Alkaloid tolerance in Manduca sexta and phylogenetically related sphingids (Lepidoptera: Sphingidae)

Abstract

Nicotine tolerance is well known for Manduca sexta. It also occurs in several other sphingids of the subfamilies Macroglossinae and Sphinginae. Only members of the subfamily Smerinthinae appear to be more susceptible to nicotine intoxication. Phylogenetic trees have been reconstructed from mitochondrial 16S rDNA and nuclear DNA to map nicotine tolerance.The nicotine binding site of both h-subunits of nicotinic acetylcholine receptors (nAChR) have been amplified and sequenced. No apparent amino acid substitution can be seen in the putative nicotine binding site of the h-subunits of nAChR from nicotine tolerant and nicotine sensitive sphingids. Thus, a simple targetsite modification can be ruled out as a cause for nicotine tolerance. This finding agrees with feeding experiments: larvae of M. sexta and other sphingids of the Macroglossinae and Sphinginae not only tolerated nicotine, but also many other alkaloids that affect neuroreceptors other than acetylcholine receptors (nAChR, mAChR).Only 10 to 20% of nicotine injected into larvae of nicotine-tolerant taxa could be recovered later as free nicotine, nicotine N-oxide or cotinine, i.e., 80 to 90% must have been converted to polar conjugates or degradation products which are not detectable with the methods applied. Usually more than 98% of the recoverable alkaloids were found in the faeces. Excretion reached a maximum 6 h after injection in tolerant taxa. Larvae of Manduca sexta, which were reared on a nicotine-rich diet, showed higher nicotine degradation and faster nicotine elimination than naïve larvae. Application of the cytochrome P450 inhibitor SKF 525A (proadifen) reduced the formation of nicotine N-oxide and the rate of alkaloid degradation. Thus, an inducible detoxification mechanism, coupled with a rapid and inducible excretion, appear to be a strategy in Sphingidae that helps them to live on host plants rich in otherwise toxic secondary metabolites.

Natural history-driven, plant-mediated RNAi-based study reveals CYP6B46 ’s role in a nicotine-mediated antipredator herbivore defense

“…This finding suggests that the CYP6B46-mediated nicotine efflux from the midgut could have evolved as a defense-specific mechanism, which may even be independent of the larvae's rapid excretion based nicotine tolerance. Whether it was a secondary innovation evolving after the basic excretory machinery was in place could be explored in other nicotinetolerant taxa within the sphingid clade by studying the role of this CYP in different species that tolerate nicotine ingestion but do not feed on nicotine-containing hostplants (8).…”

“…on the polar metabolites of nicotine, such as cotinine and the N-oxides of both nicotine and cotinine, which are commonly found in the urine and blood of human smokers (8,11,12); cytochrome P450s (CYPs) are thought to mediate nicotine's oxidation to these metabolites (8,11,(13)(14)(15), but other researchers have been unable to find the oxides in M. sexta's excretions and propose that nicotine is rapidly excreted without modification (16)(17)(18). Although this theory is widely accepted, most studies have not been able to recover all of the ingested nicotine in the frass and nicotine can be found in the hemolymph of larvae feeding on nicotine-containing diets.…”

“…hostplants respond to the herbivore attack with large increases in nicotine accumulation (7). However, the tobacco hornworm (Manduca sexta, Ms), a specialist lepidopteran herbivore that feeds on nicotine-producing Nicotiana plants, tolerates doses of nicotine that are lethal for unadapted herbivores (8). More endoparasitoid wasps (Cotesia congregata) emerged as adults from parasitized M. sexta larvae fed on low nicotine varieties of cultivated tobacco than from larvae fed on nicotine-rich varieties (9).…”

Antagonistic effects of floral scent in an insect–plant interaction

“…Moreover, upon herbivory D. wrightii plants reduce photosynthetic rates and redirect resources to storage in the roots (G. Barron-Gafford 2009, personal communication). Larvae of M. sexta develop well on D. wrightii and have physiological adaptations that allow them to feed on plants containing alkaloids that harm other insects (Glendinning 2002;Wink & Theile 2006).…”

Jasmonate-induced plant defenses and prey availability impact the preference and performance of an omnivorous stink bug, Podisus maculiventris

“…In a study where P. maculiventris was fed M. sexta prey that had been fed on artificial diet with and without plant allelochemicals, P. maculiventris performance was reduced when there was both poorquality prey and prey were scarce (Weiser and Stamp 1998). While M. sexta are not known to sequester plant secondary compounds (Self et al 1964;Wink and Theile 2002), they can have increased allelochemical concentrations in their hemolymph (Barbosa et al 1991;Strohmeyer et al 1998). This indicates that the omnivore can be affected by allelochemicals they encounter via the herbivore, and under some conditions, plant resistance and prey availability do interact to affect the omnivore.…”