Fig. 1.

Effect of allogeneic resorption on asexual propagation. (A) Depiction of controlled fusion reactions among compatible, yet genetically distinct, fused partners that result in resorption of one partner (depicted with dotted lines). (B) Time lapse imaging of a juvenile fused chimera (white arrow depicts blood vessel anastomosis between two colonies) undergoing resorption. Images were acquired at 24-h intervals starting 5 d after fusion. Moderate tissue destruction (red arrow) was first observed 6 d after fusion (Top Right) and progressed to complete stage R = IV resorption within 72 h (Bottom Right) during the takeover period (stage D) of blastogenesis (nonresorbing colony, Bottom Left). (C) Growth curves (cm²) of primary buds in a stable chimera (depicted as blue and black lines) exhibit synchronized cycles of expansion and regression (Top). Unstable chimeras exhibit growth arrest in asexually propagating tissues (Bottom). (D) High power magnification of abnormal zooids and buds (Top Right, red dotted lines) in allogeneic resorbing colonies compared with nonresorbing colonies (Top Left). (Scale bars: Top, 500 μM; Bottom, 100 μM.) (E) Hematoxylin/eosin (H&E)-stained tissue section of a resorbing 1° bud showing effacement of developing structures with associated destructive changes and infiltration of cytotoxic and phagocytic cell populations compared with nonresorbing buds of the same developmental stage as in F. (Scale bars: Left, 200 μM; Right, 100 μM.)