Turtle Shells Evolved By Recruiting Genes Involved In Limb Development

An international team of scientists has analyzed the genomes of two turtle species, shedding light on how the turtle’s shell may have evolved.

AsianScientist (Apr. 30, 2013) – An international team of scientists has analyzed the genomes of two turtle species, confirming that turtles are not primitive reptiles but are more closely related to birds and crocodiles. The turtle genomes also shed light on how the turtle’s shell may have evolved.

Turtles are good subjects for the studying evolution as they have a unique anatomy and the turtle’s shell is considered to be one of the most intriguing structures in the animal kingdom.

“Turtles are interesting because they offer an exceptional case to understand the big evolutionary changes that occurred in vertebrate history,” explains Dr. Naoki Irie, who led the study by the Joint International Turtle Genome Consortium.

In their study, published in Nature Genetics, the scientists sequenced the genomes of the Chinese soft-shell turtle (Pelodiscus sinensis) and the green sea turtle (Chelonia mydas) and studied the expression of genetic information in the developing turtle.

Their analysis confirmed a previously proposed theory that turtles are not primitive reptiles but are more closely-related to birds and crocodiles.

Based on genomic information, the researchers predict that turtles split from the bird-crocodilian group around 250 million years ago, during one of the largest extinction events ever to take place on this planet.

The study also reveals that despite their unique anatomy, turtles begin their embryonic development in much the same pattern as other vertebrates.

Rather than developing directly into a turtle-specific body shape with a shell, they first establish the vertebrates’ basic anatomy before entering a turtle-specific development phase.

During this late specialization phase, the scientists found evidence that genes typically involved in limb development were expressed in the embryonic turtle shell. This suggests that the turtle shell evolved by recruiting part of the genetic program used for developing limbs.

Another unexpected finding of the study was that turtles possess an unusually large number of olfactory receptors that are important for the sense of smell. They thus speculate that turtles may have the ability to smell a wide variety of substances.

The article can be found at: Wang et al. (2013) The Draft Genomes Of Soft-shell Turtle And Green Sea Turtle Yield Ingiths Into The Development And Evolution Of The Turtle-specific Body Plan.

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Source: RIKEN. Photo: levork (Julian Fong)/Flickr/CC.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

Yew Chung is a postdoctoral research fellow at the Duke-NUS Graduate Medical School, Singapore.

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