The origin and evolution of ARGFX homeobox loci in mammalian radiation.

BACKGROUND: Many homeobox genes show remarkable conservation between divergent animal phyla. In contrast, the ARGFX (Arginine-fifty homeobox) homeobox locus was identified in the human genome but is not present in mouse or invertebrates. Here we ask when and how this locus originated and examine its pattern of molecular evolution. RESULTS: Phylogenetic and phylogenomic analyses suggest that ARGFX originated by gene duplication from Otx1, Otx2 or Crx during early mammalian evolution, most likely on the stem lineage of the eutherians. ARGFX diverged extensively from its progenitor homeobox gene and its exons have been functional and subject to purifying selection through much of placental mammal radiation. Surprisingly, the coding region is disrupted in most mammalian genomes analysed, with human being the only mammal identified in which the full open reading frame is retained. Indeed, we describe a transcript from human testis that has the potential to encode the full deduced protein. CONCLUSIONS: The unusual pattern of evolution suggests that the ARGFX gene may encode a functional RNA or alternatively it may have 'flickered' between functional and non-functional states in the evolutionary history of mammals, particularly in the period when many mammalian lineages diverged within a relatively short time span