Transfer of Neurospora kalilo plasmids among species and strains by introgression

There are four different variants of the kalilo "family" of linear mitochon drial plasmids. This family is found in several heterothallic species and o ne pseudohomothallic species of Neurospora, as well as in one homothallic s pecies of Gelasinospora. The mode of dispersal of these plasmids is not kno wn. Horizontal transmission has proved difficult to demonstrate. Another po ssibility is transfer by introgression, and this is modelled in the present paper. We have used introgression and subsequent heterokaryosis to success fully transfer the LA-kalilo plasmid from a Haitian strain of Neurospora cr assa to the standard Oak Ridge N. crassa background, the LA-kalilo plasmid from the pseudohomothallic Neurospora tetrasperma to N. crassa, and the kal ilo plasmid from N. crassa to N. tetrasperma. Thus, introgression is shown to be a possible avenue of dispersal between species. The recipient strains were all senescent but the mechanism of this senescence is not known, It c ould be caused by the plasmids, but if so the mechanism is novel since plas mid/mtDNA junction fragments of the type found in the standard mode of mtDN A insertion could not be detected. However, mtDNA changes were observed in the senescent recipients.