Occurrence of skin alkaloids in non-dendrobatid frogs from Brazil (Bufonidae), Australia (Myobatrachidae) and Madagascar (Mantellinae)

Abstract

Several taxa of small frogs from the southern hemisphere contain alkaloids similar or identical to compounds previously known only from neotropical poison frogs of the family Dendrobatidae. Skin of the Brazilian toad Melanophryniscus moreirae (family Bufonidae) contains a new alkaloid 8-hydroxy-8-methyl-6-(5'-hydroxy-2'-methyl-hexylidene)-1-azabicycl o-[4.3.0] nonane (C16H29NO2), which is designated pumiliotoxin 267C. Such a structure is typical of the pumiliotoxin-A class of dendrobatid alkaloids. Melanophyryniscus moreirae contains smaller quantities of an alkaloid (C19H33NO3) identical in chromatographic and mass spectral properties to the dendrobatid alkaloid allopumiliotoxin 323B. Allopumiliotoxin 323B and an isomer of 267C occur with unidentified alkaloids in skin of the Australian frog Pseudophryne semimarmorata (family Myobatrachidae) and also in the skin of the Madagascan frog Mantella aurantiaca (family Ranidae, subfamily Mantellinae). In addition to new compounds, Mantella aurantiaca and M. madagascariensis also contain other alkaloids (e.g. histrionicotoxin and pumiliotoxin B) that were known previously only in dendrobatid frogs. Such alkaloids have not been detected in a phylogenetically wide array of other anuran amphibians, and the dendrobatid alkaloids thus become an evolutionary enigma. Certain of these compounds may have arisen convergently from new biosynthetic pathways in several families of frogs, or these alkaloids may represent parallel expression of shared-primitive pathways that are unexpressed or lost in related frogs.