Differential Gene Expression: successful parenting strategies for Caribbean coral

by Krti Tallam

Note from the Author:

Nia Symone Walker is a 4th year PhD Candidate at Stanford University. Nia received her degree in Organismic and Evolutionary Biology from Harvard University in 2016.  Prior to her graduate studies, Nia conducted research as a Science Educator for the Maritime Aquarium’s classrooms, supported by NOAA’s Environmental Literacy Grants Program.

Nia primarily uses genomics, genetics, and physiology techniques and applications to study climate change resilience in coral reefs. Currently, her research focus is on not just identifying, but also challenging, what makes “strong” corals by studying both coral thermal resistance and recovery.

Nia also happens to be #BlackinSTEM and #BlackinMarineScience. As a black woman in STEM, she has committed furthering access to higher education and all levels of academia for individuals from diverse and underrepresented minority backgrounds. Nia says, “the only prerequisite to pursuing anything should be a spark to get started.”

Citation: Walker, NS, Fernández, R, Sneed, JM, Paul, VJ, Giribet, G, Combosch, DJ. Differential gene expression during substrate probing in larvae of the Caribbean coral Porites astreoides. Mol Ecol. 2019; 28: 4899– 4913. https://doi.org/10.1111/mec.15265

Differential Gene Expression: successful parenting strategies for Caribbean coral

Close your eyes and think of the first time your parents dropped you off at kindergarten…or high school, or probably the biggest transition for most of us, college. It’s a moment that you become prepared for through the environment in which you were raised in at home, through parenting/guardian strategies, and through the community in which you developed in and grew from. Sessile benthic parents (a.k.a. immobile organisms that live at the bottom of the ocean), must find a way to raise their offspring into successful, fully-functioning adults. Most marine invertebrates with a benthic adult life also have a pelagic stage (a.k.a. an open-ocean stage), where they must find the most appropriate environments to settle and metamorphose. This step, known as the “dispersal step” is vital to the species’ success, as it moderates inbreeding potential and competition for resources, increasing the chances of survival for the species in the event of local extinctions. It’s not much different from humans: we, too, seek the best environments, neighborhoods, and communities to raise our young in, to ensure their success into adulthood.

Reef builders 

The scleractinian (e.g. stony or hard corals) genus Porites includes some of the most identifiable and dominant reef builders. Porites astreoides has also, in recent years, increased in abundance to become a locally dominant shallow‐water species on many Caribbean reefs and to serve as a model coral system. Given the abundance and predictable nature of the larval brooding and release schedule, Porites astreoides colonies are great candidates for studying larval settlement and metamorphosis. The species is also well-developed upon release, which allows a more holistic study of this symbiont (i.e. two different biological organisms with long-term interactions) throughout its coral life cycle. Prior research focused on larval settlement induction, response to pH levels, larval phenotypic variability, and the effects of thermal (i.e. heat) stress on larval mortality. Nia’s team realized that little has been done to understand the genetic mechanisms of settlement, so that’s where they focused this study.

Differential gene expression … wait, what?

In this study, Nia and the authors investigated the processes that connect two main life stages of corals: the open-sea planula larva and the deep-sea polyp, and the larval settlement and metamorphosis processes that connect both of them. The authors studied differential gene expression of probing (i.e. touching down and crawling) larvae and non-probing larvae in controlled environments, to understand the mechanisms influencing larval settlement. Studying planulae without distinguishing between probing and nonprobing behaviors could impact differential gene expression, as differential physiological and molecular responses are exhibited during this transition. Overall, Nia’s study sought to understand the underlying molecular components of the shift from nonprobing to substrate (i.e. the base on which an organism lives) probing, a possibly significant transitional period during larval development.

The larger picture for Porites astreoides larvae

The team discovered noteworthy processes in the holobiont (i.e. the assemblage of a host and the species living in or around it) gene expression during this complex transitional life history stage of Porites astreoides larvae. This also sheds light on the importance of distinguishing between nonprobing and substrate‐probing larvae for future larval development and settlement research.

What’s in it for the rest of the oceanic world?  Well, it connects the observed gene expression patterns to broader trends seen in other marine organisms who also go through a pelagic larval stage.  In Nia’s study, it was predicted that significantly more genes would be upregulated in probing larvae vs. nonprobing larvae, given the presumption that the substrate-probing process is increasingly more complex than the free-swimming stage. The team discovered that differentially-expressed holobiont genes showed linkage to environmental sensing and response, and this likely would aid in identifying a suitable substrate for the settlement period. 

When asked what she wants to study further with this research, Nia says there is potential to study definitive identification genes, to test whether different environmental cues (i.e. depth, light variation) can trigger the expression of alternative genes.

Krti Tallam is broadly interested in the ecology of environmental diseases, as they link to climate and anthropogenic stressors.  She studies coastal and oceanic environmental diseases that have links to human, terrestrial, and aquatic systems.   Some of her current work includes currently conducting analyses on the responses of dengue to climatic and anthropogenic stressors off of the coast of the Bay of Bengal, in India, and understanding the role of schistosomiasis in environmental reservoirs where freshwater snails serve as intermediate hosts.  Krti utilizes machine learning / deep learning, artificial intelligence, and field experiments for her work, and is interested in long-term policy implications for planetary health.  Krti is a budding scientist, an innovator, a first-generation student, a woman, a woman of color, and a proud daughter of immigrants.