Sites of Fgf signalling and perception during embryogenesis of the beetle Tribolium castaneum.

The development of multicellular embryos depends on coordinated cell-to-cell signalling events. Among the numerous cell-signalling pathways, fibroblast growth factors (FGFs) are involved in important processes during embryogenesis, such as mesoderm formation during gastrulation and growth. In vertebrates, the Fgf superfamily consists of 22 family members, whereas only few FGFs are contained in the less complex genomes of insects and worms. In the recently sequenced genome of the beetle Tribolium, we identified four Fgf family members representing three subfamilies. Tribolium has Fgf1 genes that are absent in Drosophila but known from vertebrates. By phylogenetic analysis and microsynteny to Drosophila, we further classify Tc-fgf 8 as an ancestor of pyramus and thisbe, the fly Fgf8 genes. Tc-fgf8 expression in the growth zone suggests an involvement in mesoderm formation. In the embryonic head, expression of Tc-fgf8 subdivides the brain into a larger anterior and a smaller posterior region. The Fgf Tc-branchless is expressed in the embryonic tracheal placodes and in various gland-like structures. The expression patterns of the only Tribolium Fgf receptor and the adaptor molecule Downstream-of-Fgfr are largely congruent with Tc-Fgf8 and Tc-bnl. Thus, in contrast to Drosophila, only one Fgf receptor canalises Fgf signalling in different tissues in Tribolium. Our findings significantly advance our understanding of the evolution of Fgf signalling in insects.