Mammals and birds have a specialized cardiac atrioventricular conduction system enabling rapid activation of both ventricles. This system may have evolved together with high heart rates to support their endothermic state (warm-bloodedness) and is seemingly lacking in ectothermic vertebrates from which first mammals then birds independently evolved. Here, we studied the conduction system in crocodiles (Alligator mississippiensis), the only ectothermic vertebrates with a full ventricular septum. We identified homologues of mammalian conduction system markers (Tbx3-Tbx5, Scn5a, Gja5, Nppa-Nppb) and show the presence of a functional atrioventricular bundle. The ventricular Purkinje network, however, was absent and slow ventricular conduction relied on trabecular myocardium, as it does in other ectothermic vertebrates. We propose the evolution of the atrioventricular bundle followed full ventricular septum formation prior to the development of high heart rates and endothermy. In contrast, the evolution of the ventricular Purkinje network is strongly associated with high heart rates and endothermy.

Department of Biological Sciences University of North Texas Denton United States

Department of Biomedical Engineering George Washington University Washington DC United States

Department of Clinical Genetics Academic Medical Center University of Amsterdam Amsterdam The Netherlands

Department of Medical Biology Heart Failure Research Center Academic Medical Center University of Amsterdam Amsterdam The Netherlands

Institute of Anatomy 1st Medical Faculty Charles University and Institute of Physiology Czech Academy of Sciences Prague Czech Republic

Rockefeller Wildlife Refuge Louisiana Department of Wildlife and Fisheries Grand Chenier United States

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Elife. 2018 May 08;7:e36882. doi: 10.7554/eLife.36882. PubMed

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