Heat-tolerant genes may spread through coral populations fast enough to give the marine creatures a tool to survive another 100 years of warming in our oceans.
Coral reefs are facing no shortage of threats including ocean acidification, overfishing, plastic pollution, and rising temperatures. Sea surface temperatures have been climbing on average for over a century, and ocean heat waves—which can trigger coral bleaching events—are becoming more common and severe. Scientists have long worried that as coral-killing spikes in temperature become more frequent, corals won’t have enough time to recover between bleaching events and will ultimately go extinct. But a new paper, published today in PLoS Genetics, suggests that corals might be able to adapt to another century of warming.
“Everybody knows if you take a present day coral and put it in a bucket with future [temperature] conditions, they tend to die,” says Mikhail Matz, an associate professor at the University of Texas–Austin and lead author on the new study. But given that the geographical range of corals spans many temperatures, Matz and his colleagues wondered if corals might be able to adapt as sea temperatures gradually increase.
The researchers already knew that members of at least one common species of coral on the Great Barrier Reef, Acropora millepora, possessed heat-tolerance genes. They wanted to investigate whether natural selection might take its course, spreading those heat-tolerant genes across the population and allowing corals to adapt as sea temperatures gradually increase.
The team built a model that took into account the coral’s genetic diversity and the distance that coral larvae travel before settling down, to predict how quickly heat-tolerant genes might spread. The model suggests that A. millepora has enough genetic diversity to survive another 20 to 50 generations—a timespan of 100 to 250 years.
Mikhail V. Matz, Eric A. Treml, Galina V. Aglyamova, Line K. Bay. Potential and limits for rapid genetic adaptation to warming in a Great Barrier Reef coral. PLOS Genetics, 2018; 14 (4): e1007220 DOI: 10.1371/journal.pgen.1007220 (open access)
Can genetic adaptation in reef-building corals keep pace with the current rate of sea surface warming? Here we combine population genomics, biophysical modeling, and evolutionary simulations to predict future adaptation of the common coral Acropora millepora on the Great Barrier Reef (GBR). Genomics-derived migration rates were high (0.1–1% of immigrants per generation across half the latitudinal range of the GBR) and closely matched the biophysical model of larval dispersal. Both genetic and biophysical models indicated the prevalence of southward migration along the GBR that would facilitate the spread of heat-tolerant alleles to higher latitudes as the climate warms. We developed an individual-based metapopulation model of polygenic adaptation and parameterized it with population sizes and migration rates derived from the genomic analysis. We find that high migration rates do not disrupt local thermal adaptation, and that the resulting standing genetic variation should be sufficient to fuel rapid region-wide adaptation of A. millepora populations to gradual warming over the next 20–50 coral generations (100–250 years). Further adaptation based on novel mutations might also be possible, but this depends on the currently unknown genetic parameters underlying coral thermal tolerance and the rate of warming realized. Despite this capacity for adaptation, our model predicts that coral populations would become increasingly sensitive to random thermal fluctuations such as ENSO cycles or heat waves, which corresponds well with the recent increase in frequency of catastrophic coral bleaching events.
Coral reefs worldwide are suffering high mortality from severe thermal stress episodes induced by acute ocean warming events. Under the current rate of warming, will corals be gone before the end of this century? Here we combine population genomics with biophysical and evolutionary modeling to investigate adaptive potential of a common reef-building coral from the Great Barrier Reef. To approach this task, we have developed a predictive model of polygenic adaptation in a system of multiple inter-connected populations that exist in a heterogeneous and changing environment. Applying this model to our coral species, we find that populations successfully adapt to diverse local temperatures along the range of the Great Barrier Reef despite high migrant exchange and should collectively harbor enough adaptive genetic variants to fuel region-wide thermal adaptation for another century and perhaps longer. In the same time, the model predicts that random thermal fluctuations will induce increasingly severe coral mortality episodes, which aligns well with observations over the last few decades.