Genetic diversity and phylogenetic analysis of glycoprotein GP85 of ALV-J isolates from Mainland China between 1999 and 2010: coexistence of two extremely different subgroups in layers.

Avian leukosis virus subgroup J (ALV-J), first isolated in 1989, preferentially infects meat-type birds. However, layer flocks in China have experienced outbreaks of this virus since 2008. To understand the genetic diversity of ALV-J in Chinese layers, we compared and analyzed the GP85 gene sequences of 106 ALV-J isolates that were isolated between 1999 and 2010 in Mainland China. The GP85 gene sequences of 41 layer isolates collected from 9 provinces of China between 2008 and 2010 belonged to two separate, highly diverse subgroups and were differentiated from meat-type chicken isolates. When compared to all meat-type isolates from China, Subgroup 1 exclusively contained current layer isolates and seemed to be dominant; all the isolates in this subgroup exhibited gene diversity, and many unique amino acid mutations were present. In contrast, the viruses in Subgroup 2 were perfectly conserved and shared high identity with the prototype meat-type chicken ALV-J strain HPRS-103. The two subgroups contained only two concurrent mutations at the same position. Moreover, most of the isolates in Subgroup 1 had two additional glycosylation sites (at positions 101 and 191) when compared with those in Subgroup 2. Our study provides evidence for the coexistence of two extremely different ALV-J subgroups in Chinese layers from 2008 to 2010, supporting the need for vaccine development and purification measures to prevent ALV-J infection in layers in China.