Browsing by Author "Egan, Scott P"

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    Divergent host use promotes reproductive isolation between populations of gall-forming insects
    (2020-12-07) Zhang, Linyi; Egan, Scott P
    Understanding the formation of new species, termed ‘speciation’, is one of the central issues in evolutionary biology. The role of divergent selection in initiating and promoting the speciation process, termed ‘ecological speciation’, and maintaining species differences has been under intense study in recent decades. Despite accumulating individual case studies documenting individual reproductive barriers evolving between populations due to divergent selection, a synthetic view of the ecological speciation process requires: (1.) identifying the source(s) of divergent selection, (2.) identifying the trait(s) under divergent selection, and (3.) estimating the strength of multiple reproductive barriers as a result of divergent selection. When these issues can be linked in a single system, one can take a holistic approach to understanding the evolution of species. Moreover, despite emerging research linking these micro-evolutionary processes to macro-evolutionary patterns, there are still outstanding questions regarding the role of divergent selection as a common mechanism generating biodiversity across taxonomic orders. This is a challenging question, as many other factors can vary together and obscure clear patterns. To answer this question, we need to combine both bottom up and top down approaches in a broad but controlled system where many taxa can be queried simultaneously. If this challenge can be met, we can improve our predictability of the evolutionary outcomes under divergent selection, particularly, whether populations under divergent selection can complete the speciation process, and we can test the repeatability of ecological speciation across different species and taxonomic groups at different stages in the speciation process to bridge the micro-level evolution to macro-level evolution of biodiversity. In this dissertation, we address the above challenges by, first, using a magnifiying glass approach to study how divergent host-use generates and promotes RI between populations of a host-specific gall-forming insect, Belonocnema treatae (Chapter 1, 2, 3), and, second, testing whether the same divergent host environments drive ecological and genetic divergence among host-associated populations repeatably across five additional species of gall-forming insects (Chapter 4, 5). Chapter 1 of this thesis examines two reproductive barriers that are the direct results of divergent selection: reduced immigrant viability and fecundity, the latter is a potentially important, but often overlooked reproductive barrier. We found asymmetric, but opposing reproductive barriers that combined together to reduce gene flow in both directions. In chapter 2, we also identified one important source of selection related to divergent host use: the plant immune response that affects survival. In Chapter 2, we found that intra-specific variation in host quality among individual trees within each divergent host species, rather the tree species per se, is the major driver of hybrid fitness. Hybrids vary widely in both the direction and strength of hybrid fitness relative to pure crosses across individual trees. Thus, potentially inhibiting the speciation process on some trees, while promoting reproductive isolation on others. In Chapter 3, we tested whether a critical process occurs in the evolution of strong RI in host-associated populations of B. treatae: adaptive coupling, defined as selection favors the arise of another reproductive barrier as a response towards an existing reproductive barrier. We detected signatures of asymmetric adaptive coupling: in the presence of the cost of migration- reduced immigrant fitness and hybrid viability, stronger habitat, and sexual isolation are found among sympatric populations in the direction where migration rate is higher. In Chapters 4, we expand the scope of this work to include many species of gall-forming insects (and their predators) distributed across the same two host plant environments to test general principles of speciation. We demonstrate that temporal isolation between host-associated populations is present in six gall-forming insects and three natural enemies associated with gall-forming insects, suggesting phenological differences between different host plant species cascades across multiple species at multiple trophic levels. In sum, by combining field survey, manipulation experiment and behavior observation, we found that both divergent selection and migration are critical in the evolution of RI. Our findings call for more studies on assessing factors that affect the progression of ecological speciation, such as the role of heterogeneity within each divergent environment in promoting or inhibiting RI, population demography and multiple traits under divergent selection. Only by knowing the conditions that promote or inhibit the speciation process, can we better predict the evolutionary outcomes under divergent selection.
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    The Ecology and Detection of Environmental Molecules in Aquatic Ecosystems
    (2024-04-15) Brandao Dias Ferreira Pinto, Pedro; Egan, Scott P
    Biodiversity monitoring holds great importance in both ecological research and resource management, serving as an enabling tool for understanding and preserving ecosystems. The integration of molecular tools to biodiversity monitoring enhances researchers' capabilities, contributing to a deeper understanding of patterns and ecosystem processes. Here, I expand the limits of biodiversity monitoring by contributing to the detection and interpretation of information obtained from two biological molecules recovered from environmental samples: DNA and Proteins. In the first three chapters of my thesis, I address the spatial resolution of environmental DNA (eDNA) in flowing waters. Environmental DNA is a powerful monitoring tool based in detecting organisms remotely by capturing the DNA they passively release into the environment. Despite its rapid increase in popularity, many questions remain regarding the “ecology” of eDNA, which is defined as the origin, state, fate, and transport of the molecules. Notably, most eDNA ecology experiments are performed without considering eDNA`s heterogeneous composition, usually quantifying total eDNA. Additionally, less is known about flowing water systems, where particles and eDNA are also transported downstream and subject to the dynamics of substrate trapping and resuspension. Here, I significantly expand our knowledge of eDNA ecology, providing insight into how eDNA transforms over space and time, how it interacts with other components of the environment, and how we may use this information in our favor when interpreting eDNA data. Despite eDNA being the standard for remote detection, organisms leave behind other molecules that could be used for the same purpose. Accordingly, I also explore the use of an environmental protein (eProtein) for the detection of an organism. Specifically, I use δ-endotoxins from Bacillus thuringiensis, a toxin commonly introduced into Genetically modified organisms, as a model to explore the use of eProteins for monitoring. First, I explore the fate of this proteins in streams, a system in which they have been found in significant concentrations, and where they might impact off-target organisms. Second, I explore the application of light transmission spectroscopy and functionalized gold nanoparticles to develop a novel field-ready method of eProtein detection. The developed method has higher sensitivity than competing field methods.
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    Evolutionary and Ecological Responses to the Anthropocene; The Red-Shouldered Soapberry Bug in a Rapidly Changing World
    (2022-03-01) Comerford, Mattheau S; Egan, Scott P; Fricke, Evan; Lee, Cin-ty
    The Anthropocene is defined as the current geological age during which human activity has been the dominant influence on climate and the environment.
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    The role of sampling location and depth in the performance of lake plant environmental DNA metabarcoding
    (2020-04-24) Drummond, Jenn A; Correa, Adrienne MS; Egan, Scott P
    Inventories of organisms are central to ecological research and to ecosystem management. Environmental DNA (eDNA) methods permit broad yet detailed biodiversity sampling to be performed with minimal field effort, but uncertainty remains regarding the spatial resolution necessary for effective sampling. We analyze eDNA samples from twelve temperate lakes via ITS1 rRDNA metabarcoding for aquatic and terrestrial plant taxonomic patterns across horizontal and vertical space. We detect the expected community variation (beta diversity) from lake to lake, but few community composition patterns related to offshore distance and sampling depth. Patterns included slightly elevated alpha diversity in near-shore samples, but no other significant effects. Read abundances were heavily weighted toward aquatic macrophytes, but we identified representatives of many other phylogenetically and environmentally varied taxa, including terrestrial species from surrounding areas. We conclude that freshwater plant eDNA surveys successfully capture differences among lake communities, allowing easily-accessible shore-based sampling to inform research and management.