Head-to-head linkage of carp alpha- and beta-globin genes

The alpha- and beta-globin gene variants are believed to have diverged from a single ancestral globin gene, and the divergence was primed by the duplication of the ancestral globin gene. To understand the process of gene duplication, we investigated the alpha- and beta-globin gene arrangement of a bony fish (carp). From a Southern analysis of seven previously prepared lambda phage clones (lambdaCG1-7) using radio-labelled alpha- or beta-globin gene probes, it was found that the clones included both the alpha- and beta-globin genes, and that they were located within a distance of 1 kb. Additionally, the linkage of two alpha-globin genes and two beta-globin genes in the clone lambdaCG1, 5 and 7 (e.g., alpha-beta-alpha-beta in lambdaCG5) revealed an arrangement that is different from the arrangement in higher vertebrates in which the alpha-globin and beta-globin genes generally occur at different loci. The distances between the detached alpha- to beta-globin genes were approximately 5 to 10 kb. DNA sequencing of the adjacently linked alpha- and beta-globin genes in lambdaCG3 showed that they were arranged in a head-to-head orientation. PCR amplification using primers for the internal region between the carp alpha- and beta-globin genes gave approximately 0.9-kb products from each of the clones lambdaCG1, 3, 4, 5, 6, and 7, and from the chromosomal DNA of German mirror carp, Saku carp, Suwa carp, and Yamato carp. This demonstrates the alternative arrangement of carp alpha- and beta-genes in the globin gene locus (i.e., 3'alpha5'-5'beta3'-3'alpha5'-5'beta3' in lambdaCG5), and the widespread distribution of head-to-head-linked alpha- and beta-globin genes in carp. Based on the above results, we hypothesize that the duplication of the ancestral globin gene (prior to the divergence of the a and beta forms) occurred in a head-to-head direction.