Coevolution of Mucosal Immunoglobulins and the Polymeric Immunoglobulin Receptor: Evidence That the Commensal Microbiota Provided the Driving Force

Figure 1

A model for evolution of immunoglobulin isotypes in vertebrates. The antigen-binding variable lymphocyte receptors (VLRs) expressed in extant Agnathans are thought to represent the precursors of modern immunoglobulins (Igs). Genes encoding Ig heavy and light chains apparently emerged prior to the split between cartilaginous and bony fishes, resulting in expression of Ig molecules that are characteristic of higher vertebrates. Cartilaginous fishes express an IgD-like molecule (IgW/D) and IgM, which is found evenly distributed between blood and mucosal secretions. The first evidence of a dedicated mucosal Ig isotype came with the emergence of IgT in an ancestor of modern bony fishes. Subsequent duplication of Ig constant region genes and evolution of Ig class switching mechanisms in a common ancestor of tetrapods allowed the production of multiple Ig isotypes during immune responses. Color coding indicates the presumed evolution of avian and mammalian IgA from a precursor of amphibian IgX and evolution of mammalian IgG and IgE from a precursor of amphibian IgY.