The mode of recognition of allogeneic tissue in the solitary urochordate styela plicata

The mode of allorecognition in Styela plicata was determined by establishing reciprocal first-set allografts within pairs of individuals: 27% of pairs exhibited unilateral rejection where one animal retained reciprocal grafts while the other rejected them. This frequency was far greater than the expected level of chance elimination based on control autograft loss. Furthermore, second-set grafting confirmed the compatibility allocations of all unilaterally rejected pairs. The appearance of unilateral rejection is consistent with an allorecognition mechanism involving the specific identification of foreignness. Such systems are common among vertebrates, where a difference between individuals of a single tissue haplotype is sufficient to initiate allograft rejection. However, these results conflict with recognition systems operative in other invertebrates, where a single shared haplotype yields graft compatibility.