Next, the team used a type of phylogenetic ANOVA technique, called the Expression Variance and Evolution Model, to examine changes in gene expression related to heat stress that have occurred over evolutionary time.
“In collaboration with professor Rori Rohlfs from San Francisco State University, who is a coauthor in this study, we developed a method based on a phylogenetic ANOVA that allowed us to track genes that have already diverged in expression across species in response to any given stimuli — in our case heat stress,” said Viridiana Avila-Magaña. “This approach becomes particularly relevant for coral reef research given the recent debates on adaptive potential of different coral holobionts under the threats of climate change. With this approach in mind, we were able to understand why different corals have unique physiological responses to heat stress, and how the evolution of gene expression shaped their different susceptibilities.”
Avila-Magaña explained that corals have experienced episodes of elevated temperatures through evolutionary time and understanding how gene expression has evolved in response to those events can inform corals’ responses to present-day and future warming events.
“Our goal with this research was to determine if there have been lineage-specific innovations to heat stress in corals and their algal photosymbionts, as well as whether all members, including bacterial communities, differentially contribute to holobiont robustness,” she said.
The gene-expression data revealed that the three coral holobionts did, indeed, differ in their responses and metabolic capabilities under high temperature stress. The team also found that the members of each holobiont had unique responses that influenced the holobiont’s overall ability to cope with thermal stress.
“We have uncovered more genes associated with a thermal stress response in coral holobionts than previous studies, and we also show that changes in the expression of these genes arose over evolutionary time,” said Medina.
Interestingly, the scientists concluded that the greater thermal tolerance observed in some coral holobionts, such as the starlet coral, may be due, in part, to a higher number and diversity of thermally tolerant microbes in their microbiomes, which provides redundancy in key metabolic pathways that are protective against heat stress.
“We found that some corals harbor a stable and diverse microbiome translating to a vast array of metabolic capabilities that we have shown remain active during the thermal challenge,” said Avila-Magaña. “By contrast, we found that less thermally tolerant species had reduced bacterial activity and diversity.”
Medina noted that the results stress the importance of comparative approaches across a wide range of species to better understand the diverse responses of corals to increasing sea surface temperatures.
Medina and Avila-Magaña said, “Corals have been highly impacted by climate change, and the methods we developed in our study represent an excellent tool for scientists trying to understand the adaptive potential of populations and species.”
Other authors on the paper include Susana Enríquez, professor, Universidad Nacional Autónoma de México; Bishoy Kamel, research assistant professor of biology, University of New Mexico and the Joint Genome Institute, Michael DeSalvo, University of California Merced; Roberto Iglesias-Prieto, professor of biology, Penn State; Kelly Gómez-Campo, graduate student in biology, Penn State; Hiroaki Kitano, professor, Systems Biology Institute Japan; and Rori Rohlfs, assistant professor of biology, San Francisco State University.
The National Science Foundation and the Joint Genome Institute (Department of Energy) supported this research.