FGF signaling is one of the few cell-cell signaling pathways conserved among all metazoans. The diversity of FGF gene content among different phyla suggests that evolution of FGF signaling may have participated in generating the current variety of animal forms. Vertebrates possess the greatest number of FGF genes, the functional evolution of which may have been implicated in the acquisition of vertebrate-specific morphological traits. In this study, we have investigated the roles of the FGF signal during embryogenesis of the cephalochordate amphioxus, the best proxy for the chordate ancestor. We first isolate the full FGF gene complement and determine the evolutionary relationships between amphioxus and vertebrate FGFs via phylogenetic and synteny conservation analysis. Using pharmacological treatments, we inhibit the FGF signaling pathway in amphioxus embryos in different time windows. Our results show that the requirement for FGF signaling during gastrulation is a conserved character among chordates, whereas this signal is not necessary for neural induction in amphioxus, in contrast to what is known in vertebrates. We also show that FGF signal, acting through the MAPK pathway, is necessary for the formation of the most anterior somites in amphioxus, whereas more posterior somite formation is not FGF-dependent. This result leads us to propose that modification of the FGF signal function in the anterior paraxial mesoderm in an amphioxus-like vertebrate ancestor might have contributed to the loss of segmentation in the preotic paraxial mesoderm of the vertebrate head.O nly a few cell-cell signaling molecules are known to play a major role during metazoan embryonic development. Among them, the FGFs were discovered in the mid-1970s in vertebrates. FGFs are small proteins, generally secreted, characterized by a conserved functional domain and acting through binding to their transmembrane receptors [FGF receptors (FGFRs)], causing them to homodimerize. This leads to intracellular autophosphorylation and further activation of cytoplasmic signaling cascades, such as the MAPK and the PI3K pathways (1).Sequencing of several complete metazoan genomes has shown that FGF and FGFR genes were already present in the common ancestor of diploblastic and bilaterian animals (2). In vertebrates, at least 22 genes coding for FGFs and 4 coding for their FGFRs are known. In contrast, only 3 genes coding for FGFs and 2 coding for FGFRs have been described in Drosophila (2). In the urochordate Ciona intestinalis, a member of the sister group of vertebrates, only 6 FGF genes are present, for which some orthology relationships with vertebrate FGFs are still not resolved (3). This large number of FGFs and FGFRs in vertebrates raises the question of their implication in the evolution of vertebrate-specific morphological characteristics.In vertebrates, FGF signaling is involved in different developmental processes, among which are neural and mesoderm induction in the early embryo, and somitogenesis and limb bud formation at later stages (4)....

BackgroundThe basally divergent phylogenetic position of amphioxus (Cephalochordata), as well as its conserved morphology, development and genetics, make it the best proxy for the chordate ancestor. Particularly, studies using the amphioxus model help our understanding of vertebrate evolution and development. Thus, interest for the amphioxus model led to the characterization of both the transcriptome and complete genome sequence of the American species, Branchiostoma floridae. However, recent technical improvements allowing induction of spawning in the laboratory during the breeding season on a daily basis with the Mediterranean species Branchiostoma lanceolatum have encouraged European Evo-Devo researchers to adopt this species as a model even though no genomic or transcriptomic data have been available. To fill this need we used the pyrosequencing method to characterize the B. lanceolatum transcriptome and then compared our results with the published transcriptome of B. floridae.ResultsStarting with total RNA from nine different developmental stages of B. lanceolatum, a normalized cDNA library was constructed and sequenced on Roche GS FLX (Titanium mode). Around 1.4 million of reads were produced and assembled into 70,530 contigs (average length of 490 bp). Overall 37% of the assembled sequences were annotated by BlastX and their Gene Ontology terms were determined. These results were then compared to genomic and transcriptomic data of B. floridae to assess similarities and specificities of each species.ConclusionWe obtained a high-quality amphioxus (B. lanceolatum) reference transcriptome using a high throughput sequencing approach. We found that 83% of the predicted genes in the B. floridae complete genome sequence are also found in the B. lanceolatum transcriptome, while only 41% were found in the B. floridae transcriptome obtained with traditional Sanger based sequencing. Therefore, given the high degree of sequence conservation between different amphioxus species, this set of ESTs may now be used as the reference transcriptome for the Branchiostoma genus.