Properties of DI particle resistant mutants of vesicular stomatitis virus isolated from persistent infections and from undiluted passages. 
We describe an assay procedure to quantitate relative DI resistance of a variety of DI particle resistant (Sdi-) mutants of vesicular stomatitis virus (VSV).  We show that numerous diverse Sdi- mutants of VSV are selected continuously in a stepwise manner during persistent infections, and also during serial undiluted lytic passages initiated with cloned virus.  Concurrently with the successive appearance and disappearance of different Sdi- mutants of infectious VSV, new DI particle types with altered interference properties also appear and disappear, resulting in rapid "coevolution" of virus and DI particle populations.  Complementation tests with Sdi- mutants indicate that mutations in at least two different virus factors (presumably associated with replication-encapsidation) can give rise to Sdi- mutants.  Interference studies with chimeric DI particles indicate that DI particle template RNA rather than DI particle protein determines the interference properties of DI particles interacting with Sdi- and Sdi+ mutants of helper virus.