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The massive loss of genes of a marine animal facilitated its jump to a free life

Researchers from the Evolution and Development Consolidated Research Group ( Evo-Devo ) of the Genetics Section of the Faculty of Biology of the University of Barcelona ( UB ) have deciphered one of the enigmas about the transition between free and sedentary lifestyles in ancestors Of our edge: the chordates .

The deconstruction of the heart gene network facilitated the transition to a free pelagic lifestyle, that is, in the water column and not subject to the substrate

The work, which is the cover of the journal Nature , proposes a new evolutionary scenario that helps to better understand the evolution of our phylum and to discover what the ancestor of the tunicates , the sister group of vertebrates, was like .

Specifically, it reveals that the massive losses of genes that deconstructed the heart’s gene network facilitated the transition to a free pelagic lifestyle –that is, in the water column and not subject to the substrate– from a sedentary ancestral form.

Impact of gene loss on evolution

For many years, evolutionary biology had not paid much attention to the disappearance of genes, and most studies focused on how gene duplications could lead to new biological functions. 

“However, we are increasingly identifying more gene losses that may be adaptive in nature,” explains  Cristian Cañestro , principal investigator of the Evo-Devo Group at the UB. “In other words, losing certain genes can also have evolutionary advantages that can be related to the acquisition of biological innovations in organisms.”

The massive loss of genes of a marine animal facilitated its jump to a free life

We are increasingly identifying gene losses that may be adaptive in natureCristian Cañestro

The massive loss of genes of a marine animal facilitated its jump to a free life

The phylum of the chordates is made up of the group of vertebrates, that of tunicates and that of cephalochordates. The basal position of cephalochords and the fact that they are free-living organisms, such as vertebrates, indicates that the predecessor of all chordates was also free-living.

However, in the case of tunicates there are sessile ( sea ​​squirt ) and free-living ( appendicularia ) animals . Whether the common ancestor of tunicates was a passive filter set on the seafloor, or an active swimmer, has been a key focus of debate to understand what the ancestor from which vertebrates also originated was like.

The research group is a world pioneer in the use of the species Oikopleura dioica – an organism of marine zooplankton from the group of appendiculariae – as an animal model to study the impact of gene loss as an evolutionary force. 

The results of the article reveal the existence of massive disappearances of genes in appendiculariae that millions of years ago affected the cardiopharyngeal gene network .

“Our work shows that the heart of Oikopleura dioica is homologous to that of other chordates and that, therefore, it comes from a heart that already existed in the common ancestor between humans and Oikopleura dioica “, remarks the expert. “However, it has been surprising to see that, despite having a homologous heart, many of the genes essential to make a heart both in humans and in the rest of chordates have been lost in Oikopleura dioica during a deconstruction  process – dismantling of elements – which has affected their cardiopharyngeal gene network ”.

The massive loss of genes of a marine animal facilitated its jump to a free life

Many of the genes essential to make a heart both in humans and in the rest of chordates have been lost in Oikopleura dioica during a deconstruction process. Cristian Cañestro

The massive loss of genes of a marine animal facilitated its jump to a free life

Cover of the magazine Nature. / UB

The evolutionary challenge of adapting to a free life

“Considering that vertebrates, cephalochordates and appendiculariae are free-living animals, it was postulated that the most plausible evolutionary scenario was that the tunicate ancestor was also free,” says Alfonso Ferrández-Roldán , lead author of the article.

Now, the results contradict that evolutionary scenario. “We have been able to correlate the loss of genes with the disappearance of typical characteristics of the sessile life of sea squirts. This has led us to infer that the common ancestor of all tunicates was a sessile-living organism , and not a free-living organism , as was thought up to now ”, continues the researcher. 

The article points out that the massive disappearance of genes could be linked to three evolutionary innovations that facilitated the transition from a predecessor with features similar to those of sea squirts – with motile larvae and sessile adults – towards the completely appendicular-free lifestyle.

A first adaptation was the acceleration in the process of development of the heart ( cardiogenesis ). This allowed the appendiculariae to have this organ fully operational at the beginning of their free lifestyle.

The common ancestor of all tunicates was a sessile-living organism, and not a free-living one, as was thought up to now

The second is linked to the redesign of a new structure of the heart that could improve the circulation of hemolymph – the ‘blood’ of invertebrates.

Finally, the disappearance of the pharyngeal musculature in the trunk of Oikopleura dioica would be a third evolutionary innovation on the route to active swimming life. The function that this musculature has in sessile sea squirts – it regulates the siphon through which the water enters them – would be totally unnecessary in appendicularis. 

This basic research work outlines a new evolutionary scenario to solve unknowns about the origin and evolution of appendiculariae, their phylogenetic relationship with other tunicates and the common ancestor of vertebrates.

“Our work reveals what the ancestor of tunicates was like and opens up new questions: for example, how sessile life evolved in ancestral tunicates from a free-living ancestor possibly similar to that observed in vertebrates”, Cañestro raises .

Reference:

Ferrández-Roldán, A .; et al. “Cardiopharyngeal deconstruction and ancestral tunicate sessility”.  Nature , November 2021. Doi: 10.1038 / s41586-021-04041-w Fountain: UBRights: Creative Commons.

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