Hepadnavirus evolution and molecular strategy of adaptation in a new host

In order to elucidate the mechanisms of hepadnavirus evolution in vivo and to trace the fate of known quasispecies in a single animal during the acute phase of infection, a woodchuck (Marmota monax) was infected with the hepa dnavirus woodchuck hepatitis B virus (WHV). Woodchuck 197 (W197) was inject ed intravenously with pooled sera collected from a chronic carrier that had been infected originally with a molecular clone of known genome sequence ( WHV7), Viral genome variants from both the inoculum and the follow-up sera from W197 were characterized for the presence of quasispecies related to th e WHV7 sequence. Interestingly, WHV7-related genomes were predominant 6 wee ks post-infection (p.i.), whereas a highly heterogeneous virus population w as present in the first viraemic serum (4 weeks p.i.). Using WHV7 as the pr ototype, the variability of the Pol and PreS/S regions in the first 11 week s p.i. has been calculated. The sequence population in serum collected 6 we eks p.i. was highly homogeneous, with a mean variability of 0.36% in the re gion analysed. Mean variability values ranging from 0.82% to 1.61% were fou nd in quasispecies from the other sera, The presence of possible selective pressure was analysed by means of the non-synonymous versus synonymous vari ation ratio (d(n)/d(s)), We found that the d(n)/d(s) values were stable for the S ORF (ranging from 2.6 to 3.0), whereas a wider range was observed fo r the Pol ORF (from 1.4 to 3.0). Furthermore, from the analysis of the vari ability of the codon positions for the two overlapping ORFs it was found th at, in most cases, non-synonymous mutations at position 1 of the Pol ORF (p osition 3 of the S ORF) corresponded to synonymous variation in the S (Pol) ORF, indicating independent evolution of the encoded proteins.