Allorecognition, Germline Chimerism, and Stem Cell Parasitism in the Colonial Ascidian, Botryllus schlosseri

Abstract

Many marine invertebrates have the ability to combine tissues with conspecifics and form chimeras. This ability is usually accompanied by the presence of a polymorphic self/non-self recognition system (allorecognition) that allows integration of closely related individuals, but blocks interactions between those more distantly related. The presence of a discriminatory allorecognition system suggests that there are costs and benefits to chimerism that are correlated to relatedness, but the nature of these costs and benefits is still poorly understood. Interestingly, allorecognition is found throughout the animal kingdom, with examples in nearly all the multicellular phyla; however, the role of allorecognition in these different species is not always clear. In addition, the molecular mechanisms underlying allorecognition in different species are not related, suggesting a unique origin for each species. This brings up the question as to whether there is a universal need for self/non-self recognition processes among the metazoa, or if these systems evolve in response to different selective pressures. We are studying allorecognition and chimerism in the colonial ascidian, Botryllus schlosseri. Allorecognition takes place at the tips of the extracorporeal vasculature, and can result in a parabiosis (fusion), or a rejection reaction that blocks vascular fusion. Fusion has both hypothesized benefits as well as well-documented costs. This article will focus on characterizing the costs and benefits of fusion and chimerism in Botryllus, and put them into the broader context of the role of allorecognition in the metazoa.