Changes in copy numbers of mitochondrial genes during melon leaf development reveals fewer genes than mitochondria

Authors: SHEN, JIA - Zhejiang University Of Technology; ZHANG, YUE-JIAN - Zhejiang University Of Technology; Havey, Michael; SHOU, WEI-SONG - Zhejiang University Of Technology

Submitted to: Horticulture Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/24/2019
Publication Date: 8/11/2019
Citation: Shen, J., Zhang, Y., Havey, M.J., Shou, W. 2019. Changes in copy numbers of mitochondrial genes during melon leaf development reveals fewer genes than mitochondria. Horticulture Research. 6(1):1-9. https://doi.org/10.1038/s41438-019-0177-8.
DOI: https://doi.org/10.1038/s41438-019-0177-8

Interpretive Summary: Melon is a useful plant to study mitochondrial genetics because its mitochondrial DNA (mtDNA) is s one of the largest among all plants, is structurally diverse, and shows paternal transmission. Droplet digital (dd) PCR was used in combination with flow cytometric determination of nuclear DNA quantities to determine the absolute per-cell-copy number of four mitochondrial genes across four stages of melon leaf development. The copy numbers of these mitochondrial genes varied during leaf development, and there was no correlation between copy numbers of mitochondrial genes and their transcript levels. This observation indicates that leaf cells of melon do not contain sufficient copies of mitochondrial genes to ensure that every mitochondrion possesses the entire mitochondrial genome. Our results clearly indicate that the mtDNA in melon exists as sub-genomic molecules rather than as a single master circle, and that copy numbers of individual mitochondrial genes may be independently controlled. A better understanding of the molecular mechanism(s) controlling relative prevalence and transmission of sub-genomic mtDNA molecules should provide insights about the continuity of the mitochondrial genome across generations. This research will be of interest to researchers studying the diversity and transmission of organellar DNA, and the potential to identify and select better performing organelles.

Technical Abstract: Melon is a useful plant to study mitochondrial genetics because its mitochondrial DNA (mtDNA) is one of the largest among all plants, is structurally diverse, and shows paternal transmission. We used droplet digital (dd) PCR in combination with flow cytometric determination of nuclear DNA quantities to determine the absolute per-cell-copy number of four mitochondrial genes (nad9, rps1, matR, and atp6) across four stages of melon leaf development. The copy numbers of these mitochondrial genes not only varied during leaf development, but also differed from each other, and there was no correlation between copy numbers of mitochondrial genes and their transcript levels. Gene copy numbers varied from approximately 39.5 ± 2.0 (atp6 copies in the 15th leaves) to approximately 82.2 ± 3.4 (nad9 copies in the 9th leaves), while the mean number of mitochondria varied from approximately 362.3 ± 173.6 in the 18th leaves to 459.1 ± 228.2 in the 9th leaves. This observation indicates that leaf cells of melon do not contain sufficient copies of mitochondrial genes to ensure that every mitochondrion possesses the entire mitochondrial genome. Our results clearly indicate that the mtDNA in melon exists as sub-genomic molecules rather than as a single master circle, and that copy numbers of individual mitochondrial genes may be independently controlled. A better understanding of the molecular mechanism(s) controlling relative prevalence and transmission of sub-genomic mtDNA molecules should provide insights about the continuity of the mitochondrial genome across generations.