Data from: Phylogeographic differentiation versus transcriptomic adaptation to warm temperatures in Zostera marina, a globally important seagrass

Populations distributed across a broad thermal cline are instrumental in
addressing 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 heatwave
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. We corrected for phylogenetic
differentiation among populations to discriminate neutral from adaptive
differentiation. The two southern populations recovered faster from heat
stress and showed parallel transcriptomic differentiation, as compared
with northern populations. Among 2389 differentially expressed genes, 21
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 128 of 4711
differentially expressed genes exceeding neutral expectations. Although
adaptation to warm temperatures is expected to reduce sensitivity to
heatwaves, the continued resistance of seagrass to further anthropogenic
stresses may be impaired by heat-induced downregulation of genes related
to photosynthesis, pathogen defence and stress tolerance.