Saito Y, Hirose E, Watanabe H
Shimoda Marine Research Center, University of Tsukuba, Shizuoka, Japan.
Int J Dev Biol. 1994 Jun;38(2):237-47.
In botryllids (colonial ascidians), there are two types of allorecognition: colony specificity and colony resorption. Colony specificity is manifested by fusion and rejection between two conspecific colonies. The genetic basis for this colony specificity resides in a single highly polymorphic gene locus (fusibility locus) with codominantly expressed alleles. Two colonies with no alleles in common at this locus reject each other, whereas colonies sharing at least one allele at the fusibility locus fuse and form a chimera. That is, in colony specificity, self components are distinguished from nonself components, and failure to recognize self induces rapid rejection reactions. The process of rejection in colony specificity is not uniform among all botryllid ascidians. Colony resorption can occur after the establishment of fusion between two colonies. Zooids of one partner of a chimera are resorbed more than 1 week after fusion, or, by amputation of fused blood vessels, the chimera becomes separated into the two original colonies. Colony resorption is also controlled mainly by the fusibility locus. It usually occurs in a chimera between two colonies sharing only one allele at this locus. In colony resorption, nonself determinants are recognized and chronic rejection reactions are induced resembling MHC-dependent graft rejection. Based on these findings, the fusibility locus of botryllids seems to be very similar to the MHC of vertebrates. Considering that vertebrates evolved directly from ascidians, it is likely that the fusibility locus is an ancestral form of the vertebrate MHC.
在葡萄海鞘（群体海鞘）中，存在两种异体识别类型：群体特异性和群体吸收。群体特异性表现为两个同种群体之间的融合与排斥。这种群体特异性的遗传基础在于一个单一的高度多态性基因座（融合性基因座），其等位基因呈共显性表达。在该基因座上没有共同等位基因的两个群体相互排斥，而在融合性基因座上至少共享一个等位基因的群体则会融合并形成嵌合体。也就是说，在群体特异性中，自身成分与非自身成分得以区分，无法识别自身会引发快速的排斥反应。群体特异性中的排斥过程在所有葡萄海鞘类海鞘中并不一致。群体吸收可在两个群体融合后发生。嵌合体一方的游动孢子在融合1周多后会被吸收，或者通过切断融合的血管，嵌合体可分离为两个原始群体。群体吸收也主要由融合性基因座控制。它通常发生在两个群体仅在该基因座共享一个等位基因的嵌合体中。在群体吸收过程中，非自身决定簇被识别，并引发类似于MHC依赖性移植排斥的慢性排斥反应。基于这些发现，葡萄海鞘的融合性基因座似乎与脊椎动物的MHC非常相似。鉴于脊椎动物直接从海鞘进化而来，融合性基因座很可能是脊椎动物MHC的祖先形式。
