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In the previous chapter you have
 seen what tunicates look like


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 in their natural environment, and 
you will see that their position 


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 in the tree of life has
 changed quite a lot.


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I will show you that they're our 
closest invertebrate relative.


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 Most people are not familiar 
with tunicates, which have a 


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somewhat unusual body organisation.

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Looking at this picture, people 
 may say "oh, it's a plant"


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"oh, it's a sponge", or better
"it's a sea anemone".


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But then if you tell them they might 
 have seen tunicates at a restaurant,


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eating seafood, they would say
 "oh yeah, they are mollusks!"


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And you see in these pictures, there are 
three dishes from different countries 


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where tunicates are eaten as a delicacy.

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Calling them mollusks actually fits with their
status from antiquity to the 19th century.


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 For example, Linné in the 18th century
 and Cuvier in the 19th century


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classified tunicates among 
mollusks as "shell-less bivalves".


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 It is only at the end of the 19th century
 that tunicates' position changed radically.


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In 1866, Alexander Kowalevsky carefully 
examined the embryonic development


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 of ascidians and pointed to a 
strong affinity with vertebrates.


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By careful examination, he discovered 
the existence of a notochord and 


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 a dorsal neural tube very 
similar to those of a frog. 


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This essential description allowed him to
definitely consider the chordate phylum 


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that comprises vertebrates, tunicates, 
and cephalochordates. 


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 All these animals have in common the
existence of a notochord and a dorsal 


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neural tube during embryonic life.

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You can see now on this tree of the 
Metazoans how the position of tunicates


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 has radically changed in the 19th century.

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Given the "living fossil" features of amphioxus
that you have seen in the dedicated module,


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 and the fact that other tunicates
 have a divergent body plan,


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tunicates have long been considered as
 the earliest branching group in Chordates 


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and actually it has strong implications for under
-standing the evolutionary origin of chordates.


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Possibly the ancestors of chordates were 
benthic animals like adult ascidians,


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or they might have been pelagic 
animals like ascidian larva.


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It's only at the beginning of the 21st 
century that tunicates have been placed as


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 the sister group of vertebrates. 

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Thanks to the development of sequencing
 technologies, phylogenomic approaches have


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confirmed previous morphological
 evidence and molecular data 


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that convinced the scientific community.

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So, the actual tree of Chordates

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shows that they diverged around 600m years
 ago, and the separation between vertebrates


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 and tunicates took place 550m years ago.

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 While this close association between
 vertebrates and tunicates sounds strange,


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when looking at the adult
 morphology, it is actually


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supported by the entire scientific community.
One of the possible explanations for the


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special case of tunicates is the 
fact that they are very divergent, 


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genomically and during their
 embryonic development. 


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First, their genomes have experienced 
a high rate of genomic evolution 


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and extensive rearrangements. Secondly,
their embryonic development is very fast, 


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with very few cells, but you will see that 
in more detail in the next chapter.


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We will now consider phylogenetic 
relationships within tunicates.


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High sequence divergence often leads to
long-branch attractions, and this has 


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 prevented previous molecular phylo
-genies from being strongly conclusive. 


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However better methods in phylogenomics,
 together with increased taxon sampling that


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 that was facilitated by Next Generation
Sequencing technologies,


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have now clarified the 
relationships within tunicates.


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Here you can see our current under
 -standing of tunicates' phylogeny.


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While this is not definitively conclusive,

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and people are still working on
 this phylogeny to validate it,


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 it has already some important 
evolutionary implications.


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First, you can see in red, 

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the Appendicularians are the earliest
 branching group of tunicates,


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and it means that ancestral tunicates
were more like a tadpole larva.


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Secondly, you can see in blue the Ascidians 
are not a monophyletic group, and the three


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 branches Aplousobranchia, Phlebobranchia,
and Stolidobranchia actually are valid


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classifications that were previously 
based on morphological criteria.


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The third implication is that Thaliaeans 
 that are the most bizarre tunicates and 


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 that are all pelagic, are a 
monophyletic group


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and given their position within Ascidians,
 it means that their ancestors were


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possibly ascidian-like benthic animals.

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 To conclude with the chapter, I'd like 
to summarise three key points


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 that you should remember: 

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first, chordates are made of cephalochor
-dates, tunicates, and vertebrates.


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Secondly, vertebrates and tunicates are the
 two closely-related groups that diverged


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 550 million years ago. 

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The last point is that all these animals 
 have in common during their embryonic life


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 the notochord and the dorsal neural tube.

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In the next chapter, you will see that we 
will focus on only one class of tunicates:


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 the Ascidians.