Overlapping and distinct functions provided by fgf17 , a new zebrafish member of the Fgf8/17/18 subgroup of Fgfs

Abstract

Members of the fibroblast growth factor (Fgf) family are important signaling molecules in several inductive and patterning processes, and act as brain organizer-derived signals during formation of the early vertebrate nervous system. We isolated a new member of the Fgf8/17/18 subgroup of Fgfs from the zebrafish, and studied its expression and function during somitogenesis, optic stalk and midbrain-hindbrain boundary (MHB) development. In spite of a slightly higher aminoacid similarity to Fgf8, expression analysis and mapping to a chromosome stretch that is syntenic with mammalian chromosomes shows that this gene is orthologous to mammalian Fgf17. These data provide a further example of conserved chromosomal organization between zebrafish and mammalian genomes. Using an mRNA injection assay, we show that fgf17 can act similar to fgf8 during gastrulation, when fgf17 is not normally expressed. Direct comparison of the expression patterns of fgf17 and fgf8 suggest however a possible cooperation of these Fgfs at later stages in several tissues requiring Fgf signaling. Analysis of zebrafish MHB mutants demonstrates a gene-dosage dependent requirement of fgf17 expression for the no isthmus// pax2.1 gene, showing that no isthmus/pax2.1 functions upstream of fgf17 at the MHB in a haplo-insufficient manner, similar to what has been reported for mammalian pax2 mutants. In contrast, only maintenance of fgf17 expression is disturbed at the MHB of acerebellar/fgf8 mutants. Consistent with a requirement for fgf8 function, implantation of FGF8-soaked beads induces fgf17 expression, and expression is upregulated in aussicht mutants, which display upregulation of the Fgf8 signaling pathway. Taken together, our results argue that Fgf8 and Fgf17 act as hierarchically organized signaling molecules during development of the MHB organizer and possibly other organizers in the developing nervous system.

Duplication and divergence of fgf8 functions in teleost development and evolution

“…without ambiguity, that the gene previously annotated as fgf17a (Reifers et al, 2000a) is, along with fgf8a, a duplicate of the tetrapod Fgf8 gene, and so, by zebrafish nomenclature conventions (http://zfin.org/zf_info/nomen.html), should be called fgf8b. In contrast, our phylogenetic analysis strongly supports the identification of the gene we previously called fgf17b (Cao et al, 2004) as an ortholog of the tetrapod Fgf17 gene.…”

“…This is the first evidence that fgf genes show a pattern of evolution compatible with the ray-fin fish genome duplication (Amores et al, '98; Postlethwait et al, '98;Wittbrodt et al, '98;Meyer and Schartl, '99;Christoffels et al, 2004;Meyer and Van de Peer, 2005) and suggests either a high rate of gene loss in the fgf family in the ray-fin fish or that not all fgfs have been identified in this group. Reifers et al (2000a) first identified the zebrafish fgf8b (previously fgf17a) gene by screening a zebrafish genomic library with a mouse Fgf8 cDNA probe. They assigned orthology to mouse Fgf17 based on conserved synteny for three markers surrounding fgf8b on zebrafish LG1 and Fgf17 on mouse chromosome 14 and similar expression patterns.…”

“…As expression patterns evolve independently in different lineages, as shown for the fgf8a gene here, for example, gene expression is not a good character for identifying orthologies; rather genomic structure analyses, including phylogenetic analysis and analysis of conserved syntenies are far more reliable for inferring gene histories. Analysis of conserved syntenies (Postlethwait and Jovelin unpublished results) shows that, whereas somewhat distantly linked markers used by Reifers et al (2000a) support their orthology assignments, genes immediately adjacent to fgf8b are orthologous to genes adjacent to FGF8 in human and in mouse rather than genes adjacent to FGF17. In contrast, we assigned orthology of zebrafish fgf17 based on its proximity to two genes on LG8 orthologous to two genes located very close to FGF17 on human chromosome 8 (Hsa8) (Cao et al, 2004).…”

“…Investigation of the FGF content in the urochordate Ciona intestinalis revealed only six Fgf genes, with one being clearly ancestral to the Fgf8/17/18-subfamily (Satou et al, 2002). In zebrafish, fgf17a (Reifers et al, 2000a) and fgf17b (Cao et al, 2004) were thought to be co-orthologs of the mammalian Fgf17. The zebrafish fgf17 genes exhibit different FGF8 DUPLICATION AND FUNCTIONAL DIVERGENCE patterns of expression, and probably have distinct functions.…”

“…The zebrafish fgf17 genes exhibit different FGF8 DUPLICATION AND FUNCTIONAL DIVERGENCE patterns of expression, and probably have distinct functions. Fgf17a is thought to act as an organizer of the MHB (Reifers et al, 2000a), whereas Fgf17b seems to play a role in the anteroposterior patterning of the neurectoderm (Cao et al, 2004). The zebrafish fgf17a gene was named because of conserved synteny for three loci surrounding Fgf17 on mouse chromosome 14 and fgf17a on zebrafish linkage group 1 (Reifers et al, 2000a), despite showing more sequence similarity with fgf8 (Reifers et al, 2000a;Cao et al, 2004).…”