Multimeric hemoglobin of the Australian brine shrimp Parartemia

The hemoglobin molecule of the commercially important brine shrimp Artemia sp. has been used extensively as a model for the study of molecular evoluti on. It consists of nine globin domains joined by short linker sequences, an d these domains are believed to have originated through a series of duplica tions from an original globin gene. In addition, in Artemia, two different polymers of hemoglobin, called C and T, are found which differ by 11.7% at the amino acid level and are believed to have diverged about 60 MYA. This p rovides a set of data of 18 globin domain sequences that have evolved in th e same organism. The pattern of amino acid substitution between these two p olymers is unusual, with pairs of equivalent domains displaying differences of up to 2.7-fold in total amino acid substitution. Such differences would reflect a similar range of molecular-clock rates in what appear to be dupl icate, structurally equivalent domains. In order to provide a reference out group, we sequenced the cDNA for a nine-domain hemoglobin (P) from another genus of brine shrimp, Parartemia zietziana, which differs morphologically and ecologically from Artemia and is endemic to Australia. Parartemia produ ces only one hundredth the amount of hemoglobin that Artemia produces and d oes not upregulate production in response to low oxygen partial pressure. C omparison of the globin domains at the amino acid and DNA levels suggests t hat the Artemia globin T gene has accumulated substitutions differently fro m the Parartemia P and Artemia C globin genes. We discuss the questions of accelerated evolution after duplication and possible functions for the Para rtemia globin.