A cellular basis for symbiont prevalence
Animals host a diversity of microbes, both inside and out. My research focuses on host interactions with Wolbachia bacteria, the most common known endosymbiont in nature. Wolbachia infect about half of all insects species and biocontrol programs currently use virus-blocking wMel Wolbachia from Drosophila melanogaster in transinfected mosquitoes to block transmission of deadly human diseases like dengue and Zika. My research takes an integrative approach to examine how host-symbiont interactions at the genomic and cellular levels dictate broader geographic patterns of Wolbachia prevalence and (co)evolution with insect hosts.
Transmission rate variation underlies symbiont prevalence
The rate of microbe transmission to subsequent host generations is a key determinant of symbiont prevalence and (co)evolution with hosts. Nonetheless, proximate explanations for transmission rate variation are generally unresolved. We recently found that maternal transmission of wYak Wolbachia in D. yakuba hosts declines at high altitude, where temperatures are relatively cool. Similarly, Wolbachia transmission declines when hosts are reared at low temperature in the lab, which is due to reduced Wolbachia density within host tissues. This discovery opens the door to entirely new avenues of research on the cellular and genetic basis of imperfect maternal transmission in the lab. I am currently investigating how temperature effects on cellular Wolbachia abundance during host oogenesis dictate maternal transmission efficiency and broader patterns of Wolbachia prevalence across the globe. Stay tuned for exciting new research on this front!
Symbiont effects on host fitness
Microbes have diverse effects on host physiology, behavior, and fitness. Wolbachia are intracellular bacteria residing throughout host somatic tissue, yet we have a poor understanding of how Wolbachia influence components of host fitness to rapidly spread in nature. By taking a comparative approach, we've found that divergent Wolbachia strains have pervasive effects on host physiology and behavior. Thus far, our work has demonstrated how Wolbachia alter host activity levels and thermal preference. Chelsey Caldwell, a UM undergraduate student, spearheaded the project on host thermal preference and she recently presented her findings virtually at the UM Conference on Undergraduate Research!
Evolution at the interface of toxin binding in a predator-prey arms race
Intense reciprocal selection between natural enemies, like predator and prey, can lead to arms race coevolution. I am interested in how constraints shape evolution at the interface of toxin-binding in the coevolutionary arms race between resistant garter snakes and their toxic prey, Pacific newts. My dissertation in the Brodie lab examined how molecular evolution, gene flow, and historical biogeography give rise to geographic variation in arms race dynamics between predator and prey.
Convergent evolution of a conserved protein
Two separate lineages of the common garter snake have convergently evolved resistance to tetrodotoxin (TTX), the deadly neurotoxin found in newts. We found that both snake lineages independently evolved resistance through a common series of amino acid changes to the Nav1.4 skeletal muscle sodium channel that disrupt toxin binding. In each instance, evolution proceeded through the same first-step mutation to the channel pore, suggesting the initial change had permissive effects for subsequent increases in resistance.
We further found that once TTX-resistant mutations accumulated in Nav1.4, negative trade-offs arose with sodium channel function and muscle performance. Our results indicate that costs develop as a consequence of accumulating mutations in the arms race that beneficially interfere with toxin binding. The evolutionary trajectory of Nav1.4 strikes a balance between TTX resistance and the maintenance of conserved channel function. More broadly, balancing selection and pleiotropy associated with Nav1.4 seem to explain geographic variation in snake TTX resistance across western North America.
Asymmetries in the arms race
Reciprocal selection is the hallmark of a coevolutionary relationship, but the evolutionary response of each species may not be symmetrical. We tested whether snakes and newts exhibit symmetrical evidence of local co-adaptation across their range of sympatry. We found that levels of snake resistance and newt toxins are closely matched across the landscape, implying that mosaic variation in the armaments of both species is a result of local pressures in the arms race. By the same token, phenotypic and genetic variation in snake resistance exhibit a clear signature of local adaptation. Contrary to conventional wisdom, however, landscape variation in newt toxins was best predicted by population genetic structure rather than predator resistance. Newt populations seem to structure variation in prey toxin levels, which in turn influences local levels of resistance in predators. This unexpected result implies that neutral processes like gene flow—rather than reciprocal selection—may represent the greatest source of variation across the geographic mosaic of coevolution.
Patterns of genetic diversity in sympatric lizards
Genetic variation is a fundamental requirement for evolution and adaptation. Genetic diversity should increase with effective population size and the decreasing effects of drift. For my Master's thesis in the Routman Lab, I tested how levels of genetic diversity vary in sympatric populations of lizard species in the Mojave Dessert that differ in population size and other ecological factors. At both mitochondrial and nuclear loci, we found that abundant, short-lived species had significantly higher estimates of diversity than less abundant, long-lived lizards. Higher diversity in the abundant species is likely due to a number of demographic factors, including larger effective population sizes, short generation times, and high rates of gene flow from surrounding populations.