Evolution of Pharyngeal Slits
The presence of pharyngeal slits in hemichordates led to debates of whether this structure was homologous to the slits found in chordates or a result of convergent evolution. With the placement of hemichordates and echinoderms as a sister group to chordates, a new hypothesis has emerged-suggesting that pharyngeal gill slits were present in the deuterostome ancestor . Intriguingly, extant echinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures. Comparative developmental and genetic studies of these pharyngeal structures between hemichordates and urochordates have brought about important insights regarding the evolution of the deuterostome body plan. Comparative molecular biology has revealed that the Pax1/9 genes (which encode for transcription factors) are expressed in similar patterns between hemichordates and urochordates; In vertebrates, Pax 1 and Pax 9 are expressed in the pharyngeal pouches and are important for thymus development. Interestingly applying excess retinoic acid (excess retincoic acid in vertebrates results in pharyngeal abnormalities) leads to the absence of gill slits in developing Amphioxus, suggesting that retinoic acid may act through the same mechanism in vertebrates and amphioxus. These studies indicate that the pharyngeal slits found in hemichordates and chordates are indeed homologous in a molecular sense.
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