How the turtle found its shell

How turtles evolved their unique body shape has baffled scientists for years. They are unusual, even among other armoured animals such as the armadillo, in that their shell has changed very little over the millennia.

An international team of researchers, led by Naoki Irie of the RIKEN CDB in Japan, including a researcher from King Abdulaziz University in Saudi Arabia, produced a draft sequence of the genomes of the soft-shell (Pelodiscus sinensis) and green sea (Chelonia mydas) turtles, publishing their findings in Nature Genetics.

They found that both green seas turtles and soft-shell turtles are closely related with the bird-crocodile lineage from which they diverged some 267.9–248.3 million years ago.

The olfactory receptor gene families in both turtles were highly diverse. Other genes, such as those involved in taste perception or those producing the hunger­stimulating and energy homeostasis–regulating hormone ghrelin were lost, which could help explain the turtles' low metabolic rate.

On performing whole-mount in situ hybridization for the Wnt family of genes they found that the Wnt5a gene was expressed in the growth zone of the dorsal shell. This gene is also expressed in the limbs, which supports the hypothesis that Wnt signaling may have been recruited from the limbs to trigger turtle shell development.

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