crabp and maf highlight the novelty of the amphioxus club-shaped gland

The club-shaped gland (csg) is a prominent organ during the development of amphioxus. However, the evolutionary significance of this pharyngeal structure has been a mystery because of the lack of an obvious corollary in vertebrates or other close relatives. To address the homology of the csg by molecular means, we report the cloning and expression patterns of two amphioxus genes expressed during its development, crabp and maf. Amphioxus maf is a bzip transcription factor expressed early in csg formation in the forming of the ventral duct of the gland. crabp encodes a cellular retinoic acid binding protein and is expressed widely in the csg later in its development. We compare these genes to the expression of AmphiKrox, a zinc-finger transcription factor previously reported to be expressed during csg development. Together these genes mark different spatial and temporal aspects of csg formation. However, we find little evidence to suggest homology of the csg with other organs in amphioxus or other chordates. We therefore propose that the csg can be viewed as an evolutionary novelty that probably arose within the amphioxus lineage.