The evolutionary relationships of zebrafish genes tbx6, tbx16/spadetail and mga

The T-box (Tbx) genes encode a family of transcription factors required for development of vertebrate embryos. In an attempt to discover human orthologues of the zebrafish (Danio rerio) tbx6 and tbx16/spadetail genes, we found that the most similar human (Homo sapiens) gene is the orthologue of mouse (Mus musculus) Mga (MAX gene associated). We have identified the zebrafish orthologue of Mga using analyses of sequence similarity and the orthologies of syntenic genes. Zebrafish mga maps close to dtk (developmental receptor tyrosine kinase), the orthologue of human TYRO3 (TYRO3 protein tyrosine kinase). Like its human and mouse orthologues, zebrafish mga lacks the three conserved introns within the T-box coding sequences that are characteristic of the vertebrate T-box gene family. This suggests that these genes are derived from an ancient reverse transcription event. The human genome does not appear to possess orthologues of zebrafish tbx6 or tbx16/spadetail.