Rinkevich B
National Institute of Oceanography, Haifa, Israel.
Exp Clin Immunogenet. 1996;13(2):61-9.
Allogenic colonies of the cosmopolitan ascidian Botryllus schlosseri may fuse upon contact through their blood vessels to form chimeric entities if they share at least one allele at a highly polymorphic haplotype, termed the fusibility-histocompatibility (Fu/HC) locus. Past studies have attributed several benefits to this chimerism, but none of the suggestions was confirmed by controlled laboratory studies. Furthermore, once Botryllus colonies fuse, a second allorecognition phenomenon begins which usually leads to the resorption of one partner in the chimera. Resorption is probably controlled by a multilevel hierarchial organization of the heterozygosity relatedness for several allorecognition elements (the Fu/HC and resorption loci), which also reflects the relative heterozygosity of each partner's genome. Consequently, the more heterozygotic partner will be the 'winner' in the resorption, leaving alive the most adapted genotype (sensu to the heterosis concept). However, recent studies have recorded that freely circulating stem cells from the 'subordinate' partner in the resorption phenomenon may parasitize the 'winner' for positions on the germ line. All the above studies have been performed on bichimeric entities. It is suggested here that formation of natural multichimeras in this species (resulting from an aggregated cosettlement of Fu/HC compatible colonies) produces more 'equilibrated' chimeric entities and alleviates the costs incurred through the contradicting heterosis and germ cell parasitism processes. This improves the interspecific competitive ability of the multifusion entities. Therefore, in this case, selection acts on the 'group' level, not the colony level, providing the evolutionary forces that shape the phenomenon of natural tissue transplantation in these organisms.
世界性海鞘博特氏拟菊海鞘的同种异体群体，如果在一个高度多态的单倍型（称为融合性 - 组织相容性，即Fu/HC位点）上共享至少一个等位基因，那么它们在通过血管接触时可能会融合形成嵌合实体。过去的研究认为这种嵌合现象有多种益处，但没有一个观点得到对照实验室研究的证实。此外，一旦博特氏拟菊海鞘群体融合，第二种异体识别现象就会开始，这通常会导致嵌合体中的一个伙伴被吸收。吸收可能由几个异体识别元件（Fu/HC和吸收位点）的杂合性相关性的多层次等级组织控制，这也反映了每个伙伴基因组的相对杂合性。因此，杂合性更高的伙伴将在吸收过程中成为“赢家”，留下适应性最强的基因型（从杂种优势概念的意义上来说）。然而，最近的研究记录到，在吸收现象中来自“从属”伙伴的自由循环干细胞可能会寄生在“赢家”的生殖系上以获取位置。上述所有研究都是在双嵌合实体上进行的。这里提出，该物种中天然多嵌合体的形成（由Fu/HC兼容群体的聚集共沉降产生）会产生更“平衡”的嵌合实体，并减轻因杂种优势和生殖细胞寄生过程相互矛盾而产生的成本。这提高了多融合实体的种间竞争能力。因此，在这种情况下，选择作用于“群体”水平，而不是群体水平，提供了塑造这些生物体中自然组织移植现象的进化力量。
