Head-to-head linkage of carp α- and β-globin genes

Abstract

The α- and β-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 α- and β-globin gene arrangement of a bony fish (carp). From a Southern analysis of seven previously prepared λ phage clones ( λCG1-7) using radio-labelled α- or β-globin gene probes, it was found that the clones included both the α- and β-globin genes, and that they were located within a distance of 1 kb. Additionally, the linkage of two α-globin genes and two β-globin genes in the clone λCG1, 5 and 7 (e.g., α- β– α- β in λCG5) revealed an arrangement that is different from the arrangement in higher vertebrates in which the α-globin and β-globin genes generally occur at different loci. The distances between the detached α- to β-globin genes were approximately 5 to 10 kb. DNA sequencing of the adjacently linked α- and β-globin genes in λCG3 showed that they were arranged in a head-to-head orientation. PCR amplification using primers for the internal region between the carp α- and β-globin genes gave approximately 0.9-kb products from each of the clones λCG1, 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 α- and β-genes in the globin gene locus (i.e., 3′ α 5′- 5′ β 3′– 3′ α 5′- 5′ β 3′ in λCG5), and the widespread distribution of head-to-head-linked α- and β-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 α and β forms) occurred in a head-to-head direction.