Dual evolutionary modes in the bovine globin locus.

Five bovine globin pseudogenes were subjected to sequence analysis. These genes include the three pseudogenes in the beta-type globin gene cluster as well as two allelic forms. Comparison of the sequences with those of the adult and fetal bovine globin genes shows that together they form a multigene family that was created by large-scale duplication. The structures are explained by invoking sequence exchange mediated by gene conversion. After their creation these genes evolved in a concerted fashion, exchanging sequence freely by intrachromosomal gene conversion. Subsequently, one by one, the genes were uncoupled from this exchange. This was accomplished by the creation of nonhomologies that formed barriers to gene conversion. These nonhomologies were several hundred bases in length and were formed by either deletion or by insertion of short repetitive sequences within the gene structures. In this way the genes made the transition from a rapid, coupled mode to a slow, solitary mode of evolution. Allelic gene polymorphisms were distributed inhomogeneously in the bovine globin family. It is proposed that this was due to interruption of interchromosomal gene conversion by a recent pseudogene duplication in the fetal globin gene cluster.