Resistance Is Not Futile: Widespread Convergent Evolution of Resistance to Alpha-Neurotoxic Snake Venoms in Caecilians (Amphibia: Gymnophiona)

Abstract

Predatory innovations impose reciprocal selection pressures upon prey. The evolution of snake venom alpha-neurotoxins has triggered the corresponding evolution of resistance in the post-synaptic nicotinic acetylcholine receptors of prey in a complex chemical arms race. All other things being equal, animals like caecilians (an Order of legless amphibians) are quite vulnerable to predation by fossorial elapid snakes and their powerful alpha-neurotoxic venoms; thus, they are under strong selective pressure. Here, we sequenced the nicotinic acetylcholine receptor alpha-1 subunit of 37 caecilian species, representing all currently known families of caecilians from across the Americas, Africa, and Asia, including species endemic to the Seychelles. Three types of resistance were identified: (1) steric hindrance from N-glycosylated asparagines; (2) secondary structural changes due to the replacement of proline by another amino acid; and (3) electrostatic charge repulsion of the positively charged neurotoxins, through the introduction of a positively charged amino acid into the toxin-binding site. We demonstrated that resistance to alpha-neurotoxins convergently evolved at least fifteen times across the caecilian tree (three times in Africa, seven times in the Americas, and five times in Asia). Additionally, as several species were shown to possess multiple resistance modifications acting synergistically, caecilians must have undergone at least 20 separate events involving the origin of toxin resistance. On the other hand, resistance in non-caecilian amphibians was found to be limited to five origins. Together, the mutations underlying resistance in caecilians constitute a robust signature of positive selection which strongly correlates with elapid presence through both space (sympatry with caecilian-eating elapids) and time (Cenozoic radiation of elapids). Our study demonstrates the extent of convergent evolution that can be expected when a single widespread predatory adaptation triggers parallel evolutionary arms races at a global scale.

High-Voltage Toxin’Roll: Electrostatic Charge Repulsion as a Dynamic Venom Resistance Trait in Pythonid Snakes

“…This trait was later identified as a basal trait of the mongoose/meerkat family (Herpestidae), which includes several snake-consuming species [ 37 , 38 ]. This resistance method has been found to have evolved convergently across various taxa that interact with venomous snakes, including within snakes themselves, providing self-immunity against their own venom [ 37 , 38 , 39 ]. This mutation replaces an amino acid with an asparagine (N) residue (frequently at orthosteric site positions 187 or 189), which is subsequently modified post-translationally to attach a bulky glycosylation side chain.…”

“…While acetylcholine, the small endogenous neurotransmitter, continues to bind to the orthosteric site, triggering muscle contractions, there appears to be a slight fitness disadvantage, suggested by a secondary loss of resistance in lineages that radiate outside the range of α-neurotoxic predators and are therefore no longer subjected to that predatory selection pressure [ 38 ]. Another steric hindrance strategy includes the replacement of proline (P) [ 20 ], especially at positions 194 and 197 [ 26 , 38 , 39 ]. Proline’s unique conformational properties, which induce a tight turn in the peptide chain, are critical for maintaining protein structure.…”

“…Electrostatic charge repulsion presents an alternate mechanism for countering snake venom, and, like steric hindrance, it has independently evolved in diverse groups, including both the prey (Burmese Python ( Python bivittatus )) and predators (such as the Honey Badger ( Mellivora capensis )) of venomous snakes [ 26 , 28 , 37 , 39 , 40 ]. The Honey Badger’s iconic resistance to cobra venom is linked to a singular mutation at the nAChR’s orthosteric site: tryptophan (W) at position 187 replaced by the positively charged arginine (R) [ 26 , 40 ].…”

“…The Honey Badger’s iconic resistance to cobra venom is linked to a singular mutation at the nAChR’s orthosteric site: tryptophan (W) at position 187 replaced by the positively charged arginine (R) [ 26 , 40 ]. The prey species Burmese Python has evolved resistance through the replacement of negatively charged amino acid (aspartic acid (D) or glutamic acid (E)) at positions 191 or 195 in the orthosteric site with a positively charged lysine (K) [ 26 , 28 , 39 ]. Introducing positive charges on the key receptor sites leads to electrostatic repulsion [ 26 ] due to the high concentration of positive charges on the surface of the neurotoxic peptides, which themselves evolved, in response, to facilitate the binding to the negatively charged orthosteric site residues [ 6 ].…”

A Russian Doll of Resistance: Nested Gains and Losses of Venom Immunity in Varanid Lizards

“…Steric hindrance was first described in the Egyptian Mongoose ( Herpestes ichneumon ), a predator of cobras [ 4 ], and subsequently shown to be a basal trait in the snake-eating family Herpestidae [ 25 , 26 ]. This form of resistance has been shown to have convergently evolved widely in both prey and predators of neurotoxic venomous snakes, including within the snakes themselves as a resistance against their own venoms [ 19 , 25 , 27 ]. This form of resistance evolves by mutations, typically at orthosteric positions 187 or 189, that confer a structural alteration at the orthosteric site.…”