Corals that live in hot environments express genes that protect them from heat even in moderate temperatures, according to a new study published this week in Proceedings of the National Academy of Sciences (1). By understanding gene expression levels in coral populations, researchers can better assess which of these animals will survive climate change, the biggest threat to their existence.
To understand the genetic differences between heat-resistant corals and their more sensitive counterparts, Stanford University postdoctoral fellow Daniel Barshis and colleagues looked at two populations of the coral Acropora hyacinthus from American Samoa—one that lived in a pool that regularly experienced high temperatures and another from a more moderate environment.
The team measured gene expression levels across the A. hyacinthus genome by sequencing RNA from each population under moderate and hot temperatures. Under heat stress, the researchers found that expression levels changed for hundreds of genes—including those that encoded heat shock proteins, antioxidants, oxidative stress enzymes, and cell death regulators— in both populations.
But the authors identified a different expression pattern between the two strains: under moderate temperature conditions, the hardier variety expressed 60 heat-responsive genes at higher levels than the heat-sensitive variety. This frontloaded gene expression may prepare the coral for environmental stresses and protect it from dramatic heat changes, Barshis said. "It may explain why they're more tolerant to extreme stress."
Some of these genes are involved in immune and cell death pathways, so their expression levels may determine whether coral cells undergo cell repair or death when confronted with heat stress. In addition, the authors found reduced expression of stress-indicator genes in heat-tolerant corals, indicating lower levels of physiological stress.
Right now, any disadvantages from maintaining this high baseline gene expression remain unknown. "Expressing genes all the time and making lots of protein cost energy, so you'd think that there could be some drawbacks. But we haven't seen any major costs so far," Barshis said. One possible explanation Barshish offered is that heat-tolerant corals offset this energy expenditure by reproducing less often, but there's no data to support that idea yet.
The tolerant and sensitive populations were found only a few hundred yards apart, which offers hope in terms of climate change, Barshis said. "There might be little pockets of coral that are resistant to high temperatures; if we protect those from other stresses, like overfishing and coastal development, then they might be able to persist into the future."
1. Barshis, D. J., J. T. Ladner, T. A. Oliver, F. O. Seneca, N. Traylor-Knowles, and S. R. Palumbi. 2013. Genomic basis for coral resilience to climate change. Proc. Nat. Acad. Sci. published online January 7, 2013: doi: 10.1073/pnas.1210224110.