Results of the 2-factorial RM-ANOVA for the induction phase, comparing feeding rates affected by treatment (control vs. water-borne cues/direct grazing) and time.

<p>Consumption was assessed in two-choice feeding assays using either fresh or reconstituted <i>Fucus vesiculosus</i> (n = 10).</p><p>Results of the 2-factorial RM-ANOVA for the induction phase, comparing feeding rates affected by treatment (control vs. water-borne cues/direct grazing) and time.</p

Results of two-tailed paired <i>t</i>-tests comparing isopod consumption between different 3 day intervals of the induction phase (n = 10).

<p>Significant p-values, i.e. α≤0.05, in bold.</p><p>Results of two-tailed paired <i>t</i>-tests comparing isopod consumption between different 3 day intervals of the induction phase (n = 10).</p

Mean ± SE consumption of <i>Fucus vesiculosus</i> by <i>Idotea baltica</i> in two-choice feeding assays during the induction phase (n = 10).

<p>Isopod consumption of fresh and reconstituted <i>Fucus vesiculosus</i> pieces that were previously grazed by <i>Idotea baltica</i> (A and C) or located downstream of isopod-grazed conspecifics (B and D) (grey bars) and seaweed pieces which were not exposed to grazing or located downstream of grazed <i>F. vesiculosus</i> before (controls; open bars). Asterisks indicate significant results of one-tailed paired <i>t</i>-tests comparing distribution of differences between control and grazed pieces against the null-hypothesis of no difference.</p

Mean and 95% confidence intervals of <i>Fucus vesiculosus</i> consumption by <i>Idotea baltica</i> during 3 day intervals in the induction phase (n = 10).

<p>Stippled line marks level of no consumption. Confidence intervals overlapping with stippled line indicate intervals when consumption was statistically not significantly different from zero. Different letters indicate significant difference in consumption.</p

Schematic illustration showing the allocation of <i> F. vesiculosus</i> pieces (small rectangles) for a single replicate at one of several sampling time points.

<p>Upstream and downstream compartments were separated by a polyethylene mesh and contained 28 pieces each (only 4 shown). From these, 4 pieces were allocated to feeding arenas (circles) at each of the sampling time points. Arrow indicates direction of water flow. Dashed lines indicate containers with herbivores. Solid lines indicate containers without grazers. Letters and numbers indicate control and treated (direct grazing/water-borne cues) pieces of <i>F. vesiculosus</i>, respectively. Bold letters and numbers indicate <i>F. vesiculosus</i> pieces located in upstream compartments, while all other characters indicate seaweed pieces located in downstream compartments. Pentagons indicate reconstituted food items used for feeding assays with naïve herbivores.</p

Results of one-tailed paired <i>t</i>-tests comparing controls and seaweed pieces that were previously exposed to direct grazing by <i>I. baltica</i>.

<p>Consumption by conspecific consumers was assessed in feeding assays using either fresh or reconstituted food pieces of <i>F. vesiculosus</i> (n = 10). Time = days after start the of the induction phase. Significant p-values, i.e. α≤0.05, in bold.</p><p>Results of one-tailed paired <i>t</i>-tests comparing controls and seaweed pieces that were previously exposed to direct grazing by <i>I. baltica</i>.</p

Herbivore-induced defence response in the brown seaweed <i>Fucus vesiculosus</i> (Phaeophyceae): temporal pattern and gene expression

<div><p>This study aimed to reveal the dynamics of inducible anti-herbivory traits in the brown seaweed <i>Fucus vesiculosus</i>, in response to grazing by the isopod <i>Idotea baltica</i>. As the molecular basis of defence induction in seaweeds is poorly understood, a transcriptomic approach was used to gain insight into cellular processes underlying defence induction and thus promote the mechanistic understanding of anti-herbivory responses in seaweeds. In a 27 day feeding-assayed induction experiment, temporal patterns of the induced anti-herbivory resistance of <i>F. vesiculosus</i> were assessed under laboratory conditions. Feeding assays were performed at three day intervals, using fresh and reconstituted food. Microarray hybridizations investigating the expression of genes 3 days before, as well as at the same time as, feeding assays revealed a decreased palatability of previously grazed <i>F. vesiculosus</i> pieces compared with non-grazed control pieces. Despite permanent exposure to grazers, <i>F. vesiculosus</i> palatability varied over time. Non-grazed <i>F. vesiculosus</i> pieces were significantly preferred to grazed pieces after 18 and again after 27 days of previous grazing, while their relative palatability for isopods was comparable at all other times during the experiment. Relative to controls, 562/402 genes were ≥ 1.5-fold up-/down-regulated in seaweed pieces that were grazed for 18 days, i.e. when induction of defences was detected in feeding assays. Massive reprogramming of the regulatory expression orchestra (translation, transcription) as well as up-regulation of genes involved in lipid and carbohydrate metabolism, intracellular trafficking, defence and stress response was found. At the same time, down-regulation of photosynthesis was observed in grazed seaweed pieces. This study suggests a high level of temporal variability in induced anti-herbivory traits of <i>F. vesiculosus</i> and reveals increased expression of genes with putative defensive functions in conjunction with the reallocation of resources from primary to secondary metabolism.</p></div