Browsing by Subject "Department of Ecology, Evolution and Behavior"

Now showing 1 - 9 of 9
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    Adaptive Evolution of a Blood-Clotting Gene in Venom-Resistant Opossums
    (2010-04-21) Sosa, Tim
    Action of snake venomHemolytic snake venoms—such as those found in rattlesnakes, moccasins, and lanceheads— are complex cocktails of proteases, phospholipases, and phospho-diesterases. One protein found in the venom of lancehead vipers (genus Bothrops) is botrocetin, which causes aggregation of blood platelets wherever von Willebrand Factor (vWF)and Factor VIII are present in the bloodstream (1).The blood plasma protein vWF and Factor VIII circulate freely in the blood vessel lumen during ordinary (laminar) flow. During normal blood clotting, turbulent blood flow—such as when a blood vessel ruptures—induces vWF to disengage from Factor VIII and complex with glycoprotein Ibαand collagen. This complex, by a series of reactions, aggregates platelets and fibrin to form blood clots (2). Thus, by binding vWF, botrocetin promotes inappropriate systemic clotting, reducing the ability of vWF to respond to ruptures caused by proteolytic venom proteins and promoting hemorrhage. Resistance in opossumsSeveral species of opossums in both North and South America are known to be resistant to lancehead venom—in fact, large opossums will even eat poisonous snakes (Figure 1). The mechanism by which they withstand snakebite is not known; our aim was to assess the possibility that mutations on the vWF gene may play a role in resistance.HyothesisBy calculating the rate of synonymous substitutions dSin the gene and the rate of non-synonymous substitutions dN(i.e., mutations that change the amino acid sequence of the resulting protein), we hope to detect posititve selective pressure. The ratio ω = dN/dS is a key statistic: if ω < 1, purifying selection is acting on this gene (i.e., mutations are purged); if ω = 1, no selection can be inferred; if ω> 1, positive, directional selection is acting on this gene. Our expectation is to find greater rates of substitution in the group of opossums highlighted in red in the phylogeny at right, because these are the species known to be resistant to snake venom.
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    Bloated Rhizobia: The effect of PHB storage on Bradyrhizobium japonicum mortality during desiccation
    (2011-04-13) Underbakke, Kyra
    Soil bacteria known as rhizobia infect the roots of legumes, forming nodules, where they benefit the host by fixing atmospheric nitrogen into a usable form for the plant. In return, these rhizobia use plant-derived energy to reproduce, and many species accumulate large quantities (>50% cell dry weight) of the storage lipid poly-3-hydroxybutyrate (PHB). Since high PHB stores give rhizobia up to a threefold reproductive advantage over cells with low PHB during starvation conditions, genotypes of rhizobia that are able to escape from nodules with more PHB would be expected to increase in frequency through time. However, not all strains of rhizobia synthesize large amounts of PHB. This research investigates a potential disadvantage to storing too much PHB: rhizobia that accumulate excessive amounts of PHB may be more likely to die in a drought environment. Using soybean (Glycine max) as host plants, I extracted Bradyrhizobium japonicum from nodules, desiccated the rhizobia, and measured PHB and percent of cells killed by this treatment using flow cytrometry. The results showed a positive correlation between PHB storage and percent killed by desiccation, indicating that high PHB storage may not always be selected for in rhizobial populations, despite its reproductive advantages.
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    Can we increase the nitrogen fixation efficiency of Sinorhizobium meliloti?
    (2011-04-13) Bower, Justin
    Rhizobia are soil bacteria that can grow and reproduce in nodular swellings on legume plant roots. In return for carbon resources, rhizobia provide their host plant with fixed nitrogen (N2 → NH3) for proteins, etc. In alfalfa nodules, rhizobia that fix atmospheric nitrogen lose their ability to reproduce. These non-reproductive rhizobia may divert resources from nitrogen fixation to rhizopine, which can be used as an additional carbon source for still- eproductive rhizobia in the same nodule. Consequentially, rhizopine producing rhizobia would benefit at the host’s expense and could harm agriculture. From June 2010 to March 2011, I have been measuring nitrogen-fixing efficiency to determine if rhizopine production actually reduces the quantity of nitrogen fixation in several genetically different strains of rhizobia. I measured how much carbon dioxide (from respiration) and atmospheric hydrogen (a byproduct of nitrogen fixation) were given off by nodules as a function of oxygen concentration. Of the four strains of rhizobia tested by this method, two strains synthesized rhizopines and two strains did not. The respiration and fixation data for each strain fit a linear regression with the inverse slope = nitrogen-fixation efficiency and the y-intercept = carbon cost in the absence of nitrogen fixation. My results show that one rhizopine producing strain had lower nitrogen fixation efficiency. However, this did not occur in a commonly used rhizopine producing laboratory strain. Therefore, I was able to conclude that rhizopine production may not be the most important factor controlling nitrogen-fixation efficiency.
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    Recognition of Degraded Signals in the Gray Treefrog (Hyla chrysoscelis)
    (2009-04-08) Kuczynski, Michael
    For many animals such as frogs and toads, auditory communication is critically important in conveying information regarding an organism's species, location, and quality. However, interference and distortion caused by biotic and abiotic factors in the environment can degrade acoustic signals. The amount of degradation that occurs largely depends on the distance separating the sender and receiver (degradation increases as distance increases). In this experiment, I examined how female gray treefrogs (Hyla chrysoscelis) respond to degraded male calls. I subjected females to a series of playback tests consisting of synthetic male calls that varied in amplitude modulation depth (poorly modulated calls are considered to be temporally degraded) and overall intensity (simulates different separation distances). I tested two alternative hypotheses regarding how females will respond to degraded calls at different simulated distances. One hypothesis is that females will be more tolerant of degraded calls when they are played at low intensities (simulating a large separation distance) because degradation is expected to occur in the natural environment when the signal has traveled any significant distance. The alternative hypothesis is that females will be unresponsive towards degraded calls of low intensity because it is not worth the energy investment required to travel to the "far away" male when the female is unable to accurately assess his quality due to call degradation. The results from this experiment support the hypothesis that females are more responsive to poorly modulated calls when they are played at an overall lower intensity.
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    Solving the “Cocktail Party Problem:” Fluctuating Background Noise Effects on Signal Recognition in Cope’s Grey Tree Frog
    (2011-04-13) Linehan-Skillings, Betsy
    If you have ever had trouble understanding what a person was saying to you in a noisy group setting, then you have experienced a condition known as “the cocktail-party problem.” The cocktail party problem is a condition in which an individual has trouble hearing and understanding an isolated target signal in a noisy group setting. When natural increases and decreases in the levels of background noise occur, a phenomenon known as “modulation masking release” may allow the listener to decipher the target signal during the times when background noise is lowest. Frogs breed in noisy groups known as choruses. Within these choruses, females must isolate the most attractive male call possible from the cacophony around them—thus, they are also subject to the cocktail party problem. Using simulated mating calls and modulated chorus noise, my colleagues and I attempted to study how grey tree frogs may use masking release to resolve the cocktail party problem within mating groups. I hypothesized that exposure to low-frequency modulated chorus-like noise would allow frogs to experience modulation masking release, resulting in an overall improved phonotaxis (sound response) compared to unmodulated (constant) levels of background noise. Additionally, I expected the effect of masking release to be more prominent when the mating call was longest. However, experimental results revealed that the presence of modulated chorus noise confers no significant benefit to females when they are isolating and choosing male calls with respect to flat, unmodulated noise. Future successful studies such as this one could ultimately benefit the hearing-impaired by helping audiologists to create better hearing technology.
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    Testing for female natal philopatry in Myotis lucifugus using population genetics tools
    (2010-09-13) Fernandez, Juan D.
    The little brown bat (Myotis lucifugus) has undergone much research; however, not many studies have been made to explain its dispersal behavior. Understanding its dispersal pattern will help understand how human-caused landscapes change and roost destruction affect the little brown bat. Many species of bats such as Myotis myotis show a sex-influenced dispersal pattern in which the females exhibit a natal philopatric dispersal behavior (Vonhof et. al). To test whether the little brown bat exhibited this kind of dispersal behavior using population genetics tools, was the objective of this research. A feasible method to figure out if the M. lucifugus dispersal behavior exhibited female natal philopatry was to compare the mitochondrial DNA (mtDNA) with the nuclear DNA of individuals from several maternal colonies. Since both males and females spread far away from their birthplace to mate, the genetic structure of their nuclear DNA, which is bi-parentally inherited, should be uniform among colonies. However, if this species showed natal philopatric behavior, the genetic structure of their mtDNA, which is inherited clonally from the mother to the offspring, should be similar among individuals within each maternal colony and different from other maternal colonies.