Patterns of cell differentiation in several cellular slime mold species

NOVEL INVENTION OF SPORE INDUCTION IN A SISTER SPECIES TO GROUP 4 DICTYOSTELIA

Background

Dictyostelia are soil amoebas that aggregate to form fruiting bodies with spores and stalk cells in response to starvation. Where known, species across the dictyostelid phylogeny use secreted cAMP, detected by cAMP receptors (cARs) to induce the differentiation of spores and to organize fruiting body construction. However, recent deletion of the single cAR of Polyspondylium violaceum (Pvio) left both its fruiting bodies and spores intact.

Methods

To investigate whether Pvio sporulation can occur in the absence of secreted cAMP and to explore alternative inducers in a bioassay , three prespore genes were identified and gene fusions of their promoters with the LacZ reporter gene were transformed into Pvio cells. After assessing the spatial expression pattern of the genes and the stage at which prespore gene expression initiated, the effect of cAMP and other Dictyostelium discoideum ( Ddis) signal molecules were tested on prespore gene expression in vitro.

Results

Pvio genes g4562 (psp1), g2696 (psp2) and g2380 (psp3) were identified as homologs of Ddis spore coat genes. They were first expressed around 4 h of starvation in aggregation centres and later in the posterior 4/5 th of emerging sorogens and the spore head of early fruiting bodies. Cells from dissociated 4 h aggregates and shaken in suspension for 6 h increased prespore- LacZ reporter activity 4-fold for psp1 and 6-fold for psp2, but this increase was at least 5-fold higher when cells were plated on solid substratum for 6 h to develop normally. cAMP had no effect on prespore gene induction and neither had the Pvio chemoattractant glorin nor the Ddis chemoattractants and differentiation inducers folate, c-di-GMP, DIF-1, GABA, cGMP and 8Br-cAMP.

Conclusions

The Pvio lineage uniquely evolved a novel genetic network for synthesis, detection and processing of the signal that triggers its main survival strategy.

Emerging roles for diguanylate cyclase during the evolution of soma in dictyostelia

BACKGROUND

Cyclic di-guanylate (c-di-GMP), synthesized by diguanylate cyclase, is a major second messenger in prokaryotes, where it triggers biofilm formation. The dictyostelid social amoebas acquired diguanylate cyclase (dgcA) by horizontal gene transfer. Dictyostelium discoideum (Ddis) in taxon group 4 uses c-di-GMP as a secreted signal to induce differentiation of stalk cells, the ancestral somatic cell type that supports the propagating spores. We here investigated how this role for c-di-GMP evolved in Dictyostelia by exploring dgcA function in the group 2 species Polysphondylium pallidum (Ppal) and in Polysphondylium violaceum (Pvio), which resides in a small sister clade to group 4.

RESULTS

Similar to Ddis, dgcA is upregulated after aggregation in Ppal and Pvio and predominantly expressed in the anterior region and stalks of emerging fruiting bodies. DgcA null mutants in Ppal and Pvio made fruiting bodies with very long and thin stalks and only few spores and showed delayed aggregation and larger aggregates, respectively. Ddis dgcA- cells cannot form stalks at all, but showed no aggregation defects. The long, thin stalks of Ppal and Pvio dgcA- mutants were also observed in acaA- mutants in these species. AcaA encodes adenylate cyclase A, which mediates the effects of c-di-GMP on stalk induction in Ddis. Other factors that promote stalk formation in Ddis are DIF-1, produced by the polyketide synthase StlB, low ammonia, facilitated by the ammonia transporter AmtC, and high oxygen, detected by the oxygen sensor PhyA (prolyl 4-hydroxylase). We deleted the single stlB, amtC and phyA genes in Pvio wild-type and dgcA- cells. Neither of these interventions affected stalk formation in Pvio wild-type and not or very mildly exacerbated the long thin stalk phenotype of Pvio dgcA- cells.

CONCLUSIONS

The study reveals a novel role for c-di-GMP in aggregation, while the reduced spore number in Pvio and Ppal dgcA- is likely an indirect effect, due to depletion of the cell pool by the extended stalk formation. The results indicate that in addition to c-di-GMP, Dictyostelia ancestrally used an as yet unknown factor for induction of stalk formation. The activation of AcaA by c-di-GMP is likely conserved throughout Dictyostelia.

Live cell imaging of cell movement and transdifferentiation during regeneration of an amputated multicellular body of the social amoeba Dictyostelium discoideum

The regeneration of lost body parts is a fascinating phenomenon exhibited by some multicellular organisms. In social amoebae, such as Dictyostelium discoideum, the pseudoplasmodium is a temporary migratory multicellular structure with high regeneration ability. It consists of future stalk cells (prestalk cells) at the anterior end and future spore cells (prespore cells) at the posterior end, and if amputated, the remaining cells can rapidly regenerate the lost portion within several hours. Details of this regeneration event have been extensively documented; however, little is known about the behavior of individual cells involved in this process. In this study, we performed live cell imaging of cell behavior during regeneration of the excised anterior prestalk region. We used cells that specifically express GFP in the prestalk cell lineage to examine how the prestalk region is regenerated after this region is excised. The current model of prestalk regeneration suggests that the progenitors of prestalk cells, known as anterior-like cells (ALCs), which are sparsely distributed in the prespore region, are redistributed to form the new prestalk region. However, we found that the regenerated prestalk region was formed mainly by the transdifferentiation of prespore cells surrounding the excised anterior end, with little clustering of pre-existing ALCs. Furthermore, the movement of randomly distributed labeled cells during regeneration revealed that although the posterior end was deformed and rounded in shape, the relative position of cells along the anterior-posterior axis remained largely unchanged. These results suggest that the original anterior-posterior axis is maintained in posterior bodies and that prespore cells at the anterior side transdifferentiate and regenerate the prestalk region.

Two potential evolutionary origins of the fruiting bodies of the dictyostelid slime moulds

Dictyostelium discoideum and the other dictyostelid slime moulds ('social amoebae') are popular model organisms best known for their demonstration of sorocarpic development. In this process, many cells aggregate to form a multicellular unit that ultimately becomes a fruiting body bearing asexual spores. Several other unrelated microorganisms undergo comparable processes, and in some it is evident that their multicellular development evolved from the differentiation process of encystation. While it has been argued that the dictyostelid fruiting body had similar origins, it has also been proposed that dictyostelid sorocarpy evolved from the unicellular fruiting process found in other amoebozoan slime moulds. This paper reviews the developmental biology of the dictyostelids and other relevant organisms and reassesses the two hypotheses on the evolutionary origins of dictyostelid development. Recent advances in phylogeny, genetics, and genomics and transcriptomics indicate that further research is necessary to determine whether or not the fruiting bodies of the dictyostelids and their closest relatives, the myxomycetes and protosporangids, are homologous.

Evolutionary reconstruction of pattern formation in 98 Dictyostelium species reveals that cell-type specialization by lateral inhibition is a derived trait

Background

Multicellularity provides organisms with opportunities for cell-type specialization, but requires novel mechanisms to position correct proportions of different cell types throughout the organism. Dictyostelid social amoebas display an early form of multicellularity, where amoebas aggregate to form fruiting bodies, which contain only spores or up to four additional cell-types. These cell types will form the stalk and support structures for the stalk and spore head. Phylogenetic inference subdivides Dictyostelia into four major groups, with the model organism D. discoideum residing in group 4. In D. discoideum differentiation of its five cell types is dominated by lateral inhibition-type mechanisms that trigger scattered cell differentiation, with tissue patterns being formed by cell sorting.

Results

To reconstruct the evolution of pattern formation in Dictyostelia, we used cell-type specific antibodies and promoter-reporter fusion constructs to investigate pattern formation in 98 species that represent all groupings. Our results indicate that in all early diverging Dictyostelia and most members of groups 1–3, cells differentiate into maximally two cell types, prestalk and prespore cells, with pattern formation being dominated by position-dependent transdifferentiation of prespore cells into prestalk cells. In clade 2A, prestalk and stalk cell differentiation are lost and the prespore cells construct an acellular stalk. Group 4 species set aside correct proportions of prestalk and prespore cells early in development, and differentiate into up to three more supporting cell types.

Conclusions

Our experiments show that positional transdifferentiation is the ancestral mode of pattern formation in Dictyostelia. The early specification of a prestalk population equal to the number of stalk cells is a derived trait that emerged in group 4 and a few late diverging species in the other groups. Group 4 spore masses are larger than those of other groups and the differentiation of supporting cell types by lateral inhibition may have facilitated this increase in size. The signal DIF-1, which is secreted by prespore cells, triggers differentiation of supporting cell types. The synthesis and degradation of DIF-1 were shown to be restricted to group 4. This suggests that the emergence of DIF-1 signalling caused increased cell-type specialization in this group.

Electronic supplementary material

The online version of this article (doi:10.1186/2041-9139-5-34) contains supplementary material, which is available to authorized users.

Temporal and non-permanent division of labor during sorocarp formation in the social amoeba Acytostelium subglobosum

Somatic cell differentiation is crucial for the development of multicellular organisms. While the development of a fruiting body in Dictyostelium discoideum represents a simple model of this process with separation of stalk cells from the spore lineage, that of Acytostelium subglobosum is not accompanied by cell type separation. This species produces acellular stalks and, seemingly, all aggregated amoebae become spores; however, it possesses homologs for the stalk-cell marker genes of D. discoideum. In this study, we analyzed the spatio-temporal expression of A. subglobosum orthologs for D. discoideum stalk- or spore-lineage markers to clarify the developmental process of A. subglobosum. We first found that the prespore vesicles, which contained spore coat proteins, started to accumulate in the tip region and were observed in the entire sorogen throughout later development, confirming that all A. subglobosum cells became spores. The expression of a stalk-lineage gene ortholog, As-ecmA, started at the mound stage and was prominent in the protruding sorogen. Although two spore-lineage gene orthologs, As-cotD1 and -cotD2, were likewise detected shortly after cell aggregation and increased in intensity until tip formation, their expression diminished in the protruding sorogen. Double-fluorescence staining of these prestalk and prespore marker genes revealed that the expression of these marker genes was mutually exclusive and that expression switching occurred in the early tip. Our results indicate that A. subglobosum cells become committed to the spore lineage first, and then, while keeping this commitment intact, participate in stalk formation. Instead of the permanent division of labor observed in D. discoideum, A. subglobosum produces fruiting bodies by all cells contributing to the formation of the stalk as well as forming spores.

An epithelial tissue in Dictyostelium challenges the traditional origin of metazoan multicellularity

We hypothesize that aspects of animal multicellularity originated before the divergence of metazoans from fungi and social amoebae. Polarized epithelial tissues are a defining feature of metazoans and contribute to the diversity of animal body plans. The recent finding of a polarized epithelium in the non-metazoan social amoeba Dictyostelium discoideum demonstrates that epithelial tissue is not a unique feature of metazoans, and challenges the traditional paradigm that multicellularity evolved independently in social amoebae and metazoans. An alternative view, presented here, is that the common ancestor of social amoebae, fungi, and animals spent a portion of its life cycle in a multicellular state and possessed molecular machinery necessary for forming an epithelial tissue. Some descendants of this ancestor retained multicellularity, while others reverted to unicellularity. This hypothesis makes testable predictions regarding tissue organization in close relatives of metazoans and provides a novel conceptual framework for studies of early animal evolution.

Phylogeny-wide analysis of social amoeba genomes highlights ancient origins for complex intercellular communication

Dictyostelium discoideum (DD), an extensively studied model organism for cell and developmental biology, belongs to the most derived group 4 of social amoebas, a clade of altruistic multicellular organisms. To understand genome evolution over long time periods and the genetic basis of social evolution, we sequenced the genomes of Dictyostelium fasciculatum (DF) and Polysphondylium pallidum (PP), which represent the early diverging groups 1 and 2, respectively. In contrast to DD, PP and DF have conventional telomere organization and strongly reduced numbers of transposable elements. The number of protein-coding genes is similar between species, but only half of them comprise an identifiable set of orthologous genes. In general, genes involved in primary metabolism, cytoskeletal functions and signal transduction are conserved, while genes involved in secondary metabolism, export, and signal perception underwent large differential gene family expansions. This most likely signifies involvement of the conserved set in core cell and developmental mechanisms, and of the diverged set in niche- and species-specific adaptations for defense and food, mate, and kin selection. Phylogenetic dating using a concatenated data set and extensive loss of synteny indicate that DF, PP, and DD split from their last common ancestor at least 0.6 billion years ago.

A fully resolved phylogeny of the social amoebas (Dictyostelia) based on combined SSU and ITS rDNA sequences

The dictyostelids possess a complex life cycle including aggregative and multicellular stages. They also include one of the most widely studied protistan model organisms, Dictyostelium discoideum. The current molecular phylogeny of dictyostelids is based largely on SSU (18S) rDNA sequences and shows a deep taxon consisting of four major groups, none of which correspond to the three traditional morphologically-defined genera. However, due to the generally slowly evolving nature of SSU rDNA, these data fail to resolve the majority of branches within the four groups. Given the highly morphologically mixed nature of the dictyostelid groups, it is important to resolve relationships within them. We have determined sequences for the internal transcribed spacers (ITS) of rDNA for nearly all species in the original dictyostelid global phylogeny. Phylogenetic analyses of these data, in combination with the previously determined SSU rDNA sequences, confidently resolve nearly all branches in the tree. This now fully resolved phylogeny confirms the utility of ITS for dictyostelid systematics and lays the ground work for further evolutionary study of the group.

From drought sensing to developmental control: evolution of cyclic AMP signaling in social amoebas

Amoebas and other protists commonly encyst when faced with environmental stress. Although little is known of the signaling pathways that mediate encystation, the analogous process of spore formation in dictyostelid social amoebas is better understood. In Dictyostelium discoideum, secreted cyclic AMP (cAMP) mediates the aggregation of starving amoebas and induces the differentiation of prespore cells. Intracellular cAMP acting on cAMP-dependent protein kinase (PKA) triggers the maturation of spores and prevents their germination under the prevalent conditions of high osmolality in the spore head. The osmolyte-activated adenylate cyclase, ACG, produces cAMP for prespore differentiation and inhibition of spore germination. To retrace the origin of ACG function, we investigated ACG gene conservation and function in species that span the dictyostelid phylogeny. ACG genes, osmolyte-activated ACG activity, and osmoregulation of spore germination were detected in species that represent the 4 major groups of Dictyostelia. Unlike the derived species D. discoideum, many basal Dictyostelia have retained the ancestral mechanism of encystation from solitary amoebas. In these species and in solitary amoebas, encystation is independently triggered by starvation or by high osmolality. Osmolyte-induced encystation was accompanied by an increase in cAMP and prevented by inhibition of PKA, indicating that ACG and PKA activation mediate this response. We propose that high osmolality signals drought in soil amoebas and that developmental cAMP signaling in the Dictyostelia has evolved from this stress response.

Evolution of size and pattern in the social amoebas

A fundamental goal of biology is to understand how novel phenotypes evolved through changes in existing genes. The Dictyostelia or social amoebas represent a simple form of multicellularity, where starving cells aggregate to build fruiting structures. This review summarizes efforts to provide a framework for investigating the genetic changes that generated novel morphologies in the Dictyostelia. The foundation is a recently constructed molecular phylogeny of the Dictyostelia, which was used to examine trends in the evolution of novel forms and in the divergence of genes that shape these forms. There is a major trend towards the formation of large unbranched fruiting bodies, which is correlated with the use of cyclic AMP (cAMP) as a secreted signal to coordinate cell aggregation. The role of cAMP in aggregation arose through co‐option of a pathway that originally acted to coordinate fruiting body formation. The genotypic changes that caused this innovation and the role of dynamic cAMP signaling in defining fruiting body size and pattern throughout social amoeba evolution are discussed. BioEssays 29:635–644, 2007. © 2007 Wiley Periodicals, Inc.

Folic acid stimulation of the Galpha4 G protein-mediated signal transduction pathway inhibits anterior prestalk cell development in Dictyostelium

In Dictyostelium discoideum, several G proteins are known to mediate the transduction of signals that direct chemotactic movement and regulate developmental morphogenesis. The G protein alpha subunit encoded by the Galpha4 gene has been previously shown to be required for chemotactic responses to folic acid, proper developmental morphogenesis, and spore production. In this study, cells overexpressing the wild type Galpha4 gene, due to high copy gene dosage (Galpha4HC), were found to be defective in the ability to form the anterior prestalk cell region, express prespore- and prestalk-cell specific genes, and undergo spore formation. In chimeric organisms, Galpha4HC prespore cell-specific gene expression and spore production were rescued by the presence of wild-type cells, indicating that prespore cell development in Galpha4HC cells is limited by the absence of an intercellular signal. Transplanted wild-type tips were sufficient to rescue Galpha4HC prespore cell development, suggesting that the rescuing signal originates from the anterior prestalk cells. However, the deficiencies in prestalk-specific gene expression were not rescued in the chimeric organisms. Furthermore, Galpha4HC cells were localized to the prespore region of these chimeric organisms and completely excluded from the anterior prestalk region, suggesting that the Galpha4 subunit functions cell-autonomously to prevent anterior prestalk cell development. The presence of exogenous folic acid during vegetative growth and development delayed anterior prestalk cell development in wild-type but not galpha4 null mutant aggregates, indicating that folic acid can inhibit cell-type-specific differentiation by stimulation of the Galpha4-mediated signal transduction pathway. The results of this study suggest that Galpha4-mediated signals can regulate cell-type-specific differentiation by promoting prespore cell development and inhibiting anterior prestalk-cell development.

Cell-cell signaling during Dictyostelium development

Specific proteins and peptides, as well as cAMP, are used as intercellular signals in Dictyostelium. Our understanding of the signal transduction pathways activated by these signals has been expanded by inclusion of newly characterized proteins. cAMP-dependent protein kinase (PKA) and its associated phosphodiesterase, RegA, play multiple roles in these pathways.

A way of following individual cells in the migrating slugs of Dictyostelium discoideum

In the development of the cellular slime mold Dictyostelium discoideum there is a stage in which the aggregated amoebae form a migrating slug that moves forward in a polar fashion, showing sensitive orientation to environmental cues, as well as early signs of differentiation into anterior prestalk and posterior prespore cells. Heretofore it has been difficult to follow the movement of the individual cells within the slug, but a new method is described in which small, flat (one cell thick) slugs are produced in a glass-mineral oil interface where one can follow the movement of all the cells. Observations of time-lapse videos reveal the following facts about slug migration: (i) While the posterior cells move straight forward, the anterior cells swirl about rapidly in a chaotic fashion. (ii) Turning involves shifting the high point of these hyperactive cells. (iii) Both the anterior and the posterior cells move forward on their own power as the slug moves forward. (iv) There are no visible regular oscillations within the slug. (v) The number of prestalk and prespore cells is proportional for a range of sizes of these mini-slugs. All of these observations on thin slugs are consistent with what one finds in normal, three-dimensional slugs.

Metabolic pathways for differentiation-inducing factor-1 and their regulation are conserved between closely related Dictyostelium species, but not between distant members of the family

There is suggestive evidence that a conserved signalling system involving differentiation inducing factor-1 (DIF-1) controls stalk cell differentiation in a variety of slime mould species. In the standard laboratory species, Dictyostelium discoideum, DIF-1 is first inactivated by dechlorination catalysed by DIF-1 dechlorinase, then by several hydroxylation events, so that eventually about 12 metabolites are produced. If DIF-1 is used as a signal molecule in other species, they too must be able to metabolize it. We report here that the essentials of DIF-1 metabolism are conserved in D. mucoroides, the closest relative of D. discoideum. Both the dechlorinase and hydroxylase enzymes were present in D. mucoroides, and living cells of both species produced a similar spectrum of metabolites from [3H]DIF-1. Furthermore, DIF-1 dechlorinase was induced by DIF-1, as in D. discoideum, and this induction was repressed by ammonia and cAMP. DIF-1 dechlorinase could not be detected in cell extracts from D. minutum or Polysphondylium violaceum. However, living cells of both species are able to metabolize DIF-1; P. violaceum seems to produce a small amount of the monodechlorinated compound, DIF-3, but all other metabolites from both species appear to be unique. Thus all investigated species can metabolize DIF-1, but the exact route of metabolism is not highly conserved.

Length regulation in the Dictyostelium discoideum slug is a late event

Time-lapse video light microscopy was used to study the emergence and maturation of the migratory slug from a D. discoideum aggregate. The anterior part, the tip of this simple multicellular organism, establishes migration prior to the definition of the rear, and hence the length of the slug. It was found that newly formed slugs of wild-type strain WS380B can reach lengths greater than 1 cm, yet mature slugs of this strain are rarely longer than 2-3 mumm. Often the tip extended out of the aggregation mound upon an arching pillar of cells. After the tip first touched the substratum, it commenced migration with a rapid succession of movement steps. Here we show that at the initiation of migration, a differential rate of cell movement along the developing slug axis results in a series of complicated changes, before the stable and mature shape of the slug is formed. Our results lead to new conclusions about D. discoideum slug formation and shape maintenance. Evidence is presented for regulation of slug length.

Culmination in Dictyostelium is regulated by the cAMP-dependent protein kinase

We placed a specific inhibitor of cyclic AMP-dependent protein kinase (PKA) under the control of a prestalk-specific promoter. Cells containing this construct form normally patterned slugs, but under environmental conditions that normally trigger immediate culmination, the slugs undergo prolonged migration. Slugs that eventually enter culmination do so normally but arrest as elongated, hairlike structures that contain neither stalk nor spore cells. Mutant cells do not migrate to the stalk entrance when codeveloped with wild-type cells and show greatly reduced inducibility by DIF, the stalk cell morphogen. These results suggest that the activity of PKA is necessary for the altered pattern of movement of prestalk cells at culmination and their differentiation into stalk cells. We propose a model whereby a protein repressor, under the control of PKA, inhibits precocious induction of stalk cell differentiation by DIF and so regulates the choice between slug migration and culmination.

Positively and negatively acting signals regulating stalk cell and anterior-like cell differentiation in Dictyostelium

The Dictyostelium ecmB gene encodes an extracellular matrix protein and is inducible by the stalk cell morphogen DIF. It is expressed in a subset of prestalk (pstB) cells in the slug and surrounding pstA cells first express it at culmination. A region of the ecmB promoter can direct transcription in all anterior prestalk cells, but a separate, downstream region acts to prevent its expression in pstA cells prior to culmination. This may be the site of interaction of a repressor, regulated by an extracellular antagonist to DIF. At culmination, expression of the ecmB gene also becomes greatly elevated in anterior-like cells as they move to surround the spore mass. A distal region of the ecmB promoter directs increased expression in those anterior-like cells that surmount the spore head. This divergence in gene expression suggests that anterior-like cells and anterior prestalk cells experience different inductive conditions at culmination.

Dictyostelium discoideum: a model system for cell-cell interactions in development

The cellular slime mold Dictyostelium discoideum undergoes a transition from single-celled amoebae to a multicellular organism as a natural part of its life cycle. A method of cell-cell signaling that controls chemotaxis, morphogenesis, and gene expression has developed in this organism, and a detailed understanding of this signaling system provides clues to mechanisms of intercellular communication in the development of metazoans.

The development of cell lineages: a sequential model

The concept of cell lineage and the empirical characterization of specific lineages provide valuable insight into the problems of developmental biology. Of central interest is the decision-making process that results in the diversification of cell lines. Studies of the haemopoietic system, in which stem cells can be committed to one of at least six pathways of differentiation, have suggested that the restriction of differentiation potentials is a progressive and stochastic process. We have recently proposed an alternative model which hypothesizes that lineage potentials during haemopoiesis are expressed individually and in a predetermined sequence as progenitor cells mature. The model first arises from experimental studies which show that both normal myeloid progenitor cells and a human promyeloid cell line, which are able to differentiate towards either neutrophils or monocytes, express these potentials sequentially in culture. The close linear relationship between other haemopoietic progenitor cells is inferred from collective data from studies of bipotent progenitor cells and of haemopoietic proliferative disorders. If the development of haemopoietic cell lineages shows a tendency to follow a particular program, such a mechanism is likely to operate throughout development. In this paper we consider the evidence in favour of programmed events within progenitor cells implementing diversification, and the implications of predetermined and restricted pathways of embryonic development.

Interactions between adenosine and oscillatory cAMP signaling regulate size and pattern in Dictyostelium

We present evidence for the hypothesis that in multicellular structures of Dictyostelium, production of adenosine by hydrolysis of cAMP near the tip region prevents both generation of competing tips and differentiation of prespore cells near the tip, and thus establishes a "prestalk" region. We demonstrate that adenosine affects the immunological prespore specific staining pattern in slugs in a manner opposite to cAMP:cAMP induces an increase of prespore antigen; adenosine induces a decrease. When endogenous adenosine is removed from slugs, prespore vacuoles are synthesized throughout the prestalk region. Adenosine was found to inhibit the induction of prespore differentiation by cAMP in an apparently competitive manner. It was also found that adenosine specifically increased the amount of tissue controlled by one tip, probably by inhibiting generation of competing oscillators. Removing endogenous adenosine from slugs resulted in a decrease of tip dominance.