Abstract
Because hornworts occupy a pivotal position in early land colonization as sister to other bryophytes, sister to tracheophytes, or sister to all other land plants, a renewed interest has arisen in their phylogenetic diversity, morphology, and genomes. To date, only five organellar genome sequences are available for hornworts. We sequenced the plastome (155,956 bp) and mitogenome (212,153 bp) of the hornwort Leiosporoceros dussii, the sister taxon to all hornworts. The Leiosporoceros organellar genomes show conserved gene structure and order with respect to the other hornworts and other bryophytes. Additionally, using RNA-seq data we quantified the frequency of RNA-editing events (the canonical C-to-U and the reverse editing U-to-C) in both organellar genomes. In total, 109 sites were found in the plastome and 108 in the mitogenome, respectively. The proportion of edited sites corresponds to 0.06% of the plastome and 0.05% of the mitogenome (in reference to the total genome size), in contrast to 0.58% of edited sites in the plastome of Anthoceros angustus (161,162 bp). All edited sites in the plastome and 88 of 108 sites in the mitogenome are C-to-U conversions. Twenty reverse edited sites (U-to-C conversions) were found in the mitogenome (17.8%) and none in the plastome. The low frequency of RNA editing in Leiosporoceros, which is nearly 88% less than in the plastome of Anthoceros and the mitogenome of Nothoceros, indicates that the frequency of RNA editing has fluctuated during hornwort diversification. Hornworts are a pivotal land plant group to unravel the genomic implications of RNA editing and its maintenance despite the evident evolutionary disadvantages.
Highlights
Bryophytes—liverworts, mosses and hornworts—have in common a haploid-dominant life cycle
Using high-coverage transcriptome sequencing, we studied the extent of RNA editing in both organelles of L. dussii to assess whether the apparent lack of editing in plastids and the low rate of editing in the mitochondrial nad5 gene are organellar-wide phenomena
The overall structure and gene order are identical to those of Anthoceros and Nothoceros [8,9], it differs at the IRA/large singlecopy (LSC) boundary, content of genes and pseudogenes, and intron content (Table 1, Fig 1)
Summary
Bryophytes—liverworts, mosses and hornworts—have in common a haploid-dominant life cycle. Because of their pivotal evolutionary position, bryophytes have been targets of several phylogenomic analyses to disentangle their interrelationships, resulting in three main competing and supported hypotheses. The third hypothesis identifies bryophytes as monophyletic with hornworts sister to a moss plus liverwort clade [3, 4]. From this debate, it has become evident that hornworts have a crucial place in the evolution of early land plants, prompting a renewed interest in their phylogenetic diversity, morphology, and genomes [5,6]. The Leiosporoceros genome project targets L. dussii due to its crucial phylogenetic position, small genome size (approximately 160 Mbp) and its morphological innovations that include a unique symbiotic arrangement of endophytic cyanobacteria, isobilateral tetrad development and spore architecture [5,7]
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