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Evolution and Behaviour Supervisor: Justin Gerlach What are the origins and implications of animal multicellularity? Several lineages of protists, unicellular eukaryotes, became multicellular, to form animals, fungi and plants. It is believed that there was only one origin of multicellular animals from protists, meaning that the Metazoa form a single clade. This is supported by evidence from DNA sequencing and shared derived molecular–level similarities. The origin of animals, defined as multicellular heterotrophs, was believed to have occurred 700-800 million years ago and had consequences for the evolution of characteristics in animal cells. The primary theory for the origin of multicellular animals is the colonial theory; that animals originated from the formation of colonies of genetically identical unicellular organisms. Choanoflagellates, single celled organisms with an apical flagellum and a collar of microvilli that can group into colonies, are believed to have the protists that went on to form the first multicellular animals as a common ancestor. They are the closest living outgroup to the animals, as indicated by phylogenetic trees constructed using the sequence of bases in genes from several species. (the development of a Choanoflagellate Salpingoeca rosetta colony Source: http://www.hhmi.org/research/choanoflagellates-and-origin-animals ) The way that Choanoflagellates develop into colonies gives an indication of how multicellularity could have arisen. Choanoflagellate colonies, the Proterospongia, develop through mitosis and then a failure to separate, instead of aggregation of genetically distinct organisms, so all cells in Proterospongia have the same genetic information, as seen in multicellular organisms. The Choanoflagellate cell structure is very similar to the Choanocytes of multicellular animal sponges (Poriferans), the cells which set up filter currents, suggesting that the Choanoflagellates are very close relatives to the sponges. This has been confirmed by genome sequencing, and has provided further support to the theory that Choanoflagellates descend from the unicellular ancestors of animals. There was some opposition to this theory; with the possibility that Choanoflagellates are simplified forms of Poriferans, but this was disproved by phylogenetic evidence. Sponges are basic multicellular animals, since they do not have fixed cell specialisation, and have the lowest grade of organisation amongst Evolution and Behaviour Supervisor: Justin Gerlach the animals with no organised tissues or pervading symmetry. The sponges are believed to be the basal group of metazoans from which all other animal groups emerged, since according to both molecular and morphological data sponges were the earliest diverging branch in the animal tree and the earliest identified animals in the fossil record. There are alternative theories for the origin of multicellular animals, such as the symbiotic theory; that multicellularity originated when unicellualr organisms of different species with different roles cooperated, and evolved to become dependent on one another until their genomes were incorporated into one multicellular organism. However, there is no know mechanism for the incorporation of the genomes to form a single species. The colonial theory is the one best supported by evidence and without significant flaws. Although the protists from which multicellular animals evolved already had many characteristics found in Metazoa; motility using cilia and flagella, organised cytoskeletons, receptors to enable coordinated responses to external stimuli and the ability to have sexual life cycles, there were other requirements for multicellularity. Therefore the implication of the origin of multicellularity was the evolution of additional characteristics that met these requirements. There was a requirement for the ability of cells to stick together and form a supporting matrix to maintain tissue organisation. The result of this requirement was the evolution of additional cell adhesion molecules, for example cadherin; molecules that traverse the cell surface membrane and are anchored to the cytoskeleton. Specialised cell types were required in order to increase efficiency by the division of labour, to allow the larger multicellular organisms to survive given that they have a smaller surface area to volume ratio than single celled organisms. Cell specialisation also resulted in a distinction between inside and outside layers, and the separation of the germ line and the soma. In order to be specialised different cells express a different complement of proteins, which results in different cell structures due to the shape of the cytoskeleton, and the ability to carry out different functions. Cell specialisation resulted in a far greater degree of complexity than that seen in unicellular organisms. Although some specialised cell types originated by the evolution of novel functions, more often new cells types arose by the evolution of multiple cell types from a single multifunctional cell, for example in the evolution of different cell types from a multifunctional photoreceptor cell from a box jellyfish. The cell has a photosenstivie membrane and shading pigment to make it direction sensitive and is involved in phototaxis using a motile flagellum, and evolved into separate specialised cells, with a shading pigment cell, photoreceptor cell, nerve, and locomotor ciliated cells. The different functions of the original cell are carried out by distinct cells. In order for specialisation to be effective embryogenesis is required so that the right type of cells are in their correct location. Additionally in multicellular organisms there is a requirement for cell communication for coordination of activities. In order to display these additional characteristics more genes were required to regulate the multicellular animal functions of specialisation and differentiation, cell adhesion, communication and cooperation, and embryogenesis. This can be observed in the number of genes present in multicellular animals compared to Evolution and Behaviour Supervisor: Justin Gerlach unicellular organisms. For example the yeast S. cerevisiae contain roughly 6,200 genes, while nematode worms Caenorhabditis elegans have approximately 19,000. However, the number of genes for core biological functions such as protein synthesis and metabolism is similar between unicellular yeast and animals at 2,500. The source of the genes involved in functions specific to multicellular organisms was largely from the expansion and diversification of certain gene families. An example of such a gene family is the Homeobox genes, which code for Homeodomain proteins, a transcription factor family. They are involved in control of specifying cell types, embryogenesis and body organisation. In yeast there are 8 homeodomain genes, compared to 93 in nematode worms, and around 400 in humans. The Homeobox genes expanded by gene duplication and sequence divergence to generate many distinct subfamilies in animals. Specific Homeobox gene subtypes have been identified to carry out similar functions in distantly related animals, suggesting the expansion of the gene family occurred soon after the origin of Metazoa. Another family of genes expanded in animals are those that encode cell-cell interaction proteins, such as the immunoglobulin superfamily; for example the Fibronectin III domain has 2 genes in yeast compared to 55 in nematode worms. The diversification of the different gene families and so the origin of the genes that enabled different characteristic of multicellularity occurred at different points in the evolution of animal multicellularity. For example 23 Cadherin cell adhesion protein genes were found in Choanoflagellates, in comparisoin to 17 in Arthropods, suggesting that some of the genes were lost after the origin of Metazoa in the Arthropod lineage, whereas there are 127 Cadherin genes found in Vertebrates indicating expansion after the Arthropods and Vertebrates had diverged. This also indicates that the cell adhesion molecules originated before multicellularity, hence the diversity of their genes in the Choanoflagellates, which is likely to also have been the case in their common ancestor to animals. However the Homeobox genes are consistently more numerous in Metazoa than in the Choanoflagellates, with only 2 identified in these protozoa in comparison to 31 in Poriferans and 177 in Arthropods. In conclusion, the origin of animal multicellularity is described by the colonial theory; that Choanoflagellate-like unicellular organisms formed colonies that evolved into true multicellular organisms. The implication of this event was the evolution of multicellular characteristics including cell specialisation and embryogenesis. However, whether the acquisition of the genes for these characteristics was a consequence of multicellularity, or causal its evolution is uncertain. Either there was a selective advantage to the diversification of these gene families as it improved the functioning of the multicellular organisms, or they diversified in advance of multicellularity in order to cause it. Equally, it may have been a combination of the two cases, which is indicated by the presence of diverse cell adhesion genes in Choanoflagellates, but far fewer Homeobox genes. Evidence from genome sequencing of the Choanoflagellates and early animals indicates that it is likely that there was a gradual accumulation of the genes required for multicellularity, with multicellular functions and characteristics evolving at different points in time. Evolution and Behaviour Supervisor: Justin Gerlach REFERENCES Lectures ‘Animal Diversity’ Michael Akam King et al. Nature 2008 Baurain D, Brinkmann H, Philippe H. 2007. Lack of resolution in the animal phylogeny: closely spaced cladogeneses or undetected systematic errors? Mol. Biol. Evol. 24:6–9 Botting JP, Butterfield NJ. 2005. Reconstructing early sponge relationships by using the Burgess Shale fossil Eiffelia globosa, Walcott. Proc. Natl. Acad. Sci. USA 102:1554–59 The Origins of Multicellularity and the Early History of the Genetic Toolkit For Animal Development Annual Review of Genetics (2008) Vol. 42: 235-251 Chervitz SA, Aravind L, Sherlock G, Ball CA, Koonin EV, et al. 1998. Comparison of the complete protein sets of worm and yeast: orthology and divergence. Science 282:2022–28 Margulis, Lynn (1998). Symbiotic Planet: A New Look at Evolution. New York: Basic Books. p. 160. ISBN 978-0-465-07272-9.