Fibroblast growth factors (FGFs) are essential for mesoderm induction in vertebrates and for early mesoderm formation in invertebrate chordates. However, functional studies to date do not support a role for FGF signalling in mesoderm induction in other deuterostomes (animals in which the first embryonic opening forms the anus), such as sea urchins. Thus, the ancestral role of FGF signalling during mesoderm specification in deuterostomes is unclear. On , Christopher Lowe and co-workers examine the role of FGF signalling during the early development of the hemichordate Saccoglossus kowalevskii. The researchers report that the FGF ligand fgf8/17/18 is expressed in the ectoderm overlying sites of mesoderm specification within this marine worm’s archenteron (primitive gut) endomesoderm. Mesoderm induction, they show, requires contact between the ectoderm and the endomesoderm. Moreover, loss-of-function experiments indicate that FGF ligand and receptor are both necessary for mesoderm specification. These and other results indicate that FGF signalling is required throughout mesoderm specification in hemichordates and support an ancestral role for FGF signalling in mesoderm formation in deuterostomes.
