The nucleoprotein and matrix protein segments of H5N1 influenza viruses are responsible for dominance in embryonated eggs.

Since their emergence in 1996 in southern China, highly pathogenic H5N1 avian influenza viruses have spread widely and continue to circulate in some countries. Genetic reassortment has created multiple H5N1 virus lineages, some of which are dominant in nature. However, the mechanism by which certain H5N1 influenza virus lineages (or genotypes) become dominant in avian species remains unknown. Here, we used competitive inoculation and genetic analysis of the resultant viruses to show that the nucleoprotein (NP) and matrix protein (M) segments of Fujian-like viruses (clade 2.3.4), which became predominant in southern China in mid-2006, are responsible for viral dominance in embryonated eggs. We further found that specific residues in the NP and M proteins play key roles in conferring this viral dominance; specifically, a glutamic acid at position 66 in M2 was conserved among the Fujian-like viruses. These results suggest roles for these viral proteins in influenza virus dominance.