<i>Hox3</i> Is Required for Post‐Metamorphic Heart Development in Adult <i>Ciona</i>

Although the differentiation of cardiac progenitors during embryogenesis has been characterized in detail in the ascidian Ciona robusta, heart development after metamorphosis remains unclear. The sub-terminal regions at both ends of the Ciona heart harbor pacemaker cells, as well as undifferentiated cells located within the growth zone. We performed an RNA-Seq analysis to identify transcription factors predominantly expressed in the sub-terminal regions of the Ciona heart. Among the 17 transcription factors predominantly expressed in one or both of the sub-terminal regions, Hox3 showed the strongest expression in both. Since the role of Hox3 during Ciona heart development remained unknown, we investigated the spatial expression and function of Hox3. In situ hybridization revealed the expression of Hox3 in undifferentiated cells within the growth zone at both ends of the heart tube. The TALEN-mediated disruption of Hox3 in cardiac progenitors resulted in irregularly swollen or shortened heart tubes. These results suggest that Hox3 plays a crucial role in heart tube formation by regulating the activity of growth zone cells. Similar Hox3 expression in the terminal regions of ascidian and vertebrate hearts suggests the partial conservation of cardiac patterning and pacemaker localization.