The roles of Hox genes in patterning the pharyngeal arches in zebrafish

The pharyngeal arches are one of the defining characters of the vertebrates. Comparative anatomists and morphologists have long recognized the segmental nature of the vertebrate pharyngeal region. Jaws form from the most anterior of these arches, and each arch contains skeletal elements with unique positions and shapes. Pharyngeal skeletal elements are formed from cranial neural crest cells that migrate from the neural tube during early development. My thesis work has addressed the role of Hox genes in patterning these skeletal elements in the teleost zebrafish. The Hox gene family of transcription factors are known to play important roles in specifying segmental identity along the anterior to posterior axis in metazoan embryos. I have studied the functions of zebrafish Hox paralog group 2 (PG2) and paralog group 3 (PG3) genes in pharyngeal arch patterning using both gain of function and loss of function approaches. I have found that Hox PG2 genes function redundantly to confer identity to the zebrafish second pharyngeal arch. My comparative studies have suggested that the roles of the Hox PG2 genes have diverged during the evolution of the vertebrates. Further, I have compared the regulation of Hox PG2 gene expression in the teleost and tetrapod vertebrates. I found conserved enhancer elements in teleost and tetrapod Hox PG2 gene regulatory regions, however some aspects of regulation have diverged between these vertebrates. Finally, using both loss-of-function and gain-of-function approaches I have examined the roles of zebrafish Hox PG3 genes in patterning the posterior pharyngeal arches. My results suggest that, similar to the situation in mouse, these genes do not function as homeotic selectors during pharyngeal development, or other Hox genes are performing overlapping or redundant roles. Taken together, my data have revealed both conserved and divergent Hox gene functions in pharyngeal patterning of the vertebrates.