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 many adult tissues are composed of divergence of metazoans from fungi and social amoebae. Polarized epithelial epithelia [1, 2]. Epithelial cells have a polarized organization of the plasma tissues are a defining feature of metazoans and contribute to the diversity of membrane, cytoskeleton, and cyto- animal body plans. The recent finding of a polarized epithelium in the non- plasmic organelles [2, 3]. The apical metazoan social amoeba Dictyostelium discoideum demonstrates that epi- plasma membrane faces the lumen of thelial tissue is not a unique feature of metazoans, and challenges the an organ or the outside of the organism, traditional paradigm that multicellularity evolved independently in social amoe- while the basal (or basolateral) mem- bae and metazoans. An alternative view, presented here, is that the common brane contacts the underlying tissue. Polarized epithelial sheets regulate the ancestor of social amoebae, fungi, and animals spent a portion of its life cycle directional absorption and secretion of in a multicellular state and possessed molecular machinery necessary for form- proteins and other solutes, an essential ing an epithelial tissue. Some descendants of this ancestor retained multicellu- physiological function that is often dis- larity, while others reverted to unicellularity. This hypothesis makes testable rupted in human disease [1]. In addition, predictions regarding tissue organization in close relatives of metazoans and the polarized organization of cyto- skeletal components in epithelial cells provides a novel conceptual framework for studies of early animal evolution. contributes to tissue shape changes during embryonic morphogenesis [4]. Keywords: Despite the diversity of body plans .Dictyostelium; epithelium; metazoan evolution; multicellularity and lifestyles, all metazoans share an epithelial tissue as their basic unit of organization, suggesting that the development of epithelial cell polarity Introduction organized into a three-dimensional tube, was a very early event in metazoan and is a defining feature of animal body evolution [5, 6]. A simple epithelium consists of a two- plans. An epithelium (the trophecto- Genetic, cell biological, and molecu- dimensional sheet of structurally and derm) is the first differentiated tissue lar studies in a variety of metazoans functionally polarized cells that are often formed during embryogenesis, and have shown that the formation and maintenance of polarized epithelial cells require cell-cell adhesion mediated DOI 10.1002/bies.201100187 by the cadherin-catenin complex [7]. Cadherins are transmembrane receptors 1) Program in Cancer Biology, Stanford *Corresponding author: University, Stanford, CA, USA Daniel J. Dickinson that form homophilic and heterophilic 2) Department of Biology, Stanford University, E-mail: [email protected] adhesive interactions with cadherins on Stanford, CA, USA adjacent cells, providing a spatial cue 3) Department of Molecular and Cellular Physiology, that initiates cell polarity [8]. b-Catenin Stanford University, Stanford, CA, USA 4) Department of Structural Biology, Stanford and a-catenin are cyotosolic binding University, Stanford, CA, USA partners of cadherin that transduce this adhesive cue and mediate cell polarity, yPresent address: Department of Biology, in part by directing the reorganization of University of North Carolina, Chapel Hill, NC, USA the cytoskeleton [8, 9]. Consistent with

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its fundamental role in epithelial organ- Identification of an three phases, chemotaxis has received ization, the cadherin-catenin complex is epithelial tissue in the most attention, and relatively little is conserved in all metazoans. known about the later stages of develop- Phylogenetically speaking, metazo- Dictyostelium ment. However, it is during culmination ans belong to the unikonts, a group that that the greatest degree of complexity also includes fungi, social amoebae, and D. discoideum is a representative of the in tissue organization and cell type a number of unicellular or colonial pro- social amoebae, a group of organisms specialization is observed. tists (see Fig. 2) [10, 11]. Historically, it that feed as single cells but undergo a The fruiting body consists of a rigid was thought that multicellularity evolved transition to a multicellular form upon stalk supporting a collection of spore independently in animals, fungi and starvation (Fig. 1A) [17]. Starving cells cells at the top (Fig. 1B). The defining social amoebae, and that epithelial tissue secrete cyclic AMP, which acts as a event of culmination is the production Hypotheses was a unique feature of animals [11–14]. chemoattractant and induces cells to of the stalk, which elevates the spores However, two recent studies have aggregate into spherical mounds (see and eventually facilitates their disper- established the existence of polarized [18] for a detailed description of the sal. The stalk consists of stalk cells epithelial tissue in the non-metazoan aggregation process). Once aggregation encased in a rigid tube made of cellulose social amoeba Dictyostelium discoideum is complete, the mound elongates to and extracellular matrix (ECM) proteins [15, 16]. This finding calls into question form a slug. After a period of migration, [19, 20]. During culmination, the stalk the notion of an independent origin of the slug develops into a fruiting body cells increase in volume, but because multicellularity in animals and social by a process called culmination. The they are encased in a rigid tube, their amoebae. Here, we propose and discuss developmental process can thus be div- expansion is directed in the vertical the alternative hypothesis that all uni- ided into three phases: (1) chemotaxis direction and contributes to the lifting konts evolved from an ancestor with a and aggregation; (2) slug formation and force that raises the spore head [21]. simple multicellular organization. migration; and (3) culmination. Of these 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].

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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 mem- brane, cytoskeleton, and cytoplasmic organelles. (5) Indicates directional secretion of proteins and other solutes. (6) Indicates an apical enrich- ment 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 com- plete 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 scopy data [25, 26]. These cells, which Dictyostelium requires homologs of b- located at the apex of the culminant we refer to as the tip epithelium, exhibit and a-catenin, which are necessary for [15, 16, 21–24]. Though the tip was pre- many of the hallmarks of epithelial cell epithelial organization and polarity in viously thought to play an important polarity, including: a polarized organiz- metazoans. role in culmination, its subcellular ation of the actomyosin and micro- Two distinct developmental func- organization and function had not been tubule cytoskeletons; a polarized tions have been identified for the tip studied in detail. distribution of cytoplasmic organelles; epithelium. First, the epithelial cells Confocal microscopy showed that and division of the plasma membrane secrete cellulose and ECM proteins the tip contains a highly organized cell into apical and basolateral domains directionally to form the rigid stalk tube monolayer that forms a tube surround- with distinct protein compositions on the exterior of the stalk [15]. Second, ing the apex of the stalk (Fig. 1B, C) [15], (Fig. 2) [15, 16]. Most importantly, the epithelium is an actomyosin-based consistent with earlier electron micro- polarity of the tip epithelial cells in contractile structure that applies a

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squeezing force to the stalk, thereby tance involved (Fig. 2). Perhaps most that the presence of similar-looking limiting its diameter [16]. Although importantly, Dictyostelium does not adhesion structures in Dictyostelium these two functions result from distinct have homologs of classical cadherins, and animal epithelia reflects convergent cellular activities, they both contribute which are essential for recruitment of evolution. Alternatively, sequence hom- to patterning the stalk: the squeezing b- and a-catenin to the plasma mem- ologies of junctional components other force applied by the epithelium causes brane in metazoans. Nevertheless, the than the catenins might exist but be the stalk to be tall and thin, while the catenins localize to the plasma mem- too weak to detect computationally. secreted cellulose and ECM rigidify brane in the tip epithelium [15]. Determining the molecular composition the stalk and ensure that its diameter Presumably an as yet unknown mem- of Dictyostelium cell-cell junctions remains constant even as stalk cells brane-associated protein interacts with will distinguish between these two expand. These recent descriptions of the catenins to recruit them to the possibilities. Hypotheses the subcellular organization and func- plasma membrane, and identifying this Overall, it appears that the molecu- tion of the tip epithelium support the molecule is an important goal for future lar machinery involved in epithelial cell historical view of the tip as an organizing studies. In addition, Dictyostelium lacks polarity in Dictyostelium is much sim- center for fruiting body morphogenesis ‘‘polarity proteins’’ including the PAR pler than in higher animals (Fig. 2). [21–24]. In addition, it is striking that proteins, Crumbs, and Scribble, which Therefore, studies in Dictyostelium are the tip epithelium is capable of polarized are important regulators of epithelial likely to be informative for elucidating secretion and apical enrichment of polarity in higher animals and may the core evolutionarily conserved myosin during morphogenesis, both of be necessary for the more complex machinery necessary for formation of which are important functions of meta- morphogenetic movements that meta- a polarized epithelium. b-Catenin and zoan epithelia. zoan epithelia must execute during a-catenin clearly belong to this minimal Although the recent description and morphogenesis [4, 31]. Finally, meta- machinery, since they are required for genetic analysis of the tip epithelium zoan b-catenin also plays an important epithelial polarity in both Dictyostelium were restricted to the model organism role as a transcriptional co-factor in the and metazoans [15]. Additional molecu- D. discoideum, available evidence Wnt signaling pathway [32], but despite lar components have been identified suggests that epithelia may be wide- the presence of a b-catenin homolog, in immunoprecipitates of a-catenin spread in social amoebae. Cell mono- other key Wnt pathway components from fruiting bodies, including a layers with a similar appearance to are absent in Dictyostelium, indicating Dictyostelium IQGAP homolog and its the tip epithelium have been described that the role of b-catenin in epithelial binding partner cortexillin that appear in five other dictyostelid species, repre- polarity predates its function as a signal- to act downstream of the catenins to senting three of the four major phyloge- ing protein. regulate myosin localization and tissue netic groups of social amoebae [27–30]. A more complex issue is the role of morphology [16]. Available data suggest In every case these tissues were located cell-cell junctions in the tip epithelium. that IQGAP may also have a role in adjacent to the stalk at the tip of Adherens junctions containing classical epithelial cell polarity in higher animals the culminant, suggesting a develop- cadherins and their cytosolic binding [35–37], although it is unknown whether mental function similar to that of the partners, including the catenins, medi- the putative role of IQGAP in metazoan D. discoideum tip epithelium. Moreover, ate cell-cell adhesion in animals [9]. The epithelia involves myosin regulation. homologs of b- and a-catenin are found adherens junction associates with actin Thus, experiments in Dictyostelium in every social amoeba whose genome and serves to mechanically couple the can shed light on the fundamental has been sequenced ([15] and DJD, unpub- cytoskeletons of adjacent cells, allowing requirements for epithelial cell polarity lished results). Some of these species have force transmission across a tissue [33, 34]. and reveal new molecular mechanisms fruiting body morphologies that are more In many (though not all) metazoan epi- that may be relevant in other systems. elaborate than D. discoideum,anditwill thelial cells, the adherens junction be interesting to determine whether more localizes to the boundary between complex morphogenetic movements of apical and lateral plasma membrane An epithelium in epithelia contribute to the formation of domains. Interestingly, transmission Dictyostelium may indicate these structures. electron microscopy revealed actin- associated junctional structures in an ancient origin for Dictyostelium tip epithelial cells that multicellular organization Dictyostelium has a localize to the apical-lateral boundary minimal molecular and appear to connect contractile Dictyostelium and metazoan epithelia actomyosin bundles across several cells are structurally and functionally similar machinery for epithelial cell [15, 16, 25]. Despite this apparent tissues, and importantly, they share a polarity similarity at the ultrastructural level, degree of homology at the molecular these Dictyostelium cell-cell junctions level, since the catenins are required Despite the morphological, molecular do not require either b-ora-catenin for the epithelial organization and and functional similarities between [15], or cadherins (which are absent in polarity in both systems [15, 16]. What Dictyostelium and animal epithelia, Dictyostelium) and, therefore, are most does this result mean for our under- there are also differences, as might be likely not homologous to the metazoan standing of animal origins? As men- expected given the evolutionary dis- adherens junction. One possibility is tioned above, the traditional view has

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been that social amoebae, animals, social amoebae and animals [15]. thought to have little in common at and fungi each evolved multicellularity Thus, in at least one case, proteins that the level of tissue organization, which independently [12, 13]. However, the are conserved between metazoans and is one reason that the finding of epi- Hypotheses molecular and functional similarities non-metazoans also have conserved thelial tissue in Dictyostelium was so between Dictyostelium and animal epi- functions. We expect that when more surprising. It is important, however, thelia may force us to re-examine this cases are studied in detail, additional to avoid confusing the structure of a conclusion. examples of functional similarity tissue with its physiological purpose. Is Dictyostelium an appropriate between metazoan and non-metazoan At the organismal level, epithelial tissue model system to study the origins proteins will be found. has a different purpose in Dictyostelium of multicellularity in animals? Social It is conceivable that the similar func- than in animals. Animal epithelia amoebae diverged from the metazoan tions of the catenins in Dictyostelium have functions that include forming a lineage earlier than other metazoan and animals are a product of convergent protective barrier, directional absorp- relatives (most notably choanoflagel- evolution. However, we think that the tion and secretion, and execution lates and sponges) [10], but molecular degree of molecular, morphological, of morphogenetic movements during data indicate that they may have and functional similarity between epi- development [1, 2, 4]. The function of evolved relatively little since their diver- thelial tissues in Dictyostelium and the Dictyostelium tip epithelium is to gence. Ribosomal RNA sequences from animals is very unlikely to be explained pattern the stalk, a role not obviously different social amoebae are approxi- by convergence. In light of the obser- related to that of any metazoan epi- mately as diverse as those of metazoans, vations discussed above, we propose that thelial tissue. Nevertheless, at the even though all social amoebae have at least some of the organizational prin- cellular level, Dictyostelium and animal a similar lifestyle and morphology ciples underlying animal multicellularity epithelia are very similar: they both [30, 38]. Moreover, the cellular slime predate the divergence of social amoebae have a monolayer organization, secrete mold Fonticula alba has a life cycle very from metazoans. But how are we to rec- proteins and other solutes directionally, similar to that of social amoebae, even oncile this rather provocative conclusion exhibit apical contractility, and require though it is phylogenetically more with our knowledge of other close meta- the catenins for their organization and closely related to fungi [39]. Together, zoan relatives and with the view of social polarity (Fig. 2). Thus, organismal life- these observations suggest that the amoebae, fungi, and animals as inde- style may not be a good predictor of analysis of living social amoebae can pendent ‘‘inventors’’ of multicellularity tissue structure and function, and sim- provide considerable insights into the [11–13]? We suggest that an answer to ilarly organized tissues can have quite nature of their ancient ancestors, which this apparent contradiction can be found different roles in different organisms. might have had much in common by re-thinking the nature of the transition The unikont lineages that branch with the metazoan ancestors that lived to multicellularity in metazoans. between social amoebae and animals around the same time. Of course, a cav- have unique lifestyles and ecological eat is that one cannot rely too heavily on niches, but tissue organization in these data from any single species when draw- Evidence for independent lineages has generally not been well ing conclusions about the nature of studied. Closer examination of these metazoan ancestors. Observations from origins of multicellularity is species may well reveal unexpected sim- social amoebae, choanoflagellates, as not conclusive ilarities to social amoebae or animals. well as intermediate-branching species Third, the phylogenetic clade that such as ichthyosporeans and chytrid The idea that multicellularity originated includes social amoebae, fungi, and fungi must ultimately be integrated independently in social amoebae, fungi, animals also contains some unicellular to yield a coherent understanding of and animals has been supported by organisms [10, 11], and it was considered metazoan origins. three main lines of evidence. First, these more parsimonious to postulate inde- Surprisingly, analysis of the genomes groups diverged approximately 600 pendent origins of multicellularity than of choanoflagellates, social amoebae, million years before the appearance of a loss of multicellular organization in and other close relatives of metazoans the earliest putative metazoans in the multiple unicellular lineages. We now revealed that many of the molecules fossil record [44, 45]. However, the propose an alternative model that historically thought to be restricted to absence of putative metazoans in can account for both the putative metazoans are found in these earlier- the fossil record prior to the Ediacaran homology of epithelial tissues in meta- branching lineages as well (Fig. 2) period is not definitive, as it is possible zoans and social amoebae, as well as [40–43]. Although in most cases, the that multicellular unikonts prior to the apparent loss of multicellularity in cellular functions of these proteins this time were either too small or too many unikonts. are unknown, the similar functions fragile to fossilize, or too morphologi- of the Dictyostelium and metazoan cally different from extant animals to be catenins provides one illuminating recognizable [44]. example [15, 16]. Strikingly, Dictyostelium A second argument for independent Multicellularity as a plastic a-catenin can bind directly to mouse origins of multicellularity in social trait in metazoan ancestors b-catenin, indicating that this protein- amoebae, fungi and animals has been protein interaction may have been con- the very different lifestyles of organisms A polarized epithelium is the first tissue served since the common ancestor of in these taxa. These groups were formed during metazoan development,

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and metazoan body plans at all stages of time spent as a multicellular organism Our hypothesis has significant implica- development are constructed largely similarly varies within each group. tions for views on the origins of modern from epithelial tissues. In contrast, an For example, the choanoflagellate metazoans. If it is correct, we must epithelial tissue is not ubiquitous in the Salpingoeca rosetta can exist either as regard the evolutionary event that gave Dictyostelium life cycle. Dictyostelium a single cell or as small colonies [47]. rise to metazoans not as a transition to development is initiated by an environ- The physiological significance of colony multicellularity per se, but as a tran- mental cue (nutrient deprivation), and formation is unknown, but it has been sition from facultative to constitutive an epithelial tissue forms only at the suggested that unicellular and colonial multicellularity. This event could be end of the developmental process. The states are each advantageous under viewed in one sense as an abandonment development of multicellularity facili- different environmental conditions. of the unicellular stage that character- tates spore dispersal, but is not essential Consistent with this idea, colony for- izes the life cycles of facultative multi- Hypotheses for feeding or reproduction. Indeed, mation and morphology are influenced cellular organisms (although animals social amoebae can replicate indefi- by presence of a specific bacterial prey still have unicellular gametes that may nitely as single cells, and in nature species [47]. Another group of metazoan serve as a buffer against ‘‘cheater’’ they may go many generations without relatives, the ichthyosporeans, also mutations [51]). By definition, faculta- becoming multicellular. Hence, social transitions between a unicellular tive multicellular unikonts rely on the amoebae are facultative multicellular amoeboid state and simple spherical col- unicellular state for at least one essen- organisms that adopt multicellularity onies [48, 49]. The cellular slime mold tial function; e.g., in social amoebae, only for a specific purpose. This mode Fonticula alba exhibits facultative multi- both feeding and cell proliferation are of multicellularity is different from the cellularity very similar to Dictyostelium, confined to the unicellular phase of the constitutive multicellularity that defines despite being more closely related to life cycle. Presumably, then, the tran- metazoans. fungi than to social amoebae [39]. sition to constitutive multicellularity Multicellularity in metazoans can Finally, many species of fungi exhibit was triggered by the appearance of be considered a fixed state, since all facultative multicellularity, switching new signaling pathways that allowed metazoans are multicellular throughout from unicellular (yeast-like) to multi- a greater degree of coordinated cellular their lives (with the exception of game- cellular (pseudohyphal or filamentous) movements and differentiation, thus tes). Apparently, adaptations associated growth in response to specific environ- permitting different essential functions with constitutive multicellularity have mental conditions [50]. Multicellularity (e.g. feeding and reproduction) to occur deprived animal cells of the potential in fungi is particularly plastic, having simultaneously in a single multicellular to survive and reproduce on their been lost completely and then re- individual. This view of the transition to own, or at least to effectively compete invented on several occasions [13]. constitutive multicellularity is in fact with other unicellular organisms. In Taken together, these observations not very different from other proposals contrast, facultative multicellularity in indicate that facultative multicellularity for the transition to multicellularity in social amoebae can be viewed as a is widespread in unikonts, whereas con- animals (e.g. [5, 12, 52]); the novelty of continuum: some species, including stitutive multicellularity is characteristic our hypothesis concerns the nature of D. discoideum, readily enter the multi- of metazoans. the ancestral organism in which this cellular state when subjected to We propose that the common ances- transition occurred. starvation, while other species preferen- tor of all unikonts was a facultative tially respond to starvation by forming multicellular organism in which unicellular cysts and are only rarely some aspects of cell or tissue polarity Testable predictions of the multicellular [38, 46]. The amount of were controlled by the conserved catenin time spent in the multicellular state is complex. Because of the inherently ‘‘facultative thus governed both by environmental plastic nature of facultative multicellular- multicellularity’’ hypothesis cues and by the organism’s response ity, some of the lineages descended from to those cues. Given this scenario, it is this ancestral unikont reverted to the Our hypothesis predicts that if tissue easy to imagine how a facultative multi- unicellular state, while others retained polarity mediated by the catenin cellular organism could revert to a uni- a facultative multicellular lifestyle. In complex predates the divergence of cellular state, simply by adapting to a one lineage (metazoans), multicellularity Dictyostelium from metazoans, then novel environment that favored single became constitutive. This hypothesis we should expect to find polarized cells cells.Thevariablepreferenceforaggre- accounts for the presence of an epithelial that require the catenin complex in gation versus encystation in different tissue in Dictysotelium, is consistent with some of the organisms that diverged species of social amoebae [38, 46] pro- our knowledge of unikont natural history from the metazoan lineage after social vides evidence that evolution can indeed and phylogeny, and is sensible given amoebae. Given our current knowledge modulate the extent of facultative multi- the widespread distribution of facultative of unikont phylogeny and the availabil- cellularity, and solitary amoebae such as multicellularity among unikonts. The ity of sequenced genomes, the most Acanthamoeba could be examples of idea that multicellularity existed prior informative lineages to examine would species that have abandoned multi- to metazoan origins may also help to be choanoflagellates, ichthyosporeans, cellularity entirely. explain the finding that components of and chytrid fungi (see Fig. 2). Almost Other unikonts also exhibit faculta- key metazoan signaling pathways can be nothing is known about multicellular tive multicellularity, and the amount of found in other unikonts. tissue organization in these organisms,

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all of which have been studied much the Dictyostelium tip epithelium consists 10. Steenkamp ET, Wright J, Baldauf SL. 2006. The protistan origins of animals and fungi. Mol less than Dictyostelium. However, the of a monolayer of cells with a distinct Biol Evol 23: 93–106. fact that all of these organisms have apical-basal polarity that is dependent 11. Baldauf SL. 2003. The deep roots of eukaryotes. Hypotheses homologs of a-catenin [15] is encourag- on homologs of b- and a-catenin. Science 300: 1703–6. ing, and we would predict that studying This tissue contributes to multicellular 12. King N. 2004. The unicellular ancestry of animal development. Dev Cell 7: 313–25. the function of a-catenin in these morphogenesis through both direc- 13. Medina M, Collins AG, Taylor JW, Valentine species would be likely to yield con- tional secretion of proteins [15] and JW, et al. 2003. Phylogeny of opisthokonta siderable insight into metazoan evol- apical localization of contractile acto- and the evolution of multicellularity and com- plexity in fungi and metazoa. Int J Astrobiol 2: ution. Exciting progress has been myosin structures [16]. To explain these 203–11. made toward establishing choanoflagel- results, we hypothesized that metazo- 14. Tyler S. 2003. Epithelium—the primary build- lates and ichthyosporeans as genetically ans originated from an ancestor that ing block for metazoan complexity. Integr manipulable experimental systems was facultatively multicellular, rather Comp Biol 43: 55–63. 15. Dickinson DJ, Nelson WJ, Weis WI. 2011. ([53]; I. Ruiz-Trillo, personal communi- than from a unicellular ancestor as A polarized epithelium organized by beta- and cation), and we are hopeful that future was previously proposed. The key pre- alpha-catenin predates cadherin and meta- studies will address the nature and dictions of this hypothesis are that cell zoan origins. Science 331: 1336–9. 16. Dickinson DJ, Robinson DN, Nelson WJ, genetic requirements of multicellular polarity mediated by the conserved Weis WI. 2012. a-Catenin and IQGAP1 organization in these organisms. catenin complex should utilize similar regulate myosin localization to control epithelial A second important prediction of molecular mechanisms in social amoe- tube morphogenesis in Dictyostelium. Dev Cell, in press, DOI: 10.1016/j.devcel.2012.06.008. our hypothesis is that the catenins bae and metazoans, and that catenin- 17. Urushihara H. 2008. Developmental biology should influence epithelial structure mediated tissue polarity should also of the social amoeba: history, current knowl- and polarity in Dictyostelium and meta- be observed in other unikont lineages. edge and prospects. Dev Growth Differ 50: zoans by similar mechanisms. 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