Fig 1.

The genetic makeup of reassortant viruses affects HA titer and viral growth. The hemagglutination titers of individual eggs infected with a constant dose (100 PFU) of the wild-type (wt) virus parents PR8 and Udorn were compared to those of the PR8(Ud-HA,NA,PB1) virus selected by classical reassortment of the two parental viruses (A). The hemagglutination titers (B) and infectious virus titers, determined by plaque formation in MDCK cells (C), were measured in eggs infected with a constant dose (100 PFU) of the reverse genetics-derived viruses corresponding to the PR8 and Udorn parent viruses and additional viruses containing Udorn HA and NA with or without the Udorn PB1, PB2, or PA (with the remaining genes originating from PR8). The titers of viral particles (D), measured using copy numbers of M vRNA in the allantoic fluid samples used in the assay whose results are shown in panel C, were determined by quantitative RT-PCR as a surrogate. The yields of infectious virus from individual MDCK cell cultures infected with 100 PFU of virus were determined by plaque assay of cell culture supernatants (E). In panels A, B, C, and E, each symbol represents the titer of an individual egg or culture supernatant and the line is the geometric mean titer. In panel D, each bar represents the mean particle number of all the eggs in that group and the error bar shows the standard error of the mean.