The Genetic Origin of Animal Multicellularity
The evolution of multicellular organisms from single-cell organisms is a startling turning point to understand major evolutionary transitions in developmental processes. Almost all animals develop from a single-celled zygote to a complex multicellular adult. The advantage of multicellularity is that cells can cooperate to perform particular biological roles. During embryonic development, genes within certain tissues need to be activated at precise times during development, which is called spatiotemporal regulation in order to assign cells to a specific role or fate. Studies carried out to date have shown that the genetic toolkit for animal multicellularity needs a set of gene families mainly involved in cell adhesion, cell signalling, and transcriptional regulation. The specific expression of these sets of genes on the generation of several differentiated cell types gives them unique roles for transition to multicellularity. So the question is obvious; what is the genetic basis of animal multicellularity and development? The study of gene families with key roles in multicellularity in unicellular relatives will be critical for understanding the genes’ ancestral functions and their co-option into the multicellular developmental program.
During the last decade, genome sequencing of several of the closest unicellular relatives of animals allowed a systematic reassessment of the uniqueness of the metazoan genetic toolkit. So far, the genome of four choanoflagellates, one filasterean, six ichthyosporeans and one corallochytrean have been sequenced, providing a taxon-rich evolutionary perspective. What came as a surprise was to discover that these species already had a considerable repertoire of genes and gene families involved in cell adhesion, cell signalling, and transcriptional regulation shared withanimals, including several genes that had previously been thought to be animal specific. These genomic resources demonstrated that the last common ancestor of animals must have inherited many of these genes, as they evolved in a‘unicellular’context.
Fundamental aspect of animal multicellularity: cell adhesion
The adhesion of animal cells to their neighbors and the extracellular matrix is a fundamental aspect of animal multicellularity. A few major classes of genes such as the cadherins, the integrins, the selectins (e.g., C-type lectins), and the immunoglobulin superfamily (e.g., fibronectin type III do-mains) play a key role in mediating adhesion in animal cells. Examination of the choanoflagellate proteome suggests that the gene machinery participating in adhesion in animals was likely well developed in the unicellular ancestor of animals and choanoflagellates. Most of the domains typically found in animals are present in choanoflagellates, including those of cadherins, C-type lectins, immunoglobulins, andαinte-grins. However, what is the function of such a diverse set of adhesion molecules in a unicellular organism that is not known to form cell-cell connections. Examination of the extracellular localization of two choanoflagellate cadherins reveals their presence, and colocalization with actin, at the organism’s apical collar. The choanoflagellate collar serves as a food-catching device onto which bacteria are latched and transferred toward the cell, raising the possibility that the origins of this major animal cell adhesion gene family may lie in molecules originally invented for prey capture. Several genes participating in the formation of the extracellular matrix are also well conserved and predate animal origins, including collagen, laminins, and fibronectins. Perhaps the most spectacular example of the deep, preanimal origins of some of these gene families is offered by collagens, the most abundant protein family in the mammalian body), homologs of which are found not only in choanoflagellates, but also in the animal sister kingdom, the fungi. However, integrins, one of the major receptors of collagen, are not found in fungi. Furthermore, whereas in animals integrins are functional as heterodimers constructed out of α and βsubunits, the choanoflagellate genome contains only α integrins. This finding suggests that the interaction between integrins and collagen in choanoflagellates may differ from their interaction in animals, and that its study may yield important insights about the evolution of animal cell adhesion to the extracellular matrix. A good example of a coordinated morphogenetic process in unicellular holozoans is observed during the multinucleate coenocyte stage of ichthyosporeans. During coenocyte formation,an initial mononucleated cell divides its nuclei multiple times without cell division, forming a coenocyte. When the coenocyte is mature, each nucleus is encased within an individual cell in the process of cellularisation, eventually creating a transient polarised epithelium-like layer. This process is very similar to the syncytial blastoderm stage in some arthropod embryos such as Drosophila. However, it is very unlikely to be a homologous process between ichthyosporeans and animals, ascoenocytic development is infrequent in animals and not likely to represent an ancestral state. Despite this, we can still find traces of homology in these processes. A detailed immunohistochemical and transcriptomic analysis of ichthyosporean development demonstrated that the cellularisation process involves a coordinated assembly of an actomyosin network with inward plasma membrane invaginations, asoccurs in animals. Moreover, after cellularisation, durig the transitory stage of a clonally generated polarised cell layer resembling an animal epithelium, there is an upregulation of genes activated in cell adhesion in animals. Many of the actomyosin network components or adhesion genes are older than holozoans (Arp 2/3 complex, myosin II or integrins), thus these genes likely had ancestral roles in amoeboid cell movement later deployed in ichthyosporean cellularisation.
The way of comminication in multicelullarity: cell-cell signaling
Cell communication is critical for the generation and maintenance of multicellularity in animals, and a handful of core signaling pathways, such as nuclear hormone receptors, Hedgehog, Wnt, TGF β,Notch, and receptor tyrosine kinases, are involved in its materialization. In contrast to the preanimal origin of most of the gene machinery associated with cell adhesion, the origins of signaling pathways were an animal innovation. Several of the pathways (e.g., Wnt and TGFβ) are absent from choanoflagellates, although they appear to be present in early-branching animals. Perhaps surprisingly, Wnts exhibit remarkable gene family complexity in early-branching animals; the cnidarian Nematostella vectensis contains gene representatives for at least 11 of the 12 recognized Wnt subfamilies. This complexity of Wnts in early-branching animals argues for anepisodic, pan-animal origin of this gene family, although the sudden increase in complexity may be an artifact of the lack of thorough sampling for these genes in placozoans, poriferans, or ctenophores. Nonetheless, distinct domains of certain pathways are discernible in the choanoflagellate genome (e.g., Notch, Hedgehog, and MAPK), suggesting that animal signaling molecules may have evolved, at least partially, through the shuffling and co-option of pre-existing domains. The evolutionary origin of the Hedgehog protein offers a telling example of the likely importance of this process and its potential role in the genesis of the genetic toolkit. Bilaterian Hedgehog proteins are composed of two domains, aptly known as the hedge and the hog. Choanoflagellates have only the hog domain, whereas poriferans and cnidarian proteomes contain both domains but as parts of distinct proteins, suggesting that the Hedgehog protein likely first evolved through domain shuffling in an early animal ancestor.
Regulation at transcriptional level: transcriptional regulators
Transcriptional regulation is of crucial importance in the manifestation of animal multi-cellularity and development. Here is where the protist heritage of the choanoflagellate proteome is most fully exposed, as its proteome contains the standard set of transcription factors (TFs) observed across eukaryotes, with most of the well-known animal TFs absent. In contrast, examination of the proteomes of early-branching animals shows an appreciable increase in TF family complexity, with both poriferans and cnidarians containing several representatives of the Fox, T-box, Paired, and POU families. However, transcription factor family complexity among early-branching animals is not equal; cnidarians are qualitatively (e.g., Hox class homeobox genes are present only in cnidarians) and quantitatively more complex relative to poriferans and placozoans. Further examination of the proteomes of early-branching animal phyla is likely to be crucial in understanding the origins of animal transcription factors.
The examination of the relatives of animals of several multicellular lineages has already identified several important molecular trends associated with transitions to multicellularity. The insights that we have obtained from these examinations provide a way to think about animal embryonic development. Protistan holozoa do not go through embryogenesis but nevertheless display developmental processes,using molecular tools and morphogenetic processes that resemble those of animals. Similarly, they have cell types without complex multicellularity. Furthermore, similar differentiation mechanisms are used to transition from one cell stage to another, and each of these life stages can have a specialised function. The advances in the experiments associated with transitions in individuality, the genetics of animal development have provided an important process to understand the origin of multicelluarity. The origins of some of the gene machinery that makes us multicellular can be found in our unicellular relatives, but there are still questions which need to be investigated more: how did it get there in the first place and what was its original function? How are we to reconcile the conflicting evolutionary scenarios of relationships among early-branching animals with the genesis and early evolution of the genetic toolkit? Was the genetic toolkit causal in the evolution of animal multicellularity or simply its product? What was the relative contribution of extrinsic (ecological and environmental) and intrinsic (genetic) factors in the origins of animal multi-cellularity ?
This is a review of several papers. Please check these papers if you want to learn more about animal multicellularity;