Genome Study Of Two Turtle Species Sheds Light On Evolution Of Shell
April 29, 2013

Genome Study Of Turtle Species Sheds Light On Evolution Of The Shell

Brett Smith for - Your Universe Online

With their unique outer shell and sharp beak, turtles and their evolutionary history have long been a mystery to biologists.

A Nature Genetics report on the newly sequenced genomes of the soft-shell turtle and green sea turtle shows that the iconic reptiles have a common ancestor with crocodilians and birds — diverging from these groups between 270 and 240 million years ago.

"The genome-wide phylogenetic analysis of two turtles in our project, along with two crocodile genomic data additionally, makes clear the evolutionary history of turtles in diverging from other species and settles the disputes about the phylogenetic position of reptiles,” said Zhuo Wang, the project manager from the Beijing Genomics Institute (BGI) in Shenzen, China who co-authored the report.

“The genomic analyses and embryonic gene expression profiles have been combined to reveal the fundamental (evolutionary developmental) questions on turtle evolution and development,” he said in a statement. “Besides the interesting story, the genomic data we released here will provide a platform for more scientists to initialize their genome-wide studies on turtles.”

For the study, the research team sequenced and analyzed the genomes of the two very different turtle species. They also researched various genes that may be associated with specific characteristics that are unique to turtles.

Their research uncovered olfactory receptor gene families that have allowed both turtles to develop superior olfaction ability. They also discovered that several genes involved in taste perception and certain metabolic functions are uniquely absent in turtles. In their report, the researchers suggested that the loss of these genes may be related to the reptiles´ slow metabolism.

The researchers also looked into the genes responsible for the embryonic development of turtles and compared them to those in chickens. They were able to use RNA sequencing, comparative genomics, and mathematical statistical approaches to see where the two species diverged genetically.

To understand the embryonic development of the turtle´s shell, researchers investigated miRNA expression and identified 20 key protein signaling genes. These genes allow for the formation of the carapacial ridge, an embryonic outgrowth that is responsible for shell development.

"The completed genome sequencing of soft-shell turtle and green sea turtle give an important hint to uncover the development and evolution mechanism of turtles,” said Hongyan Zhang, Regional Director of BGI Tech Solutions Co., Ltd. for Japan. “This scientific achievement is a joint effort supported by BGI's advanced sequencing technologies and excellent bioinformatics capabilities, the profound basis research background of developmental biology from RIKEN, and other partners' great contributions.”

“We are looking forward to having more collaboration with other scientists for better exploring the secret of life together in the near future,” he added.

Chinese soft-shell turtles in the study are the smallest species of soft-shell turtles, and were once quite common in pet shops.

Conversely, the green sea turtle is the largest of all the hard-shelled sea turtles. It is named after the green fat beneath its shell. It's numbers have diminished over the years to the point where officials have recently listed it as an endangered species.