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 Sout hern hybridization. However, we report in this study PCR amplification of p EM-like sequences from all but one species of Agaricus examined. Regions am plified included (1) the pEM RNA polymerase gene and (2) adjoining carboxy- termini of the DNA and RNA polymerase genes. Sequence data from the RNA pol ymerase-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 t wo of seven sequence types suggesting that functional constraints are low. A parsimony-derived phylogeny for these sequences did not match expected ph ylogenies for the host species. Recent acquisition of the plasmid is presen ted as the most likely hypothesis explaining these observations. (C) 1999 A cademic Press.