How flowering plants colonized the sea

13 May 2016

A recent genome study of the seagrass Zostera marina, published by an international
team of 35 researchers in early 2016, may prove to lay the foundations for a range of
functional ecological studies. Why this is? Associated with VIB and Ghent University,
Yves Van de Peer is happy to give a full account.

Seagrasses are the only angiosperms or ‘flowering plants’ that have ever colonized the sea. In doing so, they became the bedrock for one of our planet’s most productive coastal ecosystems. Notably, their shift to a marine lifestyle required impressive structural and physiological adaptations: an interesting research topic for both evolutionary life sciences and plant biology, to say the least.

Yves, what was the focus of your team’s research?
“We focused on Zostera marina or ‘marine eelgrass’, one of the most wide-spread seagrass species in the temperate northern hemisphere. In 2010, a genotype of Zostera marina was harvested from the northern Baltic Sea. After its DNA was extracted, it was sequenced and analyzed, revealing its complete genetic material. This is a world first: no other marine angiosperm was ever fully sequenced up until now.”

In a nutshell, what did the research reveal?
“By sequencing Zostera marina, we gathered unique insights into the genomic gains and losses that helped it achieve the necessary adaptations for a marine lifestyle. For starters, the seagrass species lost the entire repertoire of stomatal genes. Inhabiting a light-attenuated, submarine
environment, Zostera marina also lost ultraviolet resistance genes — used to sense and respond to UV damage — along with the phytochromes related to light detection.”

“What is more, we also noticed how eelgrass (re)developed specific structural traits to adjust to the salinity of full marine seawater. For instance, its cell walls contain polysaccharides. Typical of land plants, these help Zostera marina cope with dehydration and osmotic stress at low tide. Another example: the plant’s cell walls share features with those of macroalgae, crucial for taking up nutrients, regulating the exchange of oxygen and carbon dioxide, and stabilizing the number and charges of ions in the plant’s cells.”

How is this research relevant to ecological research in general?
“Nowadays, more and more people are inhabiting our planet’s coastal areas. Consequently, many ecosystems are under pressure, including seagrass beds. As a result, other ecosystems may be at risk, too. Seagrasses not only sustain harvestable fish and invertebrates like lobsters, shrimp and crabs; they also play a part in controlling erosion effects and capturing carbon dioxide. Having unraveled the genomic basis of Zostera marina’s complex adaptations to life in ocean waters, this study can advance ecological studies on how marine ecosystems might adapt to global warming.”

Olsen et al., Nature 2016


Evidence for a whole genome duplication event
By studying Zostera marina, Yves Van de Peer and his team also found evidence for a whole genome duplication (WDG) event that seems to have coincided with the Cretaceous–Paleogene (K–Pg) extinction event, 66 million years ago.
Yves: “In another recent study, we discovered that gene duplicability of core genes is highly consistent across all angiosperms. As this will also apply to Zostera marina, the seagrass species is providing further evidence for a correlation between WGD events and the K-Pg extinction event.”