MORPHOLOGY AND GENETICS OF REJECTION REACTIONS BETWEEN OOZOOIDS FROM THE TUNICATE BOTRYLLUS SCHLOSSERI

Botryllus rejection reactions were followed in pairs of oozooids placed together immediately after initiation of metamorphosis. Within twelve hours, both compatible and incompatible oozooid pairs underwent tunic fusion and initiation of ampullar tip-to-side contact. Vascular fusion followed within two days between compatible pairs, while the fusion sequence was interrupted in the incompatible pairs by a rapid cytotoxic rejection response. Events occurring within and outside the ampullae in rejections were effector responses whose consequences were separation of the ampullae and isolation of the involved tissues from the bodies of the oozooids. Genetics experiments suggested that the four distinct types of rejection reflect a hierarchy of histoincompatibility in this system.