The Origin of a Linear Mitochondrial Plasmid (mF) that Promotes Mitochondrial Fusion in Laboratory Strains of Physarum polycephalum.

Abstract

The mitochondrial-fusion promoting (mF) plasmid first found in the slime mold Physarum polycephalum is a parasitic, selfish mitochondrial plasmid, and is capable of manipulating mitochondria behavior as indicated by its name. To investigate the origin of the mF plasmid, we collected many Physarum laboratory strains as are available at present throughout the world, and classified their origin on the basis of the results of mating-type analysis and restriction endonuclease analysis of their mtDNA. They include 8 plasmodial strains obtained from mating crosses with 22 different myxamoebal strains and two sclerotia. One of the two sclerotia is originated at W. Seifriz'z laboratory in the University of Pennsylvania, and is the oldest laboratory strain in the world. The mtDNA of each strain formed a single 80-90kbp band but the mF+ strains yielded a 14-16kbp mF plasmid band in addition to this main band. Southern hybridization using labeled plasmid DNA as the probe allowed the mF- strains to be classified into two categories: simple mF- having only an mID sequence and ΔmF having some part of the mF plasmid integrated into the mtDNA. The mID sequence is almost identical to a 475-bp sequence (pID) of the mF plasmid. To confirm this finding, we designed PCR primers for amplifying certain parts of the mF plasmid. This PCR system enabled us to determine whether apparent mF- strains and sclerotia possessed the free or integrated mF plasmids. The sclerotium derived from Seifriz's laboratory is simple mF-, but the other sclerotium, which was originally derived from the North Carolina Biological supply house is complete mF+. Moreover, except this derivative of North Carolina Biological, DP12×DP13, which derived from a stock at the University of Iowa is ΔmF. Such scattered distribution of mF+ and ΔmF suggests that the mitochondria occasionally acquired the mF plasmid after the establishment of P. polycephalum as a species.