Browsing by Author "Sosa, Tim"

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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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    Gopher disturbance and plant diversity in prairie communities
    (2010-04-12) Akhter, Zoha; Gilliksen, Lorene; Swisher, Nick; Sosa, Tim
    The diversity of plant species associated with pocket gopher mound disturbance was examined at Frenchman’s Bluff, a prairie on a hill in northwestern Minnesota. Species richness and evenness between disturbed and undisturbed portions of a study area were investigated. An application of the Shannon diversity index to the collected data reveals greater diversity in areas with evidence of pocket gopher (Geomys bursarius) activity.
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    Risk Averse Foraging in Formicine Ants
    (2010-11-29) Sosa, Tim
    Ants are eusocial insects whose workers may engage in foraging as their exclusive activity for most of their adult life. Most ant communication, and thus much of the mechanism for maintenance of social organization, is thought to be chemical. When workers encounter food, they may leave pheromone trails to alert other workers to productive forage, enhancing food acquisition for the whole colony (Traniello 1989). Some ants also emit alarm pheromones in response to danger or trauma. This may elicit either swarming an attacker or escape behavior (Jackson and Ratnieks 2006). In order to examine the time-dependence of ant communication, I measured the intervals between ant visits subsequent to one worker’s discovery of a rich food source. I predicted a decreasing logarithmic curve, representing a scenario in which subsequent ants would reach the food with increasing speed. The curve would thus asymptote at some minimum interval corresponding to a maximum speed of travel for individual ants. I also examined risk awareness. By capturing foraging ants and removing them, but depriving them of the opportunity to emit alarm pheromones, I hoped to test whether ants were aware of the disappearance of compatriots without such cues. Without these cues, I expected the rate of chance encounter to determine these intervals.