FIG. 2.

Infectivity values upon passage of FMDV C-S8c1 in the absence or presence of FU or AZC. Conditions for mutagenic treatment and for determination of FMDV infectivity are detailed in Materials and Methods. Filled circles indicate passages of undiluted virus in the presence of mutagens, and squares correspond to passages in which the virus was diluted 10-fold prior to each infection (to a titer 1/10 the one indicated on the ordinate), irrespective of the virus titer obtained. Empty circles indicate serial passages of undiluted virus in the absence of mutagens (the same experimental series plotted on the two panels). No extinction of FMDV C-S8c1 has ever been observed in serial passages of viruses, even with 100- or 1,000-fold dilutions intervening between passages, in the absence of mutagens (56). Preextinction populations are indicated by arrows. The preextinction population C-S8c1AZCp10 was split into two sublineages; one was extinguished at passage 11, and the other could be further passaged and was extinguished at passage 21. Loss of infectivity was ascertained by three additional blind passages, with no evidence of infectivity or RT-PCR-amplifiable FMDV RNA in the culture supernatant from the third passage. Preextinction populations were tested in infections of BHK-21 cells in the absence of a mutagen; in all instances, titers produced were 10⁴ to 10⁶ PFU/ml. All titrations were done in triplicate. Standard deviations (not included in the plots) never exceeded 50%.