Allorecognition in Botryllus schlosseri: ultrastructural study of fusion between genetically compatible colonies.

When colonies of the ascidian Botryllus schlosseri contact each other, they can either fuse and anastomise their circulatory systems if genetically compatible, or reject and produce a series of cytotoxic spots along the contact border in the case of incompatibility. Although many studies have been devoted to the analysis of rejection, few data on fusion are available. In order to fill this gap, we started a preliminary study on the morphology of the various steps of the fusion reaction at both light and electron microscope. We were able to distinguish at least five different stages in the fusion process. In stage 1 the tunics of the facing colonies contact each other and epithelial cells of the ampullar tips appear cylindrical in shape with a cytoplasm rich in RER with enlarged cisternae containing homogeneous, finely dispersed material. In stage 2, the tunics are strictly juxtaposed and the cuticular papillae are tightly intermingled, but the two tunics are still distinguishable. In this stage, cells of the ampullar tips contain numerous membrane bound granules, with homogeneous electron-dense material, in the supranuclear (“pad”) region. Stage 3 is marked by the dissolution of the two cuticles and the local fusion of the tunics in front of the facing ampullae. Less granules are now present in the ampullar pad and some haemocytes leak out from the circulation through the ampullar tips. In stage 4, the pads of the two facing epithelial adhere and new junctional complexes are formed. Basal lamina still delimitate the ampullar lumen and appear highly folded. In stage 5 the juxtaposed epithelia open, thus permitting the communication between the vessels of the two colonies. Cell pads are progressively resorbed and cells of the ampullar tips, now lining a new vessel, return to a cubic shape. Future studies will investigate the occurrence of apoptotic events in the process of ampullar fusion in B. schlosseri.