The Increase of Simple Sequence Repeats during Diversification of Marchantiidae, An Early Land Plant Lineage, Leads to the First Known Expansion of Inverted Repeats in the Evolutionarily-Stable Structure of Liverwort Plastomes

Abstract

The chloroplast genomes of liverworts, an early land plant lineage, exhibit stable structure and gene content, however the known resources are very limited. The newly sequenced plastomes of Conocephalum, Riccia and Sphaerocarpos species revealed an increase of simple sequence repeats during the diversification of complex thalloid liverwort lineage. The presence of long TA motifs forced applying the long-read nanopore sequencing method for proper and dependable plastome assembly, since the length of dinucleotide repeats overcome the length of Illumina short reads. The accumulation of SSRs (simple sequence repeats) enabled the expansion of inverted repeats by the incorporation of rps12 and rps7 genes, which were part of large single copy (LSC) regions in the previously sequenced plastomes. The expansion of inverted repeat (IR) at the genus level is reported for the first time for non-flowering plants. Moreover, comparative analyses with remaining liverwort lineages revealed that the presence of SSR in plastomes is specific for simple thalloid species. Phylogenomic analysis resulted in trees confirming monophyly of Marchantiidae and partially congruent with previous studies, due to dataset-dependent results of Dumortiera-Reboulia relationships. Despite the lower evolutionary rate of Marchantiales plastomes, significant barcoding gap was detected, even for recently divergent holarctic Conocephalum species. The sliding window analyses revealed the presence of 18 optimal (500 bp long) barcodes that enable the molecular identification of all studied species.