Evolution of a Protochordate Allorecognition Locus.

Specificity ultimately depends on highly polymorphic genes, with populations often containing hundreds of alleles. Emergence and maintenance of this extraordinary polymorphism are unresolved issues in population genetics. Contact between adjacent colonies initiates an allorecognition reaction at juxtaposed extracorporeal blood vessels, resulting in either vascular fusion, forming a natural parabiosis, or in rejection, which prevents blood transfer between the two individuals. During positional cloning of the Fu/HC, we created F2 mapping populations using five independently isolated alleles.