Extended Data Fig. 1. Allorecognition in Botryllus schlosseri.

a, When two colonies grow close together, the ampullae reach out and interact (white arrows), and this will result in one of two outcomes: either the two ampullae will fuse b, allowing the circulation of the two colonies to interconnect (white arrows), or they will reject each other (c,d). Rejection is a localized inflammatory reaction where blood cells leak from the ampullae. c, A close-up of rejecting ampullae showing cell leakage. Once outside the circulation, cells discharge their vacuoles, initiating a prophenoloxidase pathway which eventually forms dark melanin scars, called points of rejection, or POR (c, black arrow on right; d white arrows). The ampullae then disintegrate (left of POR, top white arrow in d), and the colonies no longer interact. The reaction takes ~24–48h to occur and is controlled by a single highly polymorphic locus called the fuhc (for fusion/histocompatibility). Colonies will fuse if they share one or both alleles, and will reject if no alleles are shared. e, Lower magnification shows a single individual, consisting of multiple zooids occupying the center. The large extracorporeal vasculature and terminating ampullae are outlined (large arrows). In this experiment, a colony was placed between a compatible partner (bottom) and an incompatible partner (top). As shown, a colony can simultaneously fuse (red arrows) and reject (small black arrows, top). This demonstrates that allorecognition is spatially segregated and occur indpendently at the tips of the ampullae that are in contact.