The known diversity of dart-poison frog species has grown from 70 in the 1960s to 247 at present, with no sign that the discovery of new species will wane in the foreseeable future. Although this growth in knowledge of the diversity of this group has been accompanied by detailed investigations of many aspects of the biology of dendrobatids, their phylogenetic relationships remain poorly understood. This study was designed to test hypotheses of dendrobatid diversification by combining new and prior genotypic and phenotypic evidence in a total evidence analysis. DNA sequences were sampled for five mitochondrial and six nuclear loci (approximately 6,100 base pairs [bp]; x¯ = 3,740 bp per terminal; total dataset composed of approximately 1.55 million bp), and 174 phenotypic characters were scored from adult and larval morphology, alkaloid profiles, and behavior. These data were combined with relevant published DNA sequences. Ingroup sampling targeted several previously unsampled species, including Ar...

Phylogenetic systematics of dart-poison frogs and their relatives (amphibia: athesphatanura: dendrobatidae)

Many pufferfish of the family Tetraodontidae possess a potent neurotoxin, tetrodotoxin (TTX). In marine pufferfish species, toxicity is generally high in the liver and ovary, whereas in brackish water and freshwater species, toxicity is higher in the skin. In 1964, the toxin of the California newt was identified as TTX as well, and since then TTX has been detected in a variety of other organisms. TTX is produced primarily by marine bacteria, and pufferfish accumulate TTX via the food chain that begins with these bacteria. Consequently, pufferfish become non-toxic when they are fed TTX-free diets in an environment in which the invasion of TTX-bearing organisms is completely shut off. Although some researchers claim that the TTX of amphibians is endogenous, we believe that it also has an exogenous origin, i.e., from organisms consumed as food. TTX-bearing animals are equipped with a high tolerance to TTX, and thus retain or accumulate TTX possibly as a biologic defense substance. There have been many cases of human intoxication due to the ingestion of TTX-bearing pufferfish, mainly in Japan, China, and Taiwan, and several victims have died. Several cases of TTX intoxication due to the ingestion of small gastropods, including some lethal cases, were recently reported in China and Taiwan, revealing a serious public health issue.

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Tetrodotoxin – Distribution and Accumulation in Aquatic Organisms, and Cases of Human Intoxication

Indolizidine and quinolizidine alkaloids

Aposematism is the association, in a prey organism, of the presence of a warning signal with unprofitability to predators. The origin of aposematism is puzzling, because of its predicted low probability of establishment in a population due to the prey's increased conspicuousness. Aposematism is a widespread trait in invertebrate taxa, but, in vertebrates, it is mostly evident in amphibians, reptiles, and fishes. Poison frogs (Dendrobatidae) are one of the most well known examples of the co-occurrence of warning coloration and toxicity. This monophyletic group of mostly diurnal leaf-litter Neotropical anurans has both toxic/colorful and palatable/cryptic species. Previous studies suggested a single origin of toxicity and warning coloration, dividing the family in two discrete groups of primitively cryptic and more derived aposematic frogs. Recent molecular phylogenetic analyses using mostly aposematic taxa supported this conclusion and proposed a single tandem origin of toxicity and conspicuous warning coloration. By using expanded taxon and character sampling, we reexamined the phylogenetic correlation between the origins of toxicity and warning coloration. At least four or five independent origins of aposematism have occurred within poison frogs; by using simulations, we rejected hypotheses of one, two, or three origins of aposematism (P < 0.002). We also found that diet specialization is linked with the evolution of aposematism. Specialization on prey, such as ants and termites, may have evolved independently at least two times.

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Multiple, recurring origins of aposematism and diet specialization in poison frogs

The poison frogs (family Dendrobatidae) are terrestrial anuran amphibians displaying a wide range of coloration and toxicity. These frogs generally have been considered to be aposematic, but relatively little research has been carried out to test the predictions of this hypothesis. Here we use a comparative approach to test one prediction of the hypothesis of aposematism: that coloration will evolve in tandem with toxicity. Recently, we developed a phylogenetic hypothesis of the evolutionary relationships among representative species of poison frogs, using sequences from three regions of mitochondrial DNA. In our analysis, we use that DNAbased phylogeny and comparative analysis of independent contrasts to investigate the correlation between coloration and toxicity in the poison frog family (Dendrobatidae). Information on the toxicity of different species was obtained from the literature. Two different measures of the brightness and extent of coloration were used. (i) Twenty-four human observers were asked to rank different photos of each different species in the analysis in terms of contrast to a leaf-littered background. (ii) Color photos of each species were scanned into a computer and a computer program was used to obtain a measure of the contrast of the colors of each species relative to a leaf-littered background. Comparative analyses of the results were carried out with two different models of character evolution: gradual change, with branch lengths proportional to the amount of genetic change, and punctuational change, with all change being associated with speciation events. Comparative analysis using either method or model indicated a significant correlation between the evolution of toxicity and coloration across this family. These results are consistent with the hypothesis that coloration in this group is aposematic.

/pdf/the-evolution-of-coloration-and-toxicity-in-the-poison-frog-4q5tfwxrdk.pdf

The evolution of coloration and toxicity in the poison frog family (Dendrobatidae).

An uptake system for dietary alkaloids in poison frogs (Dendrobatidae)

Dendrobatid poison frogs readily accumulate alkaloids from diet into skin, where such compounds serve as a chemical defense against predators. Arthropods seem to be the source of decahydroquinolines (DHQs), several izidines, coccinellines, spiropyrrolizidines, pumiliotoxins (PTXs), and allopumiliotoxins (aPTXs). A DHQ iso-223F, and PTX (+)-251D were fed to poison frogs of the dendrobatid genera Dendrobates, Epipedobates, and Phyllobates. The two alkaloids were accumulated in skin unchanged except for the three species of Dendrobates, where ≈80% of accumulated PTX (+)-251D was stereoselectively hydroxylated to aPTX (+)-267A. The unnatural enantiomer PTX (-)-251D was accumulated efficiently when fed to Dendrobates auratus, but was not hydroxylated. The enantiomers of PTX 251D and their desmethyl analogs were synthesized from N-Boc-protected (-)- and (+)-proline methyl esters. Both PTX (+)-251D and aPTX (+)-267A proved to be potent convulsants in mice, with (+)-267A being ≈5-fold more toxic than (+)-251D. Both alkaloids were hyperalgesic at the site of injection. The unnatural PTX (-)-251D caused no overt effect in mice. Thus, the evolutionary development of a pumiliotoxin 7-hydroxylase would have provided frogs of the genus Dendrobates with a means of enhancing the antipredator potency of ingested PTXs.

https://www.pnas.org/content/pnas/100/19/11092.full.pdf

John F. Cover

Papers

An uptake system for dietary alkaloids in poison frogs (Dendrobatidae)

Evidence for an enantioselective pumiliotoxin 7-hydroxylase in dendrobatid poison frogs of the genus Dendrobates

Variability in alkaloid profiles in neotropical poison frogs (Dendrobatidae): genetic versus environmental determinants.

Absence of skin alkaloids in captive-raised Madagascan mantelline frogs (Mantella) and sequestration of dietary alkaloids.

Absence of tetrodotoxins in a captive-raised riparian frog, Atelopus varius