Chemical evidence for dietary toxin sequestration in the Asian snake Rhabdophis tigrinus.

摘要

Rhabdophis tigrinus (Colubridae: Natricinae) is an oviparous, bufophagous (toad-eating) snake from eastern Asia that possesses defensive integumentary glands on the neck known as nuchal glands. These glands are used in antipredator displays and typically contain bufadienolide toxins. Whereas toads are known to synthesize bufadienolide steroids from cholesterol precursors, we found that chemically undefended R. tigrinus must sequester bufadienolides from ingested toads in order to exhibit these compounds in their nuchal glands. Chemically defended females are capable of provisioning their embryos with these toxins so their unfed hatchlings possess defensive bufadienolides prior to consuming toads themselves. All of the hatchling R. tigrinus from an island with a dense population of toads were chemically defended regardless of their diet in the laboratory. In contrast, none of the hatchlings from an island lacking toads possessed bufadienolides in their nuchal glands until after they consumed bufonid prey. Proton NMR and HPLC analyses demonstrated that newly acquired bufadienolides from ingested toads can be transferred from the dam to the embryos as late as 12 days prior to oviposition, suggesting that at least some transport of toxins occurs within the oviduct. Although hatchlings are provisioned with most of the bufadienolide compounds possessed by the dam, there may be chemical selectivity to this process. In a feeding experiment with gravid R. tigrinus from various localities, most clutches had been provisioned with moderate to large quantities of bufadienolides. In the provisioned clutches, the most abundant bufadienolide in the unfed hatchlings was almost always the same compound, although it generally was not the most abundant bufadienolide in the dam. R. tigrinus is the first amniote vertebrate known to have evolved specialized defensive structures dependent upon sequestered dietary compounds, either obtained directly from prey or provisioned by the dam.