Sequence and developmental expression of amphioxus AmphiNk2-1: insights into the evolutionary origin of the vertebrate thyroid gland and forebrain

We characterized an amphioxus NK-2 homeobox gene (AmphiNk2-1), a homologue of vertebrate Nkx2-1, which is involved in the development of the central n ervous system and thyroid gland. At the early neurula stage of amphioxus, A mphiNk2-1 expression is first detected medially in the neural plate. By the mid-neurula stage, expression is localized ventrally in the nerve cord and also begins in the endoderm. During the late neurula stage, the ventral ne ural expression becomes transiently segmented posteriorly and is then down- regulated except in the cerebral vesicle at the anterior end of the central nervous system. Within the cerebral vesicle AmphiNk2-1 is expressed in a b road ventral domain, probably comprising both the floor plate and basal pla te regions: this pattern is comparable to Nkx2-1 expression in the mouse di encephalon. In the anterior part of the gut, expression becomes intense in the endostyle (the right wall of the pharynx), which is the presumed homolo gue of the vertebrate thyroid gland. More posteriorly, there is transitory expression in the midgut and hindgut. In sum, the present results help to s upport homologies (1) between the amphioxus endostyle and the vertebrate th yroid gland and (2) between the amphioxus cerebral vesicle and the vertebra te diencephalic forebrain.