Polymorphic members of the lag gene family mediate kin discrimination in Dictyostelium

Testing the coordination hypothesis: incompatibilities in aggregative development of an experimentally evolved social amoeba

Multicellular organisms that form by aggregation of cells arguably do not achieve high levels of complexity. Conflict among the cells is a widely accepted explanation for this, but an alternative hypothesis is that mixing cells of different genotypes leads to failures of coordination, which we call the "coordination hypothesis." We empirically tested the coordination hypothesis in the social amoeba Dictyostelium discoideum. We mixed D. discoideum clones that had evolved in isolation for generations and acquired mutations that have not been tested against each other by selection. To quantify the effect of incompatibilities, we measured performance in terms of the developmental traits of slug migration and spore production. Importantly, we mixed lines evolved from the same ancestor under conditions that would not select for the evolution of de novo kin recognition. Our results show no evidence of incompatibilities in four traits related to the coordinated movement of slugs toward light in the social amoeba. Spore production was higher than expected in mixtures, in apparent contradiction to the coordination hypothesis. However, we found support for coordination incompatibilities in an interaction between migration and spore production: in mixtures, fewer cells succeeded at both migrating and becoming spores.

Going against the family: Perturbation of a greenbeard pathway leads to falsebeard cheating

Greenbeards facilitate cooperation by encoding a perceptible signal, the ability to detect it, and a tendency to help others that display it. Falsebeards are hypothetical cheaters that display the signal without being altruistic. Despite many examples of greenbeards, evidence for falsebeards is scarce. The Dictyostelium discoideum tgrB1-tgrC1 allorecognition pathway encodes a greenbeard. It allows development, which yields fruiting bodies with altruistic stalks that increase spore dispersal. Here we show that cells lacking rapgapB, a tgrB1-tgrC1 signaling element, cheat by avoiding the stalk fate and generating more spores in chimeras than in pure populations. rapgapB - cells cheat only on partners with compatible tgrB1-tgrC1 allotypes, suggesting that beard display and recognition are intact but decoupled from altruism. The rapgapB - falsebeard provides a model to study greenbeard maintenance and subversion.

The greenbeard gene tgrB1 regulates altruism and cheating in Dictyostelium discoideum

Greenbeard genetic elements encode rare perceptible signals, signal recognition ability, and altruism towards others that display the same signal. Putative greenbeards have been described in various organisms but direct evidence for all the properties in one system is scarce. The tgrB1-tgrC1 allorecognition system of Dictyostelium discoideum encodes two polymorphic membrane proteins which protect cells from chimerism-associated perils. During development, TgrC1 functions as a ligand-signal and TgrB1 as its receptor, but evidence for altruism has been indirect. Here, we show that mixing wild-type and activated tgrB1 cells increases wild-type spore production and relegates the mutants to the altruistic stalk, whereas mixing wild-type and tgrB1-null cells increases mutant spore production and wild-type stalk production. The tgrB1-null cells cheat only on partners that carry the same tgrC1-allotype. Therefore, TgrB1 activation confers altruism whereas TgrB1 inactivation causes allotype-specific cheating, supporting the greenbeard concept and providing insight into the relationship between allorecognition, altruism, and exploitation.

In the social amoeba Dictyostelium discoideum, shortened stalks may limit obligate cheater success even when exploitable partners are available

Cooperation is widespread across life, but its existence can be threatened by exploitation. The rise of obligate social cheaters that are incapable of contributing to a necessary cooperative function can lead to the loss of that function. In the social amoeba Dictyostelium discoideum, obligate social cheaters cannot form dead stalk cells and in chimeras instead form living spore cells. This gives them a competitive advantage within chimeras. However, obligate cheaters of this kind have thus far not been found in nature, probably because they are often enough in clonal populations that they need to retain the ability to produce stalks. In this study we discovered an additional cost to obligate cheaters. Even when there are wild-type cells to parasitize, the chimeric fruiting bodies that result have shorter stalks and these are disadvantaged in spore dispersal. The inability of obligate cheaters to form fruiting bodies when they are on their own combined with the lower functionality of fruiting bodies when they are not represent limits on obligate social cheating as a strategy.

Crozier's paradox and kin recognition: Insights from simplified models

Crozier's paradox suggests that genetic kin recognition will not be evolutionarily stable. The problem is that more common tags (markers) are more likely to be recognised and helped. This causes common tags to increase in frequency, eliminating the genetic variability that is required for genetic kin recognition. In recent years, theoretical models have resolved Crozier's paradox in different ways, but they are based on very complicated multi-locus population genetics. Consequently, it is hard to see exactly what is going on, and whether different theoretical resolutions of Crozier's paradox lead to different types of kin discrimination. I address this by making unrealistic simplifying assumptions to produce a more tractable and understandable model of Crozier's paradox. I use this to interpret a more complex multi-locus population genetic model where I have not made the same simplifying assumptions. I explain how Crozier's paradox can be resolved, and show that only one known theoretical resolution of Crozier's paradox - multiple social encounters - leads without restrictive assumptions to the type of highly cooperative and reliable form of kin discrimination that we observe in nature. More generally, I show how adopting a methodological approach where complex models are compared with simplified ones can lead to greater understanding and accessibility.

Reduced social function in experimentally evolved Dictyostelium discoideum implies selection for social conflict in nature

Many microbes interact with one another, but the difficulty of directly observing these interactions in nature makes interpreting their adaptive value complicated. The social amoeba Dictyostelium discoideum forms aggregates wherein some cells are sacrificed for the benefit of others. Within chimaeric aggregates containing multiple unrelated lineages, cheaters can gain an advantage by undercontributing, but the extent to which wild D. discoideum has adapted to cheat is not fully clear. In this study, we experimentally evolved D. discoideum in an environment where there were no selective pressures to cheat or resist cheating in chimaeras. Dictyostelium discoideum lines grown in this environment evolved reduced competitiveness within chimaeric aggregates and reduced ability to migrate during the slug stage. By contrast, we did not observe a reduction in cell number, a trait for which selection was not relaxed. The observed loss of traits that our laboratory conditions had made irrelevant suggests that these traits were adaptations driven and maintained by selective pressures D. discoideum faces in its natural environment. Our results suggest that D. discoideum faces social conflict in nature, and illustrate a general approach that could be applied to searching for social or non-social adaptations in other microbes.

Host-parasite coevolution and the stability of genetic kin recognition

Crozier's paradox suggests that genetic kin recognition will not be evolutionarily stable. The problem is that more common tags (markers) are more likely to be recognized and helped. This causes common tags to increase in frequency, eliminating the genetic variability that is required for genetic kin recognition. Two potential solutions to this problem have been suggested: host-parasite coevolution and multiple social encounters. We show that the host-parasite coevolution hypothesis does not work as commonly assumed. Host-parasite coevolution only stabilizes kin recognition at a parasite resistance locus if parasites adapt rapidly to hosts and cause intermediate or high levels of damage (virulence). Additionally, when kin recognition is stabilized at a parasite resistance locus, this can have an additional cost of making hosts more susceptible to parasites. However, we show that if the genetic architecture is allowed to evolve, meaning natural selection can choose the recognition locus, genetic kin recognition is more likely to be stable. The reason for this is that host-parasite coevolution can maintain tag diversity at another (neutral) locus by genetic hitchhiking, allowing that other locus to be used for genetic kin recognition. These results suggest a way that host-parasite coevolution can resolve Crozier's paradox, without making hosts more susceptible to parasites. However, the opportunity for multiple social encounters may provide a more robust resolution of Crozier's paradox.

Developmental constraints enforce altruism and avert the tragedy of the commons in a social microbe

Organisms often cooperate through the production of freely available public goods. This can greatly benefit the group but is vulnerable to the "tragedy of the commons" if individuals lack the motivation to make the necessary investment into public goods production. Relatedness to groupmates can motivate individual investment because group success ultimately benefits their genes' own self-interests. However, systems often lack mechanisms that can reliably ensure that relatedness is high enough to promote cooperation. Consequently, groups face a persistent threat from the tragedy unless they have a mechanism to enforce investment when relatedness fails to provide adequate motivation. To understand the real threat posed by the tragedy and whether groups can avert its impact, we determine how the social amoeba Dictyostelium discoideum responds as relatedness decreases to levels that should induce the tragedy. We find that, while investment in public goods declines as overall within-group relatedness declines, groups avert the expected catastrophic collapse of the commons by continuing to invest, even when relatedness should be too low to incentivize any contribution. We show that this is due to a developmental buffering system that generates enforcement because insufficient cooperation perturbs the balance of a negative feedback system controlling multicellular development. This developmental constraint enforces investment under the conditions expected to be most tragic, allowing groups to avert a collapse in cooperation. These results help explain how mechanisms that suppress selfishness and enforce cooperation can arise inadvertently as a by-product of constraints imposed by selection on different traits.

Multiple social encounters can eliminate Crozier's paradox and stabilise genetic kin recognition

Crozier's paradox suggests that genetic kin recognition will not be evolutionarily stable. The problem is that more common tags (markers) are more likely to be recognised and helped. This causes common tags to increase in frequency, and hence eliminates the genetic variability that is required for genetic kin recognition. It has therefore been assumed that genetic kin recognition can only be stable if there is some other factor maintaining tag diversity, such as the advantage of rare alleles in host-parasite interactions. We show that allowing for multiple social encounters before each social interaction can eliminate Crozier's paradox, because it allows individuals with rare tags to find others with the same tag. We also show that rare tags are better indicators of relatedness, and hence better at helping individuals avoid interactions with non-cooperative cheats. Consequently, genetic kin recognition provides an advantage to rare tags that maintains tag diversity, and stabilises itself.

The evolution of extraordinary self-sacrifice

From a theoretical perspective, individuals are expected to sacrifice their welfare only when the benefits outweigh the costs. In nature, however, the costs of altruism and spite can be extreme, as in cases of irreversible sterility and self-destructive weaponry. Here we show that “extraordinary” self-sacrifice—in which actors pay costs that exceed the benefits they give or the costs they impose on recipients—can evolve in structured populations, where social actions bring secondary benefits to neighboring kin. When given information about dispersal, sedentary actors evolve extraordinary altruism towards dispersing kin. Likewise, when given information about dispersal and kinship, sedentary actors evolve extraordinary spite towards sedentary nonkin. Our results can thus be summed up by a simple rule: extraordinary self-sacrifice evolves when the actor’s neighbors are close kin and the recipient’s neighbors are not.

Trogocytosis in Unicellular Eukaryotes

Trogocytosis is a mode of internalization of a part of a live cell by nibbling and is mechanistically distinct from phagocytosis, which implies internalization of a whole cell or a particle. Trogocytosis has been demonstrated in a broad range of cell types in multicellular organisms and is also known to be involved in a plethora of functions. In immune cells, trogocytosis is involved in the "cross-dressing" between antigen presenting cells and T cells, and is thus considered to mediate intercellular communication. On the other hand, trogocytosis has also been reported in a variety of unicellular organisms including the protistan (protozoan) parasite Entamoeba histolytica. E. histolytica ingests human T cell line by trogocytosis and acquires complement resistance and cross-dresses major histocompatibility complex (MHC) class I on the cell surface. Furthermore, trogocytosis and trogocytosis-like phenomena (nibbling of a live cell, not previously described as trogocytosis) have also been reported in other parasitic protists such as Trichomonas, Plasmodium, Toxoplasma, and free-living amoebae. Thus, trogocytosis is conserved in diverse eukaryotic supergroups as a means of intercellular communication. It is depicting the universality of trogocytosis among eukaryotes. In this review, we summarize our current understanding of trogocytosis in unicellular organisms, including the history of its discovery, taxonomical distribution, roles, and molecular mechanisms.

Transcriptional milestones in Dictyostelium development

Dictyostelium development begins with single-cell starvation and ends with multicellular fruiting bodies. Developmental morphogenesis is accompanied by sweeping transcriptional changes, encompassing nearly half of the 13,000 genes in the genome. We performed time-series RNA-sequencing analyses of the wild type and 20 mutants to explore the relationships between transcription and morphogenesis. These strains show developmental arrest at different stages, accelerated development, or atypical morphologies. Considering eight major morphological transitions, we identified 1371 milestone genes whose expression changes sharply between consecutive transitions. We also identified 1099 genes as members of 21 regulons, which are groups of genes that remain coordinately regulated despite the genetic, temporal, and developmental perturbations. The gene annotations in these groups validate known transitions and reveal new developmental events. For example, DNA replication genes are tightly coregulated with cell division genes, so they are expressed in mid-development although chromosomal DNA is not replicated. Our data set includes 486 transcriptional profiles that can help identify new relationships between transcription and development and improve gene annotations. We show its utility by showing that cycles of aggregation and disaggregation in allorecognition-defective mutants involve dedifferentiation. We also show sensitivity to genetic and developmental conditions in two commonly used actin genes, act6 and act15, and robustness of the coaA gene. Finally, we propose that gpdA is a better mRNA quantitation standard because it is less sensitive to external conditions than commonly used standards. The data set is available for democratized exploration through the web application dictyExpress and the data mining environment Orange.

Cyclic AMP is dispensable for allorecognition in Dictyostelium cells overexpressing PKA-C

Allorecognition and tissue formation are interconnected processes that require signaling between matching pairs of the polymorphic transmembrane proteins TgrB1 and TgrC1 in Dictyostelium. Extracellular and intracellular cAMP signaling are essential to many developmental processes. The three adenylate cyclase genes, acaA, acrA and acgA are required for aggregation, culmination and spore dormancy, respectively, and some of their functions can be suppressed by activation of the cAMP-dependent protein kinase PKA. Previous studies have suggested that cAMP signaling might be dispensable for allorecognition and tissue formation, while others have argued that it is essential throughout development. Here, we show that allorecognition and tissue formation do not require cAMP production as long as PKA is active. We eliminated cAMP production by deleting the three adenylate cyclases and overexpressed PKA-C to enable aggregation. The cells exhibited cell polarization, tissue formation and cooperation with allotype-compatible wild-type cells, but not with incompatible cells. Therefore, TgrB1-TgrC1 signaling controls allorecognition and tissue formation, while cAMP is dispensable as long as PKA-C is overexpressed.

Search for MHC/TCR-Like Systems in Living Organisms

Highly polymorphic loci evolved many times over the history of species. These polymorphic loci are involved in three types of functions: kind recognition, self-incompatibility, and the jawed vertebrate adaptive immune system (AIS). In the first part of this perspective, we reanalyzed and described some cases of polymorphic loci reported in the literature. There is a convergent evolution within each functional category and between functional categories, suggesting that the emergence of these self/non-self recognition loci has occurred multiple times throughout the evolutionary history. Most of the highly polymorphic loci are coding for proteins that have a homophilic interaction or heterophilic interaction between linked loci, leading to self or non-self-recognition. The highly polymorphic MHCs, which are involved in the AIS have a different functional mechanism, as they interact through presented self or non-self-peptides with T cell receptors, whose diversity is generated by somatic recombination. Here we propose a mechanism called “the capacity of recognition competition mechanism” that might contribute to the evolution of MHC polymorphism. We propose that the published cases corresponding to these three biological categories represent a small part of what can be found throughout the tree of life, and that similar mechanisms will be found many times, including the one where polymorphic loci interact with somatically generated loci.

Evolution of Multicellular Complexity in The Dictyostelid Social Amoebas

Multicellularity evolved repeatedly in the history of life, but how it unfolded varies greatly between different lineages. Dictyostelid social amoebas offer a good system to study the evolution of multicellular complexity, with a well-resolved phylogeny and molecular genetic tools being available. We compare the life cycles of the Dictyostelids with closely related amoebozoans to show that complex life cycles were already present in the unicellular common ancestor of Dictyostelids. We propose frost resistance as an early driver of multicellular evolution in Dictyostelids and show that the cell signalling pathways for differentiating spore and stalk cells evolved from that for encystation. The stalk cell differentiation program was further modified, possibly through gene duplication, to evolve a new cell type, cup cells, in Group 4 Dictyostelids. Studies in various multicellular organisms, including Dictyostelids, volvocine algae, and metazoans, suggest as a common principle in the evolution of multicellular complexity that unicellular regulatory programs for adapting to environmental change serve as "proto-cell types" for subsequent evolution of multicellular organisms. Later, new cell types could further evolve by duplicating and diversifying the "proto-cell type" gene regulatory networks.

Cancer and the breakdown of multicellularity: What Dictyostelium discoideum, a social amoeba, can teach us

Ancient pathways promoting unicellularity and multicellularity are associated with cancer, the former being pro-oncogenic and the latter acting to suppress oncogenesis. However, there are only a limited number of non-vertebrate models for studying these pathways. Here, we review Dictyostelium discoideum and describe how it can be used to understand these gene networks. D. discoideum has a unicellular and multicellular life cycle, making it possible to study orthologs of cancer-associated genes in both phases. During development, differentiated amoebae form a fruiting body composed of a mass of spores that are supported atop a stalk. A portion of the cells sacrifice themselves to become non-reproductive stalk cells. Cheating disrupts the principles of multicellularity, as cheater cells alter their cell fate to preferentially become spores. Importantly, D. discoideum has gene networks and several strategies for maintaining multicellularity. Therefore, D. discoideum can help us better understand how conserved genes and pathways involved in multicellularity also influence cancer development, potentially identifying new therapeutic avenues.

Social Chemical Communication Determines Recovery From L1 Arrest via DAF-16 Activation

In a population, chemical communication determines the response of animals to changing environmental conditions, what leads to an enhanced resistance against stressors. In response to starvation, the nematode Caenorhabditis elegans arrest post-embryonic development at the first larval stage (L1) right after hatching. As arrested L1 larvae, C. elegans become more resistant to diverse stresses, allowing them to survive for several weeks expecting to encounter more favorable conditions. L1 arrested at high densities display an enhanced resistance to starvation, dependent on soluble compounds released beyond hatching and the first day of arrest. Here, we show that this chemical communication also influences recovery after prolonged periods in L1 arrest. Animals at high density recovered faster than animals at low density. We found that the density effect on survival depends on the final effector of the insulin signaling pathway, the transcription factor DAF-16. Moreover, DAF-16 activation was higher at high density, consistent with a lower expression of the insulin-like peptide DAF-28 in the neurons. The improved recovery of animals after arrest at high density depended on soluble compounds present in the media of arrested L1s. In an effort to find the nature of these compounds, we investigated the disaccharide trehalose as putative signaling molecule, since its production is enhanced during L1 arrest and it is able to activate DAF-16. We detected the presence of trehalose in the medium of arrested L1 larvae at a low concentration. The addition of this concentration of trehalose to animals arrested at low density was enough to rescue DAF-28 production and DAF-16 activation to the levels of animals arrested at high density. However, despite activating DAF-16, trehalose was not capable of reversing survival and recovery phenotypes, suggesting the participation of additional signaling molecules. With all, here we describe a molecular mechanism underlying social communication that allows C. elegans to maintain arrested L1 larvae ready to quickly recover as soon as they encounter nutrient sources.

Enforcing Cooperation in the Social Amoebae

Cooperation has been essential to the evolution of biological complexity, but many societies struggle to overcome internal conflicts and divisions. Dictyostelium discoideum, or the social amoeba, has been a useful model system for exploring these conflicts and how they can be resolved. When starved, these cells communicate, gather into groups, and build themselves into a multicellular fruiting body. Some cells altruistically die to form the rigid stalk, while the remainder sit atop the stalk, become spores, and disperse. Evolutionary theory predicts that conflict will arise over which cells die to form the stalk and which cells become spores and survive. The power of the social amoeba lies in the ability to explore how cooperation and conflict work across multiple levels, ranging from proximate mechanisms (how does it work?) to ultimate evolutionary answers (why does it work?). Recent studies point to solutions to the problem of ensuring fairness, such as the ability to suppress selfishness and to recognize and avoid unrelated individuals. This work confirms a central role for kin selection, but also suggests new explanations for how social amoebae might enforce cooperation. New approaches based on genomics are also enabling researchers to decipher for the first time the evolutionary history of cooperation and conflict and to determine its role in shaping the biology of multicellular organisms.

Wild Dictyostelium discoideum social amoebae show plastic responses to the presence of nonrelatives during multicellular development

When multiple strains of microbes form social groups, such as the multicellular fruiting bodies of Dictyostelium discoideum, conflict can arise regarding cell fate. Both fixed and plastic differences among strains can contribute to cell fate, and plastic responses may be particularly important if social environments frequently change. We used RNA-sequencing and photographic time series analysis to detect possible conflict-induced plastic differences between wild D. discoideum aggregates formed by single strains compared with mixed pairs of strains (chimeras). We found one hundred and two differentially expressed genes that were enriched for biological processes including cytoskeleton organization and cyclic AMP response (up-regulated in chimeras), and DNA replication and cell cycle (down-regulated in chimeras). In addition, our data indicate that in reference to a time series of multicellular development in the laboratory strain AX4, chimeras may be slightly behind clonal aggregates in their development. Finally, phenotypic analysis supported slower splitting of aggregates and a nonsignificant trend for larger group sizes in chimeras. The transcriptomic comparison and phenotypic analyses support discoordination among aggregate group members due to social conflict. These results are consistent with previously observed factors that affect cell fate decision in D. discoideum and provide evidence for plasticity in cAMP signaling and phenotypic coordination during development in response to social conflict in D. discoideum and similar microbial social groups.

Variation at an adhesin locus suggests sociality in natural populations of the yeast Saccharomyces cerevisiae

Microbes engage in numerous social behaviours that are critical for survival and reproduction, and that require individuals to act as a collective. Various mechanisms ensure that collectives are composed of related, cooperating cells, thus allowing for the evolution and stability of these traits, and for selection to favour traits beneficial to the collective. Since microbes are difficult to observe directly, sociality in natural populations can instead be investigated using evolutionary genetic signatures, as social loci can be evolutionary hotspots. The budding yeast has been studied for over a century, yet little is known about its social behaviour in nature. Flo11 is a highly regulated cell adhesin required for most laboratory social phenotypes; studies suggest it may function in cell recognition and its heterogeneous expression may be adaptive for collectives such as biofilms. We investigated this locus and found positive selection in the areas implicated in cell-cell interaction, suggesting selection for kin discrimination. We also found balancing selection at an upstream activation site, suggesting selection on the level of variegated gene expression. Our results suggest this model yeast is surprisingly social in natural environments and is probably engaging in various forms of sociality. By using genomic data, this research provides a glimpse of otherwise unobservable interactions.

Greenbeard Genes: Theory and Reality

Greenbeard genes were proposed as a cartoonish thought experiment to explain why altruism can be a selfish strategy from the perspective of genes. The likelihood of finding a real greenbeard gene in nature was thought to be remote because they were believed to require a set of improbable properties. Yet, despite this expectation, there is an ongoing explosion in claimed discoveries of greenbeard genes. Bringing together the latest theory and experimental findings, we argue that there is a need to dispose of the cartoon presentation of a greenbeard to refocus their burgeoning empirical study on the more fundamental concept that the thought experiment was designed to illustrate.

Conditional expression explains molecular evolution of social genes in a microbe

Conflict is thought to play a critical role in the evolution of social interactions by promoting diversity or driving accelerated evolution. However, despite our sophisticated understanding of how conflict shapes social traits, we have limited knowledge of how it impacts molecular evolution across the underlying social genes. Here we address this problem by analyzing the genome-wide impact of social interactions using genome sequences from 67 Dictyostelium discoideum strains. We find that social genes tend to exhibit enhanced polymorphism and accelerated evolution. However, these patterns are not consistent with conflict driven processes, but instead reflect relaxed purifying selection. This pattern is most likely explained by the conditional nature of social interactions, whereby selection on genes expressed only in social interactions is diluted by generations of inactivity. This dilution of selection by inactivity enhances the role of drift, leading to increased polymorphism and accelerated evolution, which we call the Red King process.

Tissue self-organization based on collective cell migration by contact activation of locomotion and chemotaxis

Despite their central role in multicellular organization, navigation rules that dictate cell rearrangement remain largely undefined. Contact between neighboring cells and diffusive attractant molecules are two of the major determinants of tissue-level patterning; however, in most cases, molecular and developmental complexity hinders one from decoding the exact governing rules of individual cell movement. A primordial example of tissue patterning by cell rearrangement is found in the social amoeba Dictyostelium discoideum where the organizing center or the "tip" self-organizes as a result of sorting of differentiating prestalk and prespore cells. By employing microfluidics and microsphere-based manipulation of navigational cues at the single-cell level, here we uncovered a previously overlooked mode of Dictyostelium cell migration that is strictly directed by cell-cell contact. The cell-cell contact signal is mediated by E-set Ig-like domain-containing heterophilic adhesion molecules TgrB1/TgrC1 that act in trans to induce plasma membrane recruitment of the SCAR complex and formation of dendritic actin networks, and the resulting cell protrusion competes with those induced by chemoattractant cAMP. Furthermore, we demonstrate that both prestalk and prespore cells can protrude toward the contact signal as well as to chemotax toward cAMP; however, when given both signals, prestalk cells orient toward the chemoattractant, whereas prespore cells choose the contact signal. These data suggest a model of cell sorting by competing juxtacrine and diffusive cues, each with potential to drive its own mode of collective cell migration.

Discrimination Experiments in Entamoeba and Evidence from Other Protists Suggest Pathogenic Amebas Cooperate with Kin to Colonize Hosts and Deter Rivals

Entamoeba histolytica is one of the least understood protists in terms of taxa, clone, and kin discrimination/recognition ability. However, the capacity to tell apart same or self (clone/kin) from different or nonself (nonclone/nonkin) has long been demonstrated in pathogenic eukaryotes like Trypanosoma and Plasmodium, free-living social amebas (Dictyostelium, Polysphondylium), budding yeast (Saccharomyces), and in numerous bacteria and archaea (prokaryotes). Kin discrimination/recognition is explained under inclusive fitness theory; that is, the reproductive advantage that genetically closely related organisms (kin) can gain by cooperating preferably with one another (rather than with distantly related or unrelated individuals), minimizing antagonism and competition with kin, and excluding genetic strangers (or cheaters = noncooperators that benefit from others' investments in altruistic cooperation). In this review, we rely on the outcomes of in vitro pairwise discrimination/recognition encounters between seven Entamoeba lineages to discuss the biological significance of taxa, clone, and kin discrimination/recognition in a range of generalist and specialist species (close or distantly related phylogenetically). We then focus our discussion on the importance of these laboratory observations for E. histolytica's life cycle, host infestation, and implications of these features of the amebas' natural history for human health (including mitigation of amebiasis).

Some chemotactic mutants can be progress through development in chimeric populations

Cell migration in response to morphogen gradients affects morphogenesis. Chemotaxis towards adenosine 3', 5'-monophosphate (cAMP) is essential for the early stage of morphogenesis in the slime mold Dictyostelium discoideum. Here, we show that D. discoideum completes morphogenesis without cAMP-chemotaxis-dependent cell migration. The extracellular cAMP gradient is believed to cause cells to form a slug-shaped multicellular structure and fruiting body. The cAMP receptor, cAR1, was not expressed at the cell surface during these stages, correlating with reduced chemotactic activity. Gβ-null cells expressing temperature sensitive Gβ are unable to generate extracellular cAMP (Jin et al., 1998) and thus unable to aggregate and exhibit proper morphogenesis under restrictive temperature. However, when mixed with wild type cells ts-Gβ expressing gβ-null cells normally aggregated and exhibited normal morphogenesis under restrictive temperature. Furthermore, cells migrated after aggregation in a mixture containing wild-type cells. KI-5 cells, which do not show aggregation or morphogenesis, spontaneously migrated to a transplanted wild-type tip and underwent normal morphogenesis and cell differentiation; this was not observed in cells lacking tgrB1and tgrC1 cells adhesion molecules. Thus, cAMP gradient-dependent cell migration may not be required for multicellular pattern formation in late Dictyostelium development.

Cellular allorecognition and its roles in Dictyostelium development and social evolution

The social amoeba Dictyostelium discoideum is a tractable model organism to study cellular allorecognition, which is the ability of a cell to distinguish itself and its genetically similar relatives from more distantly related organisms. Cellular allorecognition is ubiquitous across the tree of life and affects many biological processes. Depending on the biological context, these versatile systems operate both within and between individual organisms, and both promote and constrain functional heterogeneity. Some of the most notable allorecognition systems mediate neural self-avoidance in flies and adaptive immunity in vertebrates. D. discoideum's allorecognition system shares several structures and functions with other allorecognition systems. Structurally, its key regulators reside at a single genomic locus that encodes two highly polymorphic proteins, a transmembrane ligand called TgrC1 and its receptor TgrB1. These proteins exhibit isoform-specific, heterophilic binding across cells. Functionally, this interaction determines the extent to which co-developing D. discoideum strains co-aggregate or segregate during the aggregation phase of multicellular development. The allorecognition system thus affects both development and social evolution, as available evidence suggests that the threat of developmental cheating represents a primary selective force acting on it. Other significant characteristics that may inform the study of allorecognition in general include that D. discoideum's allorecognition system is a continuous and inclusive trait, it is pleiotropic, and it is temporally regulated.

Species recognition in social amoebae

Aggregative multicellularity requires the ability of cells to recognise conspecifics. Social amoebae are among the best studied of such organisms, but the mechanism and evolutionary background of species recognition remained to be investigated. Here we show that heterologous expression of a single Dictyostelium purpureum gene is sufficient for D. discoideum cells to efficiently make chimaeric fruiting bodies with D. purpureum cells. This gene forms a bidirectional pair with another gene on the D. purpureum genome, and they are both highly polymorphic among independent wild isolates of the same mating group that do not form chimaeric fruiting bodies with each other. These paired genes are both structurally similar to D. discoideum tgrB1/C1 pair, which is responsible for clonal discrimination within that species, suggesting that these tgr genes constitute the species recognition system that has attained a level of precision capable of discriminating between clones within a species. Analysis of the available genome sequences of social amoebae revealed that such gene pairs exist only within the clade composed of species that produce precursors of sterile stalk cells (prestalk cells), suggesting concurrent evolution of a precise allorecognition system and a new 'worker' cell-type dedicated to transporting and supporting the reproductive cells.

Clonality and non-linearity drive facultative-cooperation allele diversity

Kin discrimination describes the differential interaction of organisms with kin versus non-kin. In microorganisms, many genetic loci act as effective kin-discrimination systems, such as kin-directed help and non-kin-directed harm. Another important example is facultative cooperation, where cooperators increase their investment in group-directed cooperation with the abundance of their kin in the group. Many of these kin-discrimination loci are highly diversified, yet it remains unclear what evolutionary mechanisms maintain this diversity, and how it is affected by population structure. Here, we demonstrate the unique dependence of kin-discriminative interactions on population structure, and how this could explain facultative-cooperation allele-diversity. We show mathematically that low relatedness between microbes in non-clonal social groups is needed to maintain the diversity of facultative-cooperation alleles, while high clonality is needed to stabilize this diversity against cheating. Interestingly, we demonstrate with simulations that such population structure occurs naturally in expanding microbial colonies. Finally, analysis of experimental data of quorum-sensing mediated facultative cooperation, in Bacillus subtilis, demonstrates the relevance of our results to realistic microbial interactions, due to their intrinsic non-linear frequency dependence. Our analysis therefore stresses the impact of clonality on the interplay between exploitation and kin discrimination and portrays a way for the evolution of facultative cooperation.

Lectins modulate the microbiota of social amoebae

The social amoebaDictyostelium discoideummaintains a microbiome during multicellular development; bacteria are carried in migrating slugs and as endosymbionts within amoebae and spores. Bacterial carriage and endosymbiosis are induced by the secreted lectin discoidin I that binds bacteria, protects them from extracellular killing, and alters their retention within amoebae. This altered handling of bacteria also occurs with bacteria coated by plant lectins and leads to DNA transfer from bacteria to amoebae. Thus, lectins alter the cellular response ofD. discoideumto bacteria to establish the amoebae’s microbiome. Mammalian cells can also maintain intracellular bacteria when presented with bacteria coated with lectins, so heterologous lectins may induce endosymbiosis in animals. Our results suggest that endogenous or environmental lectins may influence microbiome homeostasis across eukaryotic phylogeny.

A Proposed Chaperone of the Bacterial Type VI Secretion System Functions To Constrain a Self-Identity Protein

The bacterium Proteus mirabilis can communicate identity through the secretion of the self-identity protein IdsD via the type VI secretion (T6S) system. IdsD secretion is essential for self-versus-nonself recognition behaviors in these populations. Here we provide an answer to the unresolved question of how the activity of a T6S substrate, such as IdsD, is regulated before secretion. We demonstrate that IdsD is found in clusters that form independently of the T6S machinery and activity. We show that the IdsC protein, which is a member of the proposed DUF4123 chaperone family, is essential for the maintenance of these clusters and of the IdsD protein itself. We provide evidence that amino acid disruptions in IdsC are sufficient to disrupt IdsD secretion but not IdsD localization into subcellular clusters, strongly supporting the notion that IdsC functions in at least two different ways: maintaining IdsD levels and secreting IdsD. We propose that IdsC, and likely other DUF4123-containing proteins, functions to regulate T6S substrates in the donor cell both by maintaining protein levels and by mediating secretion at the T6S machinery.IMPORTANCE Understanding the subcellular dynamics of self-identity proteins is crucial for developing models of self-versus-nonself recognition. We directly addressed how a bacterium restricts self-identity information before cell-cell exchange. We resolved two conflicting models for type VI secretion (T6S) substrate regulation by focusing on the self-identity protein IdsD. One model is that a cognate immunity protein binds the substrate, inhibiting activity before transport. Another model proposes that DUF4123 proteins act as chaperones in the donor cell, but no detailed molecular mechanism was previously known. We resolve this discrepancy and propose a model wherein a chaperone couples IdsD sequestration with its localization. Such a molecular mechanism restricts the communication of identity, and possibly other T6S substrates, in producing cells.

Strategic investment explains patterns of cooperation and cheating in a microbe

Contributing to cooperation is typically costly, while its rewards are often available to all members of a social group. So why should individuals be willing to pay these costs, especially if they could cheat by exploiting the investments of others? Kin selection theory broadly predicts that individuals should invest more into cooperation if their relatedness to group members is high (assuming they can discriminate kin from nonkin). To better understand how relatedness affects cooperation, we derived the ‟Collective Investment" game, which provides quantitative predictions for patterns of strategic investment depending on the level of relatedness. We then tested these predictions by experimentally manipulating relatedness (genotype frequencies) in mixed cooperative aggregations of the social amoeba Dictyostelium discoideum, which builds a stalk to facilitate spore dispersal. Measurements of stalk investment by natural strains correspond to the predicted patterns of relatedness-dependent strategic investment, wherein investment by a strain increases with its relatedness to the group. Furthermore, if overall group relatedness is relatively low (i.e., no strain is at high frequency in a group) strains face a scenario akin to the "Prisoner's Dilemma" and suffer from insufficient collective investment. We find that strains employ relatedness-dependent segregation to avoid these pernicious conditions. These findings demonstrate that simple organisms like D. discoideum are not restricted to being ‟cheaters" or ‟cooperators" but instead measure their relatedness to their group and strategically modulate their investment into cooperation accordingly. Consequently, all individuals will sometimes appear to cooperate and sometimes cheat due to the dynamics of strategic investing.

Genetic signatures of microbial altruism and cheating in social amoebas in the wild

Many microbes engage in social interactions. Some of these have come to play an important role in the study of cooperation and conflict, largely because, unlike most animals, they can be genetically manipulated and experimentally evolved. However, whereas animal social behavior can be observed and assessed in natural environments, microbes usually cannot, so we know little about microbial social adaptations in nature. This has led to some difficult-to-resolve controversies about social adaptation even for well-studied traits such as bacterial quorum sensing, siderophore production, and biofilms. Here we use molecular signatures of population genetics and molecular evolution to address controversies over the existence of altruism and cheating in social amoebas. First, we find signatures of rapid adaptive molecular evolution that are consistent with social conflict being a significant force in nature. Second, we find population-genetic signatures of purifying selection to support the hypothesis that the cells that form the sterile stalk evolve primarily through altruistic kin selection rather than through selfish direct reproduction. Our results show how molecular signatures can provide insight into social adaptations that cannot be observed in their natural context, and they support the hypotheses that social amoebas in the wild are both altruists and cheaters.

The polymorphic proteins TgrB1 and TgrC1 function as a ligand-receptor pair in Dictyostelium allorecognition

Allorecognition is a key factor in Dictyostelium development and sociality. It is mediated by two polymorphic transmembrane proteins, TgrB1 and TgrC1, which contain extracellular immunoglobulin domains. TgrB1 and TgrC1 are necessary and sufficient for allorecognition, and they carry out separate albeit overlapping functions in development, but their mechanism of action is unknown. Here, we show that TgrB1 acts as a receptor with TgrC1 as its ligand in cooperative aggregation and differentiation. The proteins bind each other in a sequence-specific manner; TgrB1 exhibits a cell-autonomous function and TgrC1 acts non-cell-autonomously. The TgrB1 cytoplasmic tail is essential for its function and it becomes phosphorylated upon association with TgrC1. Dominant mutations in TgrB1 activate the receptor function and confer partial ligand independence. These roles in development and sociality suggest that allorecognition is crucial in the integration of individual cells into a coherent organism.

The Origin of Animal Multicellularity and Cell Differentiation

Over 600 million years ago, animals evolved from a unicellular or colonial organism whose cell(s) captured bacteria with a collar complex, a flagellum surrounded by a microvillar collar. Using principles from evolutionary cell biology, we reason that the transition to multicellularity required modification of pre-existing mechanisms for extracellular matrix synthesis and cytokinesis. We discuss two hypotheses for the origin of animal cell types: division of labor from ancient plurifunctional cells and conversion of temporally alternating phenotypes into spatially juxtaposed cell types. Mechanistic studies in diverse animals and their relatives promise to deepen our understanding of animal origins and cell biology.

Bacillus subtilis Protects Public Goods by Extending Kin Discrimination to Closely Related Species

Kin discrimination systems are found in numerous communal contexts like multicellularity and are theorized to prevent exploitation of cooperative behaviors. The kin discrimination system in Bacillus subtilis differs from most other such systems because it excludes nonkin cells rather than including kin cells. Because nonkin are the target of the system, B. subtilis can potentially distinguish degrees of nonkin relatedness, not just kin versus nonkin. We examined this by testing a large strain collection of diverse Bacillus species against B. subtilis in different multicellular contexts. The effects of kin discrimination extend to nearby species, as the other subtilis clade species were treated with the same antagonism as nonkin. Species in the less-related pumilus clade started to display varied phenotypes but were mostly still discriminated against, while cereus clade members and beyond were no longer subject to kin discrimination. Seeking a reason why other species are perceived as antagonistic nonkin, we tested the ability of B. subtilis to steal communally produced surfactant from these species. We found that the species treated as nonkin were the only ones that made a surfactant that B. subtilis could utilize and that nonkin antagonism prevented such stealing when the two strains were mixed. The nonkin exclusion kin discrimination method thus allows effective protection of the cooperative behaviors prevalent in multicellularity while still permitting interactions with more distant species that are not a threat.

Multicellular systems like bacterial biofilms and swarms rely on cooperative behaviors that could be undermined by exploitative invaders. Discriminating kin from nonkin is one way to help guard against such exploitation but has thus far been examined only intraspecifically, so the phylogenetic range of this important trait is unknown. We tested whether Bacillus subtilis treats other species as nonkin by testing a single strain against a diverse collection of Bacillus isolates. We found that the species in the same clade were treated as nonkin, which then lessened in more distant relatives. Further experiments showed that these nonkin species produced a cooperative good that could be stolen by B. subtilis and that treating each other as nonkin largely prevented this exploitation. These results impact our understanding of interspecies interactions, as bacterial populations can interact only after they have diverged enough to no longer be a threat to their cooperative existences.

A polychromatic 'greenbeard' locus determines patterns of cooperation in a social amoeba

Cheaters disrupt cooperation by reaping the benefits without paying their fair share of associated costs. Cheater impact can be diminished if cooperators display a tag (‘greenbeard') and recognise and preferentially direct cooperation towards other tag carriers. Despite its popular appeal, the feasibility of such greenbeards has been questioned because the complex patterns of partner-specific cooperative behaviours seen in nature require greenbeards to come in different colours. Here we show that a locus (‘Tgr') of a social amoeba represents a polychromatic greenbeard. Patterns of natural Tgr locus sequence polymorphisms predict partner-specific patterns of cooperation by underlying variation in partner-specific protein–protein binding strength and recognition specificity. Finally, Tgr locus polymorphisms increase fitness because they help avoid potential costs of cooperating with incompatible partners. These results suggest that a polychromatic greenbeard can provide a key mechanism for the evolutionary maintenance of cooperation.

Cooperation can be stabilized against exploitation if cooperators can reliably recognize each other. Here, Gruenheit and colleagues show that different alleles of the Tgr locus of the social amoeba Dictyostelium discoideum underlie the ability of different strains to recognize and cooperate with socially compatible individuals.

A set of genes conserved in sequence and expression traces back the establishment of multicellularity in social amoebae

Background

The developmental cycle of Dictyostelid amoebae represents an early form of multicellularity with cell type differentiation. Mutant studies in the model Dictyostelium discoideum revealed that its developmental program integrates the actions of genes involved in signal transduction, adhesion, motility, autophagy and cell wall and matrix biosynthesis. However, due to functional redundancy and fail safe options not required in the laboratory, this single organism approach cannot capture all essential genes.

To understand how multicellular organisms evolved, it is essential to recognize both the conserved core features of their developmental programs and the gene modifications that instigated phenotypic innovation. For complex organisms, such as animals, this is not within easy reach, but it is feasible for less complex forms, such as the Dictyostelid social amoebas.

Results

We compared global profiles of gene expression during the development of four social amoebae species that represent 600 mya of Dictyostelia evolution, and identified orthologous conserved genes with similar developmental up-regulation of expression using three different methods. For validation, we disrupted five genes of this core set and examined the phenotypic consequences.

Conclusion

At least 71 of the developmentally regulated genes that were identified with all methods were likely to be already present in the last ancestor of all Dictyostelia. The lack of phenotypic changes in null mutants indicates that even highly conserved genes either participate in functionally redundant pathways or are necessary for developmental progression under adverse, non-standard laboratory conditions. Both mechanisms provide robustness to the developmental program, but impose a limit on the information that can be obtained from deleting single genes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3223-z) contains supplementary material, which is available to authorized users.

Contextual organismality: Beyond pattern to process in the emergence of organisms

Biologists have taken the concept of organism largely for granted. However, advances in the study of chimerism, symbiosis, bacterial‐eukaryote associations, and microbial behavior have prompted a redefinition of organisms as biological entities exhibiting low conflict and high cooperation among their parts. This expanded view identifies organisms in evolutionary time. However, the ecological processes, mechanisms, and traits that drive the formation of organisms remain poorly understood. Recognizing that organismality can be context dependent, we advocate elucidating the ecological contexts under which entities do or do not act as organisms. Here we develop a “contextual organismality” framework and provide examples of entities, such as honey bee colonies, tumors, and bacterial swarms, that can act as organisms under specific life history, resource, or other ecological circumstances. We suggest that context dependence may be a stepping stone to the development of increased organismal unification, as the most integrated biological entities generally show little context dependence. Recognizing that organismality is contextual can identify common patterns and testable hypotheses across different entities. The contextual organismality framework can illuminate timeless as well as pressing issues in biology, including topics as disparate as cancer emergence, genomic conflict, evolution of symbiosis, and the role of the microbiota in impacting host phenotype.

Progress and perspectives in signal transduction, actin dynamics, and movement at the cell and tissue level: lessons from Dictyostelium

Movement of cells and tissues is a basic biological process that is used in development, wound repair, the immune response to bacterial invasion, tumour formation and metastasis, and the search for food and mates. While some cell movement is random, directed movement stimulated by extracellular signals is our focus here. This involves a sequence of steps in which cells first detect extracellular chemical and/or mechanical signals via membrane receptors that activate signal transduction cascades and produce intracellular signals. These intracellular signals control the motile machinery of the cell and thereby determine the spatial localization of the sites of force generation needed to produce directed motion. Understanding how force generation within cells and mechanical interactions with their surroundings, including other cells, are controlled in space and time to produce cell-level movement is a major challenge, and involves many issues that are amenable to mathematical modelling.

Extracellular matrix dynamics and functions in the social amoeba Dictyostelium: A critical review

BACKGROUND

The extracellular matrix (ECM) is a dynamic complex of glycoproteins, proteoglycans, carbohydrates, and collagen that serves as an interface between mammalian cells and their extracellular environment. Essential for normal cellular homeostasis, physiology, and events that occur during development, it is also a key functionary in a number of human diseases including cancer. The social amoeba Dictyostelium discoideum secretes an ECM during multicellular development that regulates multicellularity, cell motility, cell differentiation, and morphogenesis, and provides structural support and protective layers to the resulting differentiated cell types. Proteolytic processing within the Dictyostelium ECM leads to specific bioactive factors that regulate cell motility and differentiation.

SCOPE OF REVIEW

Here we review the structure and functions of the Dictyostelium ECM and its role in regulating multicellular development. The questions and challenges that remain and how they can be answered are also discussed.

MAJOR CONCLUSIONS

The Dictyostelium ECM shares many of the features of mammalian and plant ECM, and thus presents an excellent system for studying the structure and function of the ECM.

GENERAL SIGNIFICANCE

As a genetically tractable model organism, Dictyostelium offers the potential to further elucidate ECM functions, and to possibly reveal previously unknown roles for the ECM.

Gene discovery by chemical mutagenesis and whole-genome sequencing in Dictyostelium

Whole-genome sequencing is a useful approach for identification of chemical-induced lesions, but previous applications involved tedious genetic mapping to pinpoint the causative mutations. We propose that saturation mutagenesis under low mutagenic loads, followed by whole-genome sequencing, should allow direct implication of genes by identifying multiple independent alleles of each relevant gene. We tested the hypothesis by performing three genetic screens with chemical mutagenesis in the social soil amoeba Dictyostelium discoideum Through genome sequencing, we successfully identified mutant genes with multiple alleles in near-saturation screens, including resistance to intense illumination and strong suppressors of defects in an allorecognition pathway. We tested the causality of the mutations by comparison to published data and by direct complementation tests, finding both dominant and recessive causative mutations. Therefore, our strategy provides a cost- and time-efficient approach to gene discovery by integrating chemical mutagenesis and whole-genome sequencing. The method should be applicable to many microbial systems, and it is expected to revolutionize the field of functional genomics in Dictyostelium by greatly expanding the mutation spectrum relative to other common mutagenesis methods.

Kin Discrimination in Protists: From Many Cells to Single Cells and Backwards

During four decades (1960-1990s), the conceptualization and experimental design of studies in kin recognition relied on work with multicellular eukaryotes, particularly Unikonta (including invertebrates and vertebrates) and some Bikonta (including plants). This pioneering research had an animal behavior approach. During the 2000s, work on taxa-, clone- and kin-discrimination and recognition in protists produced genetic and molecular evidence that unicellular organisms (e.g. Saccharomyces, Dictyostelium, Polysphondylium, Tetrahymena, Entamoeba and Plasmodium) could distinguish between same (self or clone) and different (diverse clones), as well as among conspecifics of close or distant genetic relatedness. Here, we discuss some of the research on the genetics of kin discrimination/recognition and highlight the scientific progress made by switching emphasis from investigating multicellular to unicellular systems (and backwards). We document how studies with protists are helping us to understand the microscopic, cellular origins and evolution of the mechanisms of kin discrimination/recognition and their significance for the advent of multicellularity. We emphasize that because protists are among the most ancient organisms on Earth, belong to multiple taxonomic groups and occupy all environments, they can be central to reexamining traditional hypotheses in the field of kin recognition, reformulating concepts, and generating new knowledge.

Entamoeba Clone-Recognition Experiments: Morphometrics, Aggregative Behavior, and Cell-Signaling Characterization

Studies on clone- and kin-discrimination in protists have proliferated during the past decade. We report clone-recognition experiments in seven Entamoeba lineages (E. invadens IP-1, E. invadens VK-1:NS, E. terrapinae, E. moshkovskii Laredo, E. moshkovskii Snake, E. histolytica HM-1:IMSS and E. dispar). First, we characterized morphometrically each clone (length, width, and cell-surface area) and documented how they differed statistically from one another (as per single-variable or canonical-discriminant analyses). Second, we demonstrated that amebas themselves could discriminate self (clone) from different (themselves vs. other clones). In mix-cell-line cultures between closely-related (E. invadens IP-1 vs. E. invadens VK-1:NS) or distant-phylogenetic clones (E. terrapinae vs. E. moshkovskii Laredo), amebas consistently aggregated with same-clone members. Third, we identified six putative cell-signals secreted by the amebas (RasGap/Ankyrin, coronin-WD40, actin, protein kinases, heat shock 70, and ubiquitin) and which known functions in Entamoeba spp. included: cell proliferation, cell adhesion, cell movement, and stress-induced encystation. To our knowledge, this is the first multi-clone characterization of Entamoeba spp. morphometrics, aggregative behavior, and cell-signaling secretion in the context of clone-recognition. Protists allow us to study cell-cell recognition from ecological and evolutionary perspectives. Modern protistan lineages can be central to studies about the origins and evolution of multicellularity.

A Combinatorial Kin Discrimination System in Bacillus subtilis

Multicellularity inherently involves a number of cooperative behaviors that are potentially susceptible to exploitation but can be protected by mechanisms such as kin discrimination. Discrimination of kin from non-kin has been observed in swarms of the bacterium Bacillus subtilis, but the underlying molecular mechanism has been unknown. We used genetic, transcriptomic, and bioinformatic analyses to uncover kin recognition factors in this organism. Our results identified many molecules involved in cell-surface modification and antimicrobial production and response. These genes varied significantly in expression level and mutation phenotype among B. subtilis strains, suggesting interstrain variation in the exact kin discrimination mechanism used. Genome analyses revealed a substantial diversity of antimicrobial genes present in unique combinations in different strains, with many likely acquired by horizontal gene transfer. The dynamic combinatorial effect derived from this plethora of kin discrimination genes creates a tight relatedness cutoff for cooperation that has likely led to rapid diversification within the species. Our data suggest that genes likely originally selected for competitive purposes also generate preferential interactions among kin, thus stabilizing multicellular lifestyles.

Allorecognition, via TgrB1 and TgrC1, mediates the transition from unicellularity to multicellularity in the social amoeba Dictyostelium discoideum

The social amoeba Dictyostelium discoideum integrates into a multicellular organism when individual starving cells aggregate and form a mound. The cells then integrate into defined tissues and develop into a fruiting body that consists of a stalk and spores. Aggregation is initially orchestrated by waves of extracellular cyclic adenosine monophosphate (cAMP), and previous theory suggested that cAMP and other field-wide diffusible signals mediate tissue integration and terminal differentiation as well. Cooperation between cells depends on an allorecognition system comprising the polymorphic adhesion proteins TgrB1 and TgrC1. Binding between compatible TgrB1 and TgrC1 variants ensures that non-matching cells segregate into distinct aggregates prior to terminal development. Here, we have embedded a small number of cells with incompatible allotypes within fields of developing cells with compatible allotypes. We found that compatibility of the allotype encoded by the tgrB1 and tgrC1 genes is required for tissue integration, as manifested in cell polarization, coordinated movement and differentiation into prestalk and prespore cells. Our results show that the molecules that mediate allorecognition in D. discoideum also control the integration of individual cells into a unified developing organism, and this acts as a gating step for multicellularity.