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Eleutherodactylus iberia |
In 1998, John W.
Daly at the National Institutes of Health noted that amphibian skin contains a
wide range of biologically active molecules and that in three decades (since
1968) more than 400 alkaloids in more than 20 structural classes have been
detected. But perhaps most surprising, the alkaloids in amphibian skins were
not made by the amphibians' cells, instead they were molecules hijacked from
the arthropods the amphibians ate. Many of these molecules are used as defense
against micro-organisms and predators. Other amphibian skin molecules are biosynthesized
by the animal's own cells. But the alkaloids particularly those that are lipid
soluble are made in the arthropod prey or possibly by symbiotic micro-organisms
and stored in the amphibian's skin glands. Twelve years after Daly's paper more
than 800 molecules have been identified.
But not all
amphibians can take molecules from their prey and use them for their own defense.
Perhaps the Neotropical poison-dart frogs (Dendrobatidae) are best known for
this ability. Poison dart frogs kept in captivity and feed a diet a fruit flies
are no longer poisonous. But there are other anurans that do this: the South
American Red bellied Toads (genus Melanophryniscus,
family Bufonidae); one genus of Madagascan Golden Frogs (Mantella, family Mantellidae); and one genus of Australian Toadlets
(Pseudophryne, family
Myobatrachidae). Minor quantities of alkaloids have also been found in the
Asian genus Limnonectes (family
Dicroglossidae) but it is unknown if they are also getting the alkaloid
molecules from their food. Thus, the frogs that do sequester alkaloids from
their prey are un-related, but they all tend to have aposematic coloration that
signals predators that they are toxic.
Now, Ariel
Rodriguez and colleagues (2010) report a 5th lineage of frogs that obtain
defensive molecules from their prey, the minute Cuban, Eleutherodactylus iberia and Eleutherodactylus
orientalis, (family Eleutherodactylidae). During recent fieldwork, the odor
of dissected specimens reminded the authors of the alkaloid-containing dendrobatid
and mantellid species. Further investigation provided conclusive evidence that these
aposematic frogs contain lipid soluble alkaloids of the same compound classes
previously reported in other alkaloid-containing frog lineages. The authors found
six pumiliotoxins and two indolizidines in E.
iberia but cannot confirm that alkaloid sequestering is characteristic for
all populations of E. Iberia. And they are unsure whether it
regularly occurs in E. orientalis and other related species of the E.
limbatus group without extensive additional effort. However, the discovery
that at least some of these frogs sequester lipid-soluble alkaloids may
contribute to the understanding of the evolutionary pathway to
alkaloid-sequestration and aposematism in amphibians. Examination of stomach
contents indicated an abundance of oribatid mites, a group of arthropods known
to contain one of the pumiliotoxins detected in E. iberia. This suggests that miniaturization and specialization to
small prey may have favored the acquisition of dietary skin alkaloids in these
amphibians.
Of interest is the discovery that oribatid mites are a common prey for
miniaturized frogs including the miniature Eleutherodactylus examined
in this study. The two alkaloids of E. iberia have previously been
detected in arthropods (mites and ants).
Additional evidence for the importance of miniaturization came from the
phylogenetic position of the poorly known Wakea madinika, a dwarf frog
from Madagascar, as the sister group of the alkaloid-containing genus Mantella,
which might indicate that the Mantella ancestor was also miniaturized.
The miniature Cuban Eleutherodactylus are largely diurnal, and
aposematic in coloration. Other species in the same clade have a much less
contrasted coloration and this group of frogs may be useful in understanding how diet, miniaturization, the ability to
sequester alkaloids, and changing daily activity patterns evolve together.
Literature
Daly, J. W.
1998. Thirty Years of Discovering Arthropod Alkaloids in Amphibian Skin. Journal of Natural Products 61:162-172.
RodrÃguez, A.,
Poth, D., S. Schulz, and M. Vences. 2010. Discovery of skin alkaloids in a
miniaturized eleutherodactylid frog from Cuba. Biology Letters doi: 10.1098/rsbl.2010.0844