Unusually similar patterns of antibody V segment diversity in distantly related marsupials

A pattern of coevolution between the V gene segments of Ig H and L chains has been noted previously by several investigators. Species with restricted germline V-H diversity tend to have limited germline V-L diversity, whereas species with high levels of germline V-H diversity have more diverse V-L gene segments. Evidence for a limited pool of V,, but diverse V-L gene segments in a South American opossum, Monodelphis domestica, is consistent with this marsupial being an exception to the pattern. To determine whether M. domestica is unique or the norm for marsupials, the V-H and V-L of an Australian possum, Trichosurus vulpecula, were characterized. The Ig repertoire in T. vulpecula is also derived from a restricted V-H pool but a diverse V-L Pool. The V-L gene segments of T. vulpecula are highly complex and contain lineages that predate the separation of marsupials and placental mammals. Thus, neither marsupial follows a pattern of coevolution of V, and VL gene segments observed in other mammals. Rather, marsupial V-H and V-L complexity appears to be evolving divergently, retaining diversity in V-H perhaps to compensate for limited V-L diversity. There is a high degree of similarity between the V-H and V-L in M. domestica and T. vulpecula, with the majority of V-L families being shared between both species. All marsupial V-H sequences isolated so far form a common clade of closely related sequences, and in contrast to the V-L genes, the V-H likely underwent a major loss of diversity early in marsupial evolution. The Journal of Immunology, 2005.