The evolution of a new cell type was associated with competition for a signaling ligand

There is presently a very limited understanding of the mechanisms that underlie the evolution of new cell types. The skeleton-forming primary mesenchyme cells (PMCs) of euechinoid sea urchins, derived from the micromeres of the 16-cell embryo, are an example of a

recently evolved cell type. All adult echinoderms have a calcite-based endoskeleton, a synapomorphy of the Ambulacraria. Only euechinoids have a micromere-PMC lineage, however, which evolved through the co-option of the adult skeletogenic program into the

embryo. During normal development, PMCs alone secrete the embryonic skeleton. Other

mesoderm cells, known as blastocoelar cells (BCs), have the potential to produce a skeleton, but a PMC-derived signal ordinarily prevents these cells from expressing a skeletogenic

fate and directs them into an alternative developmental pathway. Recently, it was shown

that vascular endothelial growth factor (VEGF) signaling plays an important role in PMC differentiation and is part of a conserved program of skeletogenesis among echinoderms.

Here, we report that VEGF signaling, acting through ectoderm-derived VEGF3 and its cognate receptor, VEGF receptor (VEGFR)-10-Ig, is also essential for the deployment of the

skeletogenic program in BCs. This VEGF-dependent program includes the activation of

aristaless-like homeobox 1 (alx1), a conserved transcriptional regulator of skeletogenic

specification across echinoderms and an example of a “terminal selector” gene that controls

cell identity. We show that PMCs control BC fate by sequestering VEGF3, thereby preventing activation of alx1 and the downstream skeletogenic network in BCs. Our findings provide

an example of the regulation of early embryonic cell fates by direct competition for a

secreted signaling ligand, a developmental mechanism that has not been widely recognized. Moreover, they reveal that a novel cell type evolved by outcompeting other embryonic

cell lineages for an essential signaling ligand that regulates the expression of a gene controlling cell identity.