Elucidation of the molecular envenomation strategy of the cone snail Conus geographus through transcriptome sequencing of its venom duct

Abstract

Background: The fish-hunting cone snail, Conus geographus, is the deadliest snail on earth. In the absence of medical intervention, 70% of human stinging cases are fatal. Although, its venom is known to consist of a cocktail of small peptides targeting different ion-channels and receptors, the bulk of its venom constituents, their sites of manufacture, relative abundances and how they function collectively in envenomation has remained unknown.

Results: We have used transcriptome sequencing to systematically elucidate the contents the C. geographus venom duct, dividing it into four segments in order to investigate each segment's mRNA contents. Three different types of calcium channel (each targeted by unrelated, entirely distinct venom peptides) and at least two different nicotinic receptors appear to be targeted by the venom. Moreover, the most highly expressed venom component is not paralytic, but causes sensory disorientation and is expressed in a different segment of the venom duct from venoms believed to cause sensory disruption. We have also identified several new toxins of interest for pharmaceutical and neuroscience research.

Conclusions: Conus geographus is believed to prey on fish hiding in reef crevices at night. Our data suggest that disorientation of prey is central to its envenomation strategy. Furthermore, venom expression profiles also suggest a sophisticated layering of venom-expression patterns within the venom duct, with disorientating and paralytic venoms expressed in different regions. Thus, our transcriptome analysis provides a new physiological framework for understanding the molecular envenomation strategy of this deadly snail.

Figure 1

The venom duct of Conus geographus.  Insert: Conus geographus shell. Main figure: schematic of its venom duct. The segments of the venom duct are labeled as Proximal (P, in blue)--‒connected to the bulb, Proximal Central (PC, in purple), Distal Central (DC, in red), and Distal (D, in green) closest to the pharynx.

Figure 2

iPath metabolism map. (a) Conus geographus; (b) Aplysia californica. Each grey dot represents a metabolite and each red line represents an enzyme acting on it. Major pathways in the map includes: 1) Glycan biosynthesis and metabolism; 2) Lipid metabolism; 3) Metabolism of terpenoids and polyketides; 4) Xenobiotics biodegradation and metabolism; 5) Carbohydrate metabolism; 6) Amino acid metabolism; 7) Energy metabolism; 8) Metabolism of cofactors and vitamins; 9) Biosynthesis of other secondary metabolites; 10) Metabolism of other amino acid; 11) Nucleotide metabolism.

Figure 3

Global relative - expression profiles of different conotoxin superfamilies. Wedge - widths proportional to expression as measured by number of aligned reads. (a) and by number of contigs (b).

Figure 4

Segment - specific relative - expression of conotoxinsuperfamilies. Wedge - widths proportional to relative expression as measured by number of aligned reads. From A to D, Proximal (P), Proximal Central (PC), Distal Central (DC) and Distal (D) segments, respectively.

Figure 5

Segment - specific expression profiles ofthe 10 most abundantconotoxin contigs. Y - axis: total percent expression for the entire venom - duct broken down by segment. X - axis: toxin ID as in Additional file 2: Table S1, with its annotation and superfamily name shown in parentheses. Expression levels are scaled to the proportion of aligned reads among the whole transcriptome, divided into four segments: Proximal (P), Proximal Central (PC), Distal Central (DC) and Distal (D).