Genetic Characteristics of Echovirus Type 9 Strains: Association of Mouse Virulence with Other Genetic Markers

Department of Virology, Hebrew University-Hadassah Medical School, Jerusalem, Israel In naturally occurring strains and laboratory selected variants of echovirus type 9, the association between virulence for infant mice (v) and the reproductive capacity at 40° (t) was of the v + t + and v - t - types. By different selection methods the two other marker combinations, v + t - and v - t + , were obtained. The frequency of t + in a t - echovirus type 9 population was 10 -5 . Of the four possible combinations between the v and c (cytopathic effect for human epithelial cell lines) markers, only three were found. The v + c + covariation could not be obtained in spite of appropriate methods of selection. Received 22 June 1968; accepted 3 October 1968.