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Background  Aerobically respiring eukaryotes usually contain four respiratory-chain complexes (complexes I-IV) and an ATP synthase (complex V). In several lineages of aerobic microbial eukaryotes, complex I has been lost, with an alternative, nuclear-encoded NADH dehydrogenase shown in certain cases to bypass complex I and oxidize NADH without proton translocation. The first loss of complex I in any multicellular eukaryote was recently reported in two studies; one sequenced the complete mitogenome of the hemiparasitic aerial mistletoe,  Viscum scurruloideum , and the other sequenced the  V. album  mitogenome. The  V. scurruloideum  study reported no significant additional loss of mitochondrial genes or genetic function, but the  V. album  study postulated that mitochondrial genes encoding all ribosomal RNAs and proteins of all respiratory complexes are either absent or pseudogenes, thus raising questions as to whether the mitogenome and oxidative respiration are functional in this plant.    Results  To determine whether these opposing conclusions about the two  Viscum  mitogenomes reflect a greater degree of reductive/degenerative evolution in  V. album  or instead result from interpretative and analytical differences, we reannotated and reanalyzed the  V. album  mitogenome and compared it with the  V. scurruloideum  mitogenome. We find that the two genomes share a complete complement of mitochondrial rRNA genes and a typical complement of genes encoding respiratory complexes II-V. Most  Viscum  mitochondrial protein genes exhibit very high levels of divergence yet are evolving under purifying, albeit relaxed selection. We discover two cases of horizontal gene transfer in  V. album  and show that the two  Viscum  mitogenomes differ by 8.6-fold in size (66 kb in  V. scurruloideum ; 565 kb in  V. album ).    Conclusions    Viscum  mitogenomes are extraordinary compared to other plant mitogenomes in terms of their wide size range, high rates of synonymous substitutions, degree of relaxed selection, and unprecedented loss of respiratory complex I. However, contrary to the initial conclusions regarding V.  album , both  Viscum  mitogenomes possess conventional sets of rRNA and, excepting complex I, respiratory genes. Both plants should therefore be able to carry out aerobic respiration. Moreover, with respect to size, the  V. scurruloideum  mitogenome has experienced a greater level of reductive evolution.    Electronic supplementary material  The online version of this article (doi:10.1186/s12870-017-0992-8) contains supplementary material, which is available to authorized users.

bmc plant biology

Comparative mitogenomics indicates respiratory competence in parasitic Viscum despite loss of complex I and extreme sequence divergence, and reveals horizontal gene transfer and remarkable variation in genome size