Annelid Perspectives Into the Role of FGF Signaling in Caudal Regeneration

Fibroblast growth factor signaling plays a crucial role in various developmental processes and is a key driver of regeneration. In annelids, this pathway is active from the earliest stages of reparative morphogenesis, yet its specific function remains unclear. Here we have functionally examined FGF signaling following the amputation of posterior segments in the marine annelid Alitta virens. We utilized the pharmacological agent SU5402 to inhibit the FGF receptor kinase at different time points. With whole-mount in situ hybridization, we analyzed the expression of regulatory genes that pattern posterior territories (cdx, evx, post2), multipotent/germ cells (vasa, piwi), mesodermal tissues (twist), and segmental boundaries (engrailed). Our findings reveal that FGF signaling is essential for blastema induction by promoting dedifferentiation and proliferation of cells at the wound site, but is not involved in posteriorization. On the contrary, this pathway is crucial for differentiation in the proximal (anterior) part of the regenerative bud, impacting its mesodermal derivatives and segment boundary formation. Comparative analysis suggests that while certain functions of FGF signaling, particularly in mesodermal patterning, are conserved across taxa, its role in posterior axis elongation appears to have evolved specifically within the vertebrate lineage. This study enhances our understanding of the evolutionary origins and functional diversification of FGF signaling in regeneration, positioning A. virens as a valuable model for exploring the complexities of regenerative biology.