Genome sequence and origin analyses of the recombinant novel IBV virulent isolate SAIBK2

Recombination between infectious bronchitis viruses (IBVs), together with point mutations, insertions, and deletions, is thought to be responsible for the emergence of new IBV variants. SAIBK2 is a nephropathogenic strain isolated from layer flocks vaccinated with live attenuated H120 vaccine in Sichuan province, China in 2011. SAIBK2 causes severe kidney lesions and results in 50 % mortality in 30-day-old specific-pathogen-free chickens (with a dose of 10(5) EID50/0.1 mL SAIBK2 per chicken). The complete genome of SAIBK2 consists of 27669 nucleotides, excluding the poly-A tail at the 3' end. SAIBK2 has the highest identity to YX10 in terms of complete genome. Phylogenetic analysis of complete sequence showed that SAIBK2 belongs to the most dominant genotype in China. Comparison and recombination analyses with other IBV strains revealed that SAIBK2 may originate from recombination events among a YX10-, a YN-, and a Mass-like strain. Furthermore, whole gene 5 and parts of nsp 3, nsp 4, nsp 16, and N genes are involved in the recombination events, and the uptake of these regions from YN and Mass strains by SAIBK2 may increase its replication efficiency and be responsible for its increased virulence in specific-pathogen-free chickens.