ORIGIN OF THE ENDOGENOUS AVIAN RETROVIRUS, RAV-O

Abstract

The genome of the endogenous chicken retrovirus, RAV-O, is carried by domestic chickens as DNA sequences integrated into host chromosomes. The domestic chicken is a domesticated form of Red Junglefowl (Gallus guallus) which belongs to the Pheasant family (Phasianidae) within the order Galliformes. To determine whether the RAV-O genome evolved from cellular genes, the presence and distribution of RAV-O related sequences within the DNA of all extant Gallus spp. and several other representative Galliform birds has been studied. DNA sequences homologous to a RAV-O cDNA were analysed both by molecular hybridisation in liquid and after electrophoresis of restriction endonuclease fragments. High levels (>90% homology) of DNA sequences related to RAV-O were detected only in the genomes of domestic chicken and its phylogenetic ancestor, the Red Junglefowl (Gallus gallus). The DNA of the three other extant Gallus spp. were found to be negative even for partially homologous RAV-O related sequences. DNA's from several other Galliform genera, however, (e.g.Phasianus, Perdix, Alectoris and Lagopus) which are only distantly related to Gallus gallus, contained varying levels of partial homology to the RAV-O cDNA. Thus it is concluded that RAV-O related sequences have not diverged together with normal host genes during Phasianid evolution; rather, it would appear that the RAV-O genome has been introduced relatively recently into the germ-line of Gallus gallus, following speciation, but prior to domestication, and independently of the related sequences found in other Galliform genera.