Introduction Plants possess three major genomes carried in the nucleus, mitochondrion, and plastids. Plant mito- chondrial genomes are much larger than those in animals and fungi. For example, melon and cu- cumber have the largest known mitochondrial genomes at 2 400 and 1 500 kb, respectively (Ward et al. 1981), similar in size to the entire ge- nome of many prokaryotes. These huge mitochon- drial genomes in plants are due in part to the transfer of DNA from the chloroplast or nucleus, and the accumulation of short repetitive DNAs (Unseld et al. 1997; Kubo et al. 2000; Lilly and Havey 2001; Notsu et al. 2002; Clifton et al. 2004; Ogihara et al. 2005). Recombination among these repetitive sequences produces rearrangements in the mitochondrial DNAs, shifting the linear order of genes even among relatively closely related species (Palmer and Hebron 1988; Fauron et al. 1995; Bartoszewski et al. 2004a). Plants regenerated from in vitro cultures may show new phenotypes; a phenomenon referred to as somaclonal variation (Larkin and Scowcroft 1981). Somaclonal variation may have been pre-existing or arose during cell culture and may be either genetic (Evans and Sharp 1983; Karp 1991) or epigenetic (Kaeppler and Phillips 1993; Kaeppler et al. 2000). Genetic variability triggered by tissue culture may result from changes in the nuclear and/or organellar (plastid and mitochon- drial) genomes. Observed changes include polyploidy, aneuploidy, deletions, inversions and translocations, activation of transposable ele- ments, and single base changes. The chloroplast genome does not appear to be significantly af- J Appl Genet 48(1), 2007, pp. 1–9 Review article The selection of mosaic (MSC) phenotype after passage of cucumber (Cucumis sativus L.) through cell culture – a method to obtain plant mitochondrial mutants Grzegorz Bartoszewski 1 , Michael J. Havey 2 , Agnieszka Zió³kowska 1 , Marek D³ugosz 1 , Stefan Malepszy 1 1 Department of Plant Genetics, Breeding and Biotechnology, Faculty of Horticulture and Landscape Architecture, Warsaw Agricultural University, Warszawa, Poland 2 Agricultural Research Service, U.S. Department of Agriculture, Vegetable Crops Unit, and Department of Horticulture 1575 Linden Dr., University of Wisconsin, Madison, USA Abstract. Mosaic (MSC) mutants of cucumber (Cucumis sativus L.) appear after passage through cell cultures. The MSC phenotype shows paternal transmission and is associated with mitochondrial DNA rearrangements. This review describes the origins and phenotypes of independently produced MSC mutants of cucumber, includ- ing current knowledge on their mitochondrial DNA rearrangements, and similarities of MSC with other plant mi- tochondrial mutants. Finally we propose that passage of cucumber through cell culture can be used as a unique and efficient method to generate mitochondrial mutants of a higher plant in a highly homozygous nuclear back- ground. Keywords: Cucumis sativus, mitochondrial mutants, mutagenesis, plant mitochondria, somaclonal variation, tissue culture. Received: August 11, 2006. Accepted: December 13, 2006. Correspondence: G. Bartoszewski, Department of Plant Genetics, Breeding and Biotechnology, Faculty of Horticulture and Landscape Architecture, Warsaw Agricultural University, Nowoursynowska 159, 02–776 Warszawa, Poland; e-mail: grzegorz_bartoszewski@sggw.pl