Predator cues reduce intraspecific trait variability in a marine dinoflagellate

Abstract

Background: Phenotypic plasticity is commonplace and enables an organism to respond to variations in the environment. Plastic responses often modify a suite of traits and can be triggered by both abiotic and biotic changes. Here we analysed the plastic response towards a grazer of two genotypes of the marine dinoflagellate Alexandrium fundyense, evaluated the similarity of this response and discuss potential strain-specific trade-offs. We compared the expression of the known inducible defensive traits paralytic shellfish toxin content, and chain length. The effectiveness of the induced defense was assessed by monitoring grazing rates in both strains.

Results: Our results show that the grazer cues diminish phenotypic variability in a population by driving the phenotype towards a common defended morphotype. We further showed that the expression of the sxtA gene that initiates the paralytic shellfish toxin biosynthesis pathway does not correlate with an observed increase in the paralytic shellfish toxin analogue saxitoxin, and that toxin induction differs in its physiological characteristics in both strains.

Conclusion: Induced defense response in Alexandrium thus can directly affect further species interactions by reducing phenotypic variation and can result in genotype-dependent ecological trade-offs.

Keywords: Alexandrium; Grazer induced defense; Intraspecific trait variation; Predator–prey interaction; Saxitoxin gene expression.

Fig. 1

Cell size distribution of the Alexandrium fundyense strains. Cell size distributions of Alex2 (A, B) and Alex5 (C, D) after exposure to direct grazing Centropages typicus (A, C) or after exposure to waterborne-cues from caged fed and caged starved C. typicus (B, D). Treatments with waterborne-cues were terminated after 48 h. The diameter of the measured particles (equivalent spherical diameter in µm) is plotted as kernel density estimation with the width corresponding to the relative occurrence of a particle in a respective size class. Box plots show the 25th, 50th and 75th percentile; the ends of the whiskers mark the 95% intervals

Fig. 2

PST contents of the Alexandrium fundyense strains and clearance rates for Centropages typicus. PST contents of Alex2 (A, B) and Alex5 (C, D). PST contents after exposure to direct grazing C. typicus individuals and respective clearance rates are given in A and C. PST contents after exposure to waterborne-cues from caged fed and caged starved C. typicus are given in B and D. Treatments with waterborne-cues were terminated after 48 h. Bars marked with an asterisk show significant differences compared to either the control at this time point (for PSTs contents) or between the treatments at different time points (for clearance rates) (ANOVA p < 0.05)

Fig. 3

PST profiles of Alex2 and Alex5. PST profiles of Alex2 after exposure to direct grazing Centropages typicus, waterborne-cues of C. typicus and the respective controls are shown in the upper panel. PST profiles of Alex5 after exposure to direct grazing C. typicus, waterborne-cues of C. typicus and the respective controls are shown in the lower panel. Shaded colors indicate confidence intervals

Fig. 4

Expression of the sxtA gene fragment. The log2 expression ratio for the putative sxtA gene fragment as determined for Alex2 from the direct grazing experiment after 48 and 72 h. Quantification of the relative expression compared to the control treatments was done using a reference spike-in gene to normalize expression level of controls and treatments. The different blue colours represent biological replicates, dots with same colours represent technical replicates