Acquisition of neural crest promoted thyroid evolution from chordate endostyle
The endostyle is an endodermal organ unique to non-vertebrate chordates except for lamprey larvae, where it serves as forerunner to the adult thyroid. Here, we examine if the acquisition of neural crest in the vertebrate lineage played a role in the elaboration of the endostyle. CM-DiI lineage tracing reveals a neural crest contribution to the endostyle, and CRISPR/Cas9 mutagenesis of key neural crest genes causes endostyle defects including formation of a single rather than bilobed structure. RNA-sequencing reveals gene profiles characteristic of embryonic neural crest cells and Schwann cell precursors in the developing endostyle. Contrasting with the prevailing view that the endostyle is an endoderm-derived organ, we propose that the acquisition of the neural crest played a critical step in promoting thyroid evolution from chordate endostyle. Overall design: To investigate the expression profiles of the lamprey endostyle, we performed bulk RNA sequencing of several developmental stages of lamprey endostyle.