Influence of Selection for Egg Production and Marek's Disease Resistance on the Incidence of Endogenous Viral Genes in White Leghorns

The influence of selection on the frequencies of endogenous viral (ev) genes related to the avian leukosis virus was studied in two genetically distinct sets of White Leghorn strains. Each set consisted of four strains: an unselected control strain, two strains selected for egg production traits, and a strain selected for Marek's disease (MD) resistance as well as egg production traits. Eight different ev genes were observed in Set I and seven in Set II, four being common to both sets. Selection for egg production traits resulted in significant changes of the frequency of four ev genes in both sets. In Set I, increased frequencies were observed for ev-4, ev-7, and ev-8; a decreased frequency for ev-9 was observed. The ev-9 gene expresses the viral envelope protein, whereas the others are transcriptionally silent, with the possible exception of ev-7. In Set II, increased frequencies were observed for the transcriptionally silent ev-8 and for ev-15, a gene which consists of a solitary long terminal repeat. Decreased frequencies were observed for ev-18, which codes for infectious endogenous virus, and for a second ev gene of unknown phenotype. In the resistance-selected strains the freque ncies of the ev genes were intermediate between those of the control strains and the strains selected for egg production traits with the exception of ev-6, which expresses the viral envelope protein, and ev-3, which expresses internal viral proteins as well as the envelope protein. The frequency of ev-6 was reduced in the resistance-selected strains of both sets, whereas the frequency of ev-3 was increased in the resistance-selected strain of Set II but not of Set I. The increased frequency of ev-3 might be related to the presence of the endogenous virus-producing gene ev-18 in strains of Set II. With this single exception, the observed changes in ev gene frequencies in the replicate selected strains within and among the two sets were consistent, indicating that they are the consequence of the selection practiced and are to a large extent independent of the genetic background.