The genome of the seagrass Zostera marina reveals angiosperm adaptation to the sea

Seagrasses colonized the sea(1) on at least three independent occasions to form the basis of one of the most productive and widespread coastal ecosystems on the planet(2). Here we report the genome of Zostera marina (L.), the first, to our knowledge, marine angiosperm to be fully sequenced. This reveals unique insights into the genomic losses and gains involved in achieving the structural and physiological adaptations required for its marine lifestyle, arguably the most severe habitat shift ever accomplished by flowering plants. Key angiosperm innovations that were lost include the entire repertoire of stomatal genes(3), genes involved in the synthesis of terpenoids and ethylene signalling, and genes for ultraviolet protection and phytochromes for far-red sensing. Seagrasses have also regained functions enabling them to adjust to full salinity. Their cell walls contain all of the polysaccharides typical of land plants, but also contain polyanionic, low-methylated pectins and sulfated galactans, a feature shared with the cell walls of all macroalgae(4) and that is important for ion homoeostasis, nutrient uptake and O-2/CO2 exchange through leaf epidermal cells. The Z. marina genome resource will markedly advance a wide range of functional ecological studies from adaptation of marine ecosystems under climate warming(5,6), to unravelling the mechanisms of osmoregulation under high salinities that may further inform our understanding of the evolution of salt tolerance in crop plants(7)

Evolution and biogeography of seagrasses

© Springer International Publishing AG, part of Springer Nature 2018. Seagrasses are an organismal biological group united by their ability to grow in marine environments. As marine flowering plants they have evolved a combined suite of adaptations multiple times enabling the four known lineages containing species of seagrass to survive, and thrive, in the sea. Unlike many other biological groups of plants however, seagrasses are all derived from a single order of flowering plants, the Alismatales. This order, being derived early in the evolution of the monocotyledons, is comprised predominantly of aquatic plants, of all forms- emergent, submerged, freshwater, estuarine and marine. A review of seagrass fossils suggests that new discoveries of seagrass fossils along with confirmation of some earlier finds lead to a clear signal that some seagrass species had a wider distribution in the past compared with today. The discovery of new fossil sites should be encouraged as this will likely produce important valuable information on the evolution of this group. In general the biogeography of seagrasses suggests that these organisms evolved successfully in the Tethys Sea of the Late Cretaceous. However, the modern division into two groups, temperate and tropical tends to suggest that at some point an ecological separation occurred in both the Northern and Southern Hemispheres. There are a disproportionately large number of temperate seagrass species in southern Australia and there is significant endemism shown in Posidonia, Amphibolis and a unique species of Halophila (H. australis). The use of genetic and genomic techniques has begun to explain these distributions but we can expect a much bigger picture to emerge in the near future