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Abstract
Seagrasses are marine angiosperms that live fully submerged in the sea. They evolved from land plant ancestors, with multiple species representing at least three independent return-to-the-sea events. This raises the question of whether these marine angiosperms followed the same adaptation pathway to allow them to live and reproduce under the hostile marine conditions. To compare the basis of marine adaptation between seagrass lineages, we generated genomic data for Halophila ovalis and compared this with recently published genomes for two members of Zosteraceae, as well as genomes of five non-marine plant species (Arabidopsis, Oryza sativa, Phoenix dactylifera, Musa acuminata, and Spirodela polyrhiza). Halophila and Zosteraceae represent two independent seagrass lineages separated by around 30 million years. Genes that were lost or conserved in both lineages were identified. All three species lost genes associated with ethylene and terpenoid biosynthesis, and retained genes related to salinity adaptation, such as those for osmoregulation. In contrast, the loss of the NADH dehydrogenase-like complex is unique to H. ovalis. Through comparison of two independent return-to-the-sea events, this study further describes marine adaptation characteristics common to seagrass families, identifies species-specific gene loss, and provides molecular evidence for convergent evolution in seagrass lineages.
Highlights
Seagrasses are a polyphyletic group of flowering plants that live fully submerged in the marine environment and form monospecific meadows resembling terrestrial grasses
Results revealed the loss of the NADH dehydrogenase-like (NDH) protein complex in H. ovalis, a characteristic that is not shared by the other two seagrass species.This study provides a more complete description of marine adaptation, and suggest a parallel convergent evolution of two independent return-to-the-sea events in seagrasses separated by 30 million years
For the seagrass reference species, 1.8% (16 727 940) and 0.5% (5 005 993) of H. ovalis reads aligned to Z. muelleri and Z. marina coding sequences (CDS), respectively
Summary
Seagrasses are a polyphyletic group of flowering plants that live fully submerged in the marine environment and form monospecific meadows resembling terrestrial grasses. The morphology of seagrasses varies among species, though common features include long, strap-shaped leaves and simple flowers. Similar in form, seagrass species represent at least three independent return-tothe-sea events (Les et al, 1997). The convergent evolution of seagrasses is characterized by common physiological and morphological features that. Seagrasses have adapted to variable quality and low levels of light, which attenuates quickly in seawater (Larkum et al, 2006; Strydom et al, 2017) and have effective osmoregulation to survive in the saline aqueous environment (Koch et al, 2007; Touchette, 2007). Seagrasses are adapted to aquatic reproduction, where the transport and capture of pollen grains is carried out on or below the water surface
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