Expression of the amphioxus Pit-1 gene (AmphiPOU1F1/Pit-1) exclusively in the developing preoral organ, a putative homolog of the vertebrate adenohypophysis

For the Florida amphioxus (Branchiostoma floridae), the full-length sequence and developmental expression of AmphiPOU1F1/Pit-1 are described. This gene, which is present in a single copy in the genome, is homologous to Pit-1 genes of vertebrates that play key roles in the development of the adenohypophysis. During amphioxus development, AmphiPOU1F1/Pit-1 transcripts are limited to Hatschek's left diverticulum and the larval tissue developing from it—namely the concave portion of the preoral organ. No other expression domains for this gene were detected during embryonic and larval development. From data currently available for hemichordates, amphioxus and ascidians, the best supported homologs for the vertebrate adenohypophysis are the preoral ciliary organ of hemichordates, preoral organ/Hatschek's pit of amphioxus and the neural gland/duct complex of ascidians. Better insights into pituitary evolution will require additional information: for invertebrate deuterostomes, more of the key pituitary genes in hemichordates and tunicates need to be studied; for the more basal groups vertebrates, it will be important to determine whether the source of the adenohypophysis is endodermal or ectodermal and to demonstrate what, if any, contribution mesodermal head coeloms might make to the developing pituitary.