Prominent transcription of zebrafish N- myc ( nmyc1) in tectal and retinal growth zones during embryonic and early larval development

Because of its oncogenic capacity and expression restricted to embryonic and newborn tissues, the N- myc proto-oncogene is suggested to play a key role in vertebrate organogenesis as well as in the control of cell proliferation and differentiation. To further approach the developmental function of N- myc, we cloned full-length zebrafish N- myc ( nmyc1) and analyzed its expression in the embryo and early larva. nmyc1 transcription is initiated at the mid-blastula stage. At somitogenesis stages, its expression was detected in the retina, midbrain, posterior hindbrain and presumptive spinal cord. nmyc1 was also transcribed in the endoderm and its derivatives as well as in branchial arches. At later developmental stages, posterior neural expression of nmyc1 was switched off, but expression remained intense in the brain, mainly in the optic tectum, cerebellar plate and dorsal rhombomere 2. Comparison of nmyc1 transcription with proliferation zones using a M phase mitotic marker revealed that nmyc1 expression is specifically associated with mitosis in the optic tectum and the retina. This result contrasts with previous studies in other vertebrates where N- myc expression can persist in differentiating cells.