title: Phylogenomics of the intracellular parasite Mikrocytos mackini reveals evidence for a mitosome in rhizaria.
abstract: Mikrocytos mackini is an intracellular protistan parasite of oysters whose position in the phylogenetic tree of eukaryotes has been a mystery for many years [1,2]. M. mackini is difficult to isolate, has not been cultured, and has no defining morphological feature. Furthermore, its only phylogenetic marker that has been successfully sequenced to date (the small subunit ribosomal RNA) is highly divergent and has failed to resolve its evolutionary position [2]. M. mackini is also one of the few eukaryotes that lacks mitochondria [1], making both its phylogenetic position and comparative analysis of mitochondrial function particularly important. Here, we have obtained transcriptomic data for M. mackini from enriched isolates and constructed a 119-gene phylogenomic data set. M. mackini proved to be among the fastest-evolving eukaryote lineages known to date, but, nevertheless, our analysis robustly placed it within Rhizaria. Searching the transcriptome for genetic evidence of a mitochondrion-related organelle (MRO) revealed only four mitochondrion-derived genes: IscS, IscU, mtHsp70, and FdxR. Interestingly, all four genes are involved in iron-sulfur cluster formation, a biochemical pathway common to other highly reduced "mitosomes" in unrelated MRO-containing lineages [7]. This is the first evidence of MRO in Rhizaria, and it suggests the parallel evolution of mitochondria to mitosomes in this supergroup.
authors.affiliation: Department of Biology, Canadian Institute for Advanced Research, University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
authors.forename: Fabien
authors.initials: F
authors.lastname: Burki
authors.forename: Nicolas
authors.initials: N
authors.lastname: Corradi
authors.forename: Roberto
authors.initials: R
authors.lastname: Sierra
authors.forename: Jan
authors.initials: J
authors.lastname: Pawlowski
authors.forename: Gary R
authors.initials: GR
authors.lastname: Meyer
authors.forename: Cathryn L
authors.initials: CL
authors.lastname: Abbott
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Keeling
mesh_terms: D000818:Animals; D018076:DNA, Complementary / Q000235:genetics / Q000378:metabolism; D016054:DNA, Protozoan / Q000235:genetics* / Q000378:metabolism; D019143:Evolution, Molecular; D020869:Gene Expression Profiling; D018503:Genome, Protozoan; D008969:Molecular Sequence Data; D010125:Ostreidae / Q000469:parasitology; D010802:Phylogeny; D056901:Rhizaria / Q000145:classification / Q000235:genetics* / Q000378:metabolism / Q000502:physiology; D017422:Sequence Analysis, DNA
chemical_list: D018076:DNA, Complementary; D016054:DNA, Protozoan