The molecular basis of allorecognition in ascidians

The process of allorecognition consists of an ability to discriminate self from non-self. This discrimination is used either to identify non-self cells and reject them (non-self histocompatibility) or to identify self cells and reject them (as in the avoidance of self-fertilization by hermaphrodites (self incompatibility). The molecular basis governing these two distinct systems has been studied recently in hermaphroditic ascidian urochordates. Harada et al.((1))) postulated two highly polymorphic self-incompatibility loci, Themis (A and B), that are transcribed from both strands, forward to yield sperm (s-) trans-membrane antigen, and reverse to yield the egg vitelline coat (v-) receptor. De Tomaso et al.((2)) characterized a candidate histocompatibility locus, encoding a highly variable immunoglobulin. Nyholm et al.((3)) isolated its candidate allorecognition receptor, fester. Only a minute similarity was found in the structure of the genes involved. It appears that ascidian harbor two very separate types of labeling and recognition genetic systems: one for self and the other for non-self. BioEssays 30:1048-1051, 2008. (C) 2008 Wiley Periodicals, Inc.