Botryllus schlosseri (tunicata) whole colony irradiation: Do senescent zooid resorption and immunological resorption involve similar recognition events?
Authors: Rinkevich Baruch , Weissman Irving L. 
Published: June 2005

The colonial tunicate Botryllus schlosseri undergoes cyclic blastogenesis where feeding zooids are senescened and resorbed and a new generation of zooids takes over the colony. When non‐identical colonies come into direct contact, they either reject each other or fuse. Fusion is usually followed by the resorption of one of the partners in the chimera (immunological resorption). The striking morphological similarities between the two resorption phenomena suggest that both may involve tissue destruction following self‐nonself recognition events. Here we attempt to modify these two events by whole colony gamma irradiation assays. Three sets of experiments were performed: 1) different doses of whole colony irradiation for determination of irradiation effects (110 colonies, up to 8,000 rads); 2) pairs of irradiated‐nonirradiated isografts of clonal replicates for the potential of reconstruction of the irradiated partners (23 pairs); 3) chimeras of irradiated‐nonirradiated partners for analysis of resorption hierarchy.

Mortality increased with the irradiation dose. All colonies exposed to more than 5,000 rads died within 19 days, while no colony died below 2,000 rads. The average mortality periods, in days, for doses of 6,000–8,000, 5,000, and 2,500–4,000 rads were 14.4 ± 3.1 (n = 24), 19.8 ± 6.0 (n = 15), and 19.6 + 5.1 (n = 22), respectively. Younger colonies (3–6 months old) may survive radiation better than older ones (more than 13 months). Many morphological alterations were recorded in irradiated colonies: ampullar contraction and/or dilation, accumulation of pigment cells within ampullae, abnormal bleeding from blood vessels, sluggish blood circulation, necrotic zones, reduction in bud number, and irregularities in zooid and system structures. With doses of 3,000–4,000 rads and above, irradiation arrested the formation of new buds and interrupted normal takeover, turning the colony into a chaotic bulk of vessels, buds, and zooid segments. Death supervened after a period of up to 1 month of poor condition, which was also characterized by loss of organization in systems. In isografts of irradiated‐nonirradiated parts, the normal subclone resorbed all zooids and buds of the irradiated one within less than 1 week, even if it was up to 13 times smaller, without showing any sign of harmful effects. Thus, the irradiated subclone is not reconstituted by sharing blood circulation with a syngeneic part. Under 2,000 rads some of the irradiated zooids within this type of union started to regenerate, and at 1,000 rads no resorption was recorded, even though the number of zooids decreased in the irradiated part. In irradiated‐nonirradiated chimeras, the irradiated partner was always resorbed within less than one week, irrespective of its place in the resorption hierarchy. It is suggested that blastogenic and immunological resorption phenomena originate from two different events (senescence and self‐nonself recognition systems), although both are expressed during the takeover phase.