Tracking the evolutionary innovations of plant terrestrialization

Rickettsial DNA and a trans-splicing rRNA group I intron in the unorthodox mitogenome of the fern Haplopteris ensiformis

Plant mitochondrial genomes can be complex owing to highly recombinant structures, lack of gene syntenies, heavy RNA editing and invasion of chloroplast, nuclear or even foreign DNA by horizontal gene transfer (HGT). Leptosporangiate ferns remained the last major plant clade without an assembled mitogenome, likely owing to a demanding combination of the above. We here present both organelle genomes now for Haplopteris ensiformis. More than 1,400 events of C-to-U RNA editing and over 500 events of reverse U-to-C edits affect its organelle transcriptomes. The Haplopteris mtDNA is gene-rich, lacking only the ccm gene suite present in ancestral land plant mitogenomes, but is highly unorthodox, indicating extraordinary recombinogenic activity. Although eleven group II introns known in disrupted trans-splicing states in seed plants exist in conventional cis-arrangements, a particularly complex structure is found for the mitochondrial rrnL gene, which is split into two parts needing reassembly on RNA level by a trans-splicing group I intron. Aside from ca. 80 chloroplast DNA inserts that complicated the mitogenome assembly, the Haplopteris mtDNA features as an idiosyncrasy 30 variably degenerated protein coding regions from Rickettiales bacteria indicative of heavy bacterial HGT on top of tRNA genes of chlamydial origin.

Pervasive genome duplications across the plant tree of life and their links to major evolutionary innovations and transitions

Whole-genome duplication (WGD) has occurred repeatedly during plant evolution and diversification, providing genetic layers for evolving new functions and phenotypes. Advances in long-read sequencing technologies have enabled sequencing and assembly of over 1000 plant genomes spanning nearly 800 species, in which a large set of ancient WGDs has been uncovered. Here, we review the recently reported WGDs that occurred in major plant lineages and key evolutionary positions, and highlight their contributions to morphological innovation and adaptive evolution. Current gaps and challenges in integrating enormous volumes of sequenced plant genomes, accurately inferring WGDs, and developing web-based analysis tools are emphasized. Looking to the future, ambitious genome sequencing projects and global efforts may substantially recapitulate the plant tree of life based on broader sampling of phylogenetic diversity, reveal much of the timetable of ancient WGDs, and address the biological significance of WGDs in plant adaptation and radiation.

Morphological characteristics and abundance of prokaryotes associated with gills in mangrove brachyuran crabs living along a tidal gradient

Due to the chemico-physical differences between air and water, the transition from aquatic life to the land poses several challenges for animal evolution, necessitating morphological, physiological and behavioural adaptations. Microbial symbiosis is known to have played an important role in eukaryote evolution, favouring host adaptation under changing environmental conditions. We selected mangrove brachyuran crabs as a model group to investigate the prokaryotes associated with the gill of crabs dwelling at different tidal levels (subtidal, intertidal and supratidal). In these animals, the gill undergoes a high selective pressure, finely regulating multiple physiological functions during both animal submersion under and emersion from the periodical tidal events. We hypothesize that similarly to other marine animals, the gills of tidal crabs are consistently colonized by prokaryotes that may quantitatively change along the environmental gradient driven by the tides. Using electron microscopy techniques, we found a thick layer of prokaryotes over the gill surfaces of all of 12 crab species from the mangrove forests of Saudi Arabia, Kenya and South Africa. We consistently observed two distinct morphotypes (rod- and spherical-shaped), positioned horizontally and/or perpendicularly to the gill surface. The presence of replicating cells indicated that the prokaryote layer is actively growing on the gill surface. Quantitative analysis of scanning electron microscopy images and the quantification of the bacterial 16S rRNA gene by qPCR revealed a higher specific abundance of prokaryote cells per gill surface area in the subtidal species than those living in the supratidal zone. Our results revealed a correlation between prokaryote colonization of the gill surfaces and the host lifestyle. This finding indicates a possible role of prokaryote partnership within the crab gills, with potential effects on animal adaptation to different levels of the intertidal gradient present in the mangrove ecosystem.