A novel cis-regulatory element drives early expression of <i>Nkx3.2</i> in the gnathostome primary jaw joint.

The acquisition of movable jaws was a major event during vertebrate evolution. The role of NK3 homeobox 2 (Nkx3.2) transcription factor in patterning the primary jaw joint of gnathostomes (jawed vertebrates) is well known, however knowledge about its regulatory mechanism is lacking. In this study, we report a proximal enhancer element of <i>Nkx3.2</i> that is deeply conserved in most gnathostomes but undetectable in the jawless hagfish and lamprey. This enhancer is active in the developing jaw joint region of the zebrafish <i>Danio rerio</i>, and was thus designated as <i>jaw joint regulatory sequence 1</i> (JRS1). We further show that JRS1 enhancer sequences from a range of gnathostome species, including a chondrichthyan and mammals, have the same activity in the jaw joint as the native zebrafish enhancer, indicating a high degree of functional conservation despite the divergence of cartilaginous and bony fish lineages or the transition of the primary jaw joint into the middle ear of mammals. Finally, we show that deletion of JRS1 from the zebrafish genome using CRISPR/Cas9 results in a significant reduction of early gene expression of <i>nkx3.2</i> and leads to a transient jaw joint deformation and partial fusion. Emergence of this <i>Nkx3.2</i> enhancer in early gnathostomes may have contributed to the origin and shaping of the articulating surfaces of vertebrate jaws.