For eons, farmers have bred plants and animals for specific qualities: to thrive in hotter climates or colder; wetter or drier; to produce more food.
Now there is growing pressure to do that with wild creatures in order to help them endure life in the Anthropocene. Nowhere is this more evident than with coral, the cornerstone of the ocean’s most productive and imperiled ecosystems.
New research offers the hope that genetic tools might help identify coral strains that can best weather a devastating disease afflicting Caribbean reefs. It also underscores the sobering fact that humanity is on a path toward needing to choose between turning an ecosystem-building animal into a domesticated crop, or watching it disappear.
Some of the most emblematic corals are already dangerously close to vanishing from the Caribbean, courtesy of a mysterious illness known as white band disease. First identified in the 1970s, the illness afflicts several species of Acropora, coral that build spiky antler-like carbonate structures in which colonies of coral polyps live. The disease can destroy coral tissue and kill entire colonies, destroying up to 95% of Acropora corals in the Caribbean. While scientists suspect a bacteria is to blame (antibiotics can combat an infection), no one has fingered the specific culprit.
Coral conservation groups have responded in part by growing Acropora corals on a massive scale. They collect corals, break them into tiny fragments, then grow those fragments into new corals. By one estimate, it could take millions of small corals to rebuild a coral reef covering less than half a square kilometer.
But that pales in comparison to what is being contemplated in Australia. There, scientists are working to develop a coral farming system that could churn out as much as 100 million corals per year, with an eye toward replacing wild corals killed by underwater heatwaves, disease and pollution.
Which begs the question: How do you increase the odds your coral crop will survive?
In the case of white band disease, restoration groups “might be farming 100 genotypes, but they don’t know which corals are the better performers. They don’t know who is highly disease resistant versus highly susceptible,” said Steven Vollmer, a marine ecologist at Northeastern University.
To address this blind spot, Vollmer teamed up with other Northeastern scientists to see if they could find genetic patterns that made some Caribbean corals better able to endure this plague.
They collected chunks of live coral from 100 different strains of staghorn coral (Acropora cervicornis), half from a Florida coral nursery run by the Coral Restoration Foundation, and half from wild reefs off the northeastern coast of Panama. They placed the coral in tanks and exposed them to a ground up slurry of coral infected by white band disease. Then they spent a week tracking how long it took for each piece of coral to show signs of illness.
The researchers compared the performance of the different corals to a detailed map of their DNA, looking for genetic patterns common to corals that best resisted infection. They found 10 regions in the coral genomes where tiny differences in the genetic code were associated with an ability to withstand white band, the scientists reported Sept. 28, in the journal Science.
For the corals being sampled, there was good news and bad. Fifteen different strains of coral were “highly disease-resistant.” But 31 scored below average, with 15 highly vulnerable to the disease. That includes seven corals being grown in the Florida program aimed at replenishing wild corals. In effect, they were trying to combat a disease by raising coral particularly sensitive to the disease.
The new findings could help counter such dead ends as groups work to scale up coral production. The team developed a tool to screen coral for their genetic vulnerability to white band disease. “We can tell you which (corals) are disease resistant with high accuracy, using as few as 10 gene variants,” said Vollmer.
What it can’t tell is whether the mutations that help a coral withstand this infection might put them at a disadvantage in some other way. Sometimes genetic mutations with an upside can also come at a cost, such as making a coral more sensitive to hot water or less fertile. Vollmer has funding to look at genetic links between disease and heat tolerance.
In a commentary published in the same issue of Science, coral scientists Laura Mydlarz of the University of Texas and Erinn Muller of Florida’s Mote Marine Laboratory say the new discovery could help coral restoration programs select for more hardy corals better prepared to cope with the dangers now battering corals. In this brave new world, linchpins of the world’s reefs won’t be so wild anymore.
“The threats to the future of these reefs are outside expected norms,” Mydlarz and Muller write. “And thus efforts to preserve them must be as well.”
“Our next NSF grant is actually going to explore the links between disease resistance and temperature tolerance. Are they the same sets of genes? Are they different sets of genes?”
Photo: Steven Vollmer’s lab identified the genetic makeups of disease-resistance coral; by Matthew Modoono/Northeastern University