Evidence for the phenotypic neutrality of the Neurospora Varkud (V) and Varkud satellite (VS) plasmids

The Varkud satellite (VS) plasmid, which requires the Varkud (V) plasmid for replication, is found in the mitochondria of several natural isolates of Neurospora. The VS transcript is sufficiently abundant that it might be expected to alter the function of mitochondria; however, previous limited characterization revealed no effect. In this work we have used genetic, biochemical and proteomic approaches to search for effects of the V and VS plasmids. We observed differences in the relative abundance of several mitochondrial proteins between plasmid-containing and plasmid-lacking natural isolates, but subsequently found these not to be due to the plasmids. We constructed a pair of iso-nuclear and iso-mitochondrial strains that differed only by the presence or absence of V and VS, and observed only subtle differences in the abundance of several mitochondrial proteins. We further attempted to detect a cryptic plasmid-related phenotype by growing this pair of strains in the presence of a variety of inhibitors of mitochondrial function or other stress conditions: this also revealed no effect of the plasmids. These observations suggest that, despite the high concentration of VS RNA in the mitochondrion, the V and VS plasmids do not cause substantial changes in the host.

► The VS plasmid encodes a very abundant mitochondrial RNA.
► We searched for evidence that the VS and V plasmids alter the function of the host Neurospora strain.
► We found only subtle differences in abundance of some mitochondrial proteins detectable on 2D IEF-SDS gels.
► We found no differences in growth rate under a wide range of stress conditions.