Brains of Jawless Fish May be More Similar to Ours Than Scientists Previously Thought

Jawless fish brains may be more similar to ours than scientists ever expected. It turns out that the complex divisions in the vertebrate brain first appeared before the evolution of jaws, more than 500 million years ago.

Most living vertebrate species have jaws, a development thought to have occurred sometime in the Paleozoic era. Jawed vertebrates, including humans, share many characteristics that have remained mostly unchanged for millennia. In fact, the brain's basic developmental plan was thought to have reached completion in jawed vertebrates because the brains of lampreys and hagfish, which are the only jawless fish alive today, seem to lack two key domains.

Now, though, researchers have brought his assumption into question. They've studied hagfish embryos using techniques derived from developmental biology.

The vertebrate brain develops from a neural tube that is divided into sections. The development of each section is very specific and is controlled by the expression of particular genes at very precise times and locations. These gene-expression patterns are highly conserved in jawed vertebrates. Lampreys, a type of jawless fish, appear to lack two brains regions common to jawed vertebrates, the cerebellum and the medial ganglionic eminenc (MGE).

In jawed vertebrates, the MGE develops from a forward section of the neural tube that expresses Nkx2.1. In this latest study, the researchers found a region in the correct location that expresses Nkx2.1. This indicates that the hagfish brain does indeed have a MGE region. And while hagfish don't have a true cerebellum, they also express the same gene for it.

"With these new findings from hagfish and lampreys, we have shown that both of the extant jawless-fish species have a rhombic lip and an MGE-the sources of the cerebellum, pallidum, and GABAergic interneurons in jawed vertebrates," said Shigeru Kuratani, one of the researchers, in a news release. "This firmly places the development of these genoarchitectural patterns back to a common ancestor shared by jawless and jawed vertebrates."

The findings are published in the journal Nature.

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