Complementary mechanisms for neurotoxin resistance in a copepod

Abstract

Toxin resistance is a recurring evolutionary response by predators feeding on toxic prey. These adaptations impact physiological interaction and community ecology. Mechanisms for resistance vary depending on the predator and the nature of the toxin. Potent neurotoxins like tetrodotoxin (TTX) and saxitoxin (STX) that are highly toxic to humans and other vertebrates, target conserved voltage-gated sodium channels (NaV) of nerve and muscle, causing paralysis. The copepod Calanus finmarchicus consumes the STX-producing dinoflagellate, Alexandrium fundyense with no effect on survival. Using transcriptomic approaches to search for the mechanism that confers resistance in C. finmarchicus, we identified splice variants of NaVs that were predicted to be toxin resistant. These were co-expressed with putatively non-resistant form in all developmental stages. However its expression was unresponsive to toxin challenge nor was there any up-regulation of genes involved in multi-xenobiotic resistance (MXR) or detoxification (phases I or II). Instead, adults consistently regulated genes encoding digestive enzymes, possibly to complement channel resistance by limiting toxin assimilation via the digestive process. The nauplii, which were more susceptible to STX, did not regulate these enzymes. This study demonstrates how deep-sequencing technology can elucidate multiple mechanisms of toxin resistance concurrently, revealing the linkages between molecular/cellular adaptations and the ecology of an organism.

The Gene-Rich Genome of the ScallopPecten maximus

“…Furthermore, it possesses the characteristic thymine residue in Domain 3 (Fig. 5, position 1425 in reference to rat sodium channel IIA), also described in the other two scallop species sequenced so far, which has been shown to confer resistance to these toxins in pufferfish, copepods and the venomous blue-ringed octopus [57][58][59]. It does not, however, have the E945D mutation seen in the softshell clam Mya arenaria and some pufferfish, which experimental evidence suggests also confers resistance [56], nor the D1663H or G1664S mutations in the blueringed octopus [62].…”

“…This annotation resulted in an initial set of 215,598 putative genes (with 32,824 genes having two or more alternative isoforms, resulting in 249,081 discrete transcript models. We filtered the initial gene set by comparing our gene models to seven previously published bivalve resources using Orthofinder2, and retained genes with orthologues shared with other species (57,574 genes, further details below). To ensure we did not discard transcribed genes absent from other bivalves but present in our resource, we also retained those genes with a good hit in the nr database (23,541 genes, diamond blastp, --more-sensitive --max-target-seqs 1 --outfmt 6 qseqid sallseqid stitle pident evalue --evalue 1e-9), a total of 81,115 genes.…”

“…Previous studies have shown that genetic mutations within the sodium channel gene, Neuron Navigator 1 (Nav1) confer immunity in taxa that accumulate saxitoxin (e.g. the soft-shell clam Mya arenaria [56]; scallop Azumapecten farreri [17]; copepods, Calanus finmarchicus and Acartia hudsonica [57]) or other similar acting neurotoxins like tetrodotoxin (TTX) (e.g. pufferfish, Tetraodon nigroviridis and Takifugu rubripes; salamanders [58][59][60][61]; and the venomous blue-ringed octopus [62]).…”

Community changes of gut microbes highlight their importance in the adaptation of copepods to toxic dinoflagellates

“…These toxins include several derivatives, with gonyautoxins (GTX) 1-4, saxitoxin (STX), and neosaxitoxin (NEO) being the most potent (Etheridge, 2010). These neurotoxins bind to voltage-gated sodium channels, preventing the passage of sodium ions and attenuating action potentials, progressively inhibit neurotransmission, relax smooth muscle, and lead to paralysis and respiratory failure in humans, as well as mussels, fishes, and seabirds (Tester et al, 2000;Hallegraeff, 2003;Turner, 2014), though several studies showed no evidence of these effects on some marine zooplankton (Van Dolah, 2000;Etheridge, 2010;Finiguerra et al, 2014;Roncalli et al, 2017). Shellfish, zooplankton, and herbivorous fish consume these toxic microalgae and act as vectors to humans directly or transfer through the food web to higher trophic levels (Van Dolah, 2000).…”

Predator cues target signaling pathways in toxic algal metabolome

“…A meta‐analysis of experiments involving marine copepods and chemically defended phytoplankton found 19 studies demonstrating adverse effects on copepods consuming Alexandrium cells, including disrupted swimming behavior, decreased feeding, and decreased fecundity and survivorship of offspring (Turner 2014). The observed toxicity has been explained in part by antagonism of voltage‐gated sodium channels by the paralytic shellfish toxins produced by many Alexandrium species (Roncalli et al 2017). However, only some zooplankton predators trigger particular defensive phenotypes (Senft‐Batoh et al 2015; Lundholm et al 2018) suggesting species‐specific recognition of predator cues.…”