Among lizards, only monitor lizards (Varanidae) have a functionally divided cardiac ventricle. The division results from the combined function of three partial septa, which may be homologous to the ventricular septum of mammals and archosaurs. We show in developing monitors that two septa, the 'muscular ridge' and 'bulbuslamelle', express the evolutionarily conserved transcription factors Tbx5, Irx1 and Irx2, orthologues of which mark the mammalian ventricular septum. Compaction of embryonic trabeculae contributes to the formation of these septa. The septa are positioned, however, to the right of the atrioventricular junction and they do not participate in the separation of incoming atrial blood streams. That separation is accomplished by the 'vertical septum', which expresses Tbx3 and Tbx5 and orchestrates the formation of the electrical conduction axis embedded in the ventricular septum. These expression patterns are more pronounced in monitors than in other lizards, and are associated with a deep electrical activation near the vertical septum, in contrast to the primitive base-to-apex activation of other lizards. We conclude that evolutionarily conserved transcriptional programmes may underlie the formation of the ventricular septa of monitors.

Department of Bioscience Zoophysiology Aarhus University 8000 Aarhus Denmark

Department of Cardiology Institute of Clinical Medicine Aarhus University Hospital Skejby 8200 Aarhus Denmark

Department of Developmental Cardiology Institute of Physiology Academy of Sciences of the Czech Republic Vídeňská 1083 142 20 Prague Czech Republic Charles University 1st Faculty of Medicine Institute of Anatomy U Nemocnice 3 128 00 Prague Czech Republic

Department of Radiology and Nuclear Medicine University of Amsterdam Amsterdam UMC Meibergdreef 9 1105AZ Amsterdam The Netherlands

University of Amsterdam Amsterdam UMC Department of Medical Biology Amsterdam Cardiovascular Sciences Meibergdreef 15 1105AZ Amsterdam The Netherlands

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