WEBVTT
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In the first video we mentioned Trichoplax
adhaerens and on the side we mentioned
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there's another two species
just recently described.
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But we know from genetic evidence
there's probably more than a dozen
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more than 100 species out there,
possibly. They just look all the same.
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We need genetics to identify them and we
need new tools to assign them to a species.
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So if we collect here at Banyuls in this
water, first, chances are we're gonna
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find Trichoplax adhaerens. But there's
a good chance we'll also find another
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species or even two or three more
species. We just don't know yet.
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There are some regions like the Mediterr
-anean sea that are very well-sampled.
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And there's other regions they have
not been sampled at all. For example,
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South America and all the coast of
Africa have not been sampled at all yet.
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So the biodiversity is unknown, please
when you go someplace to temperate
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or warm oceans, take some rocks with
biofilm and send them to our lab.
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So as I just said the biodiversity
we expect from placozoans
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derives from genetic data.
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Morphologically the most of them look
exactly the same, it's only the genetics
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that are completely different.
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But we don't have a tool
yet to assign species.
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The way we have to do it is we have to
combine morphological data, ecological data,
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behavioural data with genetic
data to assign species.
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And this genetics will require a
lot of efforts for the future.
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For now we base our systematics of
placozoans on simple markers like 16sRNA
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for example, so we have 16sRNA trees
showing a huge diversity within placozoans
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showing huge genetic distances between
different species in clades. In other words
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we don't only have different species of
placozoans - we also have different genera
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different families and probably
different orders
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of placozoans within the phylum Placozoa.
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While all placozoans are looking almost
identical, there's one huge exception
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and that is Polyplacotoma mediterranea,
and as the name says, Polyplacotoma
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it branches off in different arms
and is not just a plate
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It's more. It almost looks like growing
tentacles in all different directions.
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This species comes from rocks in the
Mediterranean where there's high current
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and so breaking waves, and this species
obviously can withstand the force of water
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of breaking waves, high water velocities.
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It's very hard to culture in the lab.
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And another most exciting
thing about this species is
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it looks to be the sister species
to all other placozoans.
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So this might be the ancestral placozoan
and we have sequenced the whole genome
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It will be exciting what this species can
add to understanding of the genetics and
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the evolution of placozoans
and the role of placozoans
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for the evolution of metazoan animals.
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And one of these transitions we're
gonna see is the Placula Hypothesis
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we have put at the end of this chapter.
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So most morphologic and genetic
evidence points to the Placula Hypothesis
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the idea that placozoans or the placula are
at the very beginning of Metazoan evolution
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So if we look at the side now of a
Trichoplax we see in the red line
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the region where we express genes
that are the animal's polarity.
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So top-bottom polarity originating
from the cells in the red area here.
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And if we look at Trichoplax now, the
feeding behaviour we have seen before
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just let's imagine Trichoplax is feeding,
forming its extracorporeal feeding cavity
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so it lifts up the centre of its body and lifts
it up more than it would do for feeding
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in an evolutionary timespin now
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and what we get is a bell shape, and if
we imagine that we need extra structures
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for a new bauplan, well one way to do it,
we could just double the regulatory genes
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the parahox gene trox-2 in
this case, for example.
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and then we might get
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uh ready to form extra structures
now that we have doubled the
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gene, we have two
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parahox genes now and one sets up polarity
the other sets up tentacles in this case
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So now we have a symmetrical bauplan,
the radial symmetric bauplan
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it's the typical cnidarian bauplan, so this
would be the most parsimonious, easiest way
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to derive the cnidarian
bauplan from the placula.
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So we have seen how we could derive the
cnidarian bauplan from a placozoan bauplan
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from the placula. But how do we derive
the sponge bauplan, the poriferans?
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Well they look completely different, they
lost all their symmetry. On the beginning
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it would be the same, we have a placula
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we have the top-bottom orientation,
we have the polarity
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we have in the middle in the red line
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the gene expression of the polarity
gene, maybe trox-2
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we have the placula and the placozoans
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lifting up the centre of the body for
feeding, we do the same thing here
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as you have seen before, we lifted up, we
formed the extracorporeal feeding cavity
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we get this bell-shaped bauplan.
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With this bell-shaped bauplan we keep
going as we have done it via the cnidarians
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but now in sharp contrast to the cnidarians
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sponges, they don't have any symmetries,
they don't build any other organs.
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So what we will do here, we just get rid of
the polarity gene, we don't need it anymore
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We don't need extra organs, we don't need
polarity anymore. We don't need symmetry,
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so we can just get rid of it. And
if you just get rid of this gene
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all we need to do is punch some holes
into the bell and if we do this, well
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we more or less have a basic
sponge bauplan and it will be
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the easiest way to explain the deviation
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of a sponge bauplan from
a placozoan bauplan.
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So that's one way to think about
deriving sponges from the placula.