Convergent Evolution of Human and Bovine Haptoglobin: Partial Duplication of the Genes

Haptoglobin (Hp) is a hemoglobin-binding plasma protein consisting of two types of chains, called α and β, which originate from a common polypeptide. In humans, but not in other mammals, Hp has been shown to occur in two allelic forms, Hp1 and Hp2, which differ in the length of the α-chain. The longer α-chain (in Hp2) seems to have arisen by an internal duplication of a gene segment coding for almost the entire α-chain of Hp1. In this article we show that Hp of cow (Bos taurus) contains an α-chain, the structure of which is similar to that of the human Hp2 α-chain. Furthermore, comparison of the structure of bovine Hp and human Hp2 suggests that the bovine gene arose by a duplication of the gene segment homologous to that duplicated in human Hp2. However, a phylogenetic analysis indicates that the two genes were formed independently. The evolutionary pressure that has led to the fixation of the Hps with a longer α-chain is not known. Reviewing Editor: Dr. Manyuan Long