Molecular epidemiology of avian leukosis virus subgroup J in 1 layer flocks in China

23 Avian leukosis virus subgroup J (ALV-J) was first isolated from meat-type 24 chickens in 1988. No field cases of ALV-J infection and tumors in layer chickens were 25 observed worldwide until 2004. However, layer flocks in China have experienced 26 outbreaks of this virus in recent years. The molecular epidemiology of ALV-J strains 27 isolated from layer flocks was investigated. The env genes of 77.8% (21/27) of the 28 ALV-J layer isolates with a high degree of genetic variation were significantly different 29 from the env genes of the prototype strain of ALV-J HPRS-103 that was observed in 30 American and Chinese strains from meat-type chickens (designated as ALV-J broiler 31 isolates). A total of 205 nucleotides were deleted from the 3’ untranslated region of 32 89.5% (17/19) of the ALV-J layer isolates. Approximately 94.7 % (16/17) of the layer 33 isolates contained a complete E element of 146-149 residues. The U3 sequences of 34 84.2 % (16/19) of the ALV-J layer isolates displayed less than 92.5% sequence 35 homology to the ALV-J broiler isolates, although the transcriptional regulatory elements 36 that are typical of avian retroviruses were highly conserved. Several unique nucleotide 37 substitutions in the env gene, the U3 region and the E element of most of the ALV-J 38 layer isolates were detected. These results suggested that the env, E element and U3 39 region in the ALV-J layer isolates have evolved rapidly and were significantly different 40 from those of the ALV-J broiler isolates. These findings will contribute to a better 41 understanding of the pathogenic mechanism of layer tumor diseases induced by ALV-J. 42