Jawless fish more like jawed fish than thought

by Mary Caperton Morton
Thursday, July 14, 2016

Lamprey eels do not have jaws; instead, they feed with a sucking mouth lined with teeth. Credit: Joanna Gilkeson, USFWS Midwest, CC BY 2.0.

Vertebrates come in all shapes and sizes, but the one thing almost all have in common are jaws, which developed early in vertebrate evolution. Jawless fish, which evolved about 530 million years ago and are thought to be more primitive than their jawed relatives, are an exception. According to a new study, however, the brains of jawless fish have more in common with the brains of jawed vertebrates than previously thought.

In vertebrate embryos, brains develop from neural tubes that are divided into sections, each with its own developmental destiny. The destiny of each section is controlled by certain genes that switch on and off at specific times, creating a pattern of gene expression, or genoarchitecture, that is shared among jawed vertebrates. Previously, scientists thought the brain development of jawless fish, including lamprey eels and hagfish, was governed by a different pattern of gene expression because they appeared to lack two brain regions — the cerebellum and the medial ganglionic eminence (MGE) — common to jawed vertebrates.

But in the new study published in Nature, Fumiaki Sugahara of RIKEN labs in Kobe, Japan, and colleagues found that lampreys and hagfish do indeed possess the genes responsible for controlling the development of the cerebellum and the MGE. Neither fish has a true cerebellum, but the hagfish was found to have its own version of an MGE, evidence that suggests that the genoarchitectural plans for the vertebrate brain date back to a common ancestor shared by jawed and jawless vertebrates.

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