aggregate, if enough aggregate they will form a motile multicellular structure called a grex, however, if only a few cells
aggregate (as occurs, for example, in water suspension) then two of the cells may fuse into a diploid cell, in a process of fertilisation,
which phagocytoses the other cells, resulting in one cell with a giant nucleus which secretes a multilayered protective wall around
itself and then divides by meiosis and becomes a dormant
macrocyst which germinates into haploid amoebae when conditions are
once again favourable. This is sexual reproduction. In
Dictyostelium discoideum, two different compatible strains are required for
fertilisation prior to macrocyst formation, but in
Dictyostelium mucoroides, sexual fusion and macrocyst formation occurs between
cells of the same strain. Alternatively, some strains form
microcysts instead - single cells enclosed by protective cellulose cell walls.

Parasexual Process

Cellular slime mould amoebae may fuse to give binucleate cells, or occasionally diploid cells. These diploid cells may divide for a
number of generations, giving rise to diploid cells. However, some genetic exchange can occur between homologous pairs of
chromosomes during this mitosis (similar to crossing-over in meiosis). At some point this diploid state become sunstable and one of
each pair of chromosomes is slowly lost over a number of amoeboid generations until haploid cells are formed again, but these cells
may have new recombinant chromosomes.

Aggregation

i) cAMP wave propagation and chemotaxis

In Dictyostelium, aggregation occurs by chemotaxis to periodic cyclic AMP (cAMP) signals released from the aggregation centre
that propagate as waves. Cyclic AMP or cyclic adenosine monophosphate is a cyclic molecule derived from ATP (adenosine
trisphosphate). Cells move chemotactically towards increasing cAMP concentrations leading to aggregation streams and
multicellular aggregates.

1.        The cAMP is detected by a
high affinity receptor, cAR1
2.        Which upon binding cAMP couples to a hetero-trimeric G protein
3.        Which liberates the bg complex
4.        Which attracts the cytosolic cAMP regulator (CRAC) to the membrane
5.        which then activates adenylyl (adenyl or adenylate) cyclise
6.        leading to cAMP synthesis
7.        cAMP is secreted and binds to the receptor again – autocatlytic feedback
8.        Binding of cAMP to the receptor leads to desensitisation
9.        cAMP is degraded extracellularly by phosphodiesterases, resensitising the receptor.

The results are
periodic oscillations of cAMP. Cells undergoing chemotaxis are elongated. The waves may appear as expanding
spirals or concentric ring waves (as predicted by mathematical models using reaction-diffusion equations). Accumulation of cells
speeds up wave propagation. This locally distorts the wave front leading to the formation of bifurcating aggregation streams – that is
branching streams of migrating cells that converge on a central position. Eventually a mound of cells develops in the centre where
the cells accumulate.

Migrating cells thus secrete cAMP, which is sensed by other migrating cells by binding to the cAR1 receptor (expressed during early
development) which are thus stimulated to synthesise and secrete more cAMP to relay the signal to other cells. This sets-up
periodic pulses or waves of cAMP, peeking in the nM range of concentrations, radiating away from the aggregation centre to which
the cells are converging. (Such a centre will be set-up once several cells move close together). The binding of cAMP to the receptor
stimulates both adenylyl cyclase and guanylyl cyclase. Adenylyl cyclase is not essential for chemotaxis but is essential for
aggregation. Guanylyl cyclase is essential for chemotaxis. There are a number of cAR receptors (cAR1 to cAR4) but cAR1 is the
high-affinity receptor active early on.

Adenylyl cyclase is a membrane protein with 12 membrane spanning helices and several regions extending below the membrane
into the cytosol, inclusing a catalytic site where ATP binds and is converted into cAMP. Adenylyl cyclase is regulated by G proteins,
both stimulatory (Gs) and inhibitory (Gi). Guanylyl cyclase manufactures cyclic gunaine monophosphate, cGMP.


Why an oscillating signal?

Each amoeba emits periodic pulses of cAMP in response to pulses detected from other cells. This tends to have a synchronising
effect and all the individual waves of cAMP, each produced by a single cell, become synchronised and summate by constructive
superposition (a property of waves) into a single larger wave. It is true that summations of many smaller waves can give rise to a
larger rhythm, however th synchronisation need not be perfect which may result in more complex wave patterns which possibly
explains observations that the wave pulses occasionally cease for a time and become continuous (as noted in Bonner, 2009).

Some have suggested that the periodic pulse acts as a pacemaker to synchronise cell activity. This is a possibility, if the cells are
timing longer term changes, such as switching genes on for later fruiting body synthesis. Developing multicellular organisms do
often use timed signals to synchronise their activity. There is growing evidence that some deep sea sponges may be using light
signals to synchronise cell activity across the sponge body during growth and wound repair. However, I am not currently aware of
any research showing the importance of a periodic pacemaker signal in
Dictyostelium.

Nevertheless, the tendency, in
Dictyostelium discoideum at least, is for a periodic signal to be generated. It has been pointed out
that the amoebae could, in principle, be attracted to a continuously emitted signal set up as a continuous concentration gradient,
growing in concentration towards the focus of aggregation, in much the same way as human neutrophils may chemotact along a
gradient towards a group of bacteria. However, there are advantages to using a periodic signal: first of all it is less demanding on
resources to emit a periodic pulse of cAMP than to secrete cAMP constantly. Additionally, it is possible that the cells reach maximum
sensitivity at the anticipated time of the next pulse, perhaps whilst secretion of their own signal is minimum, and so are in an optimal
'listening' mode. By alternating signal emission and signal 'listening' the cells could be minimising response to their own signal and
maximising their response to the signals of others. Signalling systems generally show adaptation or habituation. Consider walking
into a room with a distinctive odour (perhaps somebody forgot to take the trash out) at first the aroma is obvious, but after a few
moments you tend not to sense it, unless you walk out of the room and re-enter it after a few minutes at which point the smell hits
you again! Your olfactory system adapts and ceases to respond to a constant 'background' stimulus and is tuned to detect changes
in
stimulus intensity. Some olfactory receptors are actually GPCRs like cAR1 and it is a general property of GPCR signalling that it
adapts. To lure a cell in with a chemical 'bait' one would need either a noticeably increasing concentration of the chemical towards
the source, or a periodic signal. A periodical signal allows the receptors to reset to their original sensitivity in between pulses and
perhaps maximises the sensitivity of the system.


ii) Signals in mounds

Strain specific patterns of periodic cAMP signals occur within the mound. Some mounds are organised by single concentric ring
pacemakers, several concentric ring pacemakers, or 1,2,3,5 or multi-armed spirals. These patterns may interchange. Initially the
waves propagate fast at low frequency; later frequency increases while speed decreases. All these patterns eventually produce a
tip, which protrudes from the top of the mound. Differences and changes in wave patterns may result from alterations in the speed
of cAMP production and sensitivity to cAMP (e.g. a switch from cAR1 to the less sensitive cAR2 and cAR3 receptors).

Prestalk (pst) and prespore (psp) cells relay the signal. Prestalk cells originally form at random positions within the mound. Prestalk
cells turnover cAMP faster and have the low affinity cAR2 receptor, allowing them to relay the cAMP signal at high amplitude. The
result is an accumulation of prestalk cells in the tip, which thus becomes the sole signal-generating centre. Prespore cells express
cAR3. During this transition, localised groups of cells may form other centres, but ultimately only the centre in the tip survives.
Periodic microinjection of cAMP into mounds counteracts the endogenous signal and disrupts mound formation. The details are
more complex, with prestalk type A (pstA) cells accumulating in the tip and prestalk type B (pstB) cells accumulating in the base of
the mound, both these types form initially at random positions in the mound. Type pstA cells express ecmA markers, pstB cells
express ecmB. EcmA and EcmB are extracellular matrix proteins.

iii) Cell movement in mounds

Cell movement is directed antiparallel to the direction of wave propagation. The different patterns of cell migration are strain
specific. In spirals, cell movement is counter-rotational and cell movement is several times faster than in concentric ring patterns,
where cells may be periodically stationary.

In strain AX3, the cell speed is 10 micrometres/min during aggregation, 50 micrometres/min during mound formation. Cell velocity
increases slightly when cells enter the aggregation streams. At the aggregation centre, movement slows down and becomes
temporarily disordered. Cell movement then suddenly increases in speed and becomes highly ordered and strongly rotational.
Movement slows again at the time of tip formation. Regulation of these processes could be due to changes in cAMP receptor
expression, changes in the cytoskeleton, cell adhesion and cell-matrix interactions?

iv) Tip formation

The amoebae differentiate into two principal cell types: prestalk (pst) and prespore (psp) cells. The prestalk cells differentiate and
sort out to form the tip on the top of the mound (pstA cells at the very tip, followed by pstO cells). The mound extends into the air
and contracts at the base. Cell movement in the tip appears to be always rotational. The period suddenly increases from 2 minutes
to 4 minutes (switch from cAR1 to cAR2?). Tip formation is cAMP dependent. Low affinity receptors may allow prestalk cells to
further respond to cAMP gradients when prespore cells are adapted. Prestalk cells also move faster and are less adhesive, which
may favour migration to the centre of the cell mass.

v) Arrest in mound stage

Evidence indicates that movement up to the top of the mound requires a high motive force involving both actin and myosin. Mutants
with actin and myosin defects are unable to pass the mound stage and culminate. Prestalk cells undergo rotational migration while
the cells at the base of the mound undergo periodic upward movement. The tip contracts and the mound elongates to form a
standing slug or grex. The grex eventually becomes unstable and topples over. In
Acytostelium leptosomum several grexes usually
form from a single aggregation centre.

vi) Cell movement and signal propagation in slugs

The grex migrates at about one grex body length per hour, or 30 micrometres/min.

Most species produce an internal stalk (as a central rigid chord of cells) continuously during grex migration, but some, including
Dictyostelium discoideum form stalkless migrating slugs. In D. discoideum, prestalk cells (which are situated in the tip which is raised
in the air) and especially pstO cells undergo rotational movement around the central axis of the grex at an angle to the direction of
grex migration and at an average speed greater than that of the grex. The prespore cells move periodically forward in the direction
of grex migration and with the same average velocity as the grex. These movements are probably coordinated by rotating and
planar waves of cAMP – rotating (scroll) waves in the tip and planar waves in the main body of the grex. Models indicate that a
transition from rotating to planar waves occurs if the prespore cells are less excitable than the prestalk cells. The tip is permanently
raised above the substrate and hence prestalk cells do not provide traction, but are involved in coordinating the grex. The tip
secretes a slime sheath, which surrounds the grex and may also be secreted into the interior. This slime sheath provides a
substrate for the migrating cells. Cells at the top migrate at the same velocity as cells in the bottom of the grex.

PstA cells are formed in the anterior outer prestalk zone; pstO cells at the boundary between prestalk and prespore cells and pstAB
cells occur in the central core of the prestalk zone. The core of the rotary wave in the prestalk zone is a region of low cAMP
concentration, which activates genes in these cells. Computer simulations show that a reduction in the difference in excitability
between prespore and prestalk cells causes the rotary wave to twist upon entering the prespore zone, extending the core
throughout the whole grex, which may account for the continuous stalk formation seen in, for example
Dictyostelium mucoroides,
which deposits a horizontal stem as it crawls, before turning the tip of the stem vertical during culmination (without forming a basal
disc). Analysis of cell movements supports this hypothesis. Cell movement is faster in
D. mucoroides, but the cells follow spiral
trajectories and overall grex migration is slower. The slugs often describe a spiralling path. The cytoskeletal protein myosin is known
to be important in grex migration.

The model of Odell and Bonner, 1986

Odell and Bonner developed a model of grex migration driven by pulses of cAMP travelling back from the tip. In this model cAMP
directs individual cells though a second unidentified chemical signal is required to increase vigour of movement, with the
concentration of this second signal peeking in the inner core of the grex where cells flow fastest. In this model some cells contribute
more to grex locomotion than others, though their positions may interchange. The outermost cells, near the surface, move slowly,
but deeper layers within the grex involve cells crawling over their outer neighbours at speeds increasing with depth, creating a shear
flow. The central cells thus move fastest in an interior fountain motion that contributes most of the thrust. If the grex is stalled, by say
encountering an obstacle that prevents forwards movement, then the fountain in the central chord reverses direction. However, with
low resistance the surface cells, the slow crawlers, move backwards. The movement is driven by a group of cAMP secreting
pacemaker cells in the tip (tip-organiser cells). The flow is predicted to cause the pacemaker to occasionally be displaced
backwards, at which point it loses dominance and thus is pushed forwards again, maintaining dominance by dynamic equilibrium.
This could account for the observed pulses in the tip of migrating grexes. It would take about 15-45 minutes for cells to move the
whole length of the grex from the back to the front.

This model also predicts that most of the traction is not generated by the ventrum of the grex which is in contact with the substratum,
supported by observations in which a grex will move over a rough surface, with which it has very little of its surface in contact, with
normal speed. However, the slime sheath may be providing traction. A grex can also climb objects and descend gaps within its slime
sheath, without touching a solid surface.


Longer grexes move faster

Bonner (2006) points out that longer grexes locomote faster than shorter grexes of the same species. He suggests an analogy of
several rows of oars on a boat: the more oars the faster the boat can travel, but says he has had little positive support from the
physics community. If we imagine a muscle cell contracting: the muscle consists of a long serial chain of contractile units called
sarcomeres. Each sarcomere contracts a small amount, say one micron (one thousandth of a mm) in a second. When connected in
series, the distances moved are additive, so 1000 sarcomeres will contract by 1 mm, 50 000 by 5 cm, in one second. Thus the
longer muscle cell will contract with the greatest speed. A similar argument applies to muscle lengthening by relaxation. In the case
of the grex this kind of coupling can only work if the cells are somehow attached to one-another. We know that they are not rigidly
attached, since they can swap position quite freely, however they are loosely cemented together by slime. Thus, I would indeed
expect longer grexes to move faster. (A suitable physics analogy would be a series of springs connected in series).

I would also expect thicker grexes to move faster: since each layer of cells moves over the one beneath it, the fastest moving cells
should be in the centre (much like fluid flowing through a pipe). With more layers the central cells which reach a higher peak
velocity. Indeed,
Dictyostelium polycephalum moves about a fifth of the speed of Dictyostelium discoideum for a given length and is
a much thinner grex. It is thin and worm-like, which allows it to more easily move through narrow spaces and it crawls through soil
much more easily than the grexes of other species. The 'fluid-flow' model in which amoebae in a migrating grex behave rather like
particles of fluid flowing through a pipe, may account for this (at least in part).


Structure of the grex

The cell fate map of the grex has already been discussed (see diagram) and so has the structure of the proto-epithelium cells that
cover its surface. The anterior of the grex secretes a slime sheath which has a similar composition to the stalk tube, containing
EcmA and EcmB glycoproteins. The sheath is added to by other regions of the grex, such that it gets thicker further from the tip.
This has been suggested as a mechanical guide to help ensure cells move forwards, since the cells tend to move in the direction of
least mechanical resistance and may be constrained by the thicker and stronger sheath posteriorly. The sheath continuously
streams off the posterior end of the grex, as a hollow tube which collapses to form the slime trail.

vii) Culmination

Slugs stop migrating in order to culminate, that is produce a vertical fruiting body. Translational movement in the tip ceases, but the
rear of the grex continues moving until it is positioned beneath the tip which is pushed vertically upwards. Possibly the grex
effectively migrates up the stiffening rod of vacuolating stalk cells (which eventually form a rigid structure with the stalk tube). The
vacuolation, formation of walls and the formation of the stalk sheath by the maturing stalk cells gives the stalk rigidity. The process
is summarised as follows:

i.  Tip arrests and orients upwards
ii. The posterior moves forwards until it is beneath the prestalk tip
iii. The pst cells form the stalk tube (extracellular matrix)
iv. Cells at the apex (pst cells) move into the stalk tube anteriorly, depositing matrix, vacuolating and dying
v.  Stalk elongation continues until more-or-less all pst cells are incorporated into the stalk.

Cell movement in the tip is still rotational. The prestalk cells in the tip form the stalk. PstB cells form the inner basal disc and
rearguard cells, which develop from anterior-like cells (ACL) in the back (prespore region) of the slug form the outer basal disc and
lower cup. A second signalling centre seems to appear at the base of the culminate. The nature of this secondary signal is unclear.
At least some of the rearguard cells can move forward rapidly to form a pile of cells at the prespore-prestalk boundary, the upper
cup, joined by cells from other locations. This aggregation seems to be under the control of a second signalling centre. During
culmination prespore cells move over the pile, which ends up in the back of the slug. These cells then undergo rotational migration.

Most of the ALC cells will move up with the prespore mass to form the lower cup, others remain to form the outer part of the basal
disc. In
Acytostelium leptosomum the stalk is a narrow cellulose tube containing no cells at maturity and produces spherical spores.
The fruiting body of
Distyostelium discoideum is normally yellow in colour.

Sporulation

About 80% of the total cells form unicellular spores. The spores are elongated ovoids and have thick three-layered walls secreted
by each cell. The inner layer is cellulose, the outer and inner layers are glycoproteins, with 10 major glycoproteins contributing. The
outer glycoprotein layer is loose and easily removed, but the inner layer is covalently cross-linked. These glycoproteins are stored
in prespore vesicles (PSVs) and the prespore cells mature more-or-less in synchrony, secreting their spore coats as the PSVs fuse
with the cell membrane in exocytosis. The spores are resistant to desiccation and temperature extremes. The spores are released
when the sorus dries out and ruptures. The whole point of the life-cycle is to release the spores above the boundary layer where
they can be better dispersed. If the grex found a good place of elevation, then the height for the stalk is sufficient to clear the most
stagnant region of the boundary layer.

Directional movement of migrating slugs - taxis

The cell aggregate undergoes tip formation and upwards extension to form standing ‘fingers’ that fall over and crawl away as slugs.
Slug migration may last from one hour to nearly two weeks, depending on strain and conditions like humidity and osmolarity.
Average grex speeds are 0.2 to 2.0 mm/h (about 30 micrometres/min). The slugs are
phototactic (moving towards light),
thermotactic, rheotactic (moving towards wind currents) and acidotactic (moving in response to pH gradients). These processes
help the grex find its way out of the soil or other substrate and climb a suitably elevated object for spore dispersal.

In
thermotaxis slugs migrate towards warmth with maximum accuracy at temperatures close to the optimum growth temperature. At
higher and lower temperatures the accuracy of orientation declines, until, at several degrees either side of optimum, there is a
transition to negative thermotaxis. Sign reversals in phototaxis cause slugs to migrate at an angle either side of the direction of the
light source. Those whose direction of travel is at an angle less than the critical angle turn away from the light, while those travelling
at a greater angle turn towards the light. In wildtype slugs under most conditions the critical angle is sufficiently small that bi-
directional phototaxis is indistinguishable from unidirectional phototaxis.

Maree et al. devised a model of phototaxis, supported by empirical data, in 1999. In this model, the translucent cells of the grex act
as a lens, focusing light falling onto the tip at one side on to the cells in the tip at the far-side. (This is supported by experiment).
Light striking these cells causes a change in the cAMP waves generated by the pacemaker cells in the tip, with the waves tilting in
one direction, causing the cells in the grex to turn in response to the new gradient. This contrasts with models in which individual
cells sense the direction of the light. In this model, the light focused on the far cells causes them to produce more ammonia, which
inhibits cAMP production by the cells on this side. This is supported by empirical evidence, in which an equal concentration of
ammonia around the grex inhibits phototaxis and the tendency of the grex to avoid ammonia. Ammonia and light have no observable
effect on cell speed, so the idea is that they affect the shape of the cAMP waves.

Key points:

  • The slug tip controls slug behaviour.
  • Photosensory and thermosensory signal transduction pathways converge early and share most components.
  • Slug turning is mediated by transient lateral shifts in slug tip position. Light intensity and temperature gradients across the
    slug tip cause turning responses by altering the balance between tip activation and inhibition.
  • Tip activation signals are carried by cAMP waves (scroll-shaped), while tip inhibition signals may be borne by one or more of
    the molecules: ammonia, adenosine and Slug Turning Factor (STF). Inhibitory signals may influence cAMP-wave pacemaker
    frequency.
  • Signal transduction components: cAMP, inositol polyphosphates, cGMP, calcium ions, and cytoskeletal proteins. Mitochondria
    are also involved.

Bibliography & References

1. Rieu, J.-P., K. Tsuchiys, S. Sawai and Y. Maeda, 2003. Cell movements and traction forces during the migration of 2-dimensional
Dictyostelium slugs. J. Biol. Phys. 29: SN1-SN4.

2. Dormann, D., B. Vasiev and C.J. Weijer, 2002. Becoming multicellular by aggregation; the morphogenesis of the social amoebae
Dictyostelium discoideum. J. Biological Physics, 28:765-780.

3. Maree, A.F.M. and P. Hogeweg, 2001. How amoeboids self-organize into a fruiting body: Multicellular coordination in

Dictyostelium discoideum
. PNAS, 98: 3879–3883.

4. Maree, A.F.M., A.V. Panfilov and P. Hogeweg, 1999. Phototaxis during the slug stage of dictyostelium discoideum: a model study.
Proc. R. Soc. Lond. B, 266:1351-1360.

5. Fuchs, M., M.K. Jones and K.L. Williams, 1993. Characterisation of an epithelium-like layer of cells in the multicellular

Dictyostelium discoideum
slug. J. Cell Sci. 105:243-253.

6. Odell, G.M. and J.T. Bonner, 1986. How the
Dictyostelium discoideum grex crawls. Phil. Trans. R. Soc. Lond. B, 312: 487-525.

7. Ashworth, J.M. and J. Dee, 1975. The biology of slime moulds. The Institute of Biology's Studies in Biology no. 56. Edward Arnold
(pub).

8. Garrod, D.R. 1974. The cellular basis of movement of the migrating grex of the slime mould
Dictyostelium discoideum:
chemotactic and reaggregation behaviour of grex cells.
Embryol. exp. Morph. Vol. 32: 57-6.

9. Garrod, D.R.  1969. The Cellular basis of movement of the migrating grex of the slime mould
Dictyostelium discoideum. J. Cell
Sci
. 4: 781-798.


The following charming book is full of amazing facts about cellular slime molds and will appeal to anyone with an interest in cellular
slime molds, from lay person to the professional biologist. It lacks many of the intricate details of cell signalling networks, but gives a
fascinating and rich account of the behaviour and general biology of cellular slime molds and is packed full of fascinating facts:

Bonner, J.T. 2009. The Social Amoebae: The biology of cellular slime molds. Princeton University Press.

Cellular Slime Moulds
Left: a culminant (a mound developing a stalk and
becoming a fruiting body).
Aggregation
Above: aggregating amoebae. Thousands of the amoebae move, in pulsatile
fashion, toward an aggregation centre, forming concentric rings, spirals or
streamers (as in this case) depending on cell density. Gradually, a mound
forms in the centre.
Amoebae
Above: amoebae - single-celled shape-shifting
organisms.
Article updated: 17 Sep 2017