Getting into the world of Tunicates – Part II

By | May 7, 2021
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Tunicates are closest living relatives of vertebrates

One of Charles Darwin’s most revolutionary idea was that all living things are related to one another (to different extents) through common decent (share common ancestors), connected to each other like branches of a huge “Tree of life”.

So what can we expect at the root of this giant tree?

At the base of the tree lies a 3.8 billions year old ancestor ,which gave rise to all living forms present on earth today.Thus making us all belong to one huge family and so by understanding the evolutionary relationships between members of family will give us an idea how we, and all diverse life forms around us come into existence.

Now you might be familiar that Bilaterians are divided into Protosomes and Deutereostomes based on the fate of first opening during develoment.

Deuterostomes can be classified into : Echinodermata, Hemichordata , Xenoturbella and Chordata.

Urochordates ( Ciona , Oikopleura ) , Cephalaochordates ( Amphioxus / Lancelets) and Vertebrates makes up the chodata phylum.

If one goes by textbooks ,tunicates were always considered as the earliest offshoot of the chordate lineage, and amphioxus as the closest group to vertebrates.This is mostly due to overall morphological similarities and an apparently increased complexity in cephalochordates and vertebrates ,when compared to tunicates, which are morphologically more distinct.

The classical view of evolution of deuterostomes has been of steady, implying a smooth increase in complexity from a relatively simple and sedentary deuterostome ancestor to motile vertebrates and results were always interpreted keeping humans as central characters. But this classical textbook view was turned on its head after the work of Delsuc and colleagues (published in Nature 2006) . Now Amphioxus is viewed as the most “basal”chordate and tunicates as the sister group, or closest relatives, of the vertebrates.

Authors using assembled a phylogenomic data set of 146 nuclear genes (33,800 unambiguously aligned amino acids) from 14 deuterostomes and 24 other slowly evolving species as an outgroup have shown with strong support that that tunicates, and not cephalochordates, are our closest invertebrates relatives. furthermore they also place cephalochordates along with echinoderms ,away from both vertebrates and tunicates, but this data is not as strongly supported as tunicates – vertebrate link and hence authors are of the opinion that this hypothesis needs to be tested with additional data. However things are sure to get more clearer with phylogenetic analysis of more genes combined with an increased taxon sampling including the enigmatic xenoturbellidans, hemichordates, and a greater diversity of echinoderms.

One important aspect of this study is that it involves data from species from various taxa including tunicate oikopleura dioica ,which is particularly important because it belongs to appendicularians (or larvaceans), which are morphologically and molecularly very divergent from the ascidians previously included in studies.
Another thing which gets clear is that the deuterostome ancestor would have been motile and relatively complex, and that the sessile habits of most echinoderms and tunicates evolved later.

The outcome of study further prove the importance of both Tunicates and Amphioxus as complimentary model systems for evo devo studies. Tunicates are phylogenetically closer to vertebrates but are morphologically and molecularly highly diverged ,where as very distantly related cephalochordates (Amphioxus) importance lies in that fact that it represents closest living organism to the ancestor of all deuterostomes. Surely our understanding related to early deuterostomes and vertebrate evolution will only get better with comparative analysis of geneome sequences, as complete genome sequence is available for both Tunicates (Ciona ,Oikopleura) and Amphioxus.

In the next and final part we take a look into Hox complement in tunicates.

References:

1) Tunicates and not cephalochordates are the closest living relatives of vertebrates.
Delsuc F, Brinkmann H, Chourrout D, Philippe H.
Nature. 2006 Feb 23;439(7079):965-8.

2)Evolution: careful with that amphioxus.
Gee H.
Nature. 2006 Feb 23;439(7079):923-4.