rise to new H9N2 reassortants (Li et al., 2003), and a similar mechanism may generate novel H5N1 reassortants.

It is notable that in the short time since its emergence in 2002, genotype Z has replaced genotypes A–E, X, and Y to become dominant in both aquatic and terrestrial poultry in this region (Table 2-1). To define the genetic stability of this new gene constellation, we analysed the rates of non-synonymous (K_a) and synonymous (K_s) nucleotide substitutions in six internal gene segments of genotype Z viruses isolated in 2002–04. A K_a/K_s ratio >1 suggests evidence of positive natural selection (Presgraves et al., 2003). Of these internal genes, the M2 gene was under positive selection pressure in late 2002 to early 2003, but under less selection pressure in late 2003 to early 2004. The NS1 and NS2 genes, acquired in late 2000, were also under positive selection pressure (Supplementary Table 1).¹² These