Elephant shark genome small and slow to evolve

Elephant sharks have the smallest genome of non-bony fishes and the slowest-evolving genes among vertebrates, a study suggests.

Byrappa Venkatesh

Elephant sharks’ DNA hasn’t changed much in the 420 million years that bony fishes have been in existence, suggests a new analysis of the animal’s genome.

Callorhinchus milii is the first cartilaginous fish to have its genome fully sequenced. At 1 billion base pairs, the elephant shark has the smallest genetic code among sharks, rays and other similar fishes. A closer look at the animal’s genome suggests that the fish do not have many bones because they lack genes that switch on calcium-binding proteins, researchers report January 8 in Nature.

Elephant sharks also lack a major component of the immune system called helper T cells. The discovery suggests that acquired immunity evolved in two steps — not one, as previously thought.

Ashley Yeager is the associate news editor at Science News. She has worked at The Scientist, the Simons Foundation, Duke University and the W.M. Keck Observatory, and was the web producer for Science News from 2013 to 2015. She has a bachelor’s degree in journalism from the University of Tennessee, Knoxville, and a master’s degree in science writing from MIT.

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