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Welcome to the chapter on
sponge and animal phylogeny.
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In this chapter we are going to talk about
the fact that sponge phylogeny remains a
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very disputable subject and we're going to
talk about the fact how important it is for
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people interested in evolution of animals or
evolutionary origin of animals to understand
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what really is the phylogentic
position of sponges.
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We are going to start by going back to
Ernst Haeckel's lament from 1870 when
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he said that it is really amazing how much
doubt there is to the true nature of sponges,
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and as I said at the introduction when Ernst
Haeckel talks about the most recent times
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while he writes it in the 19th century,
here at the beginning of the 21st century
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we are still actually feeling as
much in doubt as Haeckel and his
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colleagues felt 150 years ago.
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So why is that? Well, to be quite
honest, Haeckel wasn't in doubt.
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He looked at sponges, he looked at corals,
cnidarians about which we have learnt in
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the other modules, and he said that it's
actually very simple: if you compare those
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two body plans, you are going to find
that there is fundamental agreement of
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the sponges and corals, that they
are always built from two layers,
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the endoderm and the ectoderm.
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And he also mentioned that there is this
major opening at the apical part of the body,
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which is the osculum in sponges
and the mouth in the coral polyps
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and quite recently we are not very-
in recent times we are not very much
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disputing the fact that the innermost layer
of cnidarian polyps is homologous to the
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endoderm and the mesoderm of other
animals. So the transition between cnidarians
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and bilatarians with this example
of a vertebrate shark, here, is quite
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easy to understand, but the disputable
part is the relationship between sponge
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and cnidarian body plans.
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So,
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Haeckel wasn't confused at all. He said
sponges and cnidarians basically have
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the same body plan. But Saville Kent,
another naturalist from that time,
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looked at the cell types, and especially
at the choanocytes that we have talked
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about in the previous
chapter and he said, look
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choanocytes and choanoflagellates look
so similar. He quoted his colleague
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James Clark who a few years earlier said
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that there is discovered choanoflagellates
and talked about the collars and the flagella
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and about the similarity of choanocytes
and choanoflagellates, and Saville Kent
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in 1880 said that his interpretation of
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animal and protistan relationships is
completely opposite to what is advocated
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by Professor Haeckel.
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So what Saville-Kent following
James Clark believed
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is that sponges in fact are protists.
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He said that by process of evolution there
is substantial reason to presume that
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sponges are primarily derived
from choanoflagellates.
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So this is a very different view.
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He didn't think sponges were closely
related or aligned in any way
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with cnidarians, which he believed
would be closely related to other animals.
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He thought that sponges are an independent
lineage of multicellularity derived straight
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from choanoflagellate-like ancestors.
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Now, this was the 19th century. Surely
we have come a long way from this time.
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Well, in the end of the 20th century people
started building phylogenies using individual
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genes and what has been found by Wainright
and Cavalier-Smith and their colleagues is
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that choanoflagellates clearly are the nearest
relative to animals, so that was important.
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But also they found that sponges are indeed
closer related to animals than they are to
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choanoflagellates, so the established
view from the end of the 20th century
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Is that sponges form animals together with
all those other non-sponge animals and
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choanoflagellates are the sister group, the
nearest relatives of animals that are not animals.
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Does that mean that the choanoflagllate
and choanocyte similarity doesn't matter?
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Well, it probably matters because it suggests
that if sponges are the first animals
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to evolve, then perhaps the cell
type, the choanocyte cell type
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is directly derived from the choanoflag-
ellate situation, but it seems that this
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is not something that we
can really take for granted.
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So when people started building phylogenies
built on whole genomes from individual
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genes, they produced what I like to
call a forest of phylogentic trees.
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So for example, Casey Dunn and colleagues
in 2008 published the very interesting paper
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that steered a lot of our thinking about
animal evolution, suggesting that in fact
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it is not the sponges that are the first
animals that have evolved or they are
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not the first extant lineage of animals,
but the ctenophores in fact are the
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sister group to all other animals, and
sponges evolved later than ctenophores.
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Remember ctenophores are rather complex
animals, quite similar to cnidarians with
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muscle and a nervous system, so that
was a bit disconcerting for a lot of us.
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But quite soon after that, there was another
paper that said something completely different.
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Well, they didn't address the position of
ctenophores, but they said that sponges
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are not monophyletic. So those calcareous
sponges like Sycon ciliatum and siliceous
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sponges like for example Halisarca that
we talked about, they are independent
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lineages, with the calcareous sponges
being closer related to other animals
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than the siliceous sponges. Now that was
really interesting and completely different
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to what we thought before.
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And again in 2015 there was a paper
by a group including David Pisani,
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that was published in another high-profile
journal that said actually ctenophores are
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not the sister group to all
animals, sponges are.
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But of course we didn't wait long for another
paper in 2017 that said actually ctenophores
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are the sister group to all other
animals and sponges evolved later.
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And then there was a bit of a back and forth.
Turns out that while the majority of recently
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published genome-based phylogenies
suggest that sponges are the sister group
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to all other animals, and ctenophores
evolved later, this doesn't appear to be
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really established with 100% certainty.
But you might ask, why would we care?
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Does it matter if sponges are
monophyletic or paraphyletic?
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Does it matter if ctenophores
evolved before sponges?
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Well, it does as you can see on the
example of those three different trees.
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So there are just three trees here that
I have selected because they tell very
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different stories, but there's a number
of other trees that are highly supported
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by recent literature. So
let's look at this tree.
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What does it tell us about
animal evolution?
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Well, if ctenophores were the
first animal lineage to evolve,
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then it either means that their nerves
and muscles evolved independently
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from all the other animals, which is rather
interesting, or that sponges and also
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placozoans had nerves and muscles, or
their ancestors had nerves and muscles,
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but they were later lost during evolution.
So when we think about sponges and
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having those ancestral features,
actually they don't because they
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are secondarily simplified.
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It also means that we have no
idea how first animals looked like.
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Unlikely they looked like ctenophores,
which are complex predators, but we
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basically are left with no understanding
of how first animals looked like.
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How about this tree? Well this is a tree in
which sponges are paraphyletic, meaning
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that you would have to include all other
animals with the sponges to be able to
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talk about the monophyletic sponges.
In yet another way, you can think of
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all animals being derived sponges.
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In this, if this tree was true,
it would mean that the first animals
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must have looked like sponges. It basically
means we are all sponges, just much more
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complex than other sponges, and that
the evolution of the complex cell types
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and organs, nerves and muscles
and brains happened only once.
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But what can we think about
animal evolution if this tree is true?
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Well, in this tree we have sponges being
monophyletic, meaning they share an
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ancestor that was not shared
with all the other animals.
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And then you have placozoans that are
arriving later and then ctenophores being
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close to cnidarians and
then further on bilatarians.
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So in this tree we have nerves and
muscles evolving only once, here at
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the base of bilatarians and coelenterates
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and then the first animals might have looked
like sponges but they didn't really have to.
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Can I tell you which of those trees is true?
I can tell you that I kind of think that this is
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probably the most likely tree, but I strongly
suggest that you keep your eyes open
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at the recent literature and make sure
you understand that while a lot of others
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seem very convinced that they found
the true tree, a couple of years later,
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or a couple of months later,
there might be another paper.
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So keep your eyes open for the recent or
newly emerging literature, because this is
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an exciting topic that is really
telling us or has potential to tell us
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a lot about our own ancestry.
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Now
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we know that sponges are
interesting and important,
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but we also noticed that
sponges are very diverse,
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so one of those features of those phylogenies
is that more and more we are convinced that
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sponges are monophyletic, meaning
they share a single ancestor,
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but also that those relationships within
sponges are shifting from time to time.
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Now the tree of sponge classes that I like
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and I expect that this one will remain as
a stable tree, something that people would
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continue believing is this one. In this tree
we have demosponges and hexactinellids
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together. Demosponges include Amphimedon
queenslandica, a very important sponge model
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system, and Hexactinellids or glass
sponges, we will talk about that in a
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moment, and then there is another pair
of sponge classes, Homoscleromorphs
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with Oscarella lobularis, another important
developmental biology model, and the
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calcareous sponges, about which we've
been talking a lot during this module.
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So how are those sponges characterised?
The demosponges generally have siliceous
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spicules, some of them have an organic
skeleton, and some of them have only
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an organic skeleton rather than spicules.
They have evolved into a variety of
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different body forms, shapes, and sizes,
and some even evolved to be carnivorous.
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So instead of being filter feeders,
they capture larger prey
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using modified spicules. There are some
freshwater demosponges that are greatly
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successful and present across the Earth
and they are very diverse and the most
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species-rich class of sponges.
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Hexactinellids that are closely related
to them have siliceous spicules as well,
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but they are very different than all the
other sponges and majority of other
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animals in that, while
they are products of
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almost usual embryonic development with
cell division and so on, the larvae become
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syncytial and adults are syncytial,
meaning that they don't have individual
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cells, but they form multinucleated
large cells that are forming their bodies.
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They are often deepwater and therefore
they are rather difficult as model systems.
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Now Homoscleromorphs are also
characterised by siliceous spicules.
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Some of them do not have spicules at all,
but those that have spicules have siliceous
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spicules, and some of them are very simple,
they have extremely simple body plans
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built only of epithelial cells.
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Finally, the calcareous sponges, which is
my favourite group, have calcareous spicules.
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They are built in a very similar way to the
siliceous spicules of Homoscleromorphs,
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meaning they are very simple, but
they are built of calcium carbonate,
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and they have all the different body plans,
meaning asconoid, leuconoid, and syconoid,
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despite the fact that they
are rather a small class.
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Now
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there is a variety, as there is a variety of
body plans of adults, there is also a variety
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of larval types in sponges. The most
diverse types of larvae are present in
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the largest group, in the demosponges,
and this is a beautiful, bullet-shaped
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parenchymella larva of Amphimedon. The
larvae of Hexactinellids are called trichimella
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There are very, very simple tiny larvae of
cinctoblastula, called cinctoblastula larvae
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of Homoscleromorph sponges, and in
calcareous sponges there are two different
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types of larvae, calciblastula, which are
similar to cinctoblastula, and amphiblastula
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that are characteristic to calcaraneon
sponges. They're called amphiblastula
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because they appeared to be built of
two halves, hence the term "amphi",
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the anterior part that is built of ciliated
cells and the posterior part that is built
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of non-ciliated larger cells, and as they
are one of the most simple larval types
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in sponges, we're going to be talking
about how those larvae form during
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embryonic development in the next chapter.