KirrelL, a member of the Ig-domain superfamily of adhesion proteins, is essential for fusion of primary mesenchyme cells in the sea urchin embryo

• KirrelL, an IgSF adhesion protein, is required for PMC fusion in sea urchins.
• In the absence of KirrelL, filopodial contacts do not result in membrane fusion.
• kirrelL is a member of a small, echinoid-specific gene family.
• IgSF proteins play an essential role in cell fusion in non-muscle cells.
• The PMC gene regulatory network controls a key effector of cell fusion.

In the sea urchin embryo, primary mesenchyme cells (PMCs) adhere to one another and fuse via filopodia, forming cable-like structures within which skeletal rods are deposited. Although this process was first described more than a century ago, molecules that participate in PMC adhesion and fusion have not been identified. Here we show that KirrelL, a PMC-specific, Ig domain-containing transmembrane protein, is essential for PMC fusion, probably by mediating filopodial adhesions that are a pre-requisite for subsequent membrane fusion. We show that KirrelL is not required for PMC specification, migration, or for direct filopodial contacts between PMCs. In the absence of KirrelL, however, filopodial contacts do not result in fusion. kirrelL is a member of a family of closely related, intronless genes that likely arose through an echinoid-specific gene expansion, possibly via retrotransposition. Our findings are significant in that they establish a direct linkage between the transcriptional network deployed in the PMC lineage and an effector molecule required for a critically important PMC morphogenetic process. In addition, our results point to a conserved role for Ig domain-containing adhesion proteins in facilitating cell fusion in both muscle and non-muscle cell lineages during animal development.