Important genetic origin of our senses identified – University of Innsbruck
An embryo of the tunicate Ciona intestinalis. The microscope image shows “bipolar tail neurons”, i.e. the sensory nerve cells in the tail area (green) and cells of the epidermis (magenta).
Researchers have decoded the function of a gene that is essential for the formation of nerve structures in the heads of vertebrates and their perception of the environment. In the international study that Nature was published and managed by the University of Oxford, Ute Rothbächer’s team at the Institute of Zoology at the University of Innsbruck was significantly involved.
Having a head is pretty beneficial. What actually sounds banal was tested by evolution in a long process: During the formation of the animal world in the oceans, purely invertebrate creatures initially dominated. While these were partial head structures, it was the development of a new, improved head that finally paved the way for vertebrates to succeed.
This ‘new head’ enables a wide spatial distribution and multiplication of sensory cells and thus a much better perception of the environment. This has proven to be essential for the evolution of the predatory way of life.
When external stimuli are transmitted to the vertebrate brain, the so-called ‘cranial sensory ganglia’ play an important role. These head ganglia can be thought of as nerve nodes that are distributed over the entire head and receive information from the sensory organs. How exactly these head ganglia came about was not known to science until now. A study published in the journal Nature got to the bottom of the riddle.
Prototype of Vertebrates
Ute Rothbächer’s working group at the Institute of Zoology at the University of Innsbruck was instrumental in the final phase of the research project, which was carried out in international collaboration and initiated by the University of Oxford. Their finding shows that the ‘cranial sensory ganglia’ of vertebrates arise from a genetic program that is also found in their closest surviving relatives, the tunicates. In tunicate larvae, certain sensory nerve cells are located in the tail area, the so-called ‘bipolar tail neurons’. These process external stimuli, but are also responsible for the movement of the animal. In both animal strains, the respective structures are formed by the Hmx gene.
“To a certain extent, tunicates are an evolutionary prototype for vertebrates,” explains Rothbächer. “There is a large anatomical gap between the adults of these phyla as they are adapted to ecological niches. This makes research on their evolution difficult. Common structures and mechanisms can only be recognized better in the embryonic stage – our common ancestor was probably very similar to a tunicate larva.”
The lamprey, a primitive fish resembling an eel and often referred to as a ‘living fossil’, and the tunicate served as model organisms for the study Ciona intestinalissurrounded by a yellowish tubular coat that protects the animal and filters food.
The obtained gene
Alessandro Pennati, a doctoral student in Rothbächer’s group, controls crucial data on the function of the gene Hmx in Ciona at. To do this, he uses the CRISPR-Cas9 gene technology, with which genetic sequences can be switched off in a targeted manner, and the method of transient transgenesis to read genes frequently.
The researchers find that Hmx directs the development of ‘bipolar tail neurons’ in tunicates, while in vertebrates it is instrumental in the development of ‘cranial sensory ganglia’ processes. Amazingly, lamprey Hmx gene segments that had been translated into the DNA of Ciona were inserted, similarly active as that Ciona-own Hmx.
“Hmx has been shown to be a central gene that has been conserved throughout evolution, i.e. has retained its original function and structure and was probably already found in this form in the common ancestor of vertebrates and tunicates,” explains Pennati. ‘Cranial Sensory Ganglia’ and ‘Bipolar Tail Neurons’ thus have the same evolutionary origin. Hmx was probably involved in the development of highly specialized head sensory organs in vertebrates.