The genetic basis of physiological variability among coral larvae

Ann Tarrant, Biology



Scientists have become increasingly concerned about the degree to which reef‐building corals will be able to persist and continue to build reefs in the face of increasing thermal stress and acidification associated with climate change.  A few recent studies indicate stress responses can vary greatly among individual corals of the same species.  It is still unclear how much of this variability is due to environmental effects and how much is due to genetic diversity.  For example, corals growing in an environment with more food may be healthier and better able to tolerate a given stressor.  In contrast, some coral genotypes may be better adapted to responding to the stressor.  Both processes are important to understanding how coral individuals and populations and ultimately reef ecosystems will be impacted by environmental change.  I propose to evaluate performance (survival, growth) of coral larvae with different genetic backgrounds (“families” containing larvae produced by selfand/or crossfertilization) under conditions of ambient and elevated CO2To my knowledge, this will be the first study to directly compare the performance of self‐fertilized and cross‐fertilized larvae in a coral species that naturally employs both modes of reproduction.

Many studies have experimentally documented effects of acidification on the ability of scleractinian corals to form aragonitic skeletons.  The proposed research builds on an externally‐funded project which aims to assess the effects of nutrition on the ability of juvenile corals to grow under ambient and elevated CO2.  Through the currently supported project, I will correlate variation in CO2 effects among batches of larvae with the physiological condition of the mother and the initial larval condition.  Support from OLI would enable me to add a genetic component to this study.   This new component would lead to a much stronger set of results which would strongly support a future proposal submission.   

These studies will be conducted using the hermaphroditic brooding coral Favia fragum, in which eggs produced by the mother are either fertilized by sperm from the same colony (self‐fertilized) or a neighboring colony (cross‐fertilized).  In this species, mixtures of self‐ and cross‐fertilized offspring are frequently observed within the same colony.  It has been hypothesized that self‐fertilized offspring will be less viable (self‐fertilized offspring from broadcast spawning corals often fail to develop), but this has not been tested for a brooding coral species.  I will use sensitive genetic markers (microsatellites) to assess the genetic identity (self‐ or cross‐fertilized, same or different paternal genotype) of juvenile corals at the time of spawning and after two weeks of growth under ambient or elevated CO2.  This will enable me to determine whether the relative proportion of these larval types changes over time with differential mortality.  I will also compare the growth rates between corals with different genetic backgrounds under different CO2 levels.

This study directly addresses the current OLI themes of Coral Reefs and Ocean Biodiversity.  Identification of genotypes that exhibit differential sensitivity to acidification (or any stressor) could lead to a greater understanding of compensatory mechanisms, and improve understanding of the natural genetic variation that is available to enable evolutionary selection of more tolerant genotypes.  In addition, comparing the performance of self‐ and cross‐fertilized offspring would provide insight into the potential consequences of changes in coral density and life history trade‐offs.  Following successful completion of this project, I will submit a proposal to the Division of Environmental Biology (DEB) within the National Science Foundation (NSF) to comprehensively investigate genetic and environmental influences on coral physiology and stress resistance.