An historic fish, with an intriguing¹（有趣的） past, now has had its genome sequenced, providing a wealth of information on the genetic³ changes that accompanied the adaptation from an aquatic⁴（水生的） environment to land. A team of international researchers led by Chris Amemiya, PhD, Director of Molecular⁵ Genetics at the Benaroya Research Institute at Virginia Mason (BRI) and Professor of Biology at the University of Washington, will publish "The African coelacanth（腔棘鱼） genome provides insights into tetrapod（四足动物） evolution" April 18 as the cover article in Nature. The coelacanth genome was sequenced by the Genome Center at the Broad Institute of MIT and Harvard, and analyzed⁶ by an international consortium of experts.

 

Sequencing the coelacanth genome has been a long-sought goal and a major logistical milestone⁷, says Dr. Amemiya. He and scientists throughout the world have campaigned for sequencing of the fish for over a decade. "Analysis of changes in the genome during vertebrate adaptation to land has implicated⁸ key genes⁹ that may have been involved in evolutionary¹⁰ transitions," he says. These include those regulating immunity¹¹, nitrogen excretion（排泄，分泌） and the development of fins¹², tail, ear, eye, and brain as well as those involved in sensing of odorants. The coelacanth genome will serve as a blueprint¹³ for better understanding tetrapod evolution.

 

"This is just the beginning of many analyses on what the coelacanth can teach us about the emergence¹⁴ of land vertebrates, including humans, and, combined with modern empirical approaches（经验探讨）, can lend insights into the mechanisms¹⁵ that have contributed to major evolutionary innovations," says Dr. Amemiya.

 

The coelacanth is critical to study because it is one of only two living lobe-finned fish groups that represent deep and evolutionarily informative¹⁶ lineages with respect to the land vertebrates. The other is the lungfish, which has an enormous genome that currently makes it impractical¹⁷ to sequence. The lobe-finned fishes are genealogically placed in-between the ray-finned fishes (such as goldfish and guppies) and the tetrapods − the first four-limbed vertebrates and their descendants, including living and extinct amphibians¹⁸, reptiles¹⁹, birds and mammals. A lobe-finned ancestor(s) underwent genomic changes that accompanied the transition of life in an aquatic environment to life on land. The coelacanth is undeniably a fish, however, phylogenetic analyses show that its genes are more like those of tetrapods than of ray-finned fishes. Additionally, coelacanth genes evolve at a considerably²⁰ slower rate than those of tetrapods, a fact that is coincident with its apparently²¹ slow rate of morphological（形态的） change.

 

"For evolutionary biologists the coelacanth is an iconic animal, as familiar as Darwin's finches on the Galapagos," says Toby Bradshaw, PhD, Professor and Chair, Department of Biology, University of Washington. "This paper by Chris and colleagues gives us our first comprehensive look at the coelacanth's place in our evolutionary history, and provides fascinating insights into the specific vertebrate genes involved in the critical transition from water to land − it seems that both loss and gain of gene² function were required. I find the proposed gain-of-function changes in gene regulation for limb development particularly compelling, supported by experimental evidence that the lobed²²（浅裂的） fins of the coelacanth really are akin²³ to prototypical legs. Making legs from fins is a wonderful example of Francois Jacob's observation that 'evolution is a tinkerer, not an engineer.'" Adds Gerald Nepom, MD, PhD, Director of the Benaroya Research Institute, "This work represents a major accomplishment²⁴ by a large and talented group of investigators²⁵, opening a new book of knowledge about adaptation that is now available to all scientists who want to better understand our complex genetic origins."

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