Can the product of the theta gene be a real globin?

A new member (theta 1, or psi alpha) of the alpha-globin gene family has recently been identified in a number of species. In higher primates the theta 1 gene has all the structural features apparently necessary for expression, and it appears to have long been under strong selective constraints which suggests that it could still be, or recently have been, a functional gene. No corresponding 'globin' has yet been identified, however. In some other species, galago and rabbit for example, the theta 1 and psi alpha genes have accumulated enough inactivating mutations for them to be considered genuine pseudogenes. Horses also have an alpha-like gene (psi alpha), in a 3' position identical to the other species in relation to the functional alpha genes, and this also appears to have the elements required for a functional gene. The predicted amino-acid sequence, however, suggests that any 'globin' product is likely to be non-viable because it has a number of seriously deleterious amino-acid replacements. Some of these amino-acid changes are shared with the rabbit and primate sequences, indicating that they predate the mammalian radiation, and that if indeed any of these genes are still functional, they are unlikely to be making haemoglobin.