Background for Dynamic Nature of Scientific Knowledge

Transcription

1 Background for Dynamic Nature of Scientific Knowledge General lesson information: The lesson will take a minimum of two and a half weeks to conduct: three to five days for introduction and proposal development; five to eight days for the run of the experiment; and three days for analyzing data, presentation of results and conclusions, and class discussion of results and conclusions. During the run of the experiment, difficult and confusing concepts can be revisited. A longer lesson length (~3-4 weeks) is recommended for the lower academic levels to ensure reasonably complete comprehension of the history, the hypotheses, and proper experimental procedures. Detail level, discussion depth, and direction of extensions can be adjusted to suit the academic level, needs, and interests of the students. A lot of information is presented here for the sake of being as complete as possible. It is strongly encouraged that guiding questions are asked to help students with their designs rather then telling them what to do. This is the basis of (guided) inquiry. Suggestions for the best way to take measurements, when to take measurements, use of controls, number of replicates, etc. are important; however, let the students make some mistakes so they can be discussed later when results and conclusions are presented. This is particularly useful when two or more groups want to perform the same test, but have different procedures. General Daphnia Information: Daphnia are freshwater animals (though they can handle some saltiness) that live worldwide in lakes and ponds. The common name is water flea; named for their jerky movement through water, not because they are closely related to fleas. Daphnia are kidney-like shaped and have a single eye, two branched antennae, and 4 pairs of thoracic appendages that work to clean the body and aid in movement. Their bodies are usually transparent with occasional tinting depending on their diet. Body sizes vary depending on species but usually range from 0.5 mm to approximately 6 mm. Daphnia eat free-living algae, bacteria, and fungi and are consumed by many fish, invertebrates, and bladderworts (an aquatic carnivorous plant). Their lifespan from egg to adulthood varies between species and environmental conditions but is frequently six to eight weeks. Once Daphnia hatch, it takes roughly six to ten days for them to reach maturity. Females can carry around four to twenty-two young at a time. During non-stressful times (plenty of oxygen, space, and food), populations of Daphnia consist entirely of females that reproduce parthenogenically (asexual reproduction via cloning). The young are hatched inside the body and released live. During stressful times, the females will produce eggs that will settle to the bottom of the lake or pond and that can survive harsh environmental conditions. Some of the eggs produced are male, which hatch and allow sexual reproduction (which increases genetic variation and, therefore, the probability of survival in a changing environment). Once conditions improve, the males die out and the females return to asexual reproduction. Branched antennae Eye Helmet Daphnia pulex Tail Spine Dynamic Nature of Scientific Knowledge 1

2 Proportions: It is assumed that the students have already learned proportions in math class. Thus, this lesson can help reinforce how and why scientists use them. Proportions are used to help analyze the data collected on the Daphnia. Directly measured values (e.g. tail spine length and helmet length) will not take into account the difference in sizes between individual Daphnia. For this reason, it is important that the students measure both the entire length of the body of the Daphnia and the length of the tail spine and/or helmet (depending on what they want to study). For example, if one wanted to compare leg length among humans, one would have to make a proportion from total body height of each human to the length of their legs. This will produce a value that controls for differences in height of humans (such as comparing leg length of a five-foot tall individual to one who is six feet tall). Proportion by tail spine length = TSL/BL HL=helmet length Proportion by helmet length = HL/BL BL=body length TSL=tail spine length Genetics, The Environment, and Their Interactions: Students need to have already had basic lessons in Mendelian genetics, heredity, environmental effects on traits, gene-environment interaction, and phenotypic plasticity. This lesson does not directly use these concepts; however, the students will find it easier to understand what they are studying in the lab if they have a basic understanding of these concepts and how they relate to Daphnia. The latter two concepts (geneenvironment interactions and phenotypic plasticity) may be skipped if the concepts are too advanced for the students to understand at the given time. Genetics/heredity: Basic knowledge of Mendelian genetics and heredity is sufficient. The students should notice that the Daphnia offspring look like their parent(s) when they are born. Environment: An understanding that the environment can affect how organisms look is important. A simple example would be the amount of nutrients available. Humans have grown larger and taller with more readily and consistently available food. However, depending on how complicated one would like to make the issue and to separate this from gene-environment interactions, better examples would include: dyed hair; missing limbs (from being torn or bitten off), pierced ears, pruned plants, and missing teeth. Gene-environment interactions: The phenotype (morphology, behavior, phenology, and physiology) of all organisms is a result of the environment interacting with one s genes. Neither produces a trait without the effect of the other. For example, the coat color of Siamese cats is a dramatic example of gene-environment interactions. Siamese cats usually have a light-colored torso and darker-colored paws, ears, tail, and muzzle. A mutation in one of the genes that produces pigment in Siamese cats only works well at cooler-than-normal body temperature; consequently, since the skin over the paws, ears, tail, and muzzle is usually several degrees cooler than the torso, those areas are usually darker. Dynamic Nature of Scientific Knowledge 2

3 Phenotypic plasticity: Phenotypic plasticity is the ability of a single genotype to express different phenotypes depending on varying environmental conditions. It is a type of geneenvironment interaction; except that it is the same genotype with which the environment is interacting. Adaptive phenotypic plasticity is of particular interest to researchers since this involves responses that increase fitness (reproductive output of viable offspring). An example of this is honey mesquite. It will develop as a tree if plenty of water is available and as a shrub when water is not as readily available. In this lab, the environmental condition that is interacting with the Daphnia genotype is the presence or absence of a chemical cue and the chemical cue source. Daphnia history regarding physical morphological changes: The following information regarding the study of Daphnia and their physical response to predators is as in-depth as it was thought necessary. It does not include all the information available, but enough to give a basic background. The information focuses on morphological changes (tail spine and helmet enlargement) rather than behavioral (e.g. vertical movement) and life history changes (e.g. age of reproduction and overall size). What is told to the students does not have to be as detailed as what is presented here. This lesson was designed to be used in classrooms ranging from grade 7 to college and it is left to the discretion of the instructor to determine how much detail is appropriate for the students being taught. The Summary of Daphnia Studies and Hypotheses student handout may be modified to include more or less detail. Also, the names of species are given in the descriptions, but they do not have to be used. Students can use this as an opportunity to determine if the species they have access to responds, or it can be ignored for simplicity. References are listed at the end of the lesson if more information is desired. When Daphnia species were being studied in the 1800s, it was thought that different species lived in the lakes and ponds at different times of the year because they looked different as the year progressed--during certain times of the year, the Daphnia would have large helmets and large tail spines. Near the turn of the century, it was discovered that it was the same species, but with different physical appearances. For example, in 1912 two physically different looking D. lumholtzi were discovered to be the same species (the ones with large helmets and large tail spines were found to be the same species as those with much smaller ones). The same species could develop long tail spines and an enlarged helmet. This led researchers to ask: What causes the variation in physical appearance during the course of a year in Daphnia species? In 1900, Wesenberg-lund published an article suggesting his floating theory, which explained the development of enlarged tail spines and helmets as a method to keep afloat in warm summer waters. Other researchers did not agree and proposed different hypotheses. Alternative hypotheses proposed to explain the morphological changes in Daphnia included temperature, food, light, and water turbulence. Laboratory experiments have shown Daphnia responding to these but they did not explain the changes coinciding with the seasons in lakes and ponds. In the 1970s, a scientist observed that the physical characteristics of some Daphnia species varied in response to predators. If predators are the cause of the morphological changes seen in Daphnia, then why do the Daphnia look different as the year progresses? Studies with D. galeata demonstrated that tail spine length changed with the seasons. In addition, the timing of increases in tail spine length corresponded with the change in diet of a predatory fish. That is, the fish changed their diet during the course of a year. Therefore, it was hypothesized that the enlarged tail spine and enlarged helmet of Daphnia species were a result of adaptive phenotypic plasticity since they were a defense against predators. Moreover, predators of Daphnia tend to select prey based on size (some prefer really small prey and others prefer the larger-sized Daphnia) and they have various strategies for obtaining their prey (some are visual and others are Dynamic Nature of Scientific Knowledge 3

4 tactile). Because of these differences, different predators represent different selective forces on the how Daphnia respond to protect themselves (physical, behavioral, and life history changes). A new question resulted from this: How do the Daphnia sense predators? Chemical cues were hypothesized to be the explanation. It was known at this time (1960s and 1970s) that another aquatic species, rotifers, responded to their predators chemical cues with spines that help protect them from being eaten. Chemical cues that prey can sense and that originate from their predators were called kairomones. In addition, studies from the 1960s demonstrated that some organisms responded to chemicals released by injured or killed individuals of the same species. These chemical cues, originating from the prey, were called pheromones. To test the hypothesis that Daphnia respond to pheromones (rather than the predators themselves), experiments were performed in which live Daphnia were exposed to crushed Daphnia of the same species. The Daphnia did not respond and this hypothesis was rejected and not really pursued by other researchers. Most researchers focused on the alternative hypothesis at this point the kairomone hypothesis. However, it wasn t until the 1980s that empirical evidence for predator chemical cues began appearing. Experiments with D. carinata and D. pulex demonstrated the development of enlarged helmets and neckteeth (carapace spines), respectively, when exposed to chemical substances released by their respective predators. Further research throughout the 1980s and 1990s conducted with Daphnia species exposed to predators gave further support to the kairomone hypothesis. During the late 1980s and early 1990s, another researcher revisited the pheromone hypothesis but with modifications. It was called the predator-labeling hypothesis. It was shown that other aquatic animals, such as sea anemones, fish, amphibians, insect nymphs, marine snails, and sea urchins, demonstrated this phenomenon. This hypothesis stated that chemicals released from injured or killed prey stayed in the predator and acted as a warning cue to other individuals of the same species. The chemical cue was released with the predator s waste matter. In these cases, the predator was not the source of the chemical cue (therefore, not a kairomone); rather, it was the consumed prey that was the source of the chemical cue (thus, a pheromone). It was suggested that this might be occurring with Daphnia species. This researcher performed several experiments to test this hypothesis against the kairomone hypothesis to see the behavioral responses of D. galeata. One part involved feeding worms to fish for a week, then placing those fish in the presence of D. galeata. The same fish were then fed D. galeata to see how the Daphnia responded. The reverse was also performed (fed Daphnia, then worms). The data showed that D. galeata responded to fish that were fed worms and fish that were fed Daphnia; therefore, the predatorlabeling hypothesis was unsupported and the kairomone hypothesis supported. The predatorlabeling hypothesis was not tested further at this time with Daphnia. For approximately the last decade, experimental evidence has been interpreted to support the kairomone hypothesis, which causes not only physical changes, but also behavioral changes (reduced feeding and vertical movement in the water) and life history changes (timing of reproduction). In addition, the studies also suggest that the responses of Daphnia are predator specific (not the exact same response to all the different predators of Daphnia). In 2003, a few researchers revisited the predator-labeling hypothesis. They decided that this was a reasonable alternative hypothesis because a chemical cue originating from the predator could be unreliable since predation intensity varies throughout the year. (Although, one could say that the ability to express phenotypic plasticity may allow for this unreliable predation intensity.) Two additional reasons they used to argue against the kairomone hypothesis were that sensing a fish or invertebrate (the predators of Daphnia) does not reveal any information about the fish or Dynamic Nature of Scientific Knowledge 4

5 invertebrate diet. Finally, they argued, natural selection theory would predict that kairomones would be selected against and, therefore, their presence would likely decrease or disappear entirely. They also suggested design problems with previous experiments may have caused researchers to draw incorrect conclusions regarding the source of chemical cues (supported, they say, by the conflicting results seen in experiments). One design problem suggested was that almost all previous studies used predators that were fed Daphnia (the specific prey species used in the study). Moreover, the previous study that tested the predator-labeling hypothesis fed the predacious fish worms for only a week. One week may not have been a sufficiently long enough time period to remove any labels left by previously consumed Daphnia. To account for the reported predator specific responses, these three researchers suggested the following modification to the predator-labeling hypothesis: the labeling involves the chemical cue originating in Daphnia being activated by the digestive enzymes or bacteria in the digestive tract of the predator. See the Summary of Daphnia Studies and Hypotheses student handout for details on their experiment. Sources: Agrawal, A. (2001) Phenotypic plasticity in the interactions and evolution of species. Science. 294: Black, A. R. (1993) Predator-induced phenotypic plasticity in Daphnia pulex life-history and morphological responses to notonecta and chaoborus. Limnology and Oceanography. 38(5): Jacobs, J. (1967) Temperature, food, and turbulence as natural determinants of cyclomorphosis in Daphnia. Naturwissenschaften. 54(8): 207 Lass, S. & Spaak, P. (2003) Chemically induced anti-predator defences in plankton: a review. Hydrobiologia. 491: Stabell, O. B., Ogbebo, F. & Primicerio, R. (2003) Inducible defences in Daphnia depend on latent alarm signals from conspecific prey activated in predators. Chemical Senses. 28: Stirling, G. (1995) Daphnia behavior as a bioassay of fish presence or predation. Functional Ecology. 9: Yurista, P. M. (2000) Cyclomorphosis in Daphnia lumholtzi induced by temperature. Freshwater Biology. 43(2): Dynamic Nature of Scientific Knowledge 5