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.2016 Oct 13;8(10):3090-3098.
doi: 10.1093/gbe/evw217.

Mitochondrial Genome of Palpitomonas bilix: Derived Genome Structure and Ancestral System for Cytochrome c Maturation

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Mitochondrial Genome of Palpitomonas bilix: Derived Genome Structure and Ancestral System for Cytochrome c Maturation

Yuki Nishimura et al. Genome Biol Evol..

Abstract

We here reported the mitochondrial (mt) genome of one of the heterotrophic microeukaryotes related to cryptophytes, Palpitomonas bilix The P. bilix mt genome was found to be a linear molecule composed of "single copy region" (∼16 kb) and repeat regions (∼30 kb) arranged in an inverse manner at both ends of the genome. Linear mt genomes with large inverted repeats are known for three distantly related eukaryotes (including P. bilix), suggesting that this particular mt genome structure has emerged at least three times in the eukaryotic tree of life. The P. bilix mt genome contains 47 protein-coding genes including ccmA, ccmB, ccmC, and ccmF, which encode protein subunits involved in the system for cytochrome c maturation inherited from a bacterium (System I). We present data indicating that the phylogenetic relatives of P. bilix, namely, cryptophytes, goniomonads, and kathablepharids, utilize an alternative system for cytochrome c maturation, which has most likely emerged during the evolution of eukaryotes (System III). To explain the distribution of Systems I and III in P. bilix and its phylogenetic relatives, two scenarios are possible: (i) System I was replaced by System III on the branch leading to the common ancestor of cryptophytes, goniomonads, and kathablepharids, and (ii) the two systems co-existed in their common ancestor, and lost differentially among the four descendants.

Keywords: Cryptista; cytochrome c maturation; inverted repeats; mitochondrial genome.

© The Author 2016. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.

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Figures

<sc>Fig</sc>. 1.—
Fig. 1.—
Palpitomonas bilix mitochondrial genome. (A) Putative mitochondrial (mt) genome structure. Inverted repeats are shown in blue, and the central portion of the mt genome (single copy region) is shown in yellow. The DIG probe was designed to hybridizeatp1 gene region in inverted repeats (open arrowheads). According to the monomeric linear form of theP. bilix mt genome,XbaI digestion is anticipated to produce ∼44 kb- and ∼32 kb-fragments. Likewise, ∼5.3 kb- and ∼9.6 kb-fragments are likely to be detected by Southern blot analyses followingEcoRI andSalI digestion, respectively. (B) Gene order. Protein-coding genes and ribosomal RNA genes are shown by boxes, and their putative functions are color-coded (see the inset). Transfer RNA and transfer-messenger RNA genes are shown by lines.
<sc>Fig</sc>. 2.—
Fig. 2.—
Southern blot analyses of thePalpitomonas bilix mitochondrial genome. A DIG DNA probe set withinatp1 gene region (fig. 1) was used in all the Southern blot analyses in this study. (A) Analysis of the DNA sample separated by pulsed-field gel electrophoresis (PFGE). The DNA samples with and withoutXbaI digestion were run in lanes 1 and 2, respectively. The sizes of the detected DNA fragments were found to be consistent with the predicted monomeric linear form of the mitochondrial (mt) genome (fig. 1A). Note that the precise sizes of the two DNA fragments were difficult to estimate from the PFGE image. (B). Analysis of the DNA samples digested byEcoRI andSalI. Both restriction enzymes generated the DNA fragments, the sizes of which were found to be similar to those expected from the mt genome contig reconstructed from Illumina pair-end short reads (fig. 1A).
<sc>Fig</sc>. 3.—
Fig. 3.—
Venn diagram to compare the gene content among the mitochondrial genomes ofPalpitomonas bilix and two cryptophytes (Rhodomonas salina andHemiselmis andersenii). Note thattatA,rps1, andatp4 are not annotated in the GenBank file of theR. salina mitochondrial (mt) genome (GenBank accession no. NC_002572), but listed in the genome map (fig. 1) in Hauth et al. (2005). Only functionally assignable and vertically inherited protein-coding genes are included in this diagram. Functionally unassignable genes in the three mt genomes are indicated in parentheses.
<sc>Fig</sc>. 4.—
Fig. 4.—
Scenarios for the evolution of cytochromec maturation system inPalpitomonas bilix, kathablepharids, goniomonads, and cryptophytes. The phylogenetic relationship amongst the four lineages/species is based on Yabuki et al. (2014). (A) Scenario 1 assuming no co-existence of Systems I and III. Under this assumption, the distribution of Systems I and III can be predicted as follows. Although no nuclear genome is available forPalpitomonas bilix, we can conclude that this organism lacks a HCCS gene for System III, as its mitochondrial (mt) genome containsccm genes for System I. Likewise, although no mt genome data is available for goniomonads or kathablepharids, we can conclude that the two lineages lackccm genes for System I, as HCCS genes were identified in their transcriptomic data. By combining both nuclear and mt genome data of cryptophytes, this lineage most probably possesses a single system for cytochromec maturation (i.e., System III). For each of the four lineages/species, the presence/absence (+/−) ofccm genes and a HCCS gene is indicated. This scenario assumes (i) the ancestral cell used the bacterial system (i.e., System I; designated as “sI”), and (ii) the switch from System I to System III (designated as “sIII”) occurred in the common ancestor of kathablepharids, goniomonads, and cryptophytes. (B) Scenario 2. This scenario assumes (i) the ancestral cell possessed the two evolutionarily distinct systems, and (ii) System I or System III was differentially lost on the branch leading toP. bilix and that leading to the common ancestor of kathablepharids, goniomonads, and cryptophytes. If the co-existence of Systems I and III is possible in a single cell, some uncertainties about the distribution of Systems I and III remain (represented by question marks). As no nuclear genome is available forP. bilix, it is uncertain whether a HCCS gene (System III) is absent in this organism. Likewise, no complete mt genome is available for kathablepharids or goniomonads, and the presence ofccm genes for System I in either (or both) lineages remains unclear.
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