Allorecognition and Stem Cell Parasitism: A Tale of Competition, Selfish Genes and Greenbeards in a Basal Chordate

  • Anthony W. De Tomaso


The cellular branch of the vertebrate adaptive immune system was discovered via its role in transplantation, which in turn became the assay used to discover T cells and the interaction between the T cell receptor (TCR) and polymorphic MHC proteins. During this time period, multiple invertebrate species were studied and found to have similar polymorphic transplantation responses, suggesting that the underlying mechanisms may be conserved. However, while transplantation responses are similar, the molecular mechanisms are not, and perhaps one of the biggest mysteries in evolution is that the MHC and TCR are only found in jawed vertebrates, and further that there are no orthologs, or indeed any recognizable ancestral genes, in jawless fish or invertebrates. This leads to two main questions: First, if polymorphic transplantation responses are universal, do they have a common purpose? Second, what is the origin of the key molecules (MHC, TCR, BCR) of the vertebrate adaptive immune system? We are studying a natural transplantation reaction in an invertebrate chordate, called Botryllus schlosseri, which may lend insight into these questions. Similar to the vertebrates, transplantation reactions are controlled by a single, highly polymorphic locus. In addition, in Botryllus transplantation has a well-defined role: governing the exchange of mobile germline stem cells between individuals, and we have found that this exchange can be either beneficial, or harmful. The interplay between transplantation responses and mobile stem cells in Botryllus may provide some insight into the evolution of cellular immunity in the vertebrates.


Allorecognition Germ cells Altruism Ascidian 


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© Springer International Publishing AG, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Molecular, Cellular and Developmental BiologyUniversity of California, Santa BarbaraSanta BarbaraUSA

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