Determining the Role of a Novel Sodium Channel Mutation on Tolerance to Paralytic Shellfish Toxins in the Marine Copepod Acartia hudsonica

A critical challenge for aquatic scientists is to understand how grazer populations will respond to the proliferation of toxic algal blooms. Previous studies suggest that a novel mutation in the voltage-gated sodium channel of the copepod grazer Acartia hudsonica accounts for its evolutionary adaptation to the toxic dinoflagellate Alexandrium spp. I hypothesized that expression of the mutant sodium channel isoform is adaptive for copepods challenged with toxic Alexandrium spp. To test this hypothesis I first compared fitness proxies and isoform expression measurements of individual copepods as a function of toxin dose. Since both wild-type and mutant isoforms are always expressed, I partitioned individuals into three expression groups: predominantly mutant (PMI), predominantly wild-type (PWI), or equal proportion (EI) isoforms. There was no consistent evidence that the mutant isoform was advantageous in a toxic environment for ingestion rate, egg production rate, nor gross growth efficiency. In the absence of toxic food, there was no cost associated with the mutant isoform. Lastly, no trade-offs were observed across environments. These results do not support the hypothesis that the mutant isoform is adaptive. Next, I determined if the mutant isoform was inducible or the target of selection exerted by toxic A. fundyense. Individual expression of mutant isoforms did not differ among groups fed toxic or non-toxic food after six days. During a multi-generation experiment, increases in fitness-related traits for individuals continuously exposed to toxic food were observed; however, selection for PMI individuals did not occur after four generations. These observations are not consistent with the hypothesis that the mutant isoform is associated with adaptation in A. hudsonica populations. Finally, I corroborated the laboratory experiments with field observations. There were no differences in individual isoforms among populations historically exposed and naïve to toxic Alexandrium spp., or with time within each. No consistent evidence demonstrated that a natural toxic Alexandrium spp. bloom selected for the mutant isoform. These results are consistent with the laboratory findings. This study repeatedly demonstrated that the mutant sodium channel isoform is not adaptive for Acartia hudsonica under toxic conditions. Other mechanisms of adaptation should be explored

Determining the Advantages, Costs, and Trade-Offs of a Novel Sodium Channel Mutation in the Copepod Acartia hudsonica to Paralytic Shellfish Toxins (PST).

The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium fundyense, which produce paralytic shellfish toxins (PST). Adaptation to PSTs in other organisms is caused by a mutation in the sodium channel. Recently, a mutation in the sodium channel in A. hudsonica was found. In this study, we rigorously tested for advantages, costs, and trade-offs associated with the mutant isoform of A. hudsonica under toxic and non-toxic conditions. We combined fitness with wild-type: mutant isoform ratio measurements on the same individual copepod to test our hypotheses. All A. hudsonica copepods express both the wild-type and mutant sodium channel isoforms, but in different proportions; some individuals express predominantly mutant (PMI) or wild-type isoforms (PWI), while most individuals express relatively equal amounts of each (EI). There was no consistent pattern of improved performance as a function of toxin dose for egg production rate (EPR), ingestion rate (I), and gross growth efficiency (GGE) for individuals in the PMI group relative to individuals in the PWI expression group. Neither was there any evidence to indicate a fitness benefit to the mutant isoform at intermediate toxin doses. No clear advantage under toxic conditions was associated with the mutation. Using a mixed-diet approach, there was also no observed relationship between individual wild-type: mutant isoform ratios and among expression groups, on both toxic and non-toxic diets, for eggs produced over three days. Lastly, expression of the mutant isoform did not mitigate the negative effects of the toxin. That is, the reductions in EPR from a toxic to non-toxic diet for copepods were independent of expression groups. Overall, the results did not support our hypotheses; the mutant sodium channel isoform does not appear to be related to adaptation to PST in A. hudsonica. Other potential mechanisms responsible for the adaptation are discussed

Data from: No evidence for induction or selection of mutant sodium channel expression in the copepod Acartia husdsonica challenged with the toxic dinoflagellate Alexandrium fundyense

Some species in the dinoflagellate genus Alexandrium spp. produce a suite of neurotoxins that block sodium channels, known as paralytic shellfish toxins (PST), which have deleterious effects on grazers. Populations of the ubiquitous copepod grazer Acartia hudsonica that have co-occurred with toxic Alexandrium spp. are better adapted than naïve populations. The mechanism of adaptation is currently unknown. We hypothesized that a mutation in the sodium channel could account for the grazer adaptation. We tested two hypotheses: (1) Expression of the mutant sodium channel could be induced by exposure to toxic Alexandrium fundyense; (2) in the absence of induction, selection exerted by toxic A. fundyense would favor copepods that predominantly express the mutant isoform. In the copepod A. hudsonica, both isoforms are expressed in all individuals in varying proportions. Thus, in addition to comparing expression ratios of wild-type to mutant isoforms for individual copepods, we also partitioned copepods into three groups: those that predominantly express the mutant (PMI) isoform, the wild-type (PWI) isoform, or both isoforms approximately equally (EI). There were no differences in isoform expression between individuals that were fed toxic and nontoxic food after three and 6 days; induction of mutant isoform expression did not occur. Furthermore, the hypothesis that mutant isoform expression responds to toxic food was also rejected. That is, no consistent evidence showed that the wild-type to mutant isoform ratios decreased, or that the relative proportion of PMI individuals increased, due to the consumption of toxic food over four generations. However, in the selected line that was continuously exposed to toxic food sources, egg production rate increased, which suggested that adaptation occurred but was unrelated to sodium channel isoform expression

Data from: Determining the advantages, costs, and trade-offs of a novel sodium channel mutation in the copepod Acartia hudsonica to paralytic shellfish toxins (PST)

The marine copepod Acartia hudsonica was shown to be adapted to dinoflagellate prey, Alexandrium fundyense, which produce paralytic shellfish toxins (PST). Adaptation to PSTs in other organisms is caused by a mutation in the sodium channel. Recently, a mutation in the sodium channel in A. hudsonica was found. In this study, we rigorously tested for advantages, costs, and trade-offs associated with the mutant isoform of A. hudsonica under toxic and non-toxic conditions. We combined fitness with wild-type: mutant isoform ratio measurements on the same individual copepod to test our hypotheses. All A. hudsonica copepods express both the wild-type and mutant sodium channel isoforms, but in different proportions; some individuals express predominantly mutant (PMI) or wild-type isoforms (PWI), while most individuals express relatively equal amounts of each (EI). There was no consistent pattern of improved performance as a function of toxin dose for egg production rate (EPR), ingestion rate (I), and gross growth efficiency (GGE) for individuals in the PMI group relative to individuals in the PWI expression group. Neither was there any evidence to indicate a fitness benefit to the mutant isoform at intermediate toxin doses. No clear advantage under toxic conditions was associated with the mutation. Using a mixed-diet approach, there was also no observed relationship between individual wild-type: mutant isoform ratios and among expression groups, on both toxic and non-toxic diets, for eggs produced over three days. Lastly, expression of the mutant isoform did not mitigate the negative effects of the toxin. That is, the reductions in EPR from a toxic to non-toxic diet for copepods were independent of expression groups. Overall, the results did not support our hypotheses; the mutant sodium channel isoform does not appear to be related to adaptation to PST in A. hudsonica. Other potential mechanisms responsible for the adaptation are discussed

Finiguerra et al., 2014, Ecology and Evolution, Data Sharing

This file contains all data used for plotting and statistical analyses

Finiguerra et al 2015_Data_Sharing_PLoS ONE

excel file with data

Three-day egg production on mixed-diets containing toxic or nontoxic food.

<p>Each diet consisted of 500μgC L^-1 of the standard diet (equal carbon of <i>Tetraselmis sp</i>. and <i>Thalassiosira weissfloggi</i>) mixed with 200 μgC L^-1 of either toxic <i>Alexandrium fundyense</i> or non-toxic <i>Alexandrium tamarense</i>. The data are plotted as A) within toxic and non-toxic treatments among expression groups (n = 164), and B) within expression groups between food treatments (n = 165). Abbreviations- Wild-type: Predominantly wild-type isoform expression (PWI); ~1:1: approximately equal isoform expression (EI); Mutant: predominantly mutant isoform expression (PMI). Symbol in B represents statistical differences between EPR on toxic and non-toxic food for the PMI expression group (student’s t-test, <i>p</i><0.05). Conversion to <i>f</i>moles cell^-1 can be made by multiplying pgSTX equivalents by a conversion factor of 2.54.</p

Three-day total egg production (same data as Fig 6) as a function of the log₁₀ ratio of wild-type: mutant sodium channel isoform expression in the A) non-toxic and B) toxic food treatments.

<p>No relationships were found (linear regression, <i>p</i>>0.05).</p

What Controls Toxic Phytoplankton Blooms in Long Island Sound?

TIny phytoplankton organisms known as dinoflagellates can accumulate and cause toxic blooms that can kill fish and even make people ill. It\u27s rare in Long Island Sound but can happen. What drives this phenomenon? Professor Hans G. Dam and his lab team explain in this easy-to-understand article

What Controls Toxic Phytoplankton Blooms in Long Island Sound?

TIny phytoplankton organisms known as dinoflagellates can accumulate and cause toxic blooms that can kill fish and even make people ill. It\u27s rare in Long Island Sound but can happen. What drives this phenomenon? Professor Hans G. Dam and his lab team explain in this easy-to-understand article