Dramatic dietary shift maintains sequestered toxins in chemically defended snakes

Abstract

Unlike other snakes, most species ofRhabdophispossess glands in their dorsal skin, sometimes limited to the neck, known as nucho-dorsal and nuchal glands, respectively. Those glands contain powerful cardiotonic steroids known as bufadienolides, which can be deployed as a defense against predators. Bufadienolides otherwise occur only in toads (Bufonidae) and some fireflies (Lampyrinae), which are known or believed to synthesize the toxins. The ancestral diet ofRhabdophisconsists of anuran amphibians, and we have shown previously that the bufadienolide toxins of frog-eating species are sequestered from toads consumed as prey. However, one derived clade, theRhabdophis nuchalisGroup, has shifted its primary diet from frogs to earthworms. Here we confirm that the worm-eating snakes possess bufadienolides in their nucho-dorsal glands, although the worms themselves lack such toxins. In addition, we show that the bufadienolides ofR. nuchalisGroup species are obtained primarily from fireflies. Although few snakes feed on insects, we document through feeding experiments, chemosensory preference tests, and gut contents that lampyrine firefly larvae are regularly consumed by these snakes. Furthermore, members of theR. nuchalisGroup contain compounds that resemble the distinctive bufadienolides of fireflies, but not those of toads, in stereochemistry, glycosylation, acetylation, and molecular weight. Thus, the evolutionary shift in primary prey among members of theR. nuchalisGroup has been accompanied by a dramatic shift in the source of the species’ sequestered defensive toxins.

The Nucho-dorsal Glands of Rhabdophis guangdongensis (Squamata: Colubridae: Natricinae), with Notes on Morphological Variation and Phylogeny Based on Additional Specimens

“…lateralis (Takeuchi et al, 2011(Takeuchi et al, , 2014(Takeuchi et al, , 2018. Finally, we agree with Takeuchi et al (2018) in recovering a derived clade that includes R. nuchalis, R. pentasupralabialis, and R. leonardi, known as the R. nuchalis group (Yoshida et al, 2020). Furthermore, both Takeuchi et al (2018) and we find that R. guangdongensis and R. swinhonis are more closely related to the R. nuchalis group than to other Rhabdophis.…”

Insect Collections as an Untapped Source of Bioactive Compounds—Fireflies (Coleoptera: Lampyridae) and Cardiotonic Steroids as a Proof of Concept

“…Red rectangles represent beetle specimens that tested positively for lucibufagins, blue rectangles represent beetle individuals that were devoid of these compounds. Further occurrences of bufadienolides are known from Photuris versicolor (Photurinae) [14], Diaphanes lampyroides [17], Ellychnia corrusca [16], Lucidota atra [15], Photinus ignitus, P. marginellus [9,12], and P. pyralis (Lampyrinae) [11,12]. Absence of lucibufagins was further demonstrated for Aquatica lateralis (subfamily Luciolinae) and the click beetle Ingnelater luminosus (Elateridae) [24].…”

“…Lucibufagins in fireflies were first described by Eisner et al from the north North-American species Photinus marginellus (LeConte 1852) [9]. In addition, these compounds were found in Photinus ignitus (Fall 1927) [10], Photinus pyralis (Linnaeus 1767) [11,12], Photuris versicolor (Fabricius 1798) [13,14], Lucidota atra (Olivier 1790) [15], and recently Ellychnia corrusca (Linnaeus 1767) [16] and Diaphanes lampyroides (Olivier 1891) [17]. While almost all records of lucibufagins in fireflies were made for North-American species, Tyler et al were the first who reported the occurrence of these compounds in the Eurasian species Lampyris noctiluca (Linnaeus 1767) [18].…”

“…While bufadienolides are known to occur in six plant families (Asparagaceae, Crassulaceae, Iridaceae, Melianthaceae, Ranunculaceae, and Santalaceae), their occurrence in animals is rare [22]. Besides fireflies, bufadienolides are only found in toads (Bufonidae) and snakes of the genus Rhabdophis (Colubridae) that sequester bufadienolides from toads [23] and firefly larvae [17]. Moreover, the milkweed bug Spilostethus pandurus (Heteroptera: Lygaeinae) sequesters bufadienolides from squill (Urginea maritima, Asparagaceae) [7].…”

Venom Ophthalmia and Ocular Complications Caused by Snake Venom

“…In addition to having Duvernoy’s gland to produce venom, some snakes also have another organ, the nuchal gland, to store venomous fluid ( Figure 2 B). The nuchal gland, also named nucho-dorsal gland or cervical gland, is a special defensive system in some species of snakes such as the Rhabdophis genus [ 19 , 20 ]. These snakes consume a poisonous prey and conserve the poison in their nuchal glands to spray the stored poison for defense.…”