Smith, E.J., U.S. Department of Agriculture, Avian Disease and Oncology Laboratory, East Lansing, Michigan 48823.
Levin, I., U.S. Department of Agriculture, Avian Disease and Oncology Laboratory, East Lansing, Michigan 48823.

To investigate further the sex-linked, slow-feathering (SF) locus, a DNA hybridization probe that flanks the integration site of endogenous virus ev21 was used to probe Southern blots from a variety of commercial SF White Leghorn (WL), broiler, and endangered lines. After HaeIII digestion of DNA from SF WL, the 5' and 3' provirus-cell junction fragments were seen in addition to a 2.5-kb fragment of cell sequences that was homologous with the viral integration region. The latter polymorphism, which appeared to represent sequences duplicated after integration of EV21, was designated unoccupied repeat URa. Among SF broiler crosses, the same provirus-cell junction fragments were found but the pristine region that represented the 1.6-kb proviral URb was also found. Only URb was found among rapid-feathering (RF) chickens, regardless of breed. Although there was marked (greater than 95%), agreement between the presence of ev21-cell; junction fragments and the SF phenotype among both WL and broilers, Southern blots of DNA from a few commercial SF broiler chickens lacked ev21 junction fragments but some RF revertants harbored ev21 junction fragments. These anomalies suggest that ev21 may not be the sole determinant of the SF phenotype.