Biologists have traced the origin of the head to primitive chordates

(ORDO NEWS) — Scientists have found an ancient and highly conserved gene that determines the appearance of nerve ganglia in the tail of tunicates and in the head of lampreys and more advanced animals.

For a long time, animals on Earth did without a spine, head and brain, and the cells of their nervous system were more or less evenly distributed throughout the body.

However, gradually the neurons began to concentrate in the anterior part, providing a fast and reliable connection with the senses.

Over time, this nerve ganglion developed into a full-fledged brain, enclosed in the head and connected to the body through the spinal cord in the spine – like ours. European scientists have figured out how the first and earliest steps of this process took place.

Ute Rothbächer of the University of Innsbruck and colleagues from Austria and other countries have studied the tunicates Ciona intestinalis . These are extremely primitive marine chordates, outwardly resembling rather bag-shaped sponges or coelenterates.

The features of chordates in them can be distinguished only at the larval stage of development, when they swim actively, using the tail, in which the chord is briefly formed.

The bipolar neurons of the caudal ganglion connect the sensory and motor neurons of these larvae, representing the simplest analogue of the future brain.

Scientists have found that the development of bipolar neurons in the tail in tunicates is triggered by the Hmx gene , which is involved in the early stages of the formation of the head ganglion in other, more complex chordates.

This gene contains a homeobox , a standard region that allows it to bind to DNA and regulate its expression. The homeobox serves as an important feature of genes that controls the work of many other genes, running the “built-in programs” for the development of organs and tissues.

So much depends on these control genes that they are very conservative and change little in the course of evolution.

The same Hmx determines the formation of the head nerve ganglion in lampreys , another group of primitive chordates, which, however, are already similar to “full” vertebrates.

They have a complete anterior and posterior ends and a developed notochord, and their head ganglion develops in some detail almost like a real brain.

To demonstrate the versatility of Hmx , the scientists turned it off in tunicates, replacing it with an analog obtained from lampreys.

This still led to the formation of bipolar neurons in the tail of Ciona . And vice versa: in lampreys, the Hmx gene from tunicates triggered the development of the head ganglion.

“The success of showing that Hmx is a central gene that is extremely conserved in evolution,” sums up Alessandro Pennati, one of the authors of the new work. “Perhaps its structure and functions were already determined by the common ancestor of vertebrates and tunicates.”


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