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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
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Botting JP, Butterfield NJ. 2005. Reconstructing early sponge relationships by
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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.