Sea star wasting disease has caused massive starfish die-offs along both the east and west coasts of North America. Now researchers at the University of Georgia and the University of California, Merced, have found that a genetic mutation may affect how a common species of sea star responds to the disease. Their findings, published in the online open access journal PeerJ, provide insight into the intersection of stress, environmental change and disease.
Sea star wasting disease can strike many different species. It starts with the appearance of white lesions, quickly followed by tissue decay, the loss of limbs, and eventually death. Outbreaks occurred periodically during the last century but have been happening with increasing frequency and intensity in recent years, according to study author John Wares, an associate professor in the UGA Franklin College of Arts and Sciences Department of Genetics and the Odum School of Ecology who is studying the disease.
“We’re just starting to get a handle on sea star wasting disease,” Wares said. “It’s thought to be caused by a densovirus, which is in the same group as canine parvo. And there seems to be a temperature effect; if you increase the water temperature there’s higher incidence of this disease.”
Scientists are concerned about the impacts of sea star wasting disease because starfish are keystone predators in the intertidal ecosystems they inhabit.
“When you remove sea stars from the intertidal environment, you get an immediate reduction in the relative diversity and abundance of all the other species there,” Wares said. “And diversity correlates with function, so there are many trickle-down effects for an ecosystem.”
One avenue researchers are exploring is whether any sea star populations exhibit resistance to the disease. Wares and his colleague, Lauren M. Schiebelhut of UC Merced, focused on Pisaster ochraceus, a widespread sea star that was hit hard during a major outbreak on the Pacific coast in 2013 and 2014.
Earlier research by Wares had shown that P. ochraceus had a genetic mutation of a particular sort that is often associated with disease resistance. Most organisms carry two copies of each gene region in their cells. In this type of mutation, if the gene carries two distinct copies—also called alleles—the organism has greater fitness and is better able to resist disease than if it has two identical versions of the allele.
Wares and Schiebelhut looked at samples from sea stars collected at sites from California to northern Washington before, during and after the 2013-2014 outbreak. They found that sea stars with two different alleles at a gene known as EF1A—one with the mutation and one without—had a twenty percent lower incidence of disease than those with two identical versions of the allele without this mutation. Wares said that previous work at the University of Georgia showed that offspring that carry two copies of the mutation allele die within the first few days of formation.
For a mutation that causes such a high level of mortality to persist it must also convey a major benefit, Wares explained. Boosting resistance to sea star wasting disease may be why the EF1A mutation persists in P. ochraceus.
Wares is now working on determining how the mutation helps P. ochraceus withstand disease.
“My hunch is that it’s related to how quickly an organism can respond to stress, including heat stress,” he said, adding that previous studies have shown that disease and mortality rates for marine organisms are rising as oceans warm, in part because their response to stressors alters their ability to fight pathogens. Wares plans to test this idea this summer with experiments to examine how sea stars with and without the mutation allele respond to stress at various water temperatures.
“It’s been long predicted, and we’re now seeing, massive disease outbreaks and all sorts of key populations experiencing ecosystem-changing mortality events as our oceans are warming,” Wares said. “What we’re doing is sort of triangulating these mass mortality events with the hope that some organisms have variations that will allow them to persist, and trying to gain an understanding of how these ecosystems are going to change.”
The paper, “What doesn’t kill them makes them stronger: An association between elongation factor 1-α overdominance in the sea star Pisaster ochraceus and ‘sea star wasting disease,’” is available at https://peerj.com/articles/1876/. The research was supported by the National Science Foundation and the California Sea Grant College Program.