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Dugesia tigrina
Kingdom: Animalia
Phylum: Platyhelminthes
Class: Turbellaria
Order: Tricladida
Family: Planariidae
Genus: Dugesia
Species: tigrina
Description: This species is a model example of a flatworm, phylum Platyhelminthes. It has two eye spots on its anterior end that detect light levels. The coloration of these animals is most often brown with white or yellow speckles. They can reproduce sexually, with individuals being hermaphroditic. Reproduction can also take place asexually, by fission. If this flatworm is cut in two, the body sections will regrow, and you will end up with two complete flatworms. This interesting trait has made them subjects for many different studies and experiments, ranging from ecological, to genetic and behavioral. Dugesia tigrina often swims through freshwater pond, lakes, creeks, and rivers. They can also glide across mucus they produce. Common to most members of Turbellaria, they have motile cilia, which can also be used for movement. These flatworms are carnivores that will eat what they can catch and have been known to be cannibalistic.
Distribution:
These flatworms are found in freshwater all over the world. One study done by Dr. Young mapped out exact location of where Dugesia tigrina could be found in England, as shown above. They are only present in the warmer southern half (c). They are also more common in moving water versus stagnant( a v. b). (Young).
Many scientific papers have been published about this species. Four are discussed below. The first analyzes its asexual reproductive strategy. The second uses Dugesia tigrina to examine the Pax-6 gene, responsible for nervous system development in animals. The next study conducted is a behavioral conditioning experiments using them as subject. The fourth and final paper observes the population dynamics associated with Dugesia tigrina and found some interesting trends relating to cannibalism.
1. Dugesia tigrina, as well as other flatworms, are known to be able to reproduce asexually by fission. An article in the Journal of Statically Physics examined three different species of flatworms, including Dugesia tigrina, and found through statistical analysis that each species had different and district strategies for optimizing their reproductive success. All three species reproduce asexually by spontaneously splitting themselves perpendicular to the anterior-posterior body axis to create two daughter worms, one originating from the head piece and one from the tail piece, both from a single worm. The head piece daughter contains the eyes, brain, and eating tube, whereas the tail piece daughter has to regenerate these structures from scratch. Dugesia tigrina showed to have higher head daughter investment at expense of the tail daughter, meaning the daughter from the head piece was much larger than the one formed from the tail piece. This may lead to increased survival rates for the head piece daughters and decreased survival rates for the tail piece individuals. Dugesia tigrina was never observed to have undergone reproductive events that produced more than one middle piece. This species also proved to have a reproductive waiting time of 10 days (which is rather short comparatively). This species also showed to have relatively low rates of cannibalism in this study compared to the other two species of flatworms observed. (Carter)
The figure below (d) demonstrates the asexual fission style reproduction a flatworm can undergo. It also shows other possible divisions that can be undergone by the different daughter pieces. Figure (e) illustrates the differences of the reproductive waiting times of the three species being examined. Dugesia tigrina is denoted as G and is represented by the color black. As you can see, it has a considerably shorting waiting period than the other two.
2. The Pax-6 genes are responsible for eye and central nervous system development in many animals. Other studies done on Drosophila suggest that the Pax-6 gene may be the universal control gene for eye morphogenesis. This study, presented in the Proceedings of the National Academy of Sciences, on Dugesia tigrina, found that this gene shares considerable sequence identity and conserved genomically with Pax-6 proteins from other animal phyla. Isolation of Pax-gene was done by PCR (polymerase chain reaction. Analysis shows that Dugesia tigrina’s Pax-6 gene (DtPax-6) is clustered with the other Pax-6 genes, but in the most basal position. This gene is expressed in a single transcript, but is still a bona fide Pax-gene. Several specific residues and introns were conserved, supporting the findings. Pax-genes have not been found in diploblastic animal, suggesting it is only present in triploblastic organisms. This study ends in a speculation that additional Pax genes have a correlation to more complex body plans. Observation of DtPax-6 being expressed in pigment cells and in photoreceptors provides further evidence for an early and basic function of Pax-6 genes in eye evolution. However, Pax-6 genes are not expressed in the photoreceptor cells of higher animals. This observation suggest as to how increasingly complex eyes may have been constructed during evolution. The insertion of other genes, including regulatory genes, in the developmental cascade can lead to alterations in assembly and purpose of the eye with changes beneficial to the organism being preserved and detrimental changes being selected against. (Callaerts).
Figure A shows an example of the PCR (polymerase chain reaction) used in this study to amplify a single piece of Dugesia tigrina's DNA, in this case, the Pax-6 gene. B exhibits how the specimens were prepared to be analyzed.
3. Dugesia tigrina has also been used for basic behavioral experiments, in this case, a study done by Dr. Prados, three conditioning experiments. Experiment 1 illustrated that this species was inclined to basic conditioning in that they readily developed a conditioned response to the alteration in ambient light when it was paired with an electric shock over many trials. In the second experiment, Experiment 2, the change in luminescence was presented in combination with a vibration during conditioning. Later tests showed poor conditioning of these elements compared with control groups in which the subjects were conditioned in the presence of the elements alone. In Experiment 3, pre-training for one of the elements before compound conditioning resulted in the blocking of learning about the other element entirely. In both Experiments 2 and 3, simply coupling the target stimulus with the unconditioned stimuli was not suffice to establish this element as an effective conditional stimulus. These results suggest that animals belonging to different phyla could still be ruled by similar conditioning stimuli. (Prados).
Figure 1, to the left, demonstrates the experimental set up used for this study. This apparatus enables the researchers to control every conditioning aspect. This control was key in obtaining untainted and accurate results.
4. Studies have been done not only on Dugesia tigrina genetics and reproduction, but also on their population dynamics, as in Joseph T. Armstrong, "The Population Dynamics of the Planarian, Dugesia Tigrina.” Five populations of 35 worms each (one control, one known to be cannibalistic, and three experimental) were placed in finger bowls. Reproduction was artificially increased by adding tails. They found several interesting results. There were no natural deaths of adults, implying cannibalism is used to control populations. The known cannibalism worms were found to have the highest rate of tail cannibalism. It seems smaller tails ate more likely to be eaten. This study verifies the idea that cannibalism increases with hunger. There are two contrary suggestions this experiment yields concerning the affect cannibalism has in growing D. tigrina populations. First, as stated before, hunger increases cannibalism. Second, it may become less intense due to the declining size of feeding individuals. The cannibalistic population showed to have lower reproductive rates. In conclusion, cannibalism was an important source of mortality among newborn worms in the present D. tigrina populations studied in a lab environment. (Armstrong).
Table 1. shows the final values obtained in this study. 1-0' represents the known cannibalistic control group, while 1-0 is the control group. No tails were added to either of these groups. The other three groups were the test subjects, with different number of tails added. The final population increased in correlation with an increased number of introduced tails. It is also interesting to note that the rate of cannibalism was inversely related to the number of tails added, decreasing as the tail number increased.
Bibliography
"Animal Diversity Web." ADW: Dugesia Tigrina: CLASSIFICATION. Web. 22 Nov. 2015.
Armstrong, Joseph T. "The Population Dynamics of the Planarian, Dugesia Tigrina." Ecology 45.2 (1964): 361. EBSCOhost. Web. 21 Nov. 2015.
Callaerts, P., A. M. Munoz-Marmol, S. Glardon, E. Castillo, H. Sun, W.-H. Li, W. J. Gehring, and E. Salo. "Isolation and Expression of a Pax-6 Gene in the Regenerating and Intact Planarian Dugesia(G)tigrina." Proceedings of the National Academy of Sciences 96.2 (1999): 558-63. EBSCOhost. Web. 21 Nov. 2015.
Carter, Jason, Christine Lind, M. Truong, and Eva-Marie Collins. "To Each His Own." Journal of Statistical Physics 161.1 (2015): 250- 72. EBSCOhost. Web. 21 Nov. 2015.
"Critter Catalog." BioKIDS. Web. 26 Nov. 2015.
"Dugesia Tigrina." Animal Diversity Web. Web. 30 Nov. 2015.
“Platyhelminthes." Reproduction. Web. 28 Nov. 2015.
Prados, Jose, Beatriz Alvarez, Joanna Howarth, Katharine Stewart, Claire L. Gibson, Claire V. Hutchinson, Andrew M. J. Young, and Colin Davidson. "Cue Competition Effects in the Planarian." Animal Cognition Anim Cogn 16.2 (2012): 177-86. ESCOhost. Web. 26 Nov. 2015.
Young, J.O., and T.B. Reynoldson. "Continuing Dispersal Of Freshwater Triclads (Platyhelminthes; Turbellaria) In Britain With Particular Reference To Lakes."Freshwater Biology 42.2 (1999): 247. Academic Search Premier. Web. 1 Dec. 2015.
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