Molecular characterization of endogenous viral genes of the avian leukosis virus family in an experimental population of brown-egg layers.

Retroviral DNA sequences similar to the exogenous avian leukosis virus can be found in the genome of many chicken breeds and have been identified as the ALVE family of endogenous viral (ev) genes. Most of them have been described by a restriction fragment length polymorphism (RFLP) procedure with two restriction enzymes and a full length viral probe. In order to facilitate the comparison of ALVE genes between strains, the nomenclature workshop held at the XXIV International Society for Animal Genetics Congress recommended that four enzymes and several viral subprobes be used to characterize each locus. This approach has been followed in the present study of a Rhode Island Red experimental population. A previous study had identified ev genes with the SacI and BamHI enzymes and the Rous-associated virus-2 probe (RAV-2). Chickens carrying only one ALVE locus at a time have been produced to facilitate the analysis. Additional enzymes (EcoRI, HindIII, and KpnI), the full probe RAV-2 and three viral subprobes for the gag, pol, and LTR regions have been used. In addition, a PCR diagnostic test has been used to search for homologies with the ALVE1 (= ev1), ALVE6 (= ev6) and evA loci. Currently, 12 loci have been identified precisely: three were identical to ALVE loci described previously, either in White Leghorns, ALVE6 and ALVE18 (= ev18) or in broilers (evB8). In addition, the evB8 locus was found to be identical to the evA locus previously described in brown-egg layers. Nine loci appeared specific to this Rhode Island Red population. Four of these specific loci were complete and one of them could be considered of characteristic of this population, because of its very high frequency. The remaining five specific loci showed small deletions, either in the pol region for one of them or in the env region for three of them or at the 3' long terminal repeat for one of them. Altogether, 5 out of 12 loci were structurally complete, which could suggest that deleted proviruses may have been preferentially retained.