In the previous chapter you have seen what tunicates look like
in their natural environment, and you will see that their position
in the tree of life has changed quite a lot.
I will show you that they're our closest invertebrate relative.
Most people are not familiar with tunicates, which have a
somewhat unusual body organisation.
Looking at this picture, people may say "oh, it's a plant"
"oh, it's a sponge", or better "it's a sea anemone".
But then if you tell them they might have seen tunicates at a restaurant,
eating seafood, they would say "oh yeah, they are mollusks!"
And you see in these pictures, there are three dishes from different countries
where tunicates are eaten as a delicacy.
Calling them mollusks actually fits with their status from antiquity to the 19th century.
For example, Linné in the 18th century and Cuvier in the 19th century
classified tunicates among mollusks as "shell-less bivalves".
It is only at the end of the 19th century that tunicates' position changed radically.
In 1866, Alexander Kowalevsky carefully examined the embryonic development
of ascidians and pointed to a strong affinity with vertebrates.
By careful examination, he discovered the existence of a notochord and
a dorsal neural tube very similar to those of a frog.
This essential description allowed him to definitely consider the chordate phylum
that comprises vertebrates, tunicates, and cephalochordates.
All these animals have in common the existence of a notochord and a dorsal
neural tube during embryonic life.
You can see now on this tree of the Metazoans how the position of tunicates
has radically changed in the 19th century.
Given the "living fossil" features of amphioxus that you have seen in the dedicated module,
and the fact that other tunicates have a divergent body plan,
tunicates have long been considered as the earliest branching group in Chordates
and actually it has strong implications for under -standing the evolutionary origin of chordates.
Possibly the ancestors of chordates were benthic animals like adult ascidians,
or they might have been pelagic animals like ascidian larva.
It's only at the beginning of the 21st century that tunicates have been placed as
the sister group of vertebrates.
Thanks to the development of sequencing technologies, phylogenomic approaches have
confirmed previous morphological evidence and molecular data
that convinced the scientific community.
So, the actual tree of Chordates
shows that they diverged around 600m years ago, and the separation between vertebrates
and tunicates took place 550m years ago.
While this close association between vertebrates and tunicates sounds strange,
when looking at the adult morphology, it is actually
supported by the entire scientific community. One of the possible explanations for the
special case of tunicates is the fact that they are very divergent,
genomically and during their embryonic development.
First, their genomes have experienced a high rate of genomic evolution
and extensive rearrangements. Secondly, their embryonic development is very fast,
with very few cells, but you will see that in more detail in the next chapter.
We will now consider phylogenetic relationships within tunicates.
High sequence divergence often leads to long-branch attractions, and this has
prevented previous molecular phylo -genies from being strongly conclusive.
However better methods in phylogenomics, together with increased taxon sampling that
that was facilitated by Next Generation Sequencing technologies,
have now clarified the relationships within tunicates.
Here you can see our current under -standing of tunicates' phylogeny.
While this is not definitively conclusive,
and people are still working on this phylogeny to validate it,
it has already some important evolutionary implications.
First, you can see in red,
the Appendicularians are the earliest branching group of tunicates,
and it means that ancestral tunicates were more like a tadpole larva.
Secondly, you can see in blue the Ascidians are not a monophyletic group, and the three
branches Aplousobranchia, Phlebobranchia, and Stolidobranchia actually are valid
classifications that were previously based on morphological criteria.
The third implication is that Thaliaeans that are the most bizarre tunicates and
that are all pelagic, are a monophyletic group
and given their position within Ascidians, it means that their ancestors were
possibly ascidian-like benthic animals.
To conclude with the chapter, I'd like to summarise three key points
that you should remember:
first, chordates are made of cephalochor -dates, tunicates, and vertebrates.
Secondly, vertebrates and tunicates are the two closely-related groups that diverged
550 million years ago.
The last point is that all these animals have in common during their embryonic life
the notochord and the dorsal neural tube.
In the next chapter, you will see that we will focus on only one class of tunicates:
the Ascidians.