ALBERT

Description: Background: Seagrasses (Alismatales) are the only fully marine angiosperms. Zostera marina (eelgrass) plays a crucial role in the functioning of coastal marine ecosystems and global carbon sequestration. It is the most widely studied seagrass and has become a marine model system for exploring adaptation under rapid climate change. The original draft genome (v.1.0) of the seagrass Z. marina (L.) was based on a combination of Illumina mate-pair libraries and fosmid-ends. A total of 25.55 Gb of Illumina and 0.14 Gb of Sanger sequence was obtained representing 47.7× genomic coverage. The assembly resulted in ~2000 unordered scaffolds (L50 of 486 Kb), a final genome assembly size of 203MB, 20,450 protein coding genes and 63% TE content. Here, we present an upgraded chromosome-scale genome assembly and compare v.1.0 and the new v.3.1, reconfirming previous results from Olsen et al. (2016), as well as pointing out new findings. Methods: The same high molecular weight DNA used in the original sequencing of the Finnish clone was used. A high-quality reference genome was assembled with the MECAT assembly pipeline combining PacBio long-read sequencing and Hi-C scaffolding. Results: In total, 75.97 Gb PacBio data was produced. The final assembly comprises six pseudo-chromosomes and 304 unanchored scaffolds with a total length of 260.5Mb and an N50 of 34.6 MB, showing high contiguity and few gaps (~0.5%). 21,483 protein-encoding genes are annotated in this assembly, of which 20,665 (96.2%) obtained at least one functional assignment based on similarity to known proteins. Conclusions: As an important marine angiosperm, the improved Z. marina genome assembly will further assist evolutionary, ecological, and comparative genomics at the chromosome level. The new genome assembly will further our understanding into the structural and physiological adaptations from land to marine life.

Description: Using population genomics, we reveal the global dispersal history of eelgrass (Zostera marina) from its origin in the Northwest Pacific Ocean, across to the East Pacific, Atlantic and the Mediterranean Sea. Striking differences in genetic diversity among the locations reflect past glaciations and repeated bottlenecks during Z. marina’s worldwide colonization.

Description: Currents are unique drivers of oceanic phylogeography and thus determine the distribution of marine coastal species, along with past glaciations and sea-level changes. Here we reconstruct the worldwide colonization history of eelgrass (Zostera marina L.), the most widely distributed marine flowering plant or seagrass from its origin in the Northwest Pacific, based on nuclear and chloroplast genomes. We identified two divergent Pacific clades with evidence for admixture along the East Pacific coast. Two west-to-east (trans-Pacific) colonization events support the key role of the North Pacific Current. Time-calibrated nuclear and chloroplast phylogenies yielded concordant estimates of the arrival of Z. marina in the Atlantic through the Canadian Arctic, suggesting that eelgrass-based ecosystems, hotspots of biodiversity and carbon sequestration, have only been present there for ~243 ky (thousand years). Mediterranean populations were founded ~44 kya, while extant distributions along western and eastern Atlantic shores were founded at the end of the Last Glacial Maximum (~19 kya), with at least one major refuge being the North Carolina region. The recent colonization and five- to sevenfold lower genomic diversity of the Atlantic compared to the Pacific populations raises concern and opportunity about how Atlantic eelgrass might respond to rapidly warming coastal oceans.

Description: We present chromosome-level genome assemblies from representative species of three independently evolved seagrass lineages: Posidonia oceanica, Cymodocea nodosa, Thalassia testudinum and Zostera marina. We also include a draft genome of Potamogeton acutifolius, belonging to a freshwater sister lineage to Zosteraceae. All seagrass species share an ancient whole-genome triplication, while additional whole-genome duplications were uncovered for C. nodosa, Z. marina and P. acutifolius. Comparative analysis of selected gene families suggests that the transition from submerged-freshwater to submerged-marine environments mainly involved fine-tuning of multiple processes (such as osmoregulation, salinity, light capture, carbon acquisition and temperature) that all had to happen in parallel, probably explaining why adaptation to a marine lifestyle has been exceedingly rare. Major gene losses related to stomata, volatiles, defence and lignification are probably a consequence of the return to the sea rather than the cause of it. These new genomes will accelerate functional studies and solutions, as continuing losses of the ‘savannahs of the sea’ are of major concern in times of climate change and loss of biodiversity.