Evolutionary implications of FGF and distal-less expressions during proximal-distal axis formation in the ampulla of a direct-developing ascidian, Molgula pacifica

The present results provide the first evidence of a fibroblast growth factor (FGF) family protein in a urochordate. Anti-FGF2 immunoreactive hemoblast cells were detected at day 3 of juvenile development in a direct-developing urochordate ascidian, Molgula pacifica. The detection of FGF in hemoblast cells coincided with the appearance of distal-less protein along the proximal-dis mi axis of growing ampullae. Ampullae are limb-like, fluid-filled ectodermal appendages that contain hemoblast cells and have holdfast, respiratory, and immunological functions (I). Given the evolutionary conservation of the genes encoding FGF (2), their receptors (2), and distal-less (3), the present results suggest that the formation of non-homologous ascidian appendages shares generic elements in common with proximal-distal axis formation in arthropod limbs and vertebrate limbs (4,5). The possible evolutionary implications of these findings are discussed.