Cohen C Sarah, Saito Yasunori, Weissman Irving L
Department of Developmental Biology and Hopkins Marine Station, Stanford University, Stanford, California, 94305.
Shimoda Marine Research Station, University of Tsukuba, 5-10-1 Shimoda, Shizuoka 415, Japan.
Evolution. 1998 Jun;52(3):746-756. doi: 10.1111/j.1558-5646.1998.tb03699.x.
Despite the functional and phyletic ubiquity of highly polymorphic genetic recognition systems, the evolution and maintenance of these remarkable loci remain an empirical and theoretical puzzle. Many clonal invertebrates use polymorphic genetic recognition systems to discriminate kin from unrelated individuals during behavioral interactions that mediate competition for space. Space competition may have been a selective force promoting the evolution of highly polymorphic recognition systems, or preexisting polymorphic loci may have been coopted for the purpose of mediating space competition. Ascidian species in the family Botryllidae have an allorecognition system in which fusion or rejection between neighboring colonies is controlled by allele-sharing at a single, highly polymorphic locus. The behavioral sequence involved in allorecognition varies in a species-specific fashion with some species requiring extensive intercolony tissue integration prior to the allorecognition response, while other species contact opposing colonies at only a few points on the outer surface before resolving space conflicts. Due to an apparent species-specific continuum of behavioral variation in the degree of intercolony tissue integration required for allorecognition, this system lends itself to a phylogenetic analysis of the evolution of an allorecognition system. We constructed a molecular phylogeny of the botryllids based on 18S rDNA sequence and mapped allorecognition behavioral variation onto the phylogeny. Our phylogeny shows the basal allorecognition condition for the group is the most internal form of the recognition reaction. More derived species show progressively more external allorecognition responses, and in some cases loss of some features of internal function. We suggest that external allorecognition appears to be a secondary function of a polymorphic discriminatory system that was already in place due to other selective pressures such as gamete, pathogen, or developmental cell lineage recognition.
尽管高度多态的基因识别系统在功能和系统发育上普遍存在，但这些非凡基因座的进化和维持仍然是一个经验和理论上的谜题。许多无性繁殖的无脊椎动物在介导空间竞争的行为相互作用中，利用多态基因识别系统来区分亲属和非亲属个体。空间竞争可能是推动高度多态识别系统进化的一种选择力量，或者预先存在的多态基因座可能已被用于介导空间竞争。葡萄海鞘科的海鞘物种具有一种异体识别系统，其中相邻群体之间的融合或排斥由单个高度多态基因座上的等位基因共享控制。异体识别所涉及的行为序列因物种而异，有些物种在异体识别反应之前需要广泛的群体间组织整合，而其他物种在解决空间冲突之前仅在外表面的几个点接触对立群体。由于异体识别所需的群体间组织整合程度在行为变异上存在明显的物种特异性连续体，这个系统适合对异体识别系统的进化进行系统发育分析。我们基于18S rDNA序列构建了葡萄海鞘科的分子系统发育树，并将异体识别行为变异映射到该系统发育树上。我们的系统发育树显示该群体的基础异体识别条件是识别反应的最内在形式。更多进化的物种表现出逐渐更外在的异体识别反应，在某些情况下还失去了一些内在功能的特征。我们认为，外在异体识别似乎是一个多态性鉴别系统的次要功能，该系统由于其他选择压力（如配子、病原体或发育细胞谱系识别）已经存在。
