Insights & Perspectives
Hypotheses
An epithelial tissue in Dictyostelium
challenges the traditional origin of
metazoan multicellularity
Daniel J. Dickinson1)y, W. James Nelson1)2)3) and William I. Weis1)3)4)
We hypothesize that aspects of animal multicellularity originated before the
divergence of metazoans from fungi and social amoebae. Polarized epithelial
tissues are a defining feature of metazoans and contribute to the diversity of
animal body plans. The recent finding of a polarized epithelium in the nonmetazoan social amoeba Dictyostelium discoideum demonstrates that epithelial tissue is not a unique feature of metazoans, and challenges the
traditional paradigm that multicellularity evolved independently in social amoebae and metazoans. An alternative view, presented here, is that the common
ancestor of social amoebae, fungi, and animals spent a portion of its life cycle
in a multicellular state and possessed molecular machinery necessary for forming an epithelial tissue. Some descendants of this ancestor retained multicellularity, while others reverted to unicellularity. This hypothesis makes testable
predictions regarding tissue organization in close relatives of metazoans and
provides a novel conceptual framework for studies of early animal evolution.
.
Keywords:
Dictyostelium; epithelium; metazoan evolution; multicellularity
Introduction
A simple epithelium consists of a twodimensional sheet of structurally and
functionally polarized cells that are often
organized into a three-dimensional tube,
and is a defining feature of animal body
plans. An epithelium (the trophectoderm) is the first differentiated tissue
formed during embryogenesis, and
DOI 10.1002/bies.201100187
1)
2)
3)
4)
Program in Cancer Biology, Stanford
University, Stanford, CA, USA
Department of Biology, Stanford University,
Stanford, CA, USA
Department of Molecular and Cellular Physiology,
Stanford University, Stanford, CA, USA
Department of Structural Biology, Stanford
University, Stanford, CA, USA
*Corresponding author:
Daniel J. Dickinson
E-mail: ddickins@live.unc.edu
y
Present address: Department of Biology,
University of North Carolina, Chapel Hill, NC, USA
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
many adult tissues are composed of
epithelia [1, 2]. Epithelial cells have a
polarized organization of the plasma
membrane, cytoskeleton, and cytoplasmic organelles [2, 3]. The apical
plasma membrane faces the lumen of
an organ or the outside of the organism,
while the basal (or basolateral) membrane contacts the underlying tissue.
Polarized epithelial sheets regulate the
directional absorption and secretion of
proteins and other solutes, an essential
physiological function that is often disrupted in human disease [1]. In addition,
the polarized organization of cytoskeletal components in epithelial cells
contributes to tissue shape changes
during embryonic morphogenesis [4].
Despite the diversity of body plans
and lifestyles, all metazoans share an
epithelial tissue as their basic unit
of organization, suggesting that the
development of epithelial cell polarity
was a very early event in metazoan
evolution [5, 6].
Genetic, cell biological, and molecular studies in a variety of metazoans
have shown that the formation and
maintenance of polarized epithelial
cells require cell-cell adhesion mediated
by the cadherin-catenin complex [7].
Cadherins are transmembrane receptors
that form homophilic and heterophilic
adhesive interactions with cadherins on
adjacent cells, providing a spatial cue
that initiates cell polarity [8]. b-Catenin
and a-catenin are cyotosolic binding
partners of cadherin that transduce this
adhesive cue and mediate cell polarity,
in part by directing the reorganization of
the cytoskeleton [8, 9]. Consistent with
www.bioessays-journal.com
1
Hypotheses
D. J. Dickinson et al.
its fundamental role in epithelial organization, the cadherin-catenin complex is
conserved in all metazoans.
Phylogenetically speaking, metazoans belong to the unikonts, a group that
also includes fungi, social amoebae, and
a number of unicellular or colonial protists (see Fig. 2) [10, 11]. Historically, it
was thought that multicellularity evolved
independently in animals, fungi and
social amoebae, and that epithelial tissue
was a unique feature of animals [11–14].
However, two recent studies have
established the existence of polarized
epithelial tissue in the non-metazoan
social amoeba Dictyostelium discoideum
[15, 16]. This finding calls into question
the notion of an independent origin of
multicellularity in animals and social
amoebae. Here, we propose and discuss
the alternative hypothesis that all unikonts evolved from an ancestor with a
simple multicellular organization.
Insights & Perspectives
Identification of an
epithelial tissue in
Dictyostelium
D. discoideum is a representative of the
social amoebae, a group of organisms
that feed as single cells but undergo a
transition to a multicellular form upon
starvation (Fig. 1A) [17]. Starving cells
secrete cyclic AMP, which acts as a
chemoattractant and induces cells to
aggregate into spherical mounds (see
[18] for a detailed description of the
aggregation process). Once aggregation
is complete, the mound elongates to
form a slug. After a period of migration,
the slug develops into a fruiting body
by a process called culmination. The
developmental process can thus be divided into three phases: (1) chemotaxis
and aggregation; (2) slug formation and
migration; and (3) culmination. Of these
.....
three phases, chemotaxis has received
the most attention, and relatively little is
known about the later stages of development. However, it is during culmination
that the greatest degree of complexity
in tissue organization and cell type
specialization is observed.
The fruiting body consists of a rigid
stalk supporting a collection of spore
cells at the top (Fig. 1B). The defining
event of culmination is the production
of the stalk, which elevates the spores
and eventually facilitates their dispersal. The stalk consists of stalk cells
encased in a rigid tube made of cellulose
and extracellular matrix (ECM) proteins
[19, 20]. During culmination, the stalk
cells increase in volume, but because
they are encased in a rigid tube, their
expansion is directed in the vertical
direction and contributes to the lifting
force that raises the spore head [21].
Stalk formation is orchestrated by the
Figure 1. Multicellular development and epithelial polarity in social amoebae. A: The developmental process of D. discoideum. Starvation
induces aggregation of individual amoebae to form a mound. The mound then undergoes morphogenesis, forming first a migrating slug, then
a fruiting body. A fruiting body in the process of formation is called a ‘‘culminant’’. B: Anatomy of the D. discoideum culminant. The culminant
consists of a collection of spores supported by a rigid stalk (left). The stalk is patterned by the tip, which is located at the apex of the
culminant and comprises a tubular epithelial monolayer surrounding the top of the stalk (right). The apical membrane of the tip epithelial cells
is adjacent to the stalk. Epithelial cells secrete cellulose and ECM proteins to form the stalk tube, which is the rigid exterior of the stalk. At the
same time, contractile force generated by the epithelial cells limits stalk diameter. C: Images of the tip epithelium stained for various markers,
as indicated. Note that tubulin staining is used to mark centrosomes. Scale bars represent 10 mm. Brackets indicate the tip epithelium, and
‘‘S’’ indicates the stalk. See [15] and [16] for materials and methods. Panels A and B were adapted from [16].
2
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
.....
Insights & Perspectives
D. J. Dickinson et al.
Hypotheses
Figure 2. Conservation of epithelial characters in unikonts. The table shows the presence (colored square) or absence (white square) of
the indicated characters in the indicated species. A question mark indicates that no information is available for this particular character and
species. Character descriptions and notes: (1) No molecular information about these species is available because their genomes have not yet
been sequenced. (2) Solid squares indicate constitutive multicellularity; cross-hatched squares indicates facultative multicellularity. (3) Indicates
a columnar monolayer of cells that morphologically resemble a simple epithelium. (4) Indicates a polarized organization of the plasma membrane, cytoskeleton, and cytoplasmic organelles. (5) Indicates directional secretion of proteins and other solutes. (6) Indicates an apical enrichment of myosin motor proteins, resulting in apical contractility. (7) Indicates the presence of cell-cell junctions linked to the actin cytoskeleton.
These are represented by adherens junctions in metazoans, and by junctions of unknown molecular composition in D. discoideum. See the
text for details. (8) Solid squares indicate classical cadherins that contain a cytoplasmic domain that can bind b-catenin; cross-hatched
squares indicate proteins with extracellular cadherin repeats but no classical cadherin cytoplasmic domains. (9) b-catenin orthologs are very
difficult to identify using sequence-based methods such as BLAST, due to the presence of armadillo repeats that are also found in many
other proteins. Therefore, we conservatively use solid squares to represent proteins that are known to be functionally similar to b-catenin and
cross-hatched squares to represent armadillo repeat proteins with sequence similarity to b-catenin but unknown function. (10) The a-catenin
ortholog from D. discoideum has been shown to be functionally similar to a-catenin but not the related protein vinculin [15]. Members of this
protein family from choanozoa and chytrid fungi are therefore inferred to be a-catenin-like. (11) Solid squares represent an essentially complete set of polarity proteins (see text). A few polarity proteins, including Lgl, Par-1, Par-4, and Par-5 are members of protein families that also
include non-metazoan members.
tip, which is a specialized structure
located at the apex of the culminant
[15, 16, 21–24]. Though the tip was previously thought to play an important
role in culmination, its subcellular
organization and function had not been
studied in detail.
Confocal microscopy showed that
the tip contains a highly organized cell
monolayer that forms a tube surrounding the apex of the stalk (Fig. 1B, C) [15],
consistent with earlier electron micro-
scopy data [25, 26]. These cells, which
we refer to as the tip epithelium, exhibit
many of the hallmarks of epithelial cell
polarity, including: a polarized organization of the actomyosin and microtubule cytoskeletons; a polarized
distribution of cytoplasmic organelles;
and division of the plasma membrane
into apical and basolateral domains
with distinct protein compositions
(Fig. 2) [15, 16]. Most importantly,
polarity of the tip epithelial cells in
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
Dictyostelium requires homologs of band a-catenin, which are necessary for
epithelial organization and polarity in
metazoans.
Two distinct developmental functions have been identified for the tip
epithelium. First, the epithelial cells
secrete cellulose and ECM proteins
directionally to form the rigid stalk tube
on the exterior of the stalk [15]. Second,
the epithelium is an actomyosin-based
contractile structure that applies a
3
Hypotheses
D. J. Dickinson et al.
squeezing force to the stalk, thereby
limiting its diameter [16]. Although
these two functions result from distinct
cellular activities, they both contribute
to patterning the stalk: the squeezing
force applied by the epithelium causes
the stalk to be tall and thin, while the
secreted cellulose and ECM rigidify
the stalk and ensure that its diameter
remains constant even as stalk cells
expand. These recent descriptions of
the subcellular organization and function of the tip epithelium support the
historical view of the tip as an organizing
center for fruiting body morphogenesis
[21–24]. In addition, it is striking that
the tip epithelium is capable of polarized
secretion and apical enrichment of
myosin during morphogenesis, both of
which are important functions of metazoan epithelia.
Although the recent description and
genetic analysis of the tip epithelium
were restricted to the model organism
D. discoideum, available evidence
suggests that epithelia may be widespread in social amoebae. Cell monolayers with a similar appearance to
the tip epithelium have been described
in five other dictyostelid species, representing three of the four major phylogenetic groups of social amoebae [27–30].
In every case these tissues were located
adjacent to the stalk at the tip of
the culminant, suggesting a developmental function similar to that of the
D. discoideum tip epithelium. Moreover,
homologs of b- and a-catenin are found
in every social amoeba whose genome
has been sequenced ([15] and DJD, unpublished results). Some of these species have
fruiting body morphologies that are more
elaborate than D. discoideum, and it will
be interesting to determine whether more
complex morphogenetic movements of
epithelia contribute to the formation of
these structures.
Dictyostelium has a
minimal molecular
machinery for epithelial cell
polarity
Despite the morphological, molecular
and functional similarities between
Dictyostelium and animal epithelia,
there are also differences, as might be
expected given the evolutionary dis-
4
Insights & Perspectives
tance involved (Fig. 2). Perhaps most
importantly, Dictyostelium does not
have homologs of classical cadherins,
which are essential for recruitment of
b- and a-catenin to the plasma membrane in metazoans. Nevertheless, the
catenins localize to the plasma membrane in the tip epithelium [15].
Presumably an as yet unknown membrane-associated protein interacts with
the catenins to recruit them to the
plasma membrane, and identifying this
molecule is an important goal for future
studies. In addition, Dictyostelium lacks
‘‘polarity proteins’’ including the PAR
proteins, Crumbs, and Scribble, which
are important regulators of epithelial
polarity in higher animals and may
be necessary for the more complex
morphogenetic movements that metazoan epithelia must execute during
morphogenesis [4, 31]. Finally, metazoan b-catenin also plays an important
role as a transcriptional co-factor in the
Wnt signaling pathway [32], but despite
the presence of a b-catenin homolog,
other key Wnt pathway components
are absent in Dictyostelium, indicating
that the role of b-catenin in epithelial
polarity predates its function as a signaling protein.
A more complex issue is the role of
cell-cell junctions in the tip epithelium.
Adherens junctions containing classical
cadherins and their cytosolic binding
partners, including the catenins, mediate cell-cell adhesion in animals [9]. The
adherens junction associates with actin
and serves to mechanically couple the
cytoskeletons of adjacent cells, allowing
force transmission across a tissue [33, 34].
In many (though not all) metazoan epithelial cells, the adherens junction
localizes to the boundary between
apical and lateral plasma membrane
domains. Interestingly, transmission
electron microscopy revealed actinassociated junctional structures in
Dictyostelium tip epithelial cells that
localize to the apical-lateral boundary
and appear to connect contractile
actomyosin bundles across several cells
[15, 16, 25]. Despite this apparent
similarity at the ultrastructural level,
these Dictyostelium cell-cell junctions
do not require either b- or a-catenin
[15], or cadherins (which are absent in
Dictyostelium) and, therefore, are most
likely not homologous to the metazoan
adherens junction. One possibility is
.....
that the presence of similar-looking
adhesion structures in Dictyostelium
and animal epithelia reflects convergent
evolution. Alternatively, sequence homologies of junctional components other
than the catenins might exist but be
too weak to detect computationally.
Determining the molecular composition
of Dictyostelium cell-cell junctions
will distinguish between these two
possibilities.
Overall, it appears that the molecular machinery involved in epithelial cell
polarity in Dictyostelium is much simpler than in higher animals (Fig. 2).
Therefore, studies in Dictyostelium are
likely to be informative for elucidating
the core evolutionarily conserved
machinery necessary for formation of
a polarized epithelium. b-Catenin and
a-catenin clearly belong to this minimal
machinery, since they are required for
epithelial polarity in both Dictyostelium
and metazoans [15]. Additional molecular components have been identified
in immunoprecipitates of a-catenin
from fruiting bodies, including a
Dictyostelium IQGAP homolog and its
binding partner cortexillin that appear
to act downstream of the catenins to
regulate myosin localization and tissue
morphology [16]. Available data suggest
that IQGAP may also have a role in
epithelial cell polarity in higher animals
[35–37], although it is unknown whether
the putative role of IQGAP in metazoan
epithelia involves myosin regulation.
Thus, experiments in Dictyostelium
can shed light on the fundamental
requirements for epithelial cell polarity
and reveal new molecular mechanisms
that may be relevant in other systems.
An epithelium in
Dictyostelium may indicate
an ancient origin for
multicellular organization
Dictyostelium and metazoan epithelia
are structurally and functionally similar
tissues, and importantly, they share a
degree of homology at the molecular
level, since the catenins are required
for the epithelial organization and
polarity in both systems [15, 16]. What
does this result mean for our understanding of animal origins? As mentioned above, the traditional view has
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
.....
Insights & Perspectives
social amoebae and animals [15].
Thus, in at least one case, proteins that
are conserved between metazoans and
non-metazoans also have conserved
functions. We expect that when more
cases are studied in detail, additional
examples of functional similarity
between metazoan and non-metazoan
proteins will be found.
It is conceivable that the similar functions of the catenins in Dictyostelium
and animals are a product of convergent
evolution. However, we think that the
degree of molecular, morphological,
and functional similarity between epithelial tissues in Dictyostelium and
animals is very unlikely to be explained
by convergence. In light of the observations discussed above, we propose that
at least some of the organizational principles underlying animal multicellularity
predate the divergence of social amoebae
from metazoans. But how are we to reconcile this rather provocative conclusion
with our knowledge of other close metazoan relatives and with the view of social
amoebae, fungi, and animals as independent ‘‘inventors’’ of multicellularity
[11–13]? We suggest that an answer to
this apparent contradiction can be found
by re-thinking the nature of the transition
to multicellularity in metazoans.
Evidence for independent
origins of multicellularity is
not conclusive
The idea that multicellularity originated
independently in social amoebae, fungi,
and animals has been supported by
three main lines of evidence. First, these
groups diverged approximately 600
million years before the appearance of
the earliest putative metazoans in the
fossil record [44, 45]. However, the
absence of putative metazoans in
the fossil record prior to the Ediacaran
period is not definitive, as it is possible
that multicellular unikonts prior to
this time were either too small or too
fragile to fossilize, or too morphologically different from extant animals to be
recognizable [44].
A second argument for independent
origins of multicellularity in social
amoebae, fungi and animals has been
the very different lifestyles of organisms
in these taxa. These groups were
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
thought to have little in common at
the level of tissue organization, which
is one reason that the finding of epithelial tissue in Dictyostelium was so
surprising. It is important, however,
to avoid confusing the structure of a
tissue with its physiological purpose.
At the organismal level, epithelial tissue
has a different purpose in Dictyostelium
than in animals. Animal epithelia
have functions that include forming a
protective barrier, directional absorption and secretion, and execution
of morphogenetic movements during
development [1, 2, 4]. The function of
the Dictyostelium tip epithelium is to
pattern the stalk, a role not obviously
related to that of any metazoan epithelial tissue. Nevertheless, at the
cellular level, Dictyostelium and animal
epithelia are very similar: they both
have a monolayer organization, secrete
proteins and other solutes directionally,
exhibit apical contractility, and require
the catenins for their organization and
polarity (Fig. 2). Thus, organismal lifestyle may not be a good predictor of
tissue structure and function, and similarly organized tissues can have quite
different roles in different organisms.
The unikont lineages that branch
between social amoebae and animals
have unique lifestyles and ecological
niches, but tissue organization in these
lineages has generally not been well
studied. Closer examination of these
species may well reveal unexpected similarities to social amoebae or animals.
Third, the phylogenetic clade that
includes social amoebae, fungi, and
animals also contains some unicellular
organisms [10, 11], and it was considered
more parsimonious to postulate independent origins of multicellularity than
a loss of multicellular organization in
multiple unicellular lineages. We now
propose an alternative model that
can account for both the putative
homology of epithelial tissues in metazoans and social amoebae, as well as
the apparent loss of multicellularity in
many unikonts.
Multicellularity as a plastic
trait in metazoan ancestors
A polarized epithelium is the first tissue
formed during metazoan development,
5
Hypotheses
been that social amoebae, animals,
and fungi each evolved multicellularity
independently [12, 13]. However, the
molecular and functional similarities
between Dictyostelium and animal epithelia may force us to re-examine this
conclusion.
Is Dictyostelium an appropriate
model system to study the origins
of multicellularity in animals? Social
amoebae diverged from the metazoan
lineage earlier than other metazoan
relatives (most notably choanoflagellates and sponges) [10], but molecular
data indicate that they may have
evolved relatively little since their divergence. Ribosomal RNA sequences from
different social amoebae are approximately as diverse as those of metazoans,
even though all social amoebae have
a similar lifestyle and morphology
[30, 38]. Moreover, the cellular slime
mold Fonticula alba has a life cycle very
similar to that of social amoebae, even
though it is phylogenetically more
closely related to fungi [39]. Together,
these observations suggest that the
analysis of living social amoebae can
provide considerable insights into the
nature of their ancient ancestors, which
might have had much in common
with the metazoan ancestors that lived
around the same time. Of course, a caveat is that one cannot rely too heavily on
data from any single species when drawing conclusions about the nature of
metazoan ancestors. Observations from
social amoebae, choanoflagellates, as
well as intermediate-branching species
such as ichthyosporeans and chytrid
fungi must ultimately be integrated
to yield a coherent understanding of
metazoan origins.
Surprisingly, analysis of the genomes
of choanoflagellates, social amoebae,
and other close relatives of metazoans
revealed that many of the molecules
historically thought to be restricted to
metazoans are found in these earlierbranching lineages as well (Fig. 2)
[40–43]. Although in most cases, the
cellular functions of these proteins
are unknown, the similar functions
of the Dictyostelium and metazoan
catenins provides one illuminating
example [15, 16]. Strikingly, Dictyostelium
a-catenin can bind directly to mouse
b-catenin, indicating that this proteinprotein interaction may have been conserved since the common ancestor of
D. J. Dickinson et al.
Hypotheses
D. J. Dickinson et al.
and metazoan body plans at all stages of
development are constructed largely
from epithelial tissues. In contrast, an
epithelial tissue is not ubiquitous in the
Dictyostelium life cycle. Dictyostelium
development is initiated by an environmental cue (nutrient deprivation), and
an epithelial tissue forms only at the
end of the developmental process. The
development of multicellularity facilitates spore dispersal, but is not essential
for feeding or reproduction. Indeed,
social amoebae can replicate indefinitely as single cells, and in nature
they may go many generations without
becoming multicellular. Hence, social
amoebae are facultative multicellular
organisms that adopt multicellularity
only for a specific purpose. This mode
of multicellularity is different from the
constitutive multicellularity that defines
metazoans.
Multicellularity in metazoans can
be considered a fixed state, since all
metazoans are multicellular throughout
their lives (with the exception of gametes). Apparently, adaptations associated
with constitutive multicellularity have
deprived animal cells of the potential
to survive and reproduce on their
own, or at least to effectively compete
with other unicellular organisms. In
contrast, facultative multicellularity in
social amoebae can be viewed as a
continuum: some species, including
D. discoideum, readily enter the multicellular state when subjected to
starvation, while other species preferentially respond to starvation by forming
unicellular cysts and are only rarely
multicellular [38, 46]. The amount of
time spent in the multicellular state is
thus governed both by environmental
cues and by the organism’s response
to those cues. Given this scenario, it is
easy to imagine how a facultative multicellular organism could revert to a unicellular state, simply by adapting to a
novel environment that favored single
cells. The variable preference for aggregation versus encystation in different
species of social amoebae [38, 46] provides evidence that evolution can indeed
modulate the extent of facultative multicellularity, and solitary amoebae such as
Acanthamoeba could be examples of
species that have abandoned multicellularity entirely.
Other unikonts also exhibit facultative multicellularity, and the amount of
6
Insights & Perspectives
time spent as a multicellular organism
similarly varies within each group.
For example, the choanoflagellate
Salpingoeca rosetta can exist either as
a single cell or as small colonies [47].
The physiological significance of colony
formation is unknown, but it has been
suggested that unicellular and colonial
states are each advantageous under
different environmental conditions.
Consistent with this idea, colony formation and morphology are influenced
by presence of a specific bacterial prey
species [47]. Another group of metazoan
relatives, the ichthyosporeans, also
transitions between a unicellular
amoeboid state and simple spherical colonies [48, 49]. The cellular slime mold
Fonticula alba exhibits facultative multicellularity very similar to Dictyostelium,
despite being more closely related to
fungi than to social amoebae [39].
Finally, many species of fungi exhibit
facultative multicellularity, switching
from unicellular (yeast-like) to multicellular (pseudohyphal or filamentous)
growth in response to specific environmental conditions [50]. Multicellularity
in fungi is particularly plastic, having
been lost completely and then reinvented on several occasions [13].
Taken together, these observations
indicate that facultative multicellularity
is widespread in unikonts, whereas constitutive multicellularity is characteristic
of metazoans.
We propose that the common ancestor of all unikonts was a facultative
multicellular organism in which
some aspects of cell or tissue polarity
were controlled by the conserved catenin
complex. Because of the inherently
plastic nature of facultative multicellularity, some of the lineages descended from
this ancestral unikont reverted to the
unicellular state, while others retained
a facultative multicellular lifestyle. In
one lineage (metazoans), multicellularity
became constitutive. This hypothesis
accounts for the presence of an epithelial
tissue in Dictysotelium, is consistent with
our knowledge of unikont natural history
and phylogeny, and is sensible given
the widespread distribution of facultative
multicellularity among unikonts. The
idea that multicellularity existed prior
to metazoan origins may also help to
explain the finding that components of
key metazoan signaling pathways can be
found in other unikonts.
.....
Our hypothesis has significant implications for views on the origins of modern
metazoans. If it is correct, we must
regard the evolutionary event that gave
rise to metazoans not as a transition to
multicellularity per se, but as a transition from facultative to constitutive
multicellularity. This event could be
viewed in one sense as an abandonment
of the unicellular stage that characterizes the life cycles of facultative multicellular organisms (although animals
still have unicellular gametes that may
serve as a buffer against ‘‘cheater’’
mutations [51]). By definition, facultative multicellular unikonts rely on the
unicellular state for at least one essential function; e.g., in social amoebae,
both feeding and cell proliferation are
confined to the unicellular phase of the
life cycle. Presumably, then, the transition to constitutive multicellularity
was triggered by the appearance of
new signaling pathways that allowed
a greater degree of coordinated cellular
movements and differentiation, thus
permitting different essential functions
(e.g. feeding and reproduction) to occur
simultaneously in a single multicellular
individual. This view of the transition to
constitutive multicellularity is in fact
not very different from other proposals
for the transition to multicellularity in
animals (e.g. [5, 12, 52]); the novelty of
our hypothesis concerns the nature of
the ancestral organism in which this
transition occurred.
Testable predictions of the
‘‘facultative
multicellularity’’ hypothesis
Our hypothesis predicts that if tissue
polarity mediated by the catenin
complex predates the divergence of
Dictyostelium from metazoans, then
we should expect to find polarized cells
that require the catenin complex in
some of the organisms that diverged
from the metazoan lineage after social
amoebae. Given our current knowledge
of unikont phylogeny and the availability of sequenced genomes, the most
informative lineages to examine would
be choanoflagellates, ichthyosporeans,
and chytrid fungi (see Fig. 2). Almost
nothing is known about multicellular
tissue organization in these organisms,
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
.....
Insights & Perspectives
Conclusions
We have proposed a novel intellectual
framework for the study of early metazoan evolution, based on the surprising
finding of polarized epithelial tissue
in a non-metazoan social amoeba.
Similar to metazoan simple epithelia,
the Dictyostelium tip epithelium consists
of a monolayer of cells with a distinct
apical-basal polarity that is dependent
on homologs of b- and a-catenin.
This tissue contributes to multicellular
morphogenesis through both directional secretion of proteins [15] and
apical localization of contractile actomyosin structures [16]. To explain these
results, we hypothesized that metazoans originated from an ancestor that
was facultatively multicellular, rather
than from a unicellular ancestor as
was previously proposed. The key predictions of this hypothesis are that cell
polarity mediated by the conserved
catenin complex should utilize similar
molecular mechanisms in social amoebae and metazoans, and that cateninmediated tissue polarity should also
be observed in other unikont lineages.
Advances in technology for genetic
manipulations in close metazoan
relatives will provide the experimental
tools necessary to test these predictions.
Acknowledgments
We are grateful to Nicole King and Iñaki
Ruiz-Trillo for helpful discussions. Our
work on Dictyostelium was supported by
an NSF Graduate Research Fellowship
(D. J. D.), NIH GM035527 (W. J. N.), and
NIH GM56169 (W. I. W.).
References
1. Marchiando AM, Graham WV, Turner JR.
2010. Epithelial barriers in homeostasis and
disease. Annu Rev Pathol 5: 119–44.
2. Bryant DM, Mostov KE. 2008. From cells to
organs: building polarized tissue. Nat Rev Mol
Cell Biol 9: 887–901.
3. Tanos B, Rodriguez-Boulan E. 2008. The epithelial polarity program: machineries involved
and their hijacking by cancer. Oncogene 27:
6939–57.
4. St Johnston D, Sanson B. 2011. Epithelial
polarity and morphogenesis. Curr Opin Cell
Biol 23: 540–6.
5. Nichols SA, Dirks W, Pearse JS, King N.
2006. Early evolution of animal cell signaling
and adhesion genes. Proc Natl Acad Sci USA
103: 12451–6.
6. Fahey B, Degnan BM. 2010. Origin of animal
epithelia: insights from the sponge genome.
Evol Dev 12: 601–17.
7. Nelson WJ. 2003. Adaptation of core mechanisms to generate cell polarity. Nature 422: 766.
8. Shapiro L, Weis WI. 2009. Structure and biochemistry of cadherins and catenins. Cold
Spring Harb Perspect Biol 1: a003053.
9. Nelson WJ. 2008. Regulation of cell-cell
adhesion by the cadherin-catenin complex.
Biochem Soc Trans 36: 149–55.
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.
10. Steenkamp ET, Wright J, Baldauf SL. 2006.
The protistan origins of animals and fungi. Mol
Biol Evol 23: 93–106.
11. Baldauf SL. 2003. The deep roots of eukaryotes.
Science 300: 1703–6.
12. King N. 2004. The unicellular ancestry of
animal development. Dev Cell 7: 313–25.
13. Medina M, Collins AG, Taylor JW, Valentine
JW, et al. 2003. Phylogeny of opisthokonta
and the evolution of multicellularity and complexity in fungi and metazoa. Int J Astrobiol 2:
203–11.
14. Tyler S. 2003. Epithelium—the primary building block for metazoan complexity. Integr
Comp Biol 43: 55–63.
15. Dickinson DJ, Nelson WJ, Weis WI. 2011.
A polarized epithelium organized by beta- and
alpha-catenin predates cadherin and metazoan origins. Science 331: 1336–9.
16. Dickinson DJ, Robinson DN, Nelson WJ,
Weis WI. 2012. a-Catenin and IQGAP1
regulate myosin localization to control epithelial
tube morphogenesis in Dictyostelium. Dev Cell,
in press, DOI: 10.1016/j.devcel.2012.06.008.
17. Urushihara H. 2008. Developmental biology
of the social amoeba: history, current knowledge and prospects. Dev Growth Differ 50:
S277–81.
18. Kessin RH. 2001. Dictyostelium. Cambridge:
Cambridge University Press.
19. Harrington BJ, Raper KB. 1968. Use of a
fluorescent brightener to demonstrate cellulose in the cellular slime molds. Appl Microbiol
16: 106–13.
20. McRobbie SJ, Tilly R, Blight K, Ceccarelli A,
et al. 1988. Identification and localization of
proteins encoded by two DIF-inducible genes
of Dictyostelium. Dev Biol 125: 59–63.
21. Raper KB, Fennell DI. 1952. Stalk formation
in Dictyostelium. B Torrey Bot Club 79: 25–51.
22. Bonner JT. 1944. A descriptive study of the
development of the slime mold Dictyostelium
discoideum. Am J Bot 31: 175–82.
23. Bonner JT, Cihiquoine AD, Kolderie MQ.
1955. A histochemical study of differentiation
in the cellular slime molds. J Exp Zool 130:
133–58.
24. Dormann D, Siegert F, Weijer CJ. 1996.
Analysis of cell movement during the culmination phase of Dictyostelium development.
Development 122: 761–9.
25. Grimson MJ, Coates JC, Reynolds JP,
Shipman M, et al. 2000. Adherens junctions
and beta-catenin-mediated cell signalling in a
non-metazoan organism. Nature 408: 727–31.
26. George RP, Hohl HR, Raper KB. 1972.
Ultrastructural development of stalk-producing cells in dictyostelium discoideum, a cellular slime mould. J Gen Microbiol 70: 477–89.
27. Hohl HR, Hamamoto ST, Hemmes DE. 1968.
Ultrastructural aspects of cell elongation,
cellulose synthesis, and spore differentiation
in Acytostelium leptosomum, a cellular slime
mold. Am J Bot 55: 783–96.
28. Schaap P, van der Molen L, Konijn TM. 1983.
The organisation of fruiting body formation in
Dictyostelium minutum. Cell Differ 12: 287–97.
29. Schaap P, Pinas J, Wang M. 1985. Patterns
of cell differentiation in several cellular slime
mold species. Dev Biol 111: 51–61.
30. Schaap P, Winckler T, Nelson M, AlvarezCurto E, et al. 2006. Molecular phylogeny and
evolution of morphology in the social amoebas. Science 314: 661–3.
31. Goldstein B, Macara IG. 2007. The PAR
proteins: fundamental players in animal cell
polarization. Dev Cell 13: 609–22.
7
Hypotheses
all of which have been studied much
less than Dictyostelium. However, the
fact that all of these organisms have
homologs of a-catenin [15] is encouraging, and we would predict that studying
the function of a-catenin in these
species would be likely to yield considerable insight into metazoan evolution. Exciting progress has been
made toward establishing choanoflagellates and ichthyosporeans as genetically
manipulable experimental systems
([53]; I. Ruiz-Trillo, personal communication), and we are hopeful that future
studies will address the nature and
genetic requirements of multicellular
organization in these organisms.
A second important prediction of
our hypothesis is that the catenins
should influence epithelial structure
and polarity in Dictyostelium and metazoans by similar mechanisms. The
alternative convergent evolution hypothesis makes the opposite prediction,
that the mechanisms of epithelial organization by the catenins should be
very different between metazoans and
social amoebae. One similarity between
Dictyostelium and metazoans is that
b- and a-catenin interact physically
and genetically and are required for
epithelial polarity in both systems.
However, beyond these facts, we do
not yet have a sufficiently comprehensive understanding of how the catenin
complex contributes to epithelial organization in either Dictyostelium or metazoans to be able to say whether its
mechanism of action is similar or dissimilar overall. The finding that IQGAP
homologs that act downstream of the
catenins to regulate myosin in the
Dictyostelium tip epithelium [16] is a
start, and an important next step is to
determine whether this mechanism is
also relevant in metazoan epithelia.
Additional cell biological studies of epithelial polarity in both Dictyostelium
and animal model systems are likely
to shed more light on this issue.
D. J. Dickinson et al.
Hypotheses
D. J. Dickinson et al.
32. Clevers H. 2006. Wnt/beta-catenin signaling in development and disease. Cell 127:
469–80.
33. Martin AC, Gelbart M, Fernandez-Gonzalez
R, Kaschube M, et al. 2010. Integration of
contractile forces during tissue invagination.
J Cell Biol 188: 735–49.
34. Dawes-Hoang RE, Parmar KM, Christiansen
AE, Phelps CB, et al. 2005. Folded gastrulation, cell shape change and the control of myosin localization. Development 132: 4165–78.
35. Kuroda S, Fukata M, Nakagawa M, Fujii K,
et al. 1998. Role of IQGAP1, a target of the
small GTPases Cdc42 and Rac1, in regulation
of E-cadherin- mediated cell-cell adhesion.
Science 281: 832–5.
36. Li Z, Kim SH, Higgins JM, Brenner MB,
et al. 1999. IQGAP1 and calmodulin modulate E-cadherin function. J Biol Chem 274:
37885–92.
37. Yamashiro S, Abe H, Mabuchi I. 2007.
IQGAP2 is required for the cadherin-mediated
cell-to-cell adhesion in Xenopus laevis embryos.
Dev Biol 308: 485–93.
38. Raper KB. 1984. The Dictyostelids. Princeton:
Princeton University Press.
39. Brown MW, Spiegel FW, Silberman JD.
2009. Phylogeny of the ‘‘forgotten’’ cellular
slime mold, Fonticula alba, reveals a key evol-
8
Insights & Perspectives
40.
41.
42.
43.
44.
45.
46.
47.
utionary branch within Opisthokonta. Mol Biol
Evol 26: 2699–709.
King N, Hittinger CT, Carroll SB. 2003.
Evolution of key cell signaling and adhesion
protein families predates animal origins.
Science 301: 361–3.
King N, Westbrook MJ, Young SL, Kuo A,
et al. 2008. The genome of the choanoflagellate Monosiga brevicollis and the origin of
metazoans. Nature 451: 783–8.
Ruiz-Trillo I, Lane CE, Archibald JM, Roger
AJ. 2006. Insights into the evolutionary origin
and genome architecture of the unicellular
opisthokonts Capsaspora owczarzaki and
Sphaeroforma arctica. J Eukaryot Microbiol
53: 379–84.
Abedin M, King N. 2008. The premetazoan
ancestry of cadherins. Science 319: 946–8.
Knoll AH, Carroll SB. 1999. Early animal
evolution: emerging views from comparative
biology and geology. Science 284: 2129–37.
Berney C, Pawlowski J. 2006. A molecular
time-scale for eukaryote evolution recalibrated with the continuous microfossil record.
Proc Biol Sci 273: 1867–72.
Olive EW. 1902. Monograph of the Acrasieae.
Proc Boston Soc Nat Hist 30: 451–513.
Dayel MJ, Alegado RA, Fairclough SR,
Levin TC, et al. 2011. Cell differentiation
48.
49.
50.
51.
52.
53.
.....
and morphogenesis in the colony-forming
choanoflagellate Salpingoeca rosetta. Dev
Biol 357: 73–82.
Paps J, Ruiz Trillo I. 2010. Animals and their
unicellular ancestors. In Encyclopedia of
Life Sciences. Chichester: John Wiley &
Sons, Ltd.
Ruiz-Trillo I, Burger G, Holland PWH, King
N, et al. 2007. The origins of multicellularity: a
multi-taxon genome initiative. Trends Genet
23: 113–8.
Sánchez-Martı́nez C, Pérez-Martı́n J. 2001.
Dimorphism in fungal pathogens: Candida
albicans and Ustilago maydis–similar inputs,
different outputs. Curr Opin Microbiol 4:
214–21.
Grosberg RK, Strathmann RR. 2007. The
evolution of multicellularity: a minor major
transition? Annu Rev Ecol Evol Syst 38:
621–54.
Srivastava M, Simakov O, Chapman J,
Fahey B, et al. 2010. The Amphimedon
queenslandica genome and the evolution of
animal complexity. Nature 466: 720–6.
King N, Young S, Abedin M, Carr M, et al.
2009. The choanoflagellates: heterotrophic
nanoflagellates and sister group of the
metazoa. Cold Spring Harb Protoc 2009:
pdb.emo116.
Bioessays 00: 000–000,ß 2012 WILEY Periodicals, Inc.