Genomic organization, alternative splicing, and multiple regulatory regions of the zebrafish fgf8 gene

Fgf8 is among the members of the fibroblast growth factor (FGF) family that play pivotal roles in vertebrate development. In the present study, the genomic DNA of the zebrafish fgf8 gene was cloned to elucidate the regulatory mechanism behind the temporally and spatially restricted expression of the gene in vertebrate embryos. Structural analysis revealed that the exon-intron organization of fgf8 is highly conserved during vertebrate evolution, from teleosts to mammals. Close inspection of the genomic sequence and reverse transcription-polymerase chain reaction analysis revealed that zebrafish fgf8 encodes two splicing variants, corresponding to Fgf8a and Fgf8b, among the four to seven splicing variants known in mammals. Misexpression of the two variants in zebrafish embryos following mRNA injection showed that both variants have dorsalizing activities on zebrafish embryos, with Fgf8b being more potent. Reporter gene analysis of the transcriptional regulation of zebrafish fgf8 suggested that its complicated expression pattern, which is considered essential for its multiple roles in development, is mediated by combinations of different regulatory regions in the upstream and downstream regions of the gene. Furthermore, comparison of the genomic sequence of fgf8 among different vertebrate species suggests that this regulatory mechanism is conserved during vertebrate evolution.