Postdoctoral fellow Lauren Schiebelhut. Photo: UC Merced

By Jason Alvarez
19 June 2018

(UC Merced) – In 2012, Environmental Systems graduate student Lauren Schiebelhut was collecting DNA from ochre sea stars living along the Northern California coast — part of an effort to study genetic diversity in various marine species that serve as indicators of habitat health. She had no idea that just one year later, most of the sea stars would be dead.

The culprit was sea star wasting disease (SSWD), a marine pandemic whose 2013 outbreak decimated sea star populations in waters up and down the west coast of North America. [cf. Scientists link massive starfish die-off to warming ocean. –Des]

The disease, which turns the sea star’s normally rigid body into a gooey blob, claimed 81 percent of ochre sea stars along the hundred-mile stretch of coast just north of San Francisco where Schiebelhut works — now as a postdoc in UC Merced Professor Michael Dawson’s lab.

“They go from a hard candy to a marshmallow,” Dawson said of symptomatic sea stars.

While it remains one of the worst marine pandemics ever recorded, the SSWD outbreak had an unexpected silver lining: It provided scientists with a natural experiment in evolution and an opportunity to explore how a species responds to a cataclysmic population collapse.

“We have a big-picture interest in exploring this topic as marine mass mortalities seem to be on the rise and extreme environmental events are becoming more frequent,” Schiebelhut said. “We’re interested in seeing how this keystone species — very important to the ecosystem in which it lives — responds to a major mortality event.”

Using the DNA they had gathered prior to the 2013 outbreak, Schiebelhut and Dawson compared the genomes of sea stars that were alive just before the outbreak against the genomes of outbreak survivors.

They found that outbreak survivors were genetically distinct from pre-SSWD populations, most likely the result of selective pressures imposed by the devastating disease. The findings are reported in a paper published in the Proceedings of the National Academy of Sciences.

“We explored the genetic consequences of a major mortality event in this species and saw that the mortality led to frequency shifts in certain versions of genes in the population,” Schiebelhut said. “This paper is about documenting the shift we observed in the genome.”

Sampling sea stars at 16 different sites along the Northern California coast, the researchers found substantial changes to sea star genomes following the outbreak. Certain versions of genes were found to be more prevalent in post-outbreak populations, while other versions were found less frequently than before.

These changes weren’t restricted to adults that survived the pandemic. Juvenile sea stars — which were recruited to shorelines at a rate 74 times greater than before the outbreak — were genetically more similar to the survivors than to pre-outbreak populations.

Because these changes were observed in both adults and juveniles, and at nearly all sites, the researchers concluded that the changes were not the result of random fluctuations that can sometimes give rise to population-wide genetic changes.

“Different pieces of evidence suggest selection is the likeliest explanation,” Schiebelhut said. “It’s extremely unlikely that same pattern arose in different places through just random processes.”

What Schiebelhut and Dawson think happened is that certain versions of genes were selected, becoming more common because they conferred protection against SSWD. Sea stars lacking these genes simply died off during the outbreak, allowing the protective genes to dominate in post-outbreak populations.

In other words, the sea stars were evolving.

Though they found that sea stars’ genomes had changed in response to SSWD, Schiebelhut and Dawson are still trying to understand exactly how the protective genes shielded survivors from the disease. They’re also expanding the scope of their study to include species other than Pisaster ochraceus, the ochre sea star.

Pisaster was just one species affected by this disease,” Dawson said. “We want to know whether the genetic changes we identified in Pisaster are common to other species of sea star.”

Today, five years after peak mortality, wasting disease remains a threat.

“The disease seems to flare up at certain times of the year. It might be linked to temperature and reproductive stage. There are groups looking into this,” Schiebelhut said.

And though its intensity has abated, and asymptomatic sea stars are becoming increasingly common, scientists still see mortality on the coast. Until further notice, SSWD remains a sea star killer.

Following a Devastating Pandemic, California’s Sea Stars are Evolving

By Tay Wiles
26 June 2018

(The Guardian) – Five years after a mysterious virus wiped out millions of starfish off the western coast of North America, causing them to lose legs, dissolve into fleshy goo and taking various species to the brink of disappearance, scientists have announced a remarkable reversal.

In what the authors of a new study say may be an example of evolution in action, a species appears to have evolved genetic resistance to a virus decimating it. After the peak of the epidemic, there was a 74-fold increase in the number of juveniles surviving among ochre stars, one of the species hardest hit by the sea star wasting disease, the scientists report in the Proceedings of the National Academy of Sciences.

The cause of the outbreak, which they call “one of the largest marine mass mortality events on record”, is still somewhat mysterious. Researchers theorize the disease is a densovirus, which has existed in the region for decades, but may have proliferated in response to climate change impacts such as warmer waters or ocean acidification. […]

While the new research indicates that the ochre stars have evolved resilience to the consequences of climate change, other species may face greater difficulties. “The concern is that marine disease, extreme environmental events, and the frequency of those are on the rise,” said the lead author, Lauren Schiebelhut. “If we have too many extreme events in a row, maybe that becomes more challenging for species to respond to.” [more]

The amazing return of the starfish: species triumphs over melting disease

ABSTRACT: Standing genetic variation enables or restricts a population’s capacity to respond to changing conditions, including the extreme disturbances expected to increase in frequency and intensity with continuing anthropogenic climate change. However, we know little about how populations might respond to extreme events with rapid genetic shifts, or how population dynamics may influence and be influenced by population genomic change. We use a range-wide epizootic, sea star wasting disease, that onset in mid-2013 and caused mass mortality in Pisaster ochraceus to explore how a keystone marine species responded to an extreme perturbation. We integrated field surveys with restriction site-associated DNA sequencing data to (i) describe the population dynamics of mortality and recovery, and (ii) compare allele frequencies in mature P. ochraceus before the disease outbreak with allele frequencies in adults and new juveniles after the outbreak, to identify whether selection may have occurred. We found P. ochraceus suffered 81% mortality in the study region between 2012 and 2015, and experienced a concurrent 74-fold increase in recruitment beginning in late 2013. Comparison of pre- and postoutbreak adults revealed significant allele frequency changes at three loci, which showed consistent changes across the large majority of locations. Allele frequency shifts in juvenile P. ochraceus (spawned from premortality adults) were consistent with those seen in adult survivors. Such parallel shifts suggest detectable signals of selection and highlight the potential for persistence of this change in subsequent generations, which may influence the resilience of this keystone species to future outbreaks.


Opportunities to study microevolution in wild populations are rare and challenging. Annual monitoring allowed us to capture both the prelude to and aftermath of one of the largest marine mass mortality events on record in a keystone marine species. Median mortality of 81% across populations was recorded along with significant allele frequency shifts at multiple loci in the adult population. Shifts were consistent across locations and also occurred in new recruits, with few exceptions. These results indicate a long-term species-wide change in allele frequencies will persist through future generations. Population genomic monitoring, at a time when marine diseases and mass mortalities are on the rise, will be essential for documenting rapid genetic shifts in response to chronic and extreme events.

Decimation by sea star wasting disease and rapid genetic change in a keystone species, Pisaster ochraceus



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