Protochordate allorecognition is controlled by a MHC-like gene system

Colonial tunicates, unlike vertebrates, undergo transplantation in nature. Rejection or acceptance between colonies of Botryllus is controlled by a single gene locus with multiple alleles. The same genetic region apparently maintains this polymorphism by preventing fertilization between gametes sharing alleles. The Botryllus histocompatibility system may reflect the original adaptive function of ancestral MHC genes.