The long arm of the larva: evolutionary responses to resource availability Public Deposited

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  • March 22, 2019
Creator
  • McAlister, Justin S.
    • Affiliation: College of Arts and Sciences, Department of Biology
Abstract
  • How do organisms adapt to environmental heterogeneity? We know that organisms can respond to environmental heterogeneity by expressing phenotypes that are either phenotypically plastic or constant. However, which strategy of phenotypic expression (plasticity versus constancy) will evolve depends on many factors including: the fitness costs of a given strategy, the degree of environmental heterogeneity, and the degree of association among other life history traits that are also evolving to maximize organismal fitness. Food resource availability is an environmental parameter that is frequently heterogeneous. Echinoid echinoderm larvae are one group of organisms, among many, that have been demonstrated to modify the expression of food collection structures depending on food availability. In my dissertation I examined how the aforementioned factors are associated with the evolution of plastic or constant expression of food collection structures using larval echinoids as a model system. I investigated whether plastic genotypes pay a fitness cost for expressing phenotypic plasticity of food collection structures. I reared multiple genotypes (families) of larvae of the sea urchin, Lytechinus variegatus, and examined whether the degree of plasticity of food collection structures is negatively correlated with two fitness measures: total energetic content and larval stomach length (a site of energy storage). I demonstrated genetic variation iii for plasticity among families but did not demonstrate a cost of plasticity, suggesting either that plasticity is inexpensive or that costs of plasticity are difficult to detect. I investigated whether historical changes in the availability of food resources are associated with evolved differences in the constant and/or plastic expression of food collection structures. I examined larval development of seven “geminate species pairs” of sea urchins located in coastal waters on both sides of the Isthmus of Panama. These species have been evolving in the different environments, with respect to planktonic food for larvae, of the eastern Pacific Ocean and western Caribbean Sea for the past approximately 3 million years. I demonstrated that Caribbean species have evolved to grow longer arms relative to body length than Pacific species regardless of food treatment level (a constant response), and also that none of the species have evolved phenotypic plasticity of food collection structures. I investigated whether the evolution of constancy or plasticity of different life history traits are correlated. Specifically, I examined whether evolved and experimentally induced differences in egg size, which represents an endogenous energetic resource for larvae, are associated with the expression of differences in the length and plasticity of length of larval feeding structures. Using two species from the sea urchin genus Strongylocentrotus that differ in egg size, I demonstrated that evolved and experimentally induced differences in egg size are associated with the expression of larval arm length and that evolved differences in egg size are associated with the degree of plasticity of larval arm length.
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  • In Copyright
Advisor
  • Kingsolver, Joel
Degree granting institution
  • University of North Carolina at Chapel Hill
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  • Open access
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