Widespread distribution of low-copy-number variants of mitochondrial plasmid pEM in the genus Agaricus.

The incidence of the linear mitochondrial plasmid pEM in Agaricus spp. was believed to be rare, based on visualization by gel electrophoresis and Southern hybridization. However, we report in this study PCR amplification of pEM-like sequences from all but one species of Agaricus examined. Regions amplified included (1) the pEM RNA polymerase gene and (2) adjoining carboxy-termini of the DNA and RNA polymerase genes. Sequence data from the RNA polymerase-like products support a plasmid, rather than mitochondrial, origin for these sequences. Sequence variation was low, and most differences were silent or conservative at the amino acid level. Stop codons were found in two of seven sequence types suggesting that functional constraints are low. A parsimony-derived phylogeny for these sequences did not match expected phylogenies for the host species. Recent acquisition of the plasmid is presented as the most likely hypothesis explaining these observations.