Fine Mapping of the Psm Locus of Cucumber (Cucumis sativus L.)

Mitochondria are cytoplasmic organelles that have their own DNA, serve as the powerhouses of all eukaryotic cells, and play important roles in programmed cell death and ageing. In essentially all animals, plants, and fungi, mitochondria are maternally transmitted through the egg cell. Strict maternal transmission of mitochondria reduces the possibility of selecting for better performing mitochondria, or for selecting against mutations in the mitochondrial DNA. Cucumber not only has one of the largest mitochondrial DNAs among all eukaryotes, but it is also a rare example of paternal transmission of mitochondria. Recombination among direct and indirect repeats in the mitochondrial DNA causes rearrangements associated with mosaic (MSC) phenotypes on leaves and fruits. A unique nuclear locus (Paternal sorting of mitochondria or Psm) controls sorting of paternally transmitted mitochondrial DNA in cucumber. The goal of this project was to fine-map Psm as an important step towards its eventual cloning, in order to provide insights about nuclear control of organellar-DNA sorting. We crossed single plants from cucumber plant introduction 401734 and C. sativus var. hardwickii to produce a large segregating family of 898 gametes. We analyzed Psm as a quantitative trait and mapped a major QTL between two SSR markers flanking a 459 kb region on chromosome 3. Currently available annotation of the genomic sequence in this region did not reveal a candidate gene for Psm. DNA sequence capture and/or next-generation sequencing could be cost-effective approaches to enrich sequence in the region of interest on chromosome 3 and allow analyses of cucumber lines that differ in their ability to sort mitochondria. Such an approach, together with improved annotation may lead to the eventual cloning of Psm and may allow for the selection of better performing mitochondria. Using MSC and elongated hypocotyls of cucumber, a lab exercise was developed to illustrate the transmission genetics of the nuclear and mitochondrial DNAs was developed.