Experimental Evidence of Ciguatoxin Accumulation and Depuration in Carnivorous Lionfish

Ciguatera poisoning is a food intoxication associated with the consumption of fish or shellfish contaminated, through trophic transfer, with ciguatoxins (CTXs). In this study, we developed an experimental model to assess the trophic transfer of CTXs from herbivorous parrotfish, Chlorurus microrhinos, to carnivorous lionfish, Pterois volitans. During a 6-week period, juvenile lionfish were fed naturally contaminated parrotfish fillets at a daily dose of 0.11 or 0.035 ng CTX3C equiv. g−1, as measured by the radioligand-receptor binding assay (r-RBA) or neuroblastoma cell-based assay (CBA-N2a), respectively. During an additional 6-week depuration period, the remaining fish were fed a CTX-free diet. Using r-RBA, no CTXs were detectable in muscular tissues, whereas CTXs were measured in the livers of two out of nine fish sampled during exposure, and in four out of eight fish sampled during depuration. Timepoint pooled liver samples, as analyzed by CBA-N2a, confirmed the accumulation of CTXs in liver tissues, reaching 0.89 ng CTX3C equiv. g−1 after 41 days of exposure, followed by slow toxin elimination, with 0.37 ng CTX3C equiv. g−1 measured after the 6-week depuration. These preliminary results, which need to be pursued in adult lionfish, strengthen our knowledge on CTX transfer and kinetics along the food web.

Toxin breakdown does not preclude the potential for defensive toxin use in a fruit fly

Animals that ingest toxins can themselves become toxic or unpalatable to predators and parasites. However, most animals rapidly eliminate toxins to survive toxin ingestion. It is therefore unclear how species transition from susceptibility and toxin elimination to tolerance and accumulation as chemical defense emerges. Studies of chemical defense have generally focused on species that display active toxin sequestration and target-site insensitivity mutations that permit survival without necessitating toxin metabolism. Here we investigate whether animals that presumably rely on toxin elimination for survival can also utilize ingested toxins for defense. We use the A4 and A3 Drosophila melanogaster fly strains from the Drosophila Synthetic Population Resource (DSPR), which respectively possess elevated and reduced metabolic toxin resistance. We find that ingesting nicotine increased the survival of A4 but not of A3 flies against Leptopilina heterotoma wasp parasitism. Further, we find that despite possessing enhanced toxin clearance mechanisms, A4 flies accrued more nicotine than A3 individuals. Our results suggest that enhanced metabolic detoxification can allow for greater toxin intake by offsetting the cost of toxin ingestion. Passive toxin accumulation that accompanies increased toxin intake may underlie the early origins of chemical defense.

Genomic islands of heterozygosity maintained across caribou populations despite inbreeding

Small, isolated populations are prone to inbreeding, increasing the proportion of homozygous sites across the genome that can be quantified as runs of homozygosity (ROH). Caribou (Rangifer tarandus) are declining across their range in Canada; thus, understanding the effects of inbreeding on genetic potential is pertinent for conserving small, isolated populations. We quantified ROH in high-coverage whole genomes of boreal caribou from small, isolated populations in southern Ontario, Canada, in comparison to caribou from the continuous range of Ontario, other caribou ecotypes in Canada, and western Greenland. Sampled populations presented divergent evolutionary histories, differing population sizes, and extents of isolation. We conducted BLAST searches across regions of elevated heterozygosity to identify genes that have maintained variation despite inbreeding. We found caribou from recently isolated populations in Ontario had a large proportion of their genome in long ROH. We observed even larger proportions but shorter ROH in western Greenland, indicating that inbreeding has occurred over a longer period in comparison to other populations. We observed the least inbreeding in barren-ground and eastern migratory caribou, which occur in larger population sizes than boreal caribou. Despite vastly different inbreeding extents, we found regions of high heterozygosity maintained across all populations. Within these islands of heterozygosity, we identified genes associated with immunity, signaling regulation, nucleotide binding, toxin elimination, and feeding behaviour regulation. In this study, we confirm inbreeding in isolated populations of a species at risk, but also uncover high variation in some genes maintained across divergent populations despite inbreeding, suggesting strong balancing selection.

Low susceptibility of non-target Lepidopteran maize pests to the Bt protein Cry1Ab

Transgenic Bt maize expressing the Cry1Ab toxin is poorly effective for suppressing populations of two non-target Lepidoptera,Mythimna unipunctaandHelicoverpa armigera. In order to determine the mechanisms that may be involved in this poor effectiveness, last instar larvae of the two Lepidoptera were fed with a diet containing lyophilized leaves with Bt vs non-Bt toxin for different periods; additionally, some larvae fed on Bt diet were transferred to non-Bt diet for an additional period. In the experimental larvae, we measured the growth (weight) gain from just before treatment to after the end of the treatment, and the Cry1Ab contents in the hemolymph, the peritrophic membrane and its contents and midgut epithelium. Effects of the treatments on the midgut epithelium were observed by light and transmission electron microscopy. It was seen that multiple mechanisms can be involved in the low susceptibility of the two Lepidoptera. The low content of the toxin within the peritrophic membrane 48h after ingestion indicates a high rate of toxin elimination in this space. Moreover,M. unipunctalarvae fed on the Bt diet displayed a similar growth gain index to those fed on the non-Bt diet, and showed an increasing elimination rate during the experiment. Little toxin reached the midgut epithelium, indicating a low permeability of the peritrophic membrane or a low affinity at the binding sites. Larvae fed on the Bt toxin showed rapid recovery in weight gain and in the midgut epithelium, and also showed overcompensation mechanisms.