THE ORIGIN AND LOSS OF THE UBIQUITIN-ACTIVATING ENZYME GENE ON THE MAMMALIAN Y-CHROMOSOME

Mammalian sex chromosomes are thought to be descended from a homologou s pair of autosomes: a testis-determining allele which defined the Y c hromosome arose, recombination between the nascent X and Y chromosomes became restricted and the Y chromosome gradually lost its non-essenti al genetic functions, This model was originally inferred from the occu rrence of few Y-linked genetic traits, pairing of the X and Y chromoso mes during male meiosis and, more recently, the existence of X-Y homol ogous genes, The comparative analysis of such genes is a means by whic h the validity of this model can be evaluated, One well-studied exampl e of an X-Y homologous gene is the ubiquitin activating enzyme gene (U BE1), which is X-linked with a distinct Y-linked gene in many eutheria n ('placental') and metatherian (marsupial) mammals, Nonetheless, no U BE1 homologue has yet been detected on the human Y chromosome, Here we describe a more extensive study of UBE1 homologues in primates and a prototherian mammal, the platypus, Our findings indicate that UBE1 lie s within the X-Y pairing segment of the platypus but is absent from th e human Y chromosome, having been lost from the Y chromosome during ev olution of the primate lineage, Thus UBE1 illustrates the key steps of 'autosomal to X-specific' evolution of genes on the sex chromosomes.