1. Gladstone Institute of Cardiovascular Disease, San Francisco, California 94158, USA
2. Department of Pediatrics,
3. Cardiovascular Research Institute, University of California, San Francisco, California 94158, USA
4. Division of Cardiovascular Research, Global-Edge Institute, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8503, Japan
5. Program in Stem Cell and Developmental Biology, The Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada
6. Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
7. Biology Department, Millersville University, Millersville, Pennsylvania 17551, USA
8. Institut de Recherches Cliniques de Montréal, Programme de Biologie Moléculaire, Université de Montréal, Montréal, Québec H3C 3J7, Canada
9. Department of Psychology and Program in Neuroscience, Michigan State University, East Lansing, Michigan 48824, USA
10. The Mouse Imaging Centre, The Hospital for Sick Children, Toronto, Ontario M5G 1X8, Canada
11. Department of Medical Biophysics, University of Toronto, Toronto, Ontario M5S 1A8, Canada
12. Department of Biochemistry and Biophysics, University of California, San Francisco, California 94158, USA
13. Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA
14. Department of Biochemistry, Microbiology and Immunology, University of Ottawa, Ottawa, Ontario K1H 8M5 Canada
15. Department of Biology, Swarthmore College, Swarthmore, Pennsylvania 19081, USA
16. These authors contributed equally to this work.

Correspondence to: Benoit G. Bruneau^1,2,3,5,6 Correspondence and requests for materials should be addressed to B.G.B. (Email: bbruneau@gladstone.ucsf.edu).

Abstract

The emergence of terrestrial life witnessed the need for more sophisticated circulatory systems. This has evolved in birds, mammals and crocodilians into complete septation of the heart into left and right sides, allowing separate pulmonary and systemic circulatory systems, a key requirement for the evolution of endothermy^{1, 2, 3}. However, the evolution of the amniote heart is poorly understood. Reptilian hearts have been the subject of debate in the context of the evolution of cardiac septation: do they possess a single ventricular chamber or two incompletely septated ventricles^{4, 5, 6, 7}? Here we examine heart development in the red-eared slider turtle, Trachemys scripta elegans (a chelonian), and the green anole, Anolis carolinensis (a squamate), focusing on gene expression in the developing ventricles. Both reptiles initially form a ventricular chamber that homogenously expresses the T-box transcription factor gene Tbx5. In contrast, in birds and mammals, Tbx5 is restricted to left ventricle precursors^{8, 9}. In later stages, Tbx5 expression in the turtle (but not anole) heart is gradually restricted to a distinct left ventricle, forming a left–right gradient. This suggests that Tbx5 expression was refined during evolution to pattern the ventricles. In support of this hypothesis, we show that loss of Tbx5 in the mouse ventricle results in a single chamber lacking distinct identity, indicating a requirement for Tbx5 in septation. Importantly, misexpression of Tbx5 throughout the developing myocardium to mimic the reptilian expression pattern also results in a single mispatterned ventricular chamber lacking septation. Thus ventricular septation is established by a steep and correctly positioned Tbx5 gradient. Our findings provide a molecular mechanism for the evolution of the amniote ventricle, and support the concept that altered expression of developmental regulators is a key mechanism of vertebrate evolution.

MORE ARTICLES LIKE THIS

These links to content published by NPG are automatically generated.