ETV2 induces endothelial, but not hematopoietic, lineage specification in birds

Cardiovascular system develops from the lateral plate mesoderm. Its three primary cell lineages (hematopoietic, endothelial, and muscular) are specified by the sequential actions of conserved transcriptional factors.ETV2, a master regulator of mammalian hemangioblast development, however, is absent in the chicken genome and acts downstream ofNPAS4Lin zebrafish. Here, we investigated the epistatic relationship between NPAS4L and ETV2 in avian hemangioblast development. We showed thatETV2is deleted in all 363 avian genomes analyzed. Mouse ETV2 induced LMO2, but not NPAS4L or SCL, expression in chicken mesoderm. Squamate (lizards, geckos, and snakes) genomes contain bothNPAS4LandETV2. In Madagascar ground gecko, both genes were expressed in developing hemangioblasts. Gecko ETV2 induced only LMO2 in chicken mesoderm. We propose that bothNPAS4LandETV2were present in ancestral amniote, with ETV2 acting downstream of NPAS4L in endothelial lineage specification. ETV2 may have acted as a pioneer factor by promoting chromatin accessibility of endothelial-specific genes and, in parallel withNPAS4Lloss in ancestral mammals, has gained similar function in regulating blood-specific genes.