Functions of zebrafish Hox paralog group 2 and 3 genes in hindbrain and pharyngeal arch development

The hindbrain and pharyngeal arches are series of reiterated segments that form along the anterior/posterior (A/P) axis during early vertebrate head development. Proper specification of A/P identity of these segments is important for overall head patterning. Hox genes encode homeodomain-containing transcription factors that play an important role in specifying segment identity within the hindbrain and arches. We have used both loss-of-function (morpholino-mediated knock-down) and gain-of-function (misexpression) approaches to study the roles of Hox paralog group (PG) 2 and PG3 genes during head patterning. The two zebrafish PG2 genes, hoxa2 and hoxb2, are both expressed in neural crest cells that migrate into the second pharyngeal arch. We show that these genes function redundantly to specify second pharyngeal arch identity. Loss of Hox PG2 function causes second arch structures to phenocopy those of the first arch, whereas gain of function causes a reciprocal phenotype. In both cases duplicated arch structures form a mirror-image copy of endogenous structures suggesting the existence of an “organizer” between the arches. We are currently testing candidate genes for organizing activity. The zebrafish hoxa3 and hoxb3 genes are expressed in hindbrain rhombomeres (r) 5 and 6 and in neural crest cells that migrate into the third and posterior arches. We show that knock-down of these two genes prevents differentiation of the r5- and r6-specific abducens motor neurons. We are currently investigating additional phenotypes that result from loss and gain-of-function of Hox PG3 genes.