Sequence Analyses Of The Mosaic (MSC) Mitochondrial Mutants Of Cucumber

Cucumber mitochondria have three unique attributes:  paternal transmission, a huge genome size, and appearance of strongly mosaic (MSC) phenotypes after passage through cell culture.  At 1.6 megabases (Mb), the cucumber mitochondrial DNA is one of largest among angiosperms, and is divided into one chromosome of 1.5 Mb, as well as two smaller chromosomes of 84 and 45 kilobases (Kb).  The Roche 454 platform was used to sequence mitochondrial DNAs from the highly inbred line ‘B’ and three MSC lines (MSC3, MSC12 and MSC16) independently derived from B.  Reads were aligned to the mitochondrial assembly of the cucumber cultivar ‘Calypso’.  Compared to B and Calypso, the MSC lines had five deleted regions in the 1.5 Mb chromosome.  The 85 kb chromosome was completely absent in MSC12 and MSC16 and approximately one-half was absent in MSC3.  None of the three MSC lines possessed identical deletions, indicating that they do not trace back to a single sublimon in inbred B.  Quantitative PCR supported the sequence assemblies and indicated that the MSC phenotypes are associated with unique deletions in their mitochondrial DNAs.