Introduction:
http://www.damninteresting.com/products-of-pollution/
The trematode Ribeiroia
ondatrae could be considered the Dr. Frankenstein of the parasitic realm as
many of its intermediate hosts suffer from monstrous malformations. Found
prominently throughout the United States
and the rest of North America, R. ondatrae infects three distinct hosts
at different points in its life cycle that can include: planorbid snails, amphibians,
and mammals or birds [1]. While the parasite does not pathogenically affect its
definitive hosts, mammals or birds, it is the source of bizarre limb mutations
in amphibians and the reason for infertility in its mollusk hosts [2,3]. While
environmental factors play a very clear role in the vitality of many
amphibians, many scientists are concerned with the even more severe declines in
amphibian populations due to infection by R.
ondatrae [3,4]. Simply put, a frog with five legs will most certainly not
win the race when it comes to avoiding predators.
Symbiont Description:
http://www.cracked.com/funny-3001-ribeiroia/
Ribeiroia ondatrae
is a trematode in the phylum Platyhelminthes [1]. Like most trematode parasites,
it cycles through six distinct morphological forms including: egg, miracidia,
rediae, cercariae, metacercariae and adult form [1]. The parasite is able to
thrive in a wide array of aquatic habitats, which is perhaps what makes its
rate of infection so high. For example, R.
ondatrae can live in ephemeral ponds, lakes, stock ponds, and even drainage
canals—anywhere that freshwater is found [3]. R. ondatrae goes through both sexual and asexual reproduction
depending on the host (i.e., intermediate or definitive). The adult parasite
ranges from 5,200 to 500 μm in size, similar to other trematodes of the same
genus [1]. Perhaps it is the parasite's morphological simplicity and small size
that makes the organism's effects so ominous.
Host Description:
R. ondatrae
requires three distinct hosts to complete its full life cycle. The first
intermediate host includes planorbid snail genera Planorbella and Biomphalaria
[1,3]. These particular snails are aquatic mollusks with a “ram’s horn” shape
[1]. The second intermediate host can include several different amphibians such as frogs, toads, and salamanders. Infection rates in some populations of
frogs, such as tree frogs, can be as high as 95% [2]. The third, and definitive
host, can include both mammals and birds—any organism that preys on the
amphibian intermediate hosts and can provide a suitable habitat with adequate
resources for the parasite’s metacercariae to develop into the sexually mature,
adult form [3].
http://www.science-art.com/image/?id=3561&search=1
Life Cycle:
The eggs of R.
ondatrae are dispatched into an aquatic environment through the feces of
the definitive host (mammal or bird). The eggs develop into aquatic miracidia
which are free-swimming, larval stages of the parasite [3]. These miracidia
utilize apical glands and penetration to infect the first intermediate hosts,
the planorbid snails, which provide a suitable environment, generally in the
renal glands or pulmonary veins, for the miracidia to develop into rediae that
undergo asexual reproduction in a sporocyst [3]. Often, these daughter rediae
accumulate at the birth pore and may strip the mollusks of their reproductive
organs [2].
Expelled rediae develop into free-swimming, larval cercariae,
which infect larvae of nearby amphibians [1]. These cercariae seek out areas of
the morphology of the amphibian larvae that will eventually form the hindlimbs.
Some scientists describe the embedding of subcutaneous cercariae, and later
metacercariae, into the host amphibian as “jamm[ing] a golf ball into the space
where the limb goes” [2]. As the host matures and undergoes metamorphosis, the
metacercariae cause the host to form an increase in the number of limbs
present, called supernumerary limbs [3].
With an increase in the number of limbs present, the frogs
are less adapted to avoid predation by birds and mammals, which are the
definitive hosts of S. ondatrae.
After consuming the amphibian hosts and thus, the parasite, the trematode is
able to mature in the digestive tract and undergo sexual reproduction as the
adult form [1]. The cycle begins again once the bird or mammal releases the
eggs of the parasite into an aquatic environment by means of its own feces.
Ecology:
Obviously, through the deformation it causes in its
amphibian hosts, S. ondatrae severely
affects the abundance and distribution of these organisms in their environment.
In the general population of amphibians, especially in frogs, there is a
certain level of “background abnormalities” which range from missing digits or
portions of limbs and constitutes approximately 0 to 5% of any given population
[3]. Abnormalities in frogs can be associated with a variety of causes such as
the presence of heavy metals or pesticides [4]. However, recent research has
indicated that the presence of S.
ondatrae is by far the best indicator of high rates of mutation in these
amphibians. Because infections by this parasite have significantly increased in
recent years due to excessive nitrogen runoff (which feeds a larger number of
intermediate snail hosts), it may serve as another environmental indicator that
something is not right [4].
Besides causing an adverse effect on the populations of
amphibians across the United
States, the infection in snails can lead to
certain human benefits. For example, infection by S. ondatrae in Biomphalaria
glabrata, a planorbid snail, can cause the host's complete elimination.
Consequently, B. glabrata is a
prominent host in the life cycle of human schistosomes causing a reduction in
that particular human parasite. Similarly, the rediae of S. ondatrae consume schistosome sporocytes of Schistosoma mansoni, one of the most widespread parasites causing
human schistosomes and thus, reducing the incidence of the disease [1].
An example of an extended phenotype:
http://www.cracked.com/funny-3001-ribeiroia/
Although it is still unsure as to how the mechanism occurs
in amphibians infected with S. ondatrae,
the presence of these parasites causes the amphibian to produce a larger number
of limbs than the genome of the host calls for. While the presence of the
trematode parasite extends the phenotype of the host amphibian, it does not do
so to the benefit of the host. As discussed earlier, the supernumerary limbs
cause the organisms to be more likely eaten by a bird or mammal due to
its inability to escape predation. However, by interfering with the genes of
the host, the parasite increases its own chances of being transferred to the
next host.
Some explanations as to why the presence of S. ondatrae causes an abnormal number of
limbs in amphibians are linked to infection during the larval stage of the host.
Since the cercariae infect the amphibians while it has not yet undergone
metamorphosis, the presence of the parasite may mechanically alter the
distribution of cells in the hindlimb region of the amphibians [2]. If this
occurs, the rapid growth and movement of cells during metamorphosis may
cause signals that increase the number of limbs that the organism should
produce.
http://www.youtube.com/watch?v=u3zbwWfoYmg
References:
[1] Johnson,
P. T. J., D. R. Sutherland, J. M. Kinsella, and K. D. Lunde. “Review of the
trematode genus Ribeiroia (Psilostomidae): ecology, life history and
pathogenesis with special emphasis on the amphibian malformation problem.” Advances in Parasitology 54 (2001):
192-253. Print.
[2] Jacobson, R. “Researchers battle parasite-driven frog
deformities.” PBS. Web. 5 Feb. 2012.
< http://www.pbs.org/newshour/rundown/2011/10/frog-deformities-linked-to-flatworm-parasite.html>.
[3] Johnson, P. T. J., K. B. Lunde, R. W. Haight, J.
Bowerman, and A. R. Blaustein. “Parasite (Ribeiroia
ondatrae) infection linked to amphibian malformations in the western US.” Ecological Monographs 72.2 (2002):
151-168. Print.
[4] Johnson, P. T. J., Lunde, K. B., Ritchie, E. G., Reaser,
J. K., and A. E. Launer. “Morphological abnormality patterns in a California amphibian
community.” Herpetologica 57 (2001):
336–352. Print.
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