Jueterbock A*, Franssen SU, Bergmann N, Gu J, Coyer JA, Reusch TBH, Bornberg-Bauer E , Olsen JL
Phylogeographic differentiation versus transcriptomic adaptation to warm temperatures in Zostera marina, a globally important seagrass
Molecular Ecology, 2016

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Populations distributed across a broad thermal cline are instrumental to address possible adaptation to increasing temperatures under global warming. Using a space-for-time substitution design, we tested for parallel adaptation to warm-temperatures along two independent thermal clines in Zostera marina, the most widely distributed seagrass in the temperate northern hemisphere. A North-South pair of populations was sampled along the European and North American coasts and exposed to a simulated heat-wave in a common-garden mesocosm. Transcriptomic responses under control, heat-stress and recovery were recorded in 99 RNAseq libraries with ~13,000 uniquely annotated, expressed genes. In order to discriminate neutral from adaptive differentiation, we corrected for phylogenetic differentiation among populations. The two Southern populations recovered faster from heat-stress and showed parallel transcriptomic differentiation, as compared with northern populations. Among 2,389 differentially expressed genes, 24 exceeded neutral expectations and were likely involved in parallel adaptation to warm temperatures. However, the strongest differentiation following phylogenetic correction was between the three Atlantic populations and the Mediterranean population with 170 of 4,711 differentially expressed genes exceeding neutral expectations. The Mediterranean population expressed DNA- and histone-methyltransferases at constitutively high levels. We speculate that epigenetic effects may be involved in its strong transcriptomic differentiation from the Atlantic samples. Although adaptation to warm temperatures is expected to reduce sensitivity to heat-waves, the continued resistance of Mediterranean seagrass to further anthropogenic stresses may be lowered by energetic costs involved in the constitutively high expression of heat-responsive genes.

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