THE EVOLUTION OF THE TYPE-I INTERFERONS

There are five recognized subtypes within the type I interferons (IFN) , IFN-alpha, IFN-beta, IFN-delta, IFN-omega, and IFN-tau, although oth ers may remain to be described, and the IFN-omega may have to be subdi vided further because of their evident structural complexity. Together , they constitute an ancient family of intronless genes, possibly pres ent in all vertebrates, THe IFNA/IFNB genes originated by duplication of a progenitor after the divergence of birds, most probably about 250 million years ago (MYA), The avian gene itself proceeded to duplicate to form a series of independent subtypes, The IFND, to date described only in the pig, arose from the IFNA lineage before the emergence of mammals about 180 MYA and might, therefore, be generally distributed i n present day species. The IFNB, which occurs as a single gene in prim ates and rodents, have been duplicated in some other orders. Recent ev ents have produced 10 or more genes in bovid species. The IFNA, which are clustered with the IFNW in humans and cattle, exist as multiple ge nes in all mammals so far examined as a result of a series of duplicat ion events, some of which occurred recently and, therefore, independen tly in separate mammalian lineages. The IFNW diverged from the IFNA ap proximately 130 MYA, just prior to the emergence of mammals, and have continued to duplicate since then. The IFNT, which play a role in repr oduction of ruminants, arose from an IFNW within the Artiodactyla subo rder about 36 MYA and are found only in the suborder Ruminantia, These genes have also continued to duplicate to form an extensive family. C onsequently, their involvement in early pregnancy is a feature of rumi nants and not of other mammalian species.