Unusually Similar Patterns of Antibody V Segment Diversity in Distantly Related Marsupials

Abstract

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 VH diversity tend to have limited germline VL diversity, whereas species with high levels of germline VH diversity have more diverse VL gene segments. Evidence for a limited pool of VH but diverse VL 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 VH and VL of an Australian possum, Trichosurus vulpecula, were characterized. The Ig repertoire in T. vulpecula is also derived from a restricted VH pool but a diverse VL pool. The VL 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 VH and VL gene segments observed in other mammals. Rather, marsupial VH and VL complexity appears to be evolving divergently, retaining diversity in VL perhaps to compensate for limited VH diversity. There is a high degree of similarity between the VH and VL in M. domestica and T. vulpecula, with the majority of VL families being shared between both species. All marsupial VH sequences isolated so far form a common clade of closely related sequences, and in contrast to the VL genes, the VH likely underwent a major loss of diversity early in marsupial evolution.