Fungal Genetics and Biology 43 (2006) 90–101 www.elsevier.com/locate/yfgbi 1087-1845/$ - see front matter  2005 Elsevier Inc. All rights reserved. doi:10.1016/j.fgb.2005.10.004 Asexual transmission, non-suppressiveness and meiotic extinction of small plasmid-like derivatives of the mitochondrial DNA in Neurospora crassa Georg Hausner 1 , Katherine A. Nummy, Helmut Bertrand ¤ Department of Microbiology, Michigan State University, East-Lansing, MI 48824-1101, USA Received 15 July 2005; accepted 28 October 2005 Available online 28 December 2005 Abstract For reasons that are not obvious, sets of related plasmid-like elements that consist of short segments of DNA that overlap the 5' termi- nal region of the mitochondrial large-subunit rRNA gene sometimes appear spontaneously and become ampliWed in the mitochondria of some cytochrome-deWcient and/or UV-sensitive mutants of Neurospora crassa. These elements are transmitted eYciently through hyphal anastomoses and appear to invade the mitochondria of recipient strains, but they do not cause senescence and at best cause only slight deWciencies 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 myce- lia. 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 con- tain 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.  2005 Elsevier Inc. All rights reserved. Keywords: Plasmid-like mitochondrial element; Suppressiveness; Maternal inheritance; Asexual transmission; Neurospora 1. Introduction Plasmid-like mitochondrial elements (plMEs) that are derived from regions of the mitochondrial DNA (mtDNA) have been found in several fungi. In Podospora anserina, senescence is correlated with the appearance of mtDNA- derived, circular, multimeric elements, the so-called sen DNAs (Cummings et al., 1986; Dujon and Belcour, 1989; Albert and Sellem, 2002), and in the “ragged” cytoplasmic mutants of Aspergillus amstelodami, sets of mtDNA- derived, sub-genomic, circular, multimeric molecules have been shown to co-exist with the intact wild-type mitochon- drial genome (Lazarus et al., 1980; Lazarus and Küntzel, 1981). At least in Podospora, the appearance of mtDNA- derived plasmid-like senDNAs, is not always or irrevocably associated with senescence, for the persistence of such ele- ments in longevity mutants also has been documented (Turker et al., 1987a; Silliker and Cummings, 1990; Silar et al., 1997; Jamet-Vierny et al., 1999) and their absence has been noted in aging mycelia of Podospora curvicolla (Böck- elmann and Esser, 1986). In Neurospora, mtDNA-derived plasmid-like elements have been observed in some cultures of several cytoplasmic mutants, including the non-senescing [poky] (Mannella et al., 1979) and [stp-B1] (Almasan and Mishra, 1990) strains, and the death-prone [stp-ruv] variant * Corresponding author. E-mail address: mhelmut@msu.edu (H. Bertrand). 1 Present address: Department of Microbiology, University of Manito- ba, Winnipeg, Man., Canada R3T 2N2.