Surgical fusion between incompatible colonies of the compound ascidian, Botrylloides fuscus

In the compound ascidian Botrylloides fuscus, allorecognition responses were observed at the colonial margin (natural growing edge) as necrotic reactions in the subcuticular region of the facing tunic. However, artificial cut surface contacts between incompatible colonies resulted in the formation of chimeras with common vascular systems, termed "surgical fusion." The occurrence of surgical fusion in B. fucus contrasts with rejection occurring under the same conditions in other botryllids. Together the two observations suggest that self-nonself recognition may be a specialized function of tunic cells and their precursors in the blood, and the fusion (vascular interconnection) is a different process than rejection with different effectors and recognition systems. In all pairwise combinations among 12 and 14 colonies, more than 60% of combinations resulted in fusion when colonies were assayed by growing edge contact. This rate of fusible combinations is much higher than those of the other botryllid ascidians previously studied.