Getting under the shell of the turtle genome

The genome of the western painted turtle (Chrysemys picta bellii) one of the most widespread, abundant and well-studied turtles in the world, is published this week in Genome Biology. The data show that, like turtles themselves, the rate of genome evolution is extremely slow; turtle genomes evolve at a rate that is about a third that of the human genome and a fifth that of the python, the fastest lineage analyzed.

As a group, turtles are long-lived, can withstand low temperatures including freezing solid, can survive for long periods with no oxygen, and their sex is usually determined by the temperature at which their eggs develop rather than genetically. The painted turtle is most anoxia-tolerant vertebrate and can survive up to four months under water depending on the temperature. Turtles and tortoises are also the most endangered major vertebrate group on earth, with half of all species listed as endangered. This is the first turtle, and only the second non-avian reptile genome to be sequenced, and the analysis reveals some interesting insights about these bizarre features and adaptations, many of which are only known in turtles.

The western painted turtle is a freshwater species, and the most widespread turtle native to North America. Bradley Shaffer and colleagues place the western painted turtle genome into a comparative evolutionary context, showing that turtles are more closely related to birds and crocodilians than to any other vertebrates. They also find 19 genes in the brain and 23 in the heart whose expression is increased in low oxygen conditions – including one whose expression changes nearly 130 fold. Further experiments on turtle hatchlings indicated that common microRNA was involved in freeze tolerance adaptation.

This work consistently indicates that common vertebrate regulatory networks, some of which have analogs in human diseases, are often involved in the western painted turtle achieving its extraordinary physiological capacities. The authors argue that the painted turtle may offer important insights into the management of a number of human health disorders, particularly those involved with anoxia and hypothermia. Source : hilary.glover@biomedcentral.com