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Squishy ancestors of corals had the genes to build skeletons

In a new study from KAUST, researchers set out to investigate how the soft-bodied ancestors of corals were transformed into the ecosystem builders they are today. The experts have now identified the specific genes that allowed corals to grow calcified skeletons. 

“Reef-building corals build the largest living structures in the world that provide a habitat for more than a quarter of all marine animals, and are the 

primary source of livelihood to hundreds of millions of people,” wrote the researchers.

“Their immense structures are built through calcification, i.e., the continuous

deposition of calcium carbonate that forms their skeletons.”

But while the processes involved in coral calcification are well understood, it is less clear how corals actually evolved the ability to grow calcium carbonate skeletons, explained study lead author Xin Wang.

“How did a squishy anemone-like organism begin to build reefs?” added study co-author Manuel Aranda. “Did the ‘tools’ already exist in their genetic code?”

While the earliest fossils of calcified corals are around 265 million years old, their evolution began much earlier.

“We conducted a genomic search for conserved genes that might be involved in calcification,” said Wang. “Our findings suggest that corals evolved to calcify somewhere between 308 and 265 million years ago.”

The researchers compared the genomes of six related species, including two evolutionary divergent reef-building corals, two of their closest non-calcifying relatives, and two sea anemones. The two-year analysis was conducted using the KAUST supercomputer.

“We found that the necessary proteins to make coral skeletons were already present in the soft-bodied ancestor and that various existing proteins were recruited to boost the calcifying process,” said Wang. “Essentially, we believe we’ve found the genetic toolkit for coral skeleton creation.”

In order to calcify, corals take in positively charged calcium ions from seawater. The experts pinpointed the genes responsible for transporting calcium and removing protons in the soft-bodied organisms. They showed that two of the three gene copies found in corals have been recruited to the calcifying tissue. 

The researchers also identified a gene that was duplicated in corals twice and recruited to precipitate and stabilize calcium carbonate in the initial stage of skeleton building. 

“This is a great example of how latent traits can evolve to become dominant given certain environmental pressures,” said Aranda. “Next we hope to verify which of these components is critical to calcification and investigate how coral reefs might be influenced by the changing pH of future oceans.”

The study is published in the journal Molecular Biology and Evolution.

By Chrissy Sexton, Staff Writer

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