| Summary: | Mitochondria and plastids originated through endosymbiosis, and subsequently became
reduced and integrated with the host in similar ways. Plastids spread between lineages
through further secondary or even tertiary endosymbioses, but mitochondria appear to
have originated once and have not spread between lineages. Mitochondria are also
generally lost in secondary and tertiary endosymbionts, with the single exception of the
diatom tertiary endosymbiont of dinoflagellates like Kryptoperidinium foliaceum, where
both host and endosymbiont are reported to contain mitochondria. Here, I describe the
first mitochondrial genes from this system: cytochrome c oxidase 1 (coxl), cytochrome
oxidase 3 (cox3), and cytochrome b (cob). Phylogenetic analyses demonstrated that all
characterized genes were derived from the pennate diatom endosymbiont, and not the
host. I also demonstrated that all three genes are expressed, that coxl contains spliced
group II introns, and that cob and cox3 form an operon, all like their diatom relatives.
The endosymbiont mitochondria not only retain a genome, but also express their genes,
and are therefore likely involved in electron transport. Ultrastructural examination
confirmed that the endosymbiont mitochondria retain normal tubular cristae. Overall,
these data suggest the endosymbiont mitochondria have not reduced at the genomic or
functional level. === Medicine, Faculty of === Medical Genetics, Department of === Graduate
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