A conserved regulatory program drives emergence of the lateral plate mesoderm

Cardiovascular lineages develop together with kidney, smooth muscle, and limb connective tissue progenitors from the lateral plate mesoderm (LPM). How the LPM initially emerges and how its downstream fates are molecularly interconnected remain unknown. Here, we isolated a pan-LPM enhancer in the zebrafishdraculin(drl) gene that provides specific LPM reporter activity from early gastrulation.In totolive imaging and lineage tracing ofdrl-based reporters captured the dynamic LPM emergence as lineage-restricted mesendoderm field. Thedrlpan-LPM enhancer responds to the transcription factors EomesoderminA, FoxH1, and MixL1 that combined with Smad activity drive LPM emergence. We uncovered specificdrlreporter activity in LPM-corresponding territories of several chordates including chicken, axolotl, lamprey,Ciona, and amphioxus, revealing a universal upstream LPM program. Altogether, our work provides a mechanistic framework for LPM emergence as defined progenitor field, possibly representing an ancient mesodermal cell state that predates the primordial vertebrate embryo.