Ancient vertebrate uses familiar tools to build a very different head

Jawless fish emerged 500 million years ago, 100 million years before jawed fish and well before mammals. Because they’re so unlike us, it may be difficult to fathom that the genes that create the primitive “faces” of jawless fish have anything to do with us.

It turns out they do. A collaborative team led by Stowers Investigator and Scientific Director Robb Krumlauf, PhD, has discovered that a gene network governing the vertebrate head and jaw originated in jawless animals.

Krumlauf and his colleagues report that jawless sea lampreys express an array of Hox genes reminiscent of jawed vertebrates. Like mileposts along a road, Hox genes activated along an embryo’s axis dictate where structures like arms or legs are built. “Previously, we addressed how these factors make unique structures,” explains Krumlauf. “Now, we are excited by how similar sets of genes play common roles in creating a basic structural plan.”

The team focused on Hox genes in the embryonic hindbrain, which controls head and jaw construction. To compare their expression in jawed versus jawless creatures, the team inserted fluorescent Hox “reporters” engineered from DNA of jawed animals (zebrafish or mice) into lamprey embryos. The question was whether lamprey embryos emitted embryonic signals required to switch them on.

Amazingly, lampreys exhibited hindbrain Hox reporter expression in a pattern much like jawed animals. “That means that the gene regulatory network that governs segmental patterning of the hindbrain likely evolved prior to divergence of jawed vertebrates,” says postdoctoral fellow Hugo Parker, PhD, the study’s first author.

The study was published in the September 14, 2014, advance online issue of Nature.