Sequence and developmental expression of AmphiTob, an amphioxus homolog of vertebrate Tob in the PC3/BTG1/Tob family of tumor suppressor genes.

Tob is a member of the PC3/ BTG1/Tob family of vertebrate tumor suppressor genes; its expression is known to inhibit proliferation of cells in vitro, but its possible roles during normal development have not been investigated previously. The present study concerns the structure and developmental expression of AmphiTob in an invertebrate chordate, amphioxus. This is the first investigation of any Tob gene during embryological development. The 311 amino acid AmphiTob protein is similar to vertebrate Tob but lacks the C-terminal PQ-rich domain of the latter. In early embryos of amphioxus, in situ hybridization first reveals AmphiTob expression in the hypoblast at the gastrula stage on the likely dorsal side of the embryo. During subsequent development, expression is seen in several tissues of the ectoderm, mesoderm, and endoderm. The most striking expression domains are in the developing somitic musculature and dorsal nerve cord. In the medial wall of each somite, AmphiTob is expressed strongly by cells destined to differentiate into the axial trunk muscles; this pattern persists until late in the larval stage, evidently because undifferentiated cells are continually becoming myogenic as the muscles grow. Nerve cord cells conspicuously transcribe AmphiTob from the late neurula until the early larval stage: Expression occurs in a few cells scattered along the nerve cord and in a group of cells located in the cerebral vesicle (in a region presumably homologous to the vertebrate diencephalic forebrain). During development, an intense and transitory transcription of AmphiTob may be an early event in cells exiting the cell cycle in preparation for differentiation.