Triplication of a four-gene set during evolution of the goat beta-globin locus produced three genes now expressed differentially during development

Abstract

Distinct hemoglobins are synthesized in goats at different stages of development, similar to humans. Embryonic hemoglobins (zeta 2 epsilon 2 and alpha 2 epsilon 2) are synthesized initially and are followed sequentially by fetal (alpha 2 beta F2), preadult (alpha 2 beta C2), and adult (alpha 2 beta A2) hemoglobins. To help understand the basis of these switches, the genes of the beta-globin locus have been cloned and their linkage arrangement has been determined by the isolation of lambda phage carrying overlapping inserts of genomic goat DNA. The locus extends over 120 kilobase pairs and consists of 12 genes arranged in the following order: epsilon I-epsilon II-psi beta X-beta C-epsilon III-epsilon IV-psi beta Z-beta A-epsilon V-epsilon VI-psi beta Y-beta F. Comparison of the nucleotide sequence of the 12 genes shows that the locus is organized into three homologous four-gene sets that presumably evolved by the triplication of an ancestral set of four genes (epsilon-epsilon-psi beta-beta). Interestingly, the three genes (beta C, beta A, and beta F) located at the ends of the four-gene sets are expressed at different stages of development. Therefore, the goat beta F-, beta C-, and beta A-globin genes appear to have evolved by a mechanism that includes the triplication of 40-50 kilobase pairs of DNA and the recruitment of newly formed genes for expression in fetal, preadult, and adult life.