A highly virulent mouse-adapted variant of influenza virus A/Aichi/2/68 (H3N2) was crossed either with the original A/USSR/90/77 (H1N1) influenza virus strain or with its mouse-adapted, moderately mouse virulent variant. The reassortants were characterized with respect to their genetic content and pneumovirulence for mice. The reassortants fell into three categories: avirulent, highly virulent (resembling in this respect the parent A/Aichi/2/68 virus) and moderately virulent (resembling the mouse-adapted A/USSR/90/77 parent virus). The analysis of the parental origin of the genes of 6 reassortants allowed to suggest that changes in the HA gene and in a polymerase gene (most likely, PB1) were necessary for the acquisition of virulence by the A/USSR/90/77 virus in the course of adaptation to mice, whereas the changes in two other polymerase genes as well as in the genes NA and NS were not involved. The low degree of pathogenicity characteristic of the mouse-adapted A/USSR/90/77 virus was determined by gene(s) other than HA.

Human-avian influenza reassortants possessing the HA gene of the avian parent virus were tested for their ability to replicate in MDCK cells at 37 degrees C and 31 degrees C. Both avian parent viruses, A/Duck/Ukraine/1/63 (H3N8) and A/Duck/Hoshimin/014/78 (H5N3) induced an efficient multi-cycle infection at 37 degrees C, but replicated poorly at 31 degrees C, whereas the human parent virus, MDCK-adapted variant of A/USSR/90/77 (H1N1) strain, replicated efficiently at both temperatures. The reassortant clone possessing the HA gene of A/Duck/Ukraine/1/63 virus and the other 7 genes of A/USSR/90/77 virus replicated at both temperatures almost as efficiently as the human parent virus. Among the reassortants between A/Duck/Hoshimin/014/78 and A/USSR/90/77, the clones possessing the HA and NA genes of the avian strain, or the HA, NA, NP, and NS genes of the avian strain, and the other genes of the human parent virus, replicated poorly at both temperatures, especially at 31 degrees C, whereas the reassortant possessing the HA, NA, and M genes of the avian virus replicated at both temperatures fairly efficiently. The results are discussed in connection with the limitations imposed by different genes upon avian influenza viruses' ability to replicate in mammalian cells.

RNase resistance analyses of native, heat-denatured and self-annealed L and S species of Machupo virion RNA revealed the presence of complementary sequences. Self-annealing was concentration-dependent. The data indicate, that complementary sequences were present in separate RNA strands.