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Unusually Similar Patterns of Antibody V Segment Diversity in Distantly Related Marsupials^1,2

Michelle L. Baker^*, Katherine Belov and Robert D. Miller^3,*

^* Department of Biology, University of New Mexico, Albuquerque, NM 87131; and Evolutionary Biology Unit, Australian Museum, Sydney, New South Wales, Australia

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_H 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_H and V_L gene segments observed in other mammals. Rather, marsupial V_H and V_L complexity appears to be evolving divergently, retaining diversity in V_L perhaps to compensate for limited V_H 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.

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