Colony specificity in Botrylloides leachi. I. Morphological aspects.

We studied colony specificity in the colonial ascidian Botrylloides leachi which, as in other botryllid ascidians, leads to either fusion or non-fusion between contacting colonies. Fusion requires the prior disappearance of contacting tunic cuticles and contact between facing ampullar epithelia. The epithelial cells of the ampullar tip show “pad regions” rich in ribosomes, which contribute to the synthesis of new tunic and cuticle. Blood cells, mainly phagocytes and pigment cells, increase their concentrations inside the ampullar lumen and phagocytes can cross the ampullar epithelium and enter the tunic, where they can contribute to the digestion of tunic cuticles and cells of the ampullar epithelium in order to establish a common circulation. Non-fusion reaction, as studied in the colony allorecognition assay, resembles the subcuticular rejection described in Japanese Botrylloides, characterised by limited tunic fusion, hemocyte leakage, and necrotic spots. Conversely, in the cut surface assay, a more intense cytotoxic reaction is observed along the contact border. In this case, morula cells crowd massively inside the facing ampullae, enter the tunic, and release their vacuolar contents which are probably required for the formation of necrotic spots.