A long‐term study on fused subclones in the ascidian Botryllus schlosseri: the resorption phenomenon (Protochordata: Tunicata)

Colonial tunicates have the property of forming a natural parabiotic union (chimera) in the wild with members of the same species, given that they share at least one allele at the fusibility‐histocompatibility locus. However, once such a vascular union has occurred, one of the two is often resorbed. To investigate this phenomenon in detail, 16 colonies of the colonial tunicate Botryllus schlosseri were subcloned and placed adjacent to 30 sets of fusible pairs. Subclones in each pair differed in their colour morph. Three other colonies and an additional 15 subclones of the experimental colonies served as controls for growth rate and survival. As the matched colonies grew, the adjacent colonies eventually came into contact. Fusion within a pair was established between the blood vessels, and 4–14 days later, a complete mixture of blood‐borne pigment cells was recorded. A significant correlation was found between the ratio of body size of the two colonies and the time for colour change. Fusion subsequently resulted in a partial resorption of at least one of the colonies in the pair (irrespective of the colour morph) and started in zooids lying in closest proximity to the contact zone. This process did not eliminate the blood vessels, and presumably allowed the process to extend to more distant zooids, which were gradually affected. This type of resorption was not recorded in the controls and took place in the ‘take‐over’ phase (the time when a new generation of zooids take over the colony from the previous generation). Resorption occurred when the new generation of zooids failed to develop to the mature phase, or when the new generation did not develop at all. The time scale for resorption varied from four days to six months. When larger colonies fused with smaller ones, the larger colony usually survived. When multiple subclones of two colonies were tested, usually all of the subclones from one of the colonies survived while the members of the other colony resorbed, implicating a possible genetic component. Because the fusion event usually leads to the resorption and death of at least one of the partners, it is necessary to re‐examine the proposal that fusion between closely related colonies is evolutionarily beneficial.