Dixon, G.B.; Davies, S.W.; Aglyamova, G.A.; Meyer, E.; Bay, L.K.; Matz, M.V. Genomic determinants of coral heat tolerance across latitudes. Science. 26 June 2015. DOI: 10.1126/science.1251224
It’s no secret that the globe is warming. It’s also no secret that coral reefs, some of Earth’s most diverse and alluring ecosystems, are potentially doomed. At least, that’s all the hype.
Now for the hope—scientists think that coral might be able to handle moderate increases in temperature. In other words, if we continue to curb our impact on the environment, coral might just be able to scrape by until the going gets better, despite all destruction we have already wrought on these ecosystems.
Organisms can acclimatize to heat stress by adjusting their physiologies, for example, they might alter their baseline metabolism in direct response to their environments. The big question in coral biology is whether heat tolerance can also come from heritable traits. That is, can an organism’s genetic code confer innate heat tolerance that can be passed down from one generation to the next? If this were the case, then we as humble managers of our environments could ostensibly introduce heat tolerance into an ecosystem by breeding heat tolerant strains of coral.
An elegant study recently published by Dixon et al. in Science finds the first evidence that heritable traits can lead to improved heat tolerance in coral. To address this question, they studied coral of the same species separated by a few degrees of latitude and Celsius in the Great Barrier Reef of Australia. They mated two males and females from each population to determine what effects parenthood might have on the heat responses of these populations. They further attempted to identify specific heritable traits associated with survivability of larvae faced with increasing temperatures.
After the newborns had developed into larvae, the researchers subjected them to heat stress, ramping up water temperature over the course of a little over a day to just a little bit higher than the highest temperatures encountered in the warmer of the two sampling locations. Not surprisingly, the warm-warm crosses faired the best of all, while the chance of survival in mixed crosses improved chances of survival relative to cold-cold crosses. Interestingly, those with the mother from the warmer environment faired better than those with the father from the warm environment. In other words, anyone with a warm parent found his or her situation much improved, while a strong “maternal effect” was observed for the benefit of crossing warm with cold.
The study also investigated where the variation in survivability was coming from at a molecular level. To do this, they tagged several thousand genes whose expression is correlated with heat tolerance. They found variation in both maternal and paternal genes associated with heat stress, supporting their hypothesis that heat tolerance can be inherited genetically.
Additionally, they sought to address the strong observed maternal effect on survivability. To explain it, they looked to the mitochondria – the organelle (membrane-bound unit within eukaryotic cells) in charge of producing energy for our cells. The mitochondrion itself is an incapacitated bacterium with its own DNA and cell membrane, that can’t survive on its own. Most importantly, we inherit our mitochondria exclusively from our mothers.
Once our environment heats up, so does our metabolism, and so the mitochondria have to work harder. Not surprisingly, the study found that a lot of the genetic variation had to do with both mitochondrial genes, as well as mitochondrial genes encoded within the genome of the actual coral cell itself.
In addition to asking which genes were correlated with heat shock, they also asked whether these genes actually encode capacity of an organism to deal with heat, or if those genes can be conditioned due to environmental sensitization over the course of the organism’s lifetime. To do this, they studied gene expression in the adult parental corals and compared those levels of expression to previous literature on larval gene expression. They found that adult gene expression over an extended period closely resembled that of larvae, indicating that changes in gene expression were actually due to genetic regulation, rather than conditioning.
The work done by Dixon and colleagues strongly suggests a heritable basis for coral heat tolerance. However, they caution that heat tolerance is only one factor corals must face when dealing with climate change. For example, another side effect of our global carbon dioxide emissions is ocean acidification. This study observed that genes that encode “transporters” in the heat-tolerant coral are turned down. Transporters serve many key cellular functions, which include helping the organism get rid of pollutants. Hence, while reduced transporter activity might help the organism deal with heat stress, it might also make coral more vulnerable to pollutants. Evolution is a long process of give and take, and most organisms today have arrived at an optimum over millions of years of random mutation and natural selection. Even if nature does give us some leeway, we must still do our part to curb climate change if coral are to survive.
Abrahim is a PhD student at Scripps Institution of Oceanography in San Diego where he studies marine chemical biology.