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Animal signaling systems frequently utilize multiple traits to produce and transmit a signal. These system elements may span multiple levels of organization. Functional integration of these traits may be expected to generate concordance in evolutionary pattern among system elements, such that evolutionary change in one system component is correlated with change in other system parts. This thesis investigates the evolution of multiple elements of a complex pheromone signaling system in a group of plethodontid salamanders (genus Plethodon). In this system, the transmitted signal is a multi-protein male pheromone that is delivered to a female during courtship interactions. This male courtship pheromone functions to increase female receptivity to mating. Pheromone delivery is achieved by the use of specialized morphological and behavioral characters. Two highly-conserved modes of pheromone delivery occur within this salamander clade, and each of these modes is associated with distinct morphological and behavioral traits. The transition from an ancestral to a derived mode of pheromone delivery presents a unique opportunity to address evolutionary concordance among elements of the signaling system.
I investigated the evolution of (1) pheromone-delivery behaviors and (2) multiple protein components of the courtship pheromone, across this evolutionary transition. First, I conducted courtship observations in a transitional species in which pheromone delivery includes aspects of both the ancestral and derived modes of pheromone delivery. My findings suggest that the behavioral transition in delivery mode was gradual and consisted of several changes in pheromone delivery behavior. Second, I characterized evolutionary pattern in three pheromone components across the behavioral transition. This study revealed overall concordance in evolutionary pattern between morphology, behavior and the pheromone signal, despite discordance in the type of selection acting at these different levels. However, a detailed analysis of evolutionary pattern revealed a decoupling of evolutionary changes among the three pheromone components. Overall, my results demonstrate the complexity of this pheromone delivery transition at multiple levels of organization. In particular, my findings suggest that, for both pheromone-delivery behaviors and the pheromone signal itself, the evolutionary transition was more gradual than was previously realized. These findings lend empirical insight into (1) the nature of evolutionary transitions, and (2) the evolution of functionally integrated traits that span organizational levels. |
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