Asexual transmission, non-suppressiveness and meiotic extinction of small plasmid-like derivatives of the mitochondrial DNA in Neurospora crassa

Abstract

For reasons that are not obvious, sets of related plasmid-like elements that consist of short segments of DNA that overlap the 5′ terminal region of the mitochondrial large-subunit rRNA gene sometimes appear spontaneously and become amplified in the mitochondria of some cytochrome-deficient and/or UV-sensitive mutants of Neurospora crassa. These elements are transmitted efficiently through hyphal anastomoses and appear to invade the mitochondria of recipient strains, but they do not cause senescence and at best cause only slight deficiencies in cytochromes a and b even though they are transcribed copiously. Hence, the small elements are not suppressive and, unlike large deletion derivatives of the mitochondrial chromosome, do not displace normal mtDNA molecules in vegetatively propagated mycelia. Unlike the mitochondrial chromosome, large plasmid-like mtDNA derivatives and true mitochondrial plasmids, the small plasmid-like mtDNA derivatives are rarely transmitted sexually even though they persist without selection in very high copy numbers in vegetative cells. The high copy numbers and high stability of these elements in vegetatively propagated cultures suggests that their monomers contain all the features required for their replication and transmission in the hyphae and conidia of Neurospora. However, the mt- rnl-derived molecules appear to lack a sequence or attribute required for the maintenance or transmission of mitochondrial genetic elements at some stage of the sexual reproductive cycle, including ascospore maturation and germination.