Detection and molecular characterization of recombinant avian leukosis viruses in commercial egg-type chickens in China.

Two natural recombinant avian leukosis viruses (ALVs) were isolated from Chinese commercial egg-type chickens in 2009, which suffered from haemangiomas and myelocytomas. Sequence analysis of the complete proviral genomes revealed several unique genetic characteristics of the present two isolates, demonstrating that the two viruses were derived from recombination between earlier Chinese ALV-J and endogenous virus sequences. The two recombinant viruses presented typical genetic organization of replication-competent genus Alpharetrovirus, and the gag and pol genes were well conserved with those of ALVs. The env genes of the two viruses were composed of the internal identical sequences (about 240 bp) of endogenous viruses, and the rest of the sequence belonged to subgroup J ALVs. The long terminal repeats of the two viruses were more closely related to HPRS-103 and earlier Chinese ALV-J than other subgroup ALVs, and multiple transcription regulatory elements of ALV-J were highly conserved. In addition, the two viruses shared an almost identical 3'-untranslated region (UTR) sequence with earlier Chinese ALV-J strains and the US strain 4817, containing a ~127 bp deletion in the E element region. However, further comparison with endogenous ALV indicated that the 3'-UTR sequences with ~127 bp deletion of ALV-J were most probably derived from endogenous viruses by recombination. These results suggested that the two isolates can be characterized as recombinant ALV-J with the internal env gene and 3'-UTR sequence of endogenous ALV.