Selective and Neutral Evolution in the Murine Igh-V Locus

The Igh-V locus in the mouse is comprised of at least nine families of homologous genes encoding immunoglobulin heavy chain variable (Vh) regions (Brodeur and Riblet 1984; Dildrop 1984; Brodeur et al 1985; Winter et al 1985). These gene families are, for the most part, organized into discrete clusters with little interspersion; in addition, they are highly polymorphic. These features make this locus a good system for the study of the evolutionary mechanisms and constraints operative on a complex multi-gene system. Our studies show that both selective and neutral forces contribute to the evolution of the Igh-V locus; here, we focus on the evolution of Vh family size, or copy number, and Vh family divergence.