June 25, 2010

Scientists Studying The Evolution Of Animal Limbs

A new study is shedding light on a key genetic step in the evolution of animal limbs from the fins of fish.

A team of researchers identified two genes crucial to fin development.

The researchers wrote in the journal Nature that the loss of these genes could have been an "important step" in the evolutionary transformation of fins into limbs.

Marie-Andree Akimenko from the University of Ottawa in Canada led the research.

She and her colleagues began their study by looking at the development of zebrafish embryos.  They discovered two genes coded for proteins that were important in the structure of fins.

These proteins were components of "actinotrichia," which are thread-like fibers.  These proteins are found in fish larvae and they eventually develop into the bony fin rays of mature fish.

"We found there were no [equivalent genes] in limbs, so this suggested these may have been lost in evolution," Dr Akimenko explained to BBC News.

They looked for the same family of genes in the genomes of elephant sharks to confirm this, which are a very ancient fish species.

Akimenko said this suggested that the "ancient family of genes persisted in [bony fish] and was lost when they evolved into four-footed animals."

Embryo development can provide important genetic and molecular clues about evolution. 

Scientists were able to manipulate zebrafish development in order to study these changes in more detail.  They inactivated the newly discovered genes in a developing zebrafish embryo. They found that it developed shorter "truncated" fins with no bony rays when they did this.

The scientists say that the loss of these fin rays was a key step in fin-to-limb evolution.

The team compared the development of normal zebrafish embryos with that of mouse embryos.

Akimenko told BBC, "When we compared fin development and limb development, the early steps are very similar."

"But at one point there is a divergence, and that correlates with the beginning of the expression of these genes."

Professor Jonathan Bard, a retired development biologist now working with the department of physiology, anatomy and genetic, told BBC the findings were only a very small part of the evolutionary story.

He said that this did not reveal anything about digit formation - "how the broad, multi-ray fins of fishes became transformed into the eight digits of the hand or foot plate of the first tetrapods".

"More generally," he said, "hundreds of millions of years of separate evolution divide [bony fish] and mice."

He added: "It is an interesting paper... and it will be interesting to see what the [researchers] do next."


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