Fig. 6

A conserved pattern of Hox gene expression aligns with the pharyngeal pouches. Schematic representation of Hox gene expression in the pharyngeal pouches of a generalised gnathostome throughout pharyngeal development. a Initial pharyngeal pouch formation begins with the simultaneous budding off the pharyngeal endoderm of the first and second pharyngeal pouches. Hoxb1 aligns with the second, most posterior pharyngeal pouch. b At slightly older stages, four pharyngeal pouches present. Hoxa2 is now expressed with an anterior limit in the second pouch, Hoxa3 with an anterior limit at the third pouch. Hoxb1 no longer has an anterior limit at the second pouch but is now present in the most posterior pouch, which at this stage is the fourth. c Once all pouches have developed, the anterior limit of Hoxa2 expression still aligns with the second pouch while Hoxa3 expression is no longer seen in the endoderm. Hoxb1 expression no longer aligns with the fourth pouch but rather with the most posterior pouch, which at this stage is the sixth pouch. This represents a conserved dynamic expression pattern of Hoxb1 in the most posterior pouch present of gnathostomes at any given time point during pharyngeal pouch development irrespective of the number of pouches that have formed at that time. ;(pp = pharyngeal pouch)