Hello again. So by now you should know what an ascidian looks like,
how from a fertilised egg it develops into a larva and metamorphoses into an adult.
The diagram summarises the life cycle of solitary ascidians, take a moment to look at it.
However, in this class of tunicates we can find species that also developed asexually
and form a colony of clonal animals, clonal zooids, and in this case we are
looking at colonial ascidians. In colonial species, the fertilised egg that goes
through embryogenesis, develops into a larva and metamorphoses in an adult
forms the first zooid of the colony, which is called the oozooid because of its origin
from an egg, and the oozooid starts to grow - not in size like in solitary species
- but it stays small and only part of the body starts a non-embryonic development
a form of budding, that gives rise to the other zooids.
Different species bud in different ways, and from different parts of the body.
Therefore they give rise to different- shaped columns and here in the slide
you can see some strikingly different examples of colonies.
The nature of the cells and the tissues where the budding process starts is
different in different species, and often they are not homologus.
Some budding starts from epithelia that are either pluripotent or
transdifferentiating into the new zooids. Other budding starts from populations
of mesenchymal cells that are in fact putative stem cells, and other buddings
can have a mix of these two processes.
One of the most studied species of colonial ascidians is Botryllus schlosseri
Botryllus schlosseri has been studied for more than 60 years and it became a model
for developmental biology and regeneration and it's still one of the models where we
have more data about it. In a colony of Botryllus schlosseri each petal is one zooid
and being a tunicate, the colony is embedded in this extracellular matrix,
the tunic. Each petal, each zooid is connected through a network of vessels
where mesenchymal cells are flowing through. The mode of budding adopted
by Botryllus schlosseri is called blastogenesis or parallel budding
or peribranchial budding. The buds start from one portion of one particular epithelium
which is called peribranchial epithelia that sprout from the animals.
The cells are changing shape, they are becoming cylindrical and they form a vesicle.
The vesicle is surrounded by the epidermis of the animal.
When we have this monolayered double vesicle, it's going to be the inner vesicle
that starts the organogenesis and starts the development of the new zooid.
In this species when the bud grows and it becomes an adult, it takes the place of the
parental zooids which in turn get resorbed following a highly regulated wave of apoptosis
In this way, the colony renews and recycles itself on a weekly basis.
In this video you can see a time-lapse that was filmed over one week that shows exactly
this process. Some colonial species can have more than one budding process,
and this is the case of Botryllus schlosseri. If in a colony of Botryllus, all the adults
and the buds get removed surgically by a biologist or by adverse environmental
conditions or by a predator, what is left - the tunic, the blood vessels, and the
mesenchymal cells that are flowing through - they are triggering a process
of all-body regeneration that in a few days regenerates the zooids and eventually
the entire colony. This is what we call vascular budding. In this form of budding
the starting tissue is not peribranchial epithelia but it is a group of mesenchymal
cells that trigger the process they start to proliferate, and eventually they lead to a
double vesicle just like in blastogenesis. So in Botryllus schlosseri, we have three
routes that lead to the same adult body plan. We have embryogenesis and
metamorphosis, we have blastogenesis or parellel budding, and we have vascular
budding, and these developmental pathways, they don't really reinvent
the wheel but they use the same players and they co-opt and they rewire the genes
to lead to the same body plan. Botryllus schlosseri is just an example that shows
how colonial ascidians can have different developmental pathways that start from
different tissues or from different cells and lead to the same body plan.
Also, these different developmental pathways converge in one single stage,
which is the stage of double vesicles that we described before.
The capacity of budding and therefore coloniality is distributed across the whole
subphylum of tunicates. So if we take the phylogeny that Sébastien presented in
the previous chapter, we can look when we have budding species and when we
have solitary species and we can start to plot and infer the evolutions of budding and
coloniality. For instance, if we look carefully at this phylogenetic tree,
we can see that we have six modes of budding that result
from seven convergent acquisitions and one loss.
So, a few take home messages about colonial tunicates, in particular colonial
ascidians: first, from a developmental point of view here we have different ontogeneses
that lead to the same body plan and use genes that are rewired, molecules that
are co-opted, these make tunicates, colonial tunicates, great models to study
the plasticity of developmental mechanisms behind whole-body regeneration and
budding. Then from a more evolutionary point of view, the gains and losses of
budding and therefore of coloniality within the same subphylum and the fact that the
buds arise from non-homologous tissues, suggesting that these gains have been
convergently acquired. This phylogenetic framework allows us to track the
appearance and disappearance of such a complex trait like coloniality in asexual
development and regeneration. Finally, the modular organisation, the connectivity of
the different zooids, and the clonal nature of colonial ascidians and colonial tunicates
they have important ecological consequences. For example the way a colonial animal grows
on a surface, and can compete better with other sessile animals, or the way that
tunicates and ascidians can regenerate after an injury.
So I hope you have learned something from this video, and I invite you to take
the colonial ascidians quiz.
Thank you, my cells that are in fact.