Download How has animal multicellularity evolved? The quest for the origin of

How has animal multicellularity evolved?
The quest for the origin of animal multicellularity has enormous appeal – but it is not
an easy puzzle to solve. The fossil record is of little use: molecular clock estimates place the
event at 800Ma and not only are rocks of this age very rare, but the chance of the first
multicellular organisms being preserved is minute. We may need to draw parallels with
other multicellular organisms in the tree of life. And not only do we need to consider how it
could occur on the functional/developmental level but we must also try to explain the
selection pressures that drove the evolution of multicellularity in the first place.
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What sort of organism are we talking about here? Best candidate: choanoflagellatelike protist. Outgroup to metazoa. Very similar to the choanocytes found in the most
basic animals - the sponges - reinforcing idea that similar protists played a key role in
early animal evolution. Importantly, some colonial forms e.g. Proterospongia exist.
Advantages of becoming multicellular (it has evolved 25 times independently):
o Avoiding predation e.g. study with unicellular algae Chlorella vulgaris – in
<100 generations in presence of predator, they evolved to clump together into
8-cell clusters (trade off between avoiding predation while maximising
nutrient uptake).
o Can have differentiated cell types for specialised functions
o Spatially/temporally separate different metabolic activities to improve
efficiency of food consumption
o During periods of starvation, if one cell in the group dies it can sustain the
others until conditions improve
o All these must help propagate genes into future generations – important
question is why would certain cells sacrifice themselves (as somatic cells) ‘for
the good of the whole individual’ while only a few cells (the germ cells) will
pass on their genes. The paradox is explicable by kin selection: presumable
the cells are derived from a single parent and will share at least half their
genes (depending on whether choanoflagellates are sexual or asexual).
Several obstacles need to be overcome. Altruism is susceptible to defection. ‘Conflict
mediators’ need to be employed. Might simply be a high degree of kinship and low
mutation rate, or could be more directed, involving parental control over cell
phenotype (to prevent too many daughter cells becoming germ-line cells) or a system
involving apoptotic pathways to control neighbouring cells.
Cells need to be able to adhere to one another. Can be via extracellular proteins such
as collagen, or with cell adhesion molecules (CAMs) such as cadherin. Interestingly,
choanoflagellates possess the same family of cadherin genes as the basal animal
phyla. Cadherin’s role in the protists is not clear but may be involved with capturing
bacteria for food. Cadherin adhesion complexes may have been a pre-adaptation that
facilitated the later evolution of multicellular animals.
Cell-cell communication: gap junctions are the main method in animals, where
signalling molecules are transmitted from cytoplasm to cytoplasm
Means of achieving differentiation. Probably through use of transcription factors.
Modifications of DNA-binding regions and promoter sequences can bring about
significant changes in cells quite easily. Comparative studies have shown that
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multicellular organisms possess many more genes encoding transcription factors
than their unicellular counterparts, supporting their significance.
One of the most important types of transcription factors are homeobox genes. They
produce specific transcription factors with a characteristic homeodomain and
recruitment of such genes will have been vital for creating a general organisation and
body plan. Later appearance of Hox genes in the diploblasts and their duplication
and neofunctionalisation enabled more complex body organisations to evolve.
Under certain pressures it is conceivable that a choanoflagellate-like protist would
benefit from staying in a group with its siblings, perhaps caused by altering the use
of cadherin genes, which may be achieved by modification of a transcription factor.
o Now protected from single-celled predators, the cells would have a higher
fitness than their single-celled counterparts.
o However each cell would be trying to maintain fecundity and reproduce
whilst also trying to move and catch food – not a very efficient system. Here
kin selection would facilitate reproductive altruism and cell specialisation
into a germ line and a soma.
o More complex functions - involving recruitment of other genes for cell
signalling, and other transcription factors (including homeobox genes) for
more complex body organisation and differentiation - could then develop.
Finally, we have a multicellular organism we might tentatively call an animal.
o Small colonies may not have much problem with motility and so groups of
undifferentiated protists, as we see in Proterospongia, would also be viable.
From this beginning, it is not difficult to see how very simple body plans like the
Placozoa could arise – simply sheets of cells capable of crawling along the sea bed.
This in turn could have preceded other primitive animals seen in the Ediacaran
fauna of the late Precambrian such as Dickinsonia.
Another lineage may have remained pelagic, and could have given rise to other basal
phyla such as the Cnidaria and Ctenophora.
Subsequent evolutionary arms races probably had a key role in propelling animal
evolution forwards into the Palaeozoic era, to generate the sudden diversity that
seems to appear in the ‘Cambrian Explosion’.
We are still a long way off from a comprehensive understanding of how animal
multicellularity has evolved. I expect we shall learn a lot more in the near future as wholegenome sequencing becomes cheaper and easier, and better comparative studies between
the metazoa and our sister groups become feasible. From this we will gain a deeper
understanding of the genetic toolkit responsible for one of the most significant transitions in
evolutionary history.