The mitochondrial genomes of bryophytes

Abstract

In contrast to the highly variable mitogenomes of vascular plants, the composition and architecture of mitogenomes within the three bryophyte lineages appear stable and invariant. Currently, complete mitogenomes are available from 113 bryophyte accessions of 71 genera and 28 orders. Liverworts and mosses hold a rich mitochondrial (mt) gene repertoire among land plants with 40–42 protein-coding genes, whereas hornworts maintain the smallest functional gene set among land plants, of only around two dozen protein-coding genes, with the majority of ribosomal genes pseudogenized and all cytochrome c maturase genes lost. The rRNA and tRNA genes are also conserved and rich in mosses and liverworts, whereas subject to patchy losses in hornworts. In contrast to the conserved gene set, intron content varies significantly with only one intron shared among the three bryophyte lineages. Bryophytes hold relatively compact mitogenomes with narrow size fluctuations. Among the three bryophyte lineages, intergenic spacers and repeat content are smallest in mosses, largest in hornworts, and intermediate in liverworts, mirroring their size differences and levels of structural dynamics among the three lineages. Mosses, with the least repeated sequences, show the most static genome structure; whereas hornworts, with a relatively large set of repeated sequences, experience 1–4 rearrangements; liverworts, with intermediate repeat levels, see only one structural variant that requires two inversions to gain collinearity with the mitogenome of other liverworts. Repeat sequences were evoked to explain the mt gene order rearrangements in hornwort and liverwort mitogenomes; with the latter also supported by sequencing read evidence, which suggests that the conserved mitogenome structure observed in bryophyte lineages might be shaped by low repeat recombination level, and/or along with the intensified nucleus’ surveillance. Mitochondrial RNA editing is abundant in hornworts, with medium frequency and high variation in liverwort species, and generally limited in mosses, reflecting the diversity of nuclear encoded PPR proteins that are functionally related to RNA editing processes.