Research in turtles and lizards has revealed a tantalizing clue to the evolution of the four-chambered heart and the related ability of birds and mammals to lead a warm-blooded existence. The key appears to be varied expression of the transcription factor gene Tbx5 in the ventricles. In humans and other mammals, Tbx5 levels are high in the left ventricle and low in the right. The boundary of high and low is right at the septum, which forms to separate the two ventricles. When the researchers looked at the green anole lizard, which has just a three-chambered heart, they found that Tbx5 activity was essentially the same throughout the single ventricle and stayed the same throughout heart development. In the turtle, however, which has a primitive septum that partially separates its ventricle into left and right sides, distribution of Tbx5 was gradually restricted to the left side of the ventricle, resulting in a left-right gradient of Tbx5 activity. Further experiments in genetically engineered mice conclusively showed that a sharp line demarcating an area of high levels of Tbx5 is critical to induce formation of a septum between the two ventricles. "This is the first genetic link to the evolution of two, rather than one, pumping chamber in the heart, which is a key event in the evolution of becoming warm-blooded," said Dr. Benoit Bruneau, the senior author of the study. "The gene involved, Tbx5, is also implicated in human congenital heart disease, so our results also bring insight into human disease." The work was featured as the cover story of the September 3 issue of Nature.
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