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Amphioxus FGF Signaling Predicts the Acquisition of Vertebrate Morphological Traits

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Amphioxus FGF Signaling Predicts the Acquisition of Vertebrate Morphological Traits Amphioxus FGF signaling predicts the acquisition of vertebrate morphological traits Stephanie Bertranda,1, Alain Camassesa,1,2, Ildiko Somorjaia,3, Mohamed R. Belgacema, Olivier Chabrolb, Marie-Line Escandea, Pierre Pontarottib, and Hector Escrivaa,4 aCentre National de la Recherche Scientifique, Unité Mixte de Recherche 7232, Université Pierre et Marie Curie Paris 06, Observatoire Océanologique, F-66651 Banyuls-sur-Mer, France; and bCentre National de la Recherche Scientifique, Unité Mixte de Recherche 6632, Evolutionary Biology and Modelization, University of Marseilles,13331 Marseilles, France Edited* by Walter J. Gehring, University of Basel, Basel, Switzerland, and approved April 19, 2011 (received for review September 25, 2010) FGF signaling is one of the few cell–cell signaling pathways con- lochordate amphioxus (Branchiostoma lanceolatum). Amphioxus served among all metazoans. The diversity of FGF gene content is an amenable model system to address this question because of among different phyla suggests that evolution of FGF signaling its basal position in the chordate tree. Moreover, its genomic, may have participated in generating the current variety of animal morphological, and developmental characteristics are probably forms. Vertebrates possess the greatest number of FGF genes, highly similar to those of the chordate ancestor (5). the functional evolution of which may have been implicated in We describe here the content of the amphioxus FGF genes, the acquisition of vertebrate-specific morphological traits. In this as well as their evolutionary relationships with vertebrate FGFs, study, we have investigated the roles of the FGF signal during using phylogeny and deep synteny conservation analyses. De- embryogenesis of the cephalochordate amphioxus, the best proxy tailed study of the expression patterns of all amphioxus FGFs for the chordate ancestor. We first isolate the full FGF gene com- indicates that FGF-dependent developmental processes in am- plement and determine the evolutionary relationships between phioxus are as varied as those of vertebrates. Moreover, our amphioxus and vertebrate FGFs via phylogenetic and synteny con- functional analysis shows that (i) during early development, FGF servation analysis. Using pharmacological treatments, we inhibit signal in amphioxus controls cellular movements through a MAPK-independent pathway, similar to some vertebrates; (ii) the FGF signaling pathway in amphioxus embryos in different time in contrast to what is described in vertebrates, neural and me- EVOLUTION windows. Our results show that the requirement for FGF signaling soderm induction are not FGF-dependent in amphioxus; and (iii) during gastrulation is a conserved character among chordates, a MAPK-dependent FGF signal is necessary during gastrulation whereas this signal is not necessary for neural induction in amphi- for the formation of the rostral but not the posterior somites, oxus, in contrast to what is known in vertebrates. We also show where it is instead required for their normal maturation. Overall, that FGF signal, acting through the MAPK pathway, is necessary our work shows that the role of the FGF signaling pathway for the formation of the most anterior somites in amphioxus, during neural and mesoderm induction is a specific vertebrate whereas more posterior somite formation is not FGF-dependent. acquisition. In addition, changes in the requirement for the FGF This result leads us to propose that modification of the FGF signal signal during somitogenesis, particularly at the rostral level, could function in the anterior paraxial mesoderm in an amphioxus-like explain the appearance of the “new head” in vertebrates (6). vertebrate ancestor might have contributed to the loss of segmen- tation in the preotic paraxial mesoderm of the vertebrate head. Results Amphioxus FGF Gene Content. BLAST searches on the Bran- evo-devo | somitogenesis | gene family chiostoma floridae genome for FGF and FGFR genes resulted in the identification of eight putative genes containing an FGF nly a few cell–cell signaling molecules are known to play domain and one FGFR coding gene. The corresponding full- Oa major role during metazoan embryonic development. length cDNAs were cloned in B. lanceolatum. All the predicted Among them, the FGFs were discovered in the mid-1970s in ver- FGF proteins contain a highly conserved FGF domain, and five tebrates. FGFs are small proteins, generally secreted, character- contain a putative signal peptide in their N-terminal region ized by a conserved functional domain and acting through binding (Table S1). The unique FGFR gene was previously described as to their transmembrane receptors [FGF receptors (FGFRs)], the ortholog of the four vertebrate FGFRs (7). We used phylo- causing them to homodimerize. This leads to intracellular auto- genetic reconstructions based on the alignment of the FGF phosphorylation and further activation of cytoplasmic signaling domains to assign the orthology relationships of the amphioxus 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 Author contributions: S.B., A.C., I.S., P.P., and H.E. designed research; S.B., A.C., I.S., M.R.B., ancestor of diploblastic and bilaterian animals (2). In vertebrates, O.C., M.-L.E., and H.E. performed research; S.B., I.S., P.P., and H.E. analyzed data; and S.B., at least 22 genes coding for FGFs and 4 coding for their FGFRs I.S., and H.E. wrote the paper. are known. In contrast, only 3 genes coding for FGFs and 2 coding The authors declare no conflict of interest. for FGFRs have been described in Drosophila (2). In the uro- *This Direct Submission article had a prearranged editor. chordate Ciona intestinalis, a member of the sister group of ver- Data deposition: The sequences reported in this paper have been deposited in the Gen- tebrates, only 6 FGF genes are present, for which some orthology Bank database (accession nos. EU606032.1, EU606035.1, EU606036.1, EU606033.1, relationships with vertebrate FGFs are still not resolved (3). This EU606034.1, EU606038.1, HM854710, EU606037.1, HM854709, HM359129, HM359124, large number of FGFs and FGFRs in vertebrates raises the HM359125, and HM359126). question of their implication in the evolution of vertebrate-specific 1S.B. and A.C. contributed equally to this work. morphological characteristics. 2Present address: Institut de Génétique Moléculaire de Montpellier, Centre National de la In vertebrates, FGF signaling is involved in different devel- Recherche Scientifique, Unité Mixte de Recherche 5535, 34293 Montpellier Cedex 5, opmental processes, among which are neural and mesoderm France. induction in the early embryo, and somitogenesis and limb bud 3Present address: Departament de Genètica, Facultat de Biologia, Universitat de Barce- formation at later stages (4). To investigate the evolution of lona, E08028 Barcelona, Spain. FGF signaling pathway function during embryogenesis at the 4To whom correspondence should be addressed. E-mail: [email protected]. invertebrate chordate-to-vertebrate transition, we undertook This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. a study of the embryonic role of this pathway in the cepha- 1073/pnas.1014235108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1014235108 PNAS Early Edition | 1of6 Downloaded by guest on September 29, 2021 FGF genes. No matter the method used (maximum likelihood or visible in the mesoderm at the midneurula stage (Fig. 2D). Bayesian inference), only three FGF genes have a well-sup- FGFA expression in the anterior dorsal mesendoderm is only ported position in the phylogenetic tree (Fig. S1), namely, FGF1/ observed at the early midneurula stage (Fig. 2M). FGFE is also 2, FGF8/17/18, and FGF9/16/20, which are placed at the base of expressed in a very restricted mesodermal domain, the first left the vertebrate FGF1/2, FGF8/17/18, and FGF9/16/20 paralogy somite, and only very transiently in midlate neurula embryos groups, respectively. The other five genes were named FGFA, (Fig. 2Q). Four amphioxus FGFs, including FGF8/17/18, FGF9/ FGFB, FGFC, FGFD, and FGFE. To go further, we looked into 16/20, FGFA, and FGFE, are expressed in the central nervous synteny conservation of the FGF and FGFR gene loci between system, although the expression is very dynamic. The first three amphioxus and vertebrates (Fig. 1). For FGF1/2 (Fig. 1A), are detected in the anterior neural plate and cerebral vesicle but FGF8/17/18 (Fig. 1D), and FGFR (Fig. 1F), synteny conserva- at different developmental stages (Fig. 2V). FGF9/16/20 is the tion with vertebrates reinforces their orthology relationships. We earliest neurally expressed gene, with restricted expression in the also found amphioxus/vertebrate synteny conservation for presumptive neural plate of the gastrulating embryo (Fig. 2 G FGFA, FGFB, and FGFC genes (Fig. 1 B, C, and E), indicating and H). It is thereafter expressed segmentally in the neural tube that they are orthologs of the FGF3/7/10/22, FGF4/5/6, and until the late neurula stage (Fig. 2 J and K); at that time, FGFE FGF19/21/23 vertebrate paralogy groups, respectively. expression starts to be observed in some neurons up until the larval stage (Fig. 2 R and S). With respect to the endoderm, FGFs Show a Dynamic Expression Pattern During Embryogenesis. We specific expression in the gut is first observed for FGFC in the analyzed the expression pattern of FGFR and the eight FGF late neurula stage embryo (Fig. 2T). Subsequently, in the larva, genes from the eight-cell stage to the larval stage. The FGFR FGF9/16/20, FGFA, and FGFE are expressed in the midgut gene is ubiquitously expressed at all the studied stages, except in region, with the first two also being expressed in the anus (Fig. 2 L the epidermis, with a higher expression level in the mesoderm and P). The pharyngeal endoderm is the embryonic region show- (Fig. S2). Expression of two of the FGF genes (FGFB and ing the most complex FGF expression combination (Fig.
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