Abstract
SummaryThe mitochondrial DNA of diplonemid and kinetoplastid protists is known for its suite of bizarre features, including the presence of concatenated circular molecules, extensive trans‐splicing and various forms of RNA editing. Here we report on the existence of another remarkable characteristic: hyper‐inflated DNA content. We estimated the total amount of mitochondrial DNA in four kinetoplastid species (Trypanosoma brucei, Trypanoplasma borreli, Cryptobia helicis, and Perkinsela sp.) and the diplonemid Diplonema papillatum. Staining with 4′,6‐diamidino‐2‐phenylindole and RedDot1 followed by color deconvolution and quantification revealed massive inflation in the total amount of DNA in their organelles. This was further confirmed by electron microscopy. The most extreme case is the ∼260 Mbp of DNA in the mitochondrion of Diplonema, which greatly exceeds that in its nucleus; this is, to our knowledge, the largest amount of DNA described in any organelle. Perkinsela sp. has a total mitochondrial DNA content ~6.6× greater than its nuclear genome. This mass of DNA occupies most of the volume of the Perkinsela cell, despite the fact that it contains only six protein‐coding genes. Why so much DNA? We propose that these bloated mitochondrial DNAs accumulated by a ratchet‐like process. Despite their excessive nature, the synthesis and maintenance of these mtDNAs must incur a relatively low cost, considering that diplonemids are one of the most ubiquitous and speciose protist groups in the ocean. © 2018 The Authors. IUBMB Life published by Wiley Periodicals, Inc. on behalf of International Union of Biochemistry and Molecular Biology., 70(12):1267–1274, 2018
Highlights
Mitochondrial genomes come in all shapes and sizes
The mitochondrial genomes of excavate protists are unusual in other ways
While jakobids harbor the most gene-rich mitochondrial DNA known [6], it is the mitochondrial genomes of Euglenozoa—a diverse group of aerobic excavates composed of photosynthetic Euglenida, heterotrophic free-living Diplonemea, and parasitic Kinetoplastea—that exhibit a range of unique features
Summary
Mitochondrial genomes come in all shapes and sizes. The mitochondrial genomes of flowering plants range from ~200 kb to over 10 Mb, have very low gene densities, undergo frequent rearrangements associated with the gain or loss of intergenic regions [2,3,4] and are capable of acquiring, via horizontal gene transfer, whole mitochondrial genomes from other organisms [5]. The mitochondrial genomes of excavate protists are unusual in other ways. While jakobids harbor the most gene-rich mitochondrial DNA known [6], it is the mitochondrial genomes of Euglenozoa—a diverse group of aerobic excavates composed of photosynthetic Euglenida, heterotrophic free-living Diplonemea, and parasitic Kinetoplastea—that exhibit a range of unique features. The mitochondrial DNA (or kinetoplast DNA) of the human parasite Trypanosoma brucei is amongst the best-studied non-nuclear genomes.
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