Plastic gene expression in a reef coral

Microarray analysis reveals transcriptional plasticity in the reef building coral Acropora millepora.
Bay L, Ulstrup K, Nielsen H, Jarmer H, Goffard N, Willis B, Miller D, VAN Oppen M.
Mol Ecol. 2009 Jul;18(14):3062-75.

We investigated variation in transcript abundance in the scleractinian coral, Acropora millepora, within and between populations characteristically exposed to different turbidity regimes and hence different levels of light and suspended particulate matter. We examined phenotypic plasticity by comparing levels of gene expression between source populations and following 10 days of acclimatization to a laboratory environment. Analyses of variance revealed that 0.05% of genes were differentially expressed between source populations, 1.32% following translocation into a common laboratory and 0.07% in the interaction (source population-dependent responses to translocation). Functional analyses identified an over-representation of differentially expressed genes associated with metabolism and fluorescence categories (primarily downregulated), and environmental information processing (primarily upregulated) following translocation to a lower light and turbidity environment. Such metabolic downregulation may indicate nonoxidative stress, hibernation or caloric restriction associated with the changed environmental conditions. Green fluorescent protein-related genes were the most differentially expressed and were exclusively downregulated; however, green fluorescent protein levels remained unchanged following translocation. Photophysiological responses of corals from both locations were characterized by a decline when introduced to the common laboratory environment but remained healthy (F(v)/F(m) > 0.6). Declines in total lipid content following translocation were the greatest for inshore corals, suggesting that turbid water corals have a strong reliance on heterotrophic feeding.

An Ecological Microarray Study of Coral Bleaching

Seneca F, Foret S, Goffard N, Smith C, Grasso L, Hayward D, Saint R, van Oppen M, Ball E, Miller D.

In: 11th International Coral Reef Symposium. Fort Lauderdale (USA); 2008. In: 11th Pacific Science Inter-congress: PSI 20009. Tahiti (French Polynesia); 2009

Reef building corals live close to their upper thermal tolerance limit and prolonged exposure to temperatures exceeding 31°C induces coral bleaching – the expulsion of Symbiodinium sp. which is often the first step toward mass mortality. Current projections suggest that average tropical ocean temperatures could warm by 1-3°C by the end of this century, so unless corals have the capacity for adaptation to anthropogenically induced climate change, those species that survive are likely to undergo dramatic shifts in distribution patterns. To investigate coral stress responses at a fundamental level we used microarrays of approximately 17,000 expressed sequence tags (ESTs) from the hermatypic coral Acropora millepora to attempt to identify genes responsible for individual fitness and the capacity to survive. Bleaching responses have traditionally been investigated largely by sub jecting corals to acute thermal stress in vitro.
Our approach has focussed on several coral colonies growing in a single bay that have been sampled in situ through a natural bleaching episode and the subsequent recovery period. During the sampling period, water temperature was continuously monitored (at 15 min intervals) and symbiont density recorded at monthly intervals as a measure of bleaching status. Individual colonies differed dramatically in their overall responses to similar environmental conditions – the extent of reduction of symbiont density varied considerably and, whereas some colonies recovered after the summer period, others died. Microarray experiments on a subset of colonies, which showed similar patterns of symbiont loss, identified a large number of genes with expression significantly correlated to decreases in symbiont density. The implications of these experiments in terms of understanding the mechanisms by which corals respond during bleaching episodes will be discussed.