Distinct gene structure and expression pattern of Smad nuclear interacting protein 1 in amphioxus and zebrafish.

A Smad nuclear interacting protein 1 (SNIP1) homologous gene was identified in amphioxus. Phylogenic analysis showed that SNIP1 proteins from different species share a highly conserved FHA domain at the C-terminus, but their N-terminus varies remarkably. The genomic structure of SNIP1 varies in different species, especially at the 5' end. Through in situ hybridization, we studied SNIP1 expression patterns in amphioxus and zebrafish embryos. Amphioxus SNIP1 transcripts were specifically located in the notochord in larval and adult stages. In zebrafish, however, snip1 transcripts were specifically located not only in the notochord, but also in the rhombencephalic ventricle, otic vesicles and pectoral fin buds. This is the first report of SNIP1 expression pattern in early development, which clearly shows different expression patterns between invertebrates and vertebrates. Previous studies reported that it is the N-terminal domain of human and mouse SNIP1 that functions to inhibit both TGF-ß and the NF-κB pathways. Therefore, it is most likely that the modification of SNIP1 expression pattern is related to the remarkable evolution in the N-terminal sequence. In addition, the difference in SNIP1 expression pattern between amphioxus and zebrafish implies the role of SNIP1 in the vertebrate body structural innovation of brain, otic vesicles and pectoral fins.