Ancient origin of the integrin-mediated adhesion and signaling machinery

Alternative mechanisms for talin to mediate integrin function

Cell-matrix adhesion is essential for building animals, promoting tissue cohesion, and enabling cells to migrate and resist mechanical force. Talin is an intracellular protein that is critical for linking integrin extracellular-matrix receptors to the actin cytoskeleton. A key question raised by structure-function studies is whether talin, which is critical for all integrin-mediated adhesion, acts in the same way in every context. We show that distinct combinations of talin domains are required for each of three different integrin functions during Drosophila development. The partial function of some mutant talins requires vinculin, indicating that recruitment of vinculin allows talin to duplicate its own activities. The different requirements are best explained by alternative mechanisms of talin function, with talin using one or both of its integrin-binding sites. We confirmed these alternatives by showing that the proximity between the second integrin-binding site and integrins differs, suggesting that talin adopts different orientations relative to integrins. Finally, we show that vinculin and actomyosin activity help change talin’s orientation. These findings demonstrate that the mechanism of talin function differs in each developmental context examined. The different arrangements of the talin molecule relative to integrins suggest that talin is able to sense different force vectors, either parallel or perpendicular to the membrane. This provides a paradigm for proteins whose apparent uniform function is in fact achieved by a variety of distinct mechanisms involving different molecular architectures.

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Integrin function requires distinct sets of talin domains in three different tissues

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Vinculin helps talin retain function when domains are removed

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Talin IBS2 is separated from integrins in muscle but not wing adhesion sites

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Vinculin and actomyosin contribute to separating IBS2 from integrins

Epithelial sodium transport and its control by aldosterone: the story of our internal environment revisited

Transcription and translation require a high concentration of potassium across the entire tree of life. The conservation of a high intracellular potassium was an absolute requirement for the evolution of life on Earth. This was achieved by the interplay of P- and V-ATPases that can set up electrochemical gradients across the cell membrane, an energetically costly process requiring the synthesis of ATP by F-ATPases. In animals, the control of an extracellular compartment was achieved by the emergence of multicellular organisms able to produce tight epithelial barriers creating a stable extracellular milieu. Finally, the adaptation to a terrestrian environment was achieved by the evolution of distinct regulatory pathways allowing salt and water conservation. In this review we emphasize the critical and dual role of Na(+)-K(+)-ATPase in the control of the ionic composition of the extracellular fluid and the renin-angiotensin-aldosterone system (RAAS) in salt and water conservation in vertebrates. The action of aldosterone on transepithelial sodium transport by activation of the epithelial sodium channel (ENaC) at the apical membrane and that of Na(+)-K(+)-ATPase at the basolateral membrane may have evolved in lungfish before the emergence of tetrapods. Finally, we discuss the implication of RAAS in the origin of the present pandemia of hypertension and its associated cardiovascular diseases.

Emergence and subsequent functional specialization of kindlins during evolution of cell adhesiveness

Kindlins are integrin-interacting proteins essential for integrin-mediated cell adhesiveness. In this study, we focused on the evolutionary origin and functional specialization of kindlins as a part of the evolutionary adaptation of cell adhesive machinery. Database searches revealed that many members of the integrin machinery (including talin and integrins) existed before kindlin emergence in evolution. Among the analyzed species, all metazoan lineages—but none of the premetazoans—had at least one kindlin-encoding gene, whereas talin was present in several premetazoan lineages. Kindlin appears to originate from a duplication of the sequence encoding the N-terminal fragment of talin (the talin head domain) with a subsequent insertion of the PH domain of separate origin. Sequence analysis identified a member of the actin filament-associated protein 1 (AFAP1) superfamily as the most likely origin of the kindlin PH domain. The functional divergence between kindlin paralogues was assessed using the sequence swap (chimera) approach. Comparison of kindlin 2 (K2)/kindlin 3 (K3) chimeras revealed that the F2 subdomain, in particular its C-terminal part, is crucial for the differential functional properties of K2 and K3. The presence of this segment enables K2 but not K3 to localize to focal adhesions. Sequence analysis of the C-terminal part of the F2 subdomain of K3 suggests that insertion of a variable glycine-rich sequence in vertebrates contributed to the loss of constitutive K3 targeting to focal adhesions. Thus emergence and subsequent functional specialization of kindlins allowed multicellular organisms to develop additional tissue-specific adaptations of cell adhesiveness.

Insights into the early evolution of animal calcium signaling machinery: a unicellular point of view

The basic principles of Ca(2+) regulation emerged early in prokaryotes. Ca(2+) signaling acquired more extensive and varied functions when life evolved into multicellular eukaryotes with intracellular organelles. Animals, fungi and plants display differences in the mechanisms that control cytosolic Ca(2+) concentrations. The aim of this review is to examine recent findings from comparative genomics of Ca(2+) signaling molecules in close unicellular relatives of animals and in common unicellular ancestors of animals and fungi. Also discussed are the evolution and origins of the sperm-specific CatSper channel complex, cation/Ca(2+) exchangers and four-domain voltage-gated Ca(2+) channels. Newly identified evolutionary evidence suggests that the distinct Ca(2+) signaling machineries in animals, plants and fungi likely originated from an ancient Ca(2+) signaling machinery prior to early eukaryotic radiation.

Integrins of the starlet sea anemone Nematostella vectensis

Integrins are extracellular matrix receptors composed of α and β subunits. Here we describe two α subunits and four β subunits from the starlet sea anemone Nematostella vectensis. Phylogenetic analysis suggests that the α subunits are most closely related to RGD- and LDV-dependent α subunits of chordates. The β subunits cluster with the previously described β integrins of the hard coral Acropora millepora. The expression of one of the α subunits and three of the β subunits was confirmed by reverse transcription PCR and in situ hybridization. The α subunit is primarily expressed in cells near muscles, by a subset of gastrodermal cells, and in the gonad. The three β subunits each have distinctive patterns of expression: one is concentrated in the gonad and mesenteric filament, another is found in a subset of cells in the epidermis of the oral region and in a subset of gastrodermal cells in the mesenteries, and a third is expressed widely. Changes in expression were also studied 48 h after horizontal transection by quantitative reverse transcription PCR and in situ hybridization. One of the β subunits is expressed 8-fold higher during regeneration, and its expression is observed in cells within both the epidermis and the gastrodermis at the site of regeneration. Our observations confirm that complex patterns of integrin expression were already present in basal metazoans. The integrins expressed in the gonads may play roles in mediating sperm-egg interactions in N. vectensis, while others may play a role in regulating proliferation during regeneration.

Early chordate origin of the vertebrate integrin αI domains

Half of the 18 human integrins α subunits have an inserted αI domain yet none have been observed in species that have diverged prior to the appearance of the urochordates (ascidians). The urochordate integrin αI domains are not human orthologues but paralogues, but orthologues of human αI domains extend throughout later-diverging vertebrates and are observed in the bony fish with duplicate isoforms. Here, we report evidence for orthologues of human integrins with αI domains in the agnathostomes (jawless vertebrates) and later diverging species. Sequence comparisons, phylogenetic analyses and molecular modeling show that one nearly full-length sequence from lamprey and two additional fragments include the entire integrin αI domain region, have the hallmarks of collagen-binding integrin αI domains, and we show that the corresponding recombinant proteins recognize the collagen GFOGER motifs in a metal dependent manner, unlike the α1I domain of the ascidian C. intestinalis. The presence of a functional collagen receptor integrin αI domain supports the origin of orthologues of the human integrins with αI domains prior to the earliest diverging extant vertebrates, a domain that has been conserved and diversified throughout the vertebrate lineage.

Pre-metazoan origins and evolution of the cadherin adhesome

Vertebrate adherens junctions mediate cell–cell adhesion via a “classical” cadherin–catenin “core” complex, which is associated with and regulated by a functional network of proteins, collectively named the cadherin adhesome (“cadhesome”). The most basal metazoans have been shown to conserve the cadherin–catenin “core”, but little is known about the evolution of the cadhesome. Using a bioinformatics approach based on both sequence and structural analysis, we have traced the evolution of this larger network in 26 organisms, from the uni-cellular ancestors of metazoans, through basal metazoans, to vertebrates. Surprisingly, we show that approximately 70% of the cadhesome, including proteins with similarity to the catenins, predate metazoans. We found that the transition to multicellularity was accompanied by the appearance of a small number of adaptor proteins, and we show how these proteins may have helped to integrate pre-metazoan sub-networks via PDZ domain–peptide interactions. Finally, we found the increase in network complexity in higher metazoans to have been driven primarily by expansion of paralogs. In summary, our analysis helps to explain how the complex protein network associated with cadherin at adherens junctions first came together in the first metazoan and how it evolved into the even more complex mammalian cadhesome.

Parvin-ILK: An intimate relationship

Integrin-linked kinase (ILK), PINCH and Parvin proteins form the IPP-complex that has been established as a core component of the integrin-actin link. Our recent genetic studies on Drosophila parvin, reveal that loss of function mutant defects phenocopy those observed upon loss of ILK or PINCH in the muscle and the wing, strengthening the notion that these proteins function together in the organism. Our work identified that ILK is necessary and sufficient for parvin subcellular localization, corroborating previous data indicating a direct association between these two proteins. Further genetic epistasis analysis of the IPP-complex assembly at integrin adhesion sites reveals that depending on the cell context each component is required differently. At the muscle attachment sites of the embryo, ILK is placed upstream in the hierarchy of genetic interactions required for the IPP-complex assembly. By contrast, in the wing epithelium the three proteins are mutually interdependent. Finally, we uncovered a novel property for the CH1-domain of parvin: its recruitment at the integrin-containing junctions in an ILK-dependent manner. Apparently, this ability of the CH1-domain is controlled by the inter-CH linker region. Thus, an intramolecular interaction within parvin could serve as a putative regulatory mechanism controlling the ILK-Parvin interaction.

Bacterial influences on animal origins

Animals evolved in seas teeming with bacteria, yet the influences of bacteria on animal origins are poorly understood. Comparisons among modern animals and their closest living relatives, the choanoflagellates, suggest that the first animals used flagellated collar cells to capture bacterial prey. The cell biology of prey capture, such as cell adhesion between predator and prey, involves mechanisms that may have been co-opted to mediate intercellular interactions during the evolution of animal multicellularity. Moreover, a history of bacterivory may have influenced the evolution of animal genomes by driving the evolution of genetic pathways for immunity and facilitating lateral gene transfer. Understanding the interactions between bacteria and the progenitors of animals may help to explain the myriad ways in which bacteria shape the biology of modern animals, including ourselves.

Cell substratum adhesion during early development of Dictyostelium discoideum

Vegetative and developed amoebae of Dictyostelium discoideum gain traction and move rapidly on a wide range of substrata without forming focal adhesions. We used two independent assays to quantify cell-substrate adhesion in mutants and in wild-type cells as a function of development. Using a microfluidic device that generates a range of hydrodynamic shear stress, we found that substratum adhesion decreases at least 10 fold during the first 6 hr of development of wild type cells. This result was confirmed using a single-cell assay in which cells were attached to the cantilever of an atomic force probe and allowed to adhere to untreated glass surfaces before being retracted. Both of these assays showed that the decrease in substratum adhesion was dependent on the cAMP receptor CAR1 which triggers development. Vegetative cells missing talin as the result of a mutation in talA exhibited slightly reduced adhesive properties compared to vegetative wild-type cells. In sharp contrast to wild-type cells, however, these talA mutant cells did not show further reduction of adhesion during development such that after 5 hr of development they were significantly more adhesive than developed wild type cells. In addition, both assays showed that substrate adhesion was reduced in 0 hr cells when the actin cytoskeleton was disrupted by latrunculin. Consistent with previous observations, substrate adhesion was also reduced in 0 hr cells lacking the membrane proteins SadA or SibA as the result of mutations in sadA or sibA. However, there was no difference in the adhesion properties between wild type AX3 cells and these mutant cells after 6 hr of development, suggesting that neither SibA nor SadA play an essential role in substratum adhesion during aggregation. Our results provide a quantitative framework for further studies of cell substratum adhesion in Dictyostelium.

Molecular basis of kindlin-2 binding to integrin-linked kinase pseudokinase for regulating cell adhesion

Integrin-linked kinase (ILK) is a distinct intracellular adaptor essential for integrin-mediated cell-extracellular matrix adhesion, cell spreading, and migration. Acting as a major docking platform in focal adhesions, ILK engages many proteins to dynamically link integrins with the cytoskeleton, but the underlying mechanism remains elusive. Here, we have characterized the interaction of ILK with kindlin-2, a key regulator for integrin bidirectional signaling. We show that human kindlin-2 binds to human ILK with high affinity. Using systematic mapping approaches, we have identified a major ILK binding site involving a 20-residue fragment (residues 339-358) in kindlin-2. NMR-based analysis reveals a helical conformation of this fragment that utilizes its leucine-rich surface to recognize the ILK pseudokinase domain in a mode that is distinct from another ILK pseudokinase domain binding protein, α-parvin. Structure-based mutational experiments further demonstrate that the kindlin-2 binding to ILK is crucial for the kindlin-2 localization to focal adhesions and cell spreading (integrin outside-in signaling) but dispensable for the kindlin-2-mediated integrin activation (integrin inside-out signaling). These data define a specific mode of the kindlin-2/ILK interaction with mechanistic implications as to how it spatiotemporally mediates integrin signaling and cell adhesion.

How to find a leucine in a haystack? Structure, ligand recognition and regulation of leucine-aspartic acid (LD) motifs

LD motifs (leucine-aspartic acid motifs) are short helical protein-protein interaction motifs that have emerged as key players in connecting cell adhesion with cell motility and survival. LD motifs are required for embryogenesis, wound healing and the evolution of multicellularity. LD motifs also play roles in disease, such as in cancer metastasis or viral infection. First described in the paxillin family of scaffolding proteins, LD motifs and similar acidic LXXLL interaction motifs have been discovered in several other proteins, whereas 16 proteins have been reported to contain LDBDs (LD motif-binding domains). Collectively, structural and functional analyses have revealed a surprising multivalency in LD motif interactions and a wide diversity in LDBD architectures. In the present review, we summarize the molecular basis for function, regulation and selectivity of LD motif interactions that has emerged from more than a decade of research. This overview highlights the intricate multi-level regulation and the inherently noisy and heterogeneous nature of signalling through short protein-protein interaction motifs.

Moving towards a paradigm: common mechanisms of chemotactic signaling in Dictyostelium and mammalian leukocytes

Chemotaxis, or directed migration of cells along a chemical gradient, is a highly coordinated process that involves gradient sensing, motility, and polarity. Most of our understanding of chemotaxis comes from studies of cells undergoing amoeboid-type migration, in particular the social amoeba Dictyostelium discoideum and leukocytes. In these amoeboid cells the molecular events leading to directed migration can be conceptually divided into four interacting networks: receptor/G protein, signal transduction, cytoskeleton, and polarity. The signal transduction network occupies a central position in this scheme as it receives direct input from the receptor/G protein network, as well as feedback from the cytoskeletal and polarity networks. Multiple overlapping modules within the signal transduction network transmit the signals to the actin cytoskeleton network leading to biased pseudopod protrusion in the direction of the gradient. The overall architecture of the networks, as well as the individual signaling modules, is remarkably conserved between Dictyostelium and mammalian leukocytes, and the similarities and differences between the two systems are the subject of this review.

The genomic and cellular foundations of animal origins

The first animals arose more than six hundred million years ago, yet they left little impression in the fossil record. Nonetheless, the cell biology and genome composition of the first animal, the Urmetazoan, can be reconstructed through the study of phylogenetically relevant living organisms. Comparisons among animals and their unicellular and colonial relatives reveal that the Urmetazoan likely possessed a layer of epithelium-like collar cells, preyed on bacteria, reproduced by sperm and egg, and developed through cell division, cell differentiation, and invagination. Although many genes involved in development, body patterning, immunity, and cell-type specification evolved in the animal stem lineage or after animal origins, several gene families critical for cell adhesion, signaling, and gene regulation predate the origin of animals. The ancestral functions of these and other genes may eventually be revealed through studies of gene and genome function in early-branching animals and their closest non-animal relatives.

The others: our biased perspective of eukaryotic genomes

Understanding the origin and evolution of the eukaryotic cell and the full diversity of eukaryotes is relevant to many biological disciplines. However, our current understanding of eukaryotic genomes is extremely biased, leading to a skewed view of eukaryotic biology. We argue that a phylogeny-driven initiative to cover the full eukaryotic diversity is needed to overcome this bias. We encourage the community: (i) to sequence a representative of the neglected groups available at public culture collections, (ii) to increase our culturing efforts, and (iii) to embrace single cell genomics to access organisms refractory to propagation in culture. We hope that the community will welcome this proposal, explore the approaches suggested, and join efforts to sequence the full diversity of eukaryotes.

Making the connection: ciliary adhesion complexes anchor basal bodies to the actin cytoskeleton

Cilia have been associated with diverse developmental and physiological processes, and defects in cilia underlie a number of genetic conditions. Several lines of evidence support a critical role of the actin cytoskeleton in ciliogenesis and ciliary function. Here, we show that well-characterized focal adhesion (FA) proteins, including FAK, Paxillin, and Vinculin, associate with the basal bodies of multiciliated cells and form complexes (CAs) that interact with the actin cytoskeleton. FAK downregulation leads to ciliogenesis defects similar to those observed when the actin cytoskeleton is disrupted, including defects in basal body migration, docking, and spacing, suggesting that CAs link basal bodies to the actin cytoskeleton. The important role of FA proteins in ciliogenesis leads us to propose that evolutionarily FA proteins, many of which are found in primitive flagellated unicellular eukaryotes, may have originally evolved to perform functions at flagella and were later co-opted for use in cell adhesion.

The predicted secretomes of Monosiga brevicollis and Capsaspora owczarzaki, close unicellular relatives of metazoans, reveal new insights into the evolution of the metazoan extracellular matrix

The extracellular matrix (ECM) is a major mediator of multi-cellularity in the metazoa. Multiple ECM proteins are conserved from sponges to human, raising questions about the evolutionary origin of ECM. Choanoflagellates are the closest unicellular relatives of the metazoa and proteins with domains characteristic of metazoan ECM proteins have been identified from the genome-predicted proteome of the choanoflagellate Monosiga brevicollis. However, a systematic analysis of M. brevicollis secretory signal peptide-containing proteins with ECM domains has been lacking. We analysed all predicted secretory signal-peptide-containing proteins of M. brevicollis for ECM domains. Nine domains that are widespread in metazoan ECM proteins are represented, with EGF, fibronectin III, laminin G, and von Willebrand Factor_A domains being the most numerous. Three proteins contain more than one category of ECM domain, however, no proteins correspond to the domain architecture of metazoan ECM proteins. The fibronectin III domains are all present within glycoside hydrolases and none contain an integrin-binding motif. Glycosaminoglycan-binding motifs identified in animal thrombospondin type 1 domains are conserved in some M. brevicollis representatives of this domain, whereas there is little evidence of conservation of glycosaminoglycan-binding motifs in the laminin G domains. The identified proteins were compared with the predicted secretory ECM domain-containing proteins of the integrin-expressing filasterean, Capsaspora owczarzaki. C. owczarzaki encodes a smaller number of secretory, ECM domain-containing proteins and only EGF, fibronectin type III and laminin G domains are represented. The M. brevicollis and C. owczarzaki proteins have distinct domain architectures and all proteins differ in their domain architecture to metazoan ECM proteins. These identifications provide a basis for future experiments to validate the extracellular location of these proteins and uncover their functions in choanoflagellates and C. owczarzaki. The data strengthen the model that ECM proteins are metazoan-specific and evolved as innovations in the last common metazoan ancestor.

Regulation of Src and Csk nonreceptor tyrosine kinases in the filasterean Ministeria vibrans

The development of the phosphotyrosine-based signaling system predated the evolution of multicellular animals. Single-celled choanoflagellates, the closest living relatives to metazoans, possess numerous tyrosine kinases, including Src family nonreceptor tyrosine kinases. Choanoflagellates also have Csk (C-terminal Src kinase), the enzyme that regulates Src in metazoans; however, choanoflagellate Csk kinases fail to repress the cognate Src. Here, we have cloned and characterized Src and Csk kinases from Ministeria vibrans, a filasterean (the sister group to metazoans and choanoflagellates). The two Src kinases (MvSrc1 and MvSrc2) are enzymatically active Src kinases, although they have low activity toward mammalian cellular proteins. Unexpectedly, MvSrc2 has significant Ser/Thr kinase activity. The Csk homologue (MvCsk) is enzymatically inactive and fails to repress MvSrc activity. We suggest that the low activity of MvCsk is due to sequences in the SH2-kinase interface, and we show that a point mutation in this region partially restores MvCsk activity. The inactivity of filasterean Csk kinases is consistent with a model in which the stringent regulation of Src family kinases arose more recently in evolution, after the split between choanoflagellates and multicellular animals.

Molecular evolution of the Yap/Yorkie proto-oncogene and elucidation of its core transcriptional program

Throughout Metazoa, developmental processes are controlled by a surprisingly limited number of conserved signaling pathways. Precisely how these signaling cassettes were assembled in early animal evolution remains poorly understood, as do the molecular transitions that potentiated the acquisition of their myriad developmental functions. Here we analyze the molecular evolution of the proto-oncogene yes-associated protein (Yap)/Yorkie, a key effector of the Hippo signaling pathway that controls organ size in both Drosophila and mammals. Based on heterologous functional analysis of evolutionarily distant Yap/Yorkie orthologs, we demonstrate that a structurally distinct interaction interface between Yap/Yorkie and its partner TEAD/Scalloped became fixed in the eumetazoan common ancestor. We then combine transcriptional profiling of tissues expressing phylogenetically diverse forms of Yap/Yorkie with ChIP-seq validation to identify a common downstream gene expression program underlying the control of tissue growth in Drosophila. Intriguingly, a subset of the newly identified Yorkie target genes are also induced by Yap in mammalian tissues, thus revealing a conserved Yap-dependent gene expression signature likely to mediate organ size control throughout bilaterian animals. Combined, these experiments provide new mechanistic insights while revealing the ancient evolutionary history of Hippo signaling.

The analysis of eight transcriptomes from all poriferan classes reveals surprising genetic complexity in sponges

Sponges (Porifera) are among the earliest evolving metazoans. Their filter-feeding body plan based on choanocyte chambers organized into a complex aquiferous system is so unique among metazoans that it either reflects an early divergence from other animals prior to the evolution of features such as muscles and nerves, or that sponges lost these characters. Analyses of the Amphimedon and Oscarella genomes support this view of uniqueness-many key metazoan genes are absent in these sponges-but whether this is generally true of other sponges remains unknown. We studied the transcriptomes of eight sponge species in four classes (Hexactinellida, Demospongiae, Homoscleromorpha, and Calcarea) specifically seeking genes and pathways considered to be involved in animal complexity. For reference, we also sought these genes in transcriptomes and genomes of three unicellular opisthokonts, two sponges (A. queenslandica and O. carmela), and two bilaterian taxa. Our analyses showed that all sponge classes share an unexpectedly large complement of genes with other metazoans. Interestingly, hexactinellid, calcareous, and homoscleromorph sponges share more genes with bilaterians than with nonbilaterian metazoans. We were surprised to find representatives of most molecules involved in cell-cell communication, signaling, complex epithelia, immune recognition, and germ-lineage/sex, with only a few, but potentially key, absences. A noteworthy finding was that some important genes were absent from all demosponges (transcriptomes and the Amphimedon genome), which might reflect divergence from main-stem lineages including hexactinellids, calcareous sponges, and homoscleromorphs. Our results suggest that genetic complexity arose early in evolution as shown by the presence of these genes in most of the animal lineages, which suggests sponges either possess cryptic physiological and morphological complexity and/or have lost ancestral cell types or physiological processes.

Phylogenetic analysis of CDK and cyclin proteins in premetazoan lineages

Background

The molecular history of animal evolution from single-celled ancestors remains a major question in biology, and little is known regarding the evolution of cell cycle regulation during animal emergence. In this study, we conducted a comprehensive evolutionary analysis of CDK and cyclin proteins in metazoans and their unicellular relatives.

Results

Our analysis divided the CDK family into eight subfamilies. Seven subfamilies (CDK1/2/3, CDK5, CDK7, CDK 20, CDK8/19, CDK9, and CDK10/11) are conserved in metazoans and fungi, with the remaining subfamily, CDK4/6, found only in eumetazoans. With respect to cyclins, cyclin C, H, L, Y subfamilies, and cyclin K and T as a whole subfamily, are generally conserved in animal, fungi, and amoeba Dictyostelium discoideum. In contrast, cyclin subfamilies B, A, E, and D, which are cell cycle-related, have distinct evolutionary histories. The cyclin B subfamily is generally conserved in D. discoideum, fungi, and animals, whereas cyclin A and E subfamilies are both present in animals and their unicellular relatives such as choanoflagellate Monosiga brevicollis and filasterean Capsaspora owczarzaki, but are absent in fungi and D. discoideum. Although absent in fungi and D. discoideum, cyclin D subfamily orthologs can be found in the early-emerging, non-opisthokont apusozoan Thecamonas trahens. Within opisthokonta, the cyclin D subfamily is conserved only in eumetazoans, and is absent in fungi, choanoflagellates, and the basal metazoan Amphimedon queenslandica.

Conclusions

Our data indicate that the CDK4/6 subfamily and eumetazoans emerged simultaneously, with the evolutionary conservation of the cyclin D subfamily also tightly linked with eumetazoan appearance. Establishment of the CDK4/6-cyclin D complex may have been the key step in the evolution of cell cycle control during eumetazoan emergence.

Evolution of integrin I domains

In humans, an ~200-residue "inserted" I domain, a von Willebrand factor A domain (vWFA), buds out from the β-propeller domain in 9 of 18 integrin α subunits. The vWFA domain is not unique to the α subunit as it is an integral part of all integrin β subunits and many other proteins. The βI domain has always been a component of integrins but the αI domain makes its appearance relatively late, in early chordates, since it is found in tunicates and later diverging species. The tunicate αI domains are distinct from the human collagen and leukocyte recognizing integrin α subunits, but fragments of integrins from agnathastomes suggest that the human-type αI domains arose in an ancestor of the very first vertebrate species. The rise of integrins with αI domains parallels the enormous changes in body plan and systemic development of the chordate line that began some 550 million or more years ago.

A broad genomic survey reveals multiple origins and frequent losses in the evolution of respiratory hemerythrins and hemocyanins

Hemerythrins and hemocyanins are respiratory proteins present in some of the most ecologically diverse animal lineages; however, the precise evolutionary history of their enzymatic domains (hemerythrin, hemocyanin M, and tyrosinase) is still not well understood. We survey a wide dataset of prokaryote and eukaryote genomes and RNAseq data to reconstruct the phylogenetic origins of these proteins. We identify new species with hemerythrin, hemocyanin M, and tyrosinase domains in their genomes, particularly within animals, and demonstrate that the current distribution of respiratory proteins is due to several events of lateral gene transfer and/or massive gene loss. We conclude that the last common metazoan ancestor had at least two hemerythrin domains, one hemocyanin M domain, and six tyrosinase domains. The patchy distribution of these proteins among animal lineages can be partially explained by physiological adaptations, making these genes good targets for investigations into the interplay between genomic evolution and physiological constraints.

Regulated aggregative multicellularity in a close unicellular relative of metazoa

The evolution of metazoans from their unicellular ancestors was one of the most important events in the history of life. However, the cellular and genetic changes that ultimately led to the evolution of multicellularity are not known. In this study, we describe an aggregative multicellular stage in the protist Capsaspora owczarzaki, a close unicellular relative of metazoans. Remarkably, transition to the aggregative stage is associated with significant upregulation of orthologs of genes known to establish multicellularity and tissue architecture in metazoans. We further observe transitions in regulated alternative splicing during the C. owczarzaki life cycle, including the deployment of an exon network associated with signaling, a feature of splicing regulation so far only observed in metazoans. Our results reveal the existence of a highly regulated aggregative stage in C. owczarzaki and further suggest that features of aggregative behavior in an ancestral protist may had been co-opted to develop some multicellular properties currently seen in metazoans.

DOI:http://dx.doi.org/10.7554/eLife.01287.001

When living things made from many cells evolved from single-celled ancestors, it was a breakthrough in the history of life—and one that has occurred more than once. In fact, multicellular life has evolved independently at least 25 times, in groups as diverse as animals, fungi, plants, slime molds and seaweeds. There are broadly two ways to become multicellular. The most complex multicellular species, such as animals, will replicate a single cell, over and over, without separating the resultant cells. However, in species that are only occasionally multicellular, free-living cells tend instead to join together in one mass of many cells.

Evolution is constrained by its raw materials; so looking at the living relatives of a given species, or group, can lead to a better understanding of its evolution because its relatives contain clues about its ancestors. To gain insights into how animal multicellular life might have began; Sebé-Pedrós et al. studied the life cycle of the amoeboid organism Capsaspora owczarzaki. Found within the bodies of freshwater snails, this single-celled amoeba is a close relative of multicellular animals and could resemble one of their earliest ancestors.

At certain stages of the life cycle Sebé-Pedrós et al. noticed that individual amoebae gathered together to form a multicellular mass—something that had not been seen before in such a close relative of the animals. Moreover, the genes that ‘switched on’ when the amoebae began to aggregate are also found in animals; where, together with other genes, they control development and the formation of tissues. Sebé-Pedrós et al. suggest that the first multicellular animals could have recycled the genes that control the aggregation of single-celled species: in other words, genes that once controlled the changes that happen at different times in a life cycle, now control the changes that develop between different tissues at the same time.

Sebé-Pedrós et al. also observed that alternative splicing—a process that allows different proteins to be made from a single gene—occurs via two different mechanisms during the life cycle of Capsaspora. Most of the time Capsaspora employs a form of alternative splicing that is often seen in plants and fungi, and only rarely in animals; for the rest of the time it uses a form of alternative splicing similar to that used by animal cells.

The evolution of complex alternative splicing mechanisms is a hallmark feature of multicellular animals. The exploitation of two major forms of alternative splicing in Capsaspora could thus reflect an important transition during evolution that resulted in an increased diversity of proteins and in more complex gene regulation. Such a transition may ultimately have paved the way for the increased specialization of cell types seen in animals.

This glimpse into the possible transitions in gene regulation that contributed to the birth of multicellular animals indicates that the single-celled ancestor of the animals was likely more complex than previously thought. Future analyses of the animals’ close relatives may further improve our understanding of how single-celled organisms became multicellular animals.

DOI:http://dx.doi.org/10.7554/eLife.01287.002

The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution

An understanding of ctenophore biology is critical for reconstructing events that occurred early in animal evolution. Toward this goal, we have sequenced, assembled, and annotated the genome of the ctenophore Mnemiopsis leidyi. Our phylogenomic analyses of both amino acid positions and gene content suggest that ctenophores rather than sponges are the sister lineage to all other animals. Mnemiopsis lacks many of the genes found in bilaterian mesodermal cell types, suggesting that these cell types evolved independently. The set of neural genes in Mnemiopsis is similar to that of sponges, indicating that sponges may have lost a nervous system. These results present a newly supported view of early animal evolution that accounts for major losses and/or gains of sophisticated cell types, including nerve and muscle cells.

Drosophila importin-7 functions upstream of the Elmo signaling module to mediate the formation and stability of muscle attachments

Establishment and maintenance of stable muscle attachments is essential for coordinated body movement. Studies in Drosophila have pioneered a molecular understanding of the morphological events in the conserved process of muscle attachment formation, including myofiber migration, muscle-tendon signaling, and stable junctional adhesion between muscle cells and their corresponding target insertion sites. In both Drosophila and vertebrate models, integrin complexes play a key role in the biogenesis and stability of muscle attachments through the interactions of integrins with extracellular matrix (ECM) ligands. We show that Drosophila importin-7 (Dim7) is an upstream regulator of the conserved Elmo-Mbc→Rac signaling pathway in the formation of embryonic muscle attachment sites (MASs). Dim7 is encoded by the moleskin (msk) locus and was identified as an Elmo-interacting protein. Both Dim7 and Elmo localize to the ends of myofibers coincident with the timing of muscle-tendon attachment in late myogenesis. Phenotypic analysis of elmo mutants reveal muscle attachment defects similar to those previously described for integrin mutants. Furthermore, Elmo and Dim7 interact both biochemically and genetically in the developing musculature. The muscle detachment phenotype resulting from mutations in the msk locus can be rescued by components in the Elmo signaling pathway, including the Elmo-Mbc complex, an activated Elmo variant, or a constitutively active form of Rac. In larval muscles, the localization of Dim7 and activated Elmo to the sites of muscle attachment is attenuated upon RNAi knockdown of integrin heterodimer complex components. Our results show that integrins function as upstream signals to mediate Dim7-Elmo enrichment to the MASs.

Phylogenomics demonstrates that breviate flagellates are related to opisthokonts and apusomonads

Most eukaryotic lineages belong to one of a few major groups. However, several protistan lineages have not yet been robustly placed in any of these groups. Both the breviates and apusomonads are two such lineages that appear to be related to the Amoebozoa and Opisthokonta (i.e. the 'unikonts' or Amorphea); however, their precise phylogenetic positions remain unclear. Here, we describe a novel microaerophilic breviate, Pygsuia biforma gen. nov. sp. nov., isolated from a hypoxic estuarine sediment. Ultrastructurally, this species resembles the breviate genera Breviata and Subulatomonas but has two cell morphologies, adherent and swimming. Phylogenetic analyses of the small sub-unit rRNA gene show that Pygsuia is the sister to the other breviates. We constructed a 159-protein supermatrix, including orthologues identified in RNA-seq data from Pygsuia. Phylogenomic analyses of this dataset show that breviates, apusomonads and Opisthokonta form a strongly supported major eukaryotic grouping we name the Obazoa. Although some phylogenetic methods disagree, the balance of evidence suggests that the breviate lineage forms the deepest branch within Obazoa. We also found transcripts encoding a nearly complete integrin adhesome from Pygsuia, indicating that this protein complex involved in metazoan multicellularity may have evolved earlier in eukaryote evolution than previously thought.

Insights into the origin of metazoan filopodia and microvilli

Filopodia are fine actin-based cellular projections used for both environmental sensing and cell motility, and they are essential organelles for metazoan cells. In this study, we reconstruct the origin of metazoan filopodia and microvilli. We first report on the evolutionary assembly of the filopodial molecular toolkit and show that homologs of many metazoan filopodial components, including fascin and myosin X, were already present in the unicellular or colonial progenitors of metazoans. Furthermore, we find that the actin crosslinking protein fascin localizes to filopodia-like structures and microvilli in the choanoflagellate Salpingoeca rosetta. In addition, homologs of filopodial genes in the holozoan Capsaspora owczarzaki are upregulated in filopodia-bearing cells relative to those that lack them. Therefore, our findings suggest that proteins essential for metazoan filopodia and microvilli are functionally conserved in unicellular and colonial holozoans and that the last common ancestor of metazoans bore a complex and specific filopodial machinery.

The genome of the anaerobic fungus Orpinomyces sp. strain C1A reveals the unique evolutionary history of a remarkable plant biomass degrader

Anaerobic gut fungi represent a distinct early-branching fungal phylum (Neocallimastigomycota) and reside in the rumen, hindgut, and feces of ruminant and nonruminant herbivores. The genome of an anaerobic fungal isolate, Orpinomyces sp. strain C1A, was sequenced using a combination of Illumina and PacBio single-molecule real-time (SMRT) technologies. The large genome (100.95 Mb, 16,347 genes) displayed extremely low G+C content (17.0%), large noncoding intergenic regions (73.1%), proliferation of microsatellite repeats (4.9%), and multiple gene duplications. Comparative genomic analysis identified multiple genes and pathways that are absent in Dikarya genomes but present in early-branching fungal lineages and/or nonfungal Opisthokonta. These included genes for posttranslational fucosylation, the production of specific intramembrane proteases and extracellular protease inhibitors, the formation of a complete axoneme and intraflagellar trafficking machinery, and a near-complete focal adhesion machinery. Analysis of the lignocellulolytic machinery in the C1A genome revealed an extremely rich repertoire, with evidence of horizontal gene acquisition from multiple bacterial lineages. Experimental analysis indicated that strain C1A is a remarkable biomass degrader, capable of simultaneous saccharification and fermentation of the cellulosic and hemicellulosic fractions in multiple untreated grasses and crop residues examined, with the process significantly enhanced by mild pretreatments. This capability, acquired during its separate evolutionary trajectory in the rumen, along with its resilience and invasiveness compared to prokaryotic anaerobes, renders anaerobic fungi promising agents for consolidated bioprocessing schemes in biofuels production.

Before the endless forms: embodied model of transition from single cells to aggregates to ecosystem engineering

The emergence of complex multicellular systems and their associated developmental programs is one of the major problems of evolutionary biology. The advantages of cooperation over individuality seem well known but it is not clear yet how such increase of complexity emerged from unicellular life forms. Current multicellular systems display a complex cell-cell communication machinery, often tied to large-scale controls of body size or tissue homeostasis. Some unicellular life forms are simpler and involve groups of cells cooperating in a tissue-like fashion, as it occurs with biofilms. However, before true gene regulatory interactions were widespread and allowed for controlled changes in cell phenotypes, simple cellular colonies displaying adhesion and interacting with their environments were in place. In this context, models often ignore the physical embedding of evolving cells, thus leaving aside a key component. The potential for evolving pre-developmental patterns is a relevant issue: how far a colony of evolving cells can go? Here we study these pre-conditions for morphogenesis by using CHIMERA, a physically embodied computational model of evolving virtual organisms in a pre-Mendelian world. Starting from a population of identical, independent cells moving in a fluid, the system undergoes a series of changes, from spatial segregation, increased adhesion and the development of generalism. Eventually, a major transition occurs where a change in the flow of nutrients is triggered by a sub-population. This ecosystem engineering phenomenon leads to a subsequent separation of the ecological network into two well defined compartments. The relevance of these results for evodevo and its potential ecological triggers is discussed.

Ancient complexity, opisthokont plasticity, and discovery of the 11th subfamily of Arf GAP proteins

The organelle paralogy hypothesis is one model for the acquisition of nonendosymbiotic organelles, generated from molecular evolutionary analyses of proteins encoding specificity in the membrane traffic system. GTPase activating proteins (GAPs) for the ADP-ribosylation factor (Arfs) GTPases are additional regulators of the kinetics and fidelity of membrane traffic. Here we describe molecular evolutionary analyses of the Arf GAP protein family. Of the 10 subfamilies previously defined in humans, we find that 5 were likely present in the last eukaryotic common ancestor. Of the 3 most recently derived subfamilies, 1 was likely present in the ancestor of opisthokonts (animals and fungi) and apusomonads (flagellates classified as the sister lineage to opisthokonts), while 2 arose in the holozoan lineage. We also propose to have identified a novel ancient subfamily (ArfGAPC2), present in diverse eukaryotes but which is lost frequently, including in the opisthokonts. Surprisingly few ancient domains accompanying the ArfGAP domain were identified, in marked contrast to the extensively decorated human Arf GAPs. Phylogenetic analyses of the subfamilies reveal patterns of single and multiple gene duplications specific to the Holozoa, to some degree mirroring evolution of Arf GAP targets, the Arfs. Conservation, and lack thereof, of various residues in the ArfGAP structure provide contextualization of previously identified functional amino acids and their application to Arf GAP biology in general. Overall, our results yield insights into current Arf GAP biology, reveal complexity in the ancient eukaryotic ancestor and integrate the Arf GAP family into a proposed mechanism for the evolution of nonendosymbiotic organelles.

Metazoan-like signaling in a unicellular receptor tyrosine kinase

Background

Receptor tyrosine kinases (RTKs) are crucial components of signal transduction systems in multicellular animals. Surprisingly, numerous RTKs have been identified in the genomes of unicellular choanoflagellates and other protists. Here, we report the first biochemical study of a unicellular RTK, namely RTKB2 from Monosiga brevicollis.

Results

We cloned, expressed, and purified the RTKB2 kinase, and showed that it is enzymatically active. The activity of RTKB2 is controlled by autophosphorylation, as in metazoan RTKs. RTKB2 possesses six copies of a unique domain (designated RM2) in its C-terminal tail. An isolated RM2 domain (or a synthetic peptide derived from the RM2 sequence) served as a substrate for RTKB2 kinase. When phosphorylated, the RM2 domain bound to the Src homology 2 domain of MbSrc1 from M. brevicollis. NMR structural studies of the RM2 domain indicated that it is disordered in solution.

Conclusions

Our results are consistent with a model in which RTKB2 activation stimulates receptor autophosphorylation within the RM2 domains. This leads to recruitment of Src-like kinases (and potentially other M. brevicollis proteins) and further phosphorylation, which may serve to increase or dampen downstream signals. Thus, crucial features of signal transduction circuitry were established prior to the evolution of metazoans from their unicellular ancestors.

The revised classification of eukaryotes

This revision of the classification of eukaryotes, which updates that of Adl et al. [J. Eukaryot. Microbiol. 52 (2005) 399], retains an emphasis on the protists and incorporates changes since 2005 that have resolved nodes and branches in phylogenetic trees. Whereas the previous revision was successful in re-introducing name stability to the classification, this revision provides a classification for lineages that were then still unresolved. The supergroups have withstood phylogenetic hypothesis testing with some modifications, but despite some progress, problematic nodes at the base of the eukaryotic tree still remain to be statistically resolved. Looking forward, subsequent transformations to our understanding of the diversity of life will be from the discovery of novel lineages in previously under-sampled areas and from environmental genomic information.

Development of ichthyosporeans sheds light on the origin of metazoan multicellularity

To understand the mechanisms involved in the transition from protists to multicellular animals (metazoans), studying unicellular relatives of metazoans is as important as studying metazoans themselves. However, investigations remain poor on the closest unicellular (or colonial) relatives of Metazoa, i.e., choanoflagellates, filastereans and ichthyosporeans. Molecular-level analyses on these protists have been severely limited by the lack of transgenesis tools. Their genomes, however, contain several key genes encoding proteins important for metazoan development and multicellularity, including those involved in cell–cell communication, cell proliferation, cell differentiation, and tissue growth control. Tools to analyze their functions in a molecular level are awaited. Here we report techniques of cell transformation and gene silencing developed for the first time in a close relative of metazoans, the ichthyosporean Creolimax fragrantissima. We propose C. fragrantissima as a model organism to investigate the origin of metazoan multicellularity. By transgenesis, we demonstrate that its colony develops from a fully-grown multinucleate syncytium, in which nuclear divisions are strictly synchronized. It has been hypothesized that metazoan multicellular development initially occurred in the course of evolution through successive rounds of cell division, which were not necessarily be synchronized, or alternatively through cell aggregation. Our findings point to another possible mechanism for the evolution of animal multicellularity, namely, cellularization of a syncytium in which nuclear divisions are synchronized. We believe that further studies on the development of ichthyosporeans by the use of our methodologies will provide novel insights into the origin of metazoan multicellularity.

The Capsaspora genome reveals a complex unicellular prehistory of animals

To reconstruct the evolutionary origin of multicellular animals from their unicellular ancestors, the genome sequences of diverse unicellular relatives are essential. However, only the genome of the choanoflagellate Monosiga brevicollis has been reported to date. Here we completely sequence the genome of the filasterean Capsaspora owczarzaki, the closest known unicellular relative of metazoans besides choanoflagellates. Analyses of this genome alter our understanding of the molecular complexity of metazoans' unicellular ancestors showing that they had a richer repertoire of proteins involved in cell adhesion and transcriptional regulation than previously inferred only with the choanoflagellate genome. Some of these proteins were secondarily lost in choanoflagellates. In contrast, most intercellular signalling systems controlling development evolved later concomitant with the emergence of the first metazoans. We propose that the acquisition of these metazoan-specific developmental systems and the co-option of pre-existing genes drove the evolutionary transition from unicellular protists to metazoans.

An investigation into plasmolysis in the oomycete Achlya bisexualis reveals that membrane–wall attachment points are sensitive to peptides containing the sequence RGD and that cell wall deposition can occur despite retraction of the protoplast

The structure and function of membrane–wall attachment sites in walled cells, and how these relate to animal focal adhesions, is an area that is poorly understood. In view of this, we investigated how membrane–wall attachments that form upon plasmolysis, respond to peptides that disrupt animal focal adhesions. The degree of cytoplasmic disruption during plasmolysis was also investigated. Upon hyperosmotic challenge, the protoplast in hyphae of the oomycete Achlya bisexualis typically retracted incompletely due to membrane–wall attachments. The inclusion, in the plasmolysing solution, of peptides containing the sequence RGD disrupted these attachments in a dose-dependent manner. In some hyphae, protoplast retraction stopped temporarily at attachment points — upon resumption of retraction, material was left that traced the outline of the static protoplast. Staining of this material with fluorescence brightener indicated the presence of cellulose, which suggests that wall deposition was able to occur despite plasmolysis. The F-actin cytoskeleton was disrupted during plasmolysis; peripheral F-actin staining was observed, but there was no distinct F-actin cap; staining was more diffuse; and there were fewer plaques compared with nonplasmolysed hyphae. Our data indicate that membrane–wall attachment points are sensitive to RGD-containing peptides and that wall deposition continues despite protoplast retraction and F-actin disruption.

Lack of Csk-mediated negative regulation in a unicellular SRC kinase

Phosphotyrosine-based signaling plays a vital role in cellular communication in multicellular organisms. Unexpectedly, unicellular choanoflagellates (the closest phylogenetic group to metazoans) possess numbers of tyrosine kinases that are comparable to those in complex metazoans. Here, we have characterized tyrosine kinases from the filasterean Capsaspora owczarzaki, a unicellular protist representing the sister group to choanoflagellates and metazoans. Two Src-like tyrosine kinases have been identified in C. owczarzaki (CoSrc1 and CoSrc2), both of which have the arrangement of SH3, SH2, and catalytic domains seen in mammalian Src kinases. In Capsaspora cells, CoSrc1 and CoSrc2 localize to punctate structures in filopodia that may represent primordial focal adhesions. We have cloned, expressed, and purified both enzymes. CoSrc1 and CoSrc2 are active tyrosine kinases. Mammalian Src kinases are normally regulated in a reciprocal fashion by autophosphorylation in the activation loop (which increases activity) and by Csk-mediated phosphorylation of the C-terminal tail (which inhibits activity). Similar to mammalian Src kinases, the enzymatic activities of CoSrc1 and CoSrc2 are increased by autophosphorylation in the activation loop. We have identified a Csk-like kinase (CoCsk) in the genome of C. owczarzaki. We cloned, expressed, and purified CoCsk and found that it has no measurable tyrosine kinase activity. Furthermore, CoCsk does not phosphorylate or regulate CoSrc1 or CoSrc2 in cells or in vitro, and CoSrc1 and CoSrc2 are active in Capsaspora cell lysates. Thus, the function of Csk as a negative regulator of Src family kinases appears to have arisen with the emergence of metazoans.

ELMO domains, evolutionary and functional characterization of a novel GTPase-activating protein (GAP) domain for Arf protein family GTPases

The human family of ELMO domain-containing proteins (ELMODs) consists of six members and is defined by the presence of the ELMO domain. Within this family are two subclassifications of proteins, based on primary sequence conservation, protein size, and domain architecture, deemed ELMOD and ELMO. In this study, we used homology searching and phylogenetics to identify ELMOD family homologs in genomes from across eukaryotic diversity. This demonstrated not only that the protein family is ancient but also that ELMOs are potentially restricted to the supergroup Opisthokonta (Metazoa and Fungi), whereas proteins with the ELMOD organization are found in diverse eukaryotes and thus were likely the form present in the last eukaryotic common ancestor. The segregation of the ELMO clade from the larger ELMOD group is consistent with their contrasting functions as unconventional Rac1 guanine nucleotide exchange factors and the Arf family GTPase-activating proteins, respectively. We used unbiased, phylogenetic sorting and sequence alignments to identify the most highly conserved residues within the ELMO domain to identify a putative GAP domain within the ELMODs. Three independent but complementary assays were used to provide an initial characterization of this domain. We identified a highly conserved arginine residue critical for both the biochemical and cellular GAP activity of ELMODs. We also provide initial evidence of the function of human ELMOD1 as an Arf family GAP at the Golgi. These findings provide the basis for the future study of the ELMOD family of proteins and a new avenue for the study of Arf family GTPases.

Innate non-specific cell substratum adhesion

Adhesion of motile cells to solid surfaces is necessary to transmit forces required for propulsion. Unlike mammalian cells, Dictyostelium cells do not make integrin mediated focal adhesions. Nevertheless, they can move rapidly on both hydrophobic and hydrophilic surfaces. We have found that adhesion to such surfaces can be inhibited by addition of sugars or amino acids to the buffer. Treating whole cells with αlpha-mannosidase to cleave surface oligosaccharides also reduces adhesion. The results indicate that adhesion of these cells is mediated by van der Waals attraction of their surface glycoproteins to the underlying substratum. Since glycoproteins are prevalent components of the surface of most cells, innate adhesion may be a common cellular property that has been overlooked.

The evolution of metazoan extracellular matrix

The modular domain structure of extracellular matrix (ECM) proteins and their genes has allowed extensive exon/domain shuffling during evolution to generate hundreds of ECM proteins. Many of these arose early during metazoan evolution and have been highly conserved ever since. Others have undergone duplication and divergence during evolution, and novel combinations of domains have evolved to generate new ECM proteins, particularly in the vertebrate lineage. The recent sequencing of several genomes has revealed many details of this conservation and evolution of ECM proteins to serve diverse functions in metazoa.

Science Signaling Podcast: 1 May 2012

Receptor tyrosine kinases existed in the unicellular ancestors of multicellular animals and underwent diversification in the metazoan lineage.