Phenotypes of rotavirus reassortants depend upon the recipient genetic background

We have previously characterized the biological and immunological properties of a simian rotavirus SA11 variant (4F) with an altered genome segment 4. The SA11-4F variant formed large plaques in the presence of protease, formed small clear plaques in the absence of protease, and grew to high titer in the presence of protease when compared to our standard wild type (SA11 clone 3). To determine the genome segment of the rotavirus SA11 variant 4F that encoded the unique protease-associated phenotypes of the variant, reassortants were generated that segregated the outer capsid genes of 4F onto a genetic background derived from either the bovine rotavirus B223 or our standard SA11 wild type (clone 3), both of which have contrasting protease-associated phenotypes. The parental and reassortant viruses were examined to determine which genes from the 4F variant encoded the ability (i) to form large plaques in the presence of protease, (ii) to form small clear plaques in the absence of exogenous protease, and (iii) to grow to significantly higher titer in the presence of protease. These phenotypes could be transferred to a clone 3 genetic background by a single genome segment from the 4F variant segment 4. However, in the 4F/B223 reassortants a different and unexpected situation was found. On a B223 genetic background the same phenotypes segregated with a combination of a minimum of two 4F genome segments, segments 4 and 9. These results indicate that the recipient genetic background onto which the genes of a donor rotavirus are reassorted can affect the phenotypes conferred by the presence of the donor segments. Thus, the results of segregation mapping experiments using reassortant viruses should be interpreted with caution.