Interview with the author: Heritable variation in bleaching responses and its functional genomic basis in reef-building corals

Given how quickly the environment is changing, a major question in evolutionary biology and conservation is the degree to which natural populations will be able to adapt to rapidly changing conditions. Corals, which form the foundation of biodiverse marine systems, are particularly threatened by rising ocean temperatures, with many species susceptible to coral bleaching. Understanding how genetic variation in natural populations is linked to coral bleaching responses may help us better understand and predict how increases in ocean temperatures will affect coral populations, and by extension, the health of our oceans.

In this blog post, we go behind the scenes with Dr. Katherine E. Dziedzic to talk about their recent publication in Molecular Ecology on heritable variation in bleaching response in a reef-building coral. Find out more about Dr. Dziedzic’s research by checking out their website or following them on twitter: @cnidiariangal

Photo from Katherine Dziedzic

What led to your interest in this topic / what was the motivation for this study? 
One of the most pressing questions for coral reef ecosystems is whether they are going to be able to adapt to changing ocean conditions, specifically rising ocean temperatures. Previous studies have shown variation in bleaching susceptibility across coral species, populations and geographic regions, but estimates of heritability of thermal tolerance only existed for coral larvae using controlled genetic crosses. When I started graduate school, I was interested in the same questions, but wanted to focus on adult corals that were already experiencing the effects of climate change. I wanted to determine if there was heritable variation in thermal tolerance in natural populations of corals, and therefore set out to use quantitative genetics and genomics to explore mechanisms of adaptation in a population of corals in the Caribbean.

What difficulties did you run into along the way? 
Field studies can be tricky, especially when trying to recreate environmental conditions and conduct heat stress experiments in lab settings. I chose Orbicella faveolata as the focal species for this experiment because there were genomic resources available (e.g. linkage map and genome), and it is a major reef building coral in the Caribbean. However, this species wasn’t as abundant as I had initially expected and therefore wasn’t able to collect the number of colonies I had initially intended (100 vs. 43 used in the experiment). Additionally, when I sequenced my RNAseq libraries I had low coverage and had to re-sequence my libraries multiple times. This took a lot of time and resources, but it made the results better.

What is the biggest or most surprising finding from this study? 
My co-authors and myself were surprised to see the estimate of heritability of thermal tolerance so high (h2 = 0.58). Heritability of thermal tolerance has been shown to be high in coral larvae, but no estimates existed for adults prior to this study. This was surprising, but we were excited to find hope for possible adaptation in this population. Also, when we explored gene expression across contrasting phenotypes, we found an interesting pattern where heat-tolerant corals expressed genes at higher levels constitutively and down-regulated these genes during heat stress. This pattern has been documented before, but it was surprising to see it as the dominant pattern of expression for all heat-tolerant corals in this study.

Moving forward, what are the next steps for this research?
I would love to expand this analysis to other coral species as well as other populations of corals to see if there is heritable variation in other reefs. Through genomic studies and population genetic surveys, we can use methods, like those used in this study, to determine which coral species have the capacity to adapt, and which populations globally and regionally have enough genetic variation for selection to act on. Using this type of information, we can determine which conservation actions (i.e. assisted evolution, restoration of certain reefs, resilience-based management, etc.)  can be put in place to protect and preserve these populations and thus the genetic variation in thermal tolerance.

What would your message be for students about to start their first research projects in this topic? 
If possible, I would suggest doing preliminary trials with temperature treatments and the species of interest to ensure that your stress temperature shows enough phenotypic variation and is biologically relevant for the area in which they were collected. I would also practice library preps and sequencing a small number of samples to ensure no issues with sequencing – troubleshooting ahead of time can really save a lot of time later when you’re in full production mode. And of course, enjoy your time in the field! If you have the opportunity to do scientific diving, don’t forget to look up and enjoy the view! It’s hard not to become over focused on the research project, so make sure you take a moment to appreciate the beauty.

Photo from Katherine Dziedzic

What have you learned about science over the course of this project? 
I’ve learned that science is not a straight, smooth path – there are twists and turns, speed bumps, and dead-ends. And that things fail and it is totally okay! Most of my research, particularly this project, ended up on Plan C or D, and I was really discouraged when I had to re-start or re-do aspects of it. But having to continually troubleshoot the experimental design and sequencing issues have been valuable teaching moments and have helped me become a better scientist. We only see and evaluate the final product, but it’s important to recognize and appreciate the messy part in the middle – the part that made the final product what it is. Science is messy, but that’s what makes is so fun and rewarding!

Describe the significance of this research for the general scientific community in one sentence.
This study contributes to the growing body of evidence that natural populations of coral reefs possess genetic variation in thermal stress responses that may potentially support adaptive responses to rising ocean temperatures.

Describe the significance of this research for your scientific community in one sentence.
This study provides crucial data for models aiming to predict corals’ adaptation to ocean warming, and identifies genetic markers for thermal tolerance that may be useful for restoration efforts as conservation biologists work to reverse the global degradation of coral populations resulting from changing ocean conditions.

Dziedzic, K. E., Elder, H., Tavalire, H., & Meyer, E. (2019). Heritable variation in bleaching responses and its functional genomic basis in reef-building corals (Oribicella faveolata). Molecular Ecology, 28(9), 2238-2253.

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