Regeneration in the tail fan of crayfish

Summary The tail fan of a crayfish consists of the caudal end of the body, the telson, and the most caudal limbs, the uropods. We investigated the positional information in these structures with grafting operations. The uropods are biramous; they bifurcate to a lateral exopodite and a medial endopodite. After the distal part of a uropod ramus was grafted to the stump of a ramus, medio-lateral or dorso-ventral mismatch of surfaces provoked the production of supernumerary distal parts. Proximo-distal intercalation between exopodite and endopodite yielded a mosaic ramus. The results show that the two rami contain equivalent ramus fields in congruent orientation. The exopodite consists of basal and distal segments; each of these segments seems to have an equivalent segmental field.The telson regenerated an ablated distal portion poorly, unlike the limbs of crayfish. After the posterior lobe of the telson was inverted dorso-ventrally and grafted into the telson stump, supernumerary posterior lobes regenerated dorsal and ventral to the graft. Thus the dorsal and ventral surfaces of the telson embody different positional information. A grafted uropod endopodite or exopodite healed to the telson, but dorsoventral inversion of the graft did not provoke the formation of supernumerary structures at the graft-host boundary. Because supernumeraries did not form, the relations between positional information in the telson (a body axis structure) and the uropod (a limb) remain unclear.