Research

Research in the lab is varied but generally focuses on a few key themes, including plant-insect interactions, toxin sequestration, chemical ecology, and integrative biology of monarch butterflies. Specific areas of emphasis are highlighted below. Trainees will have access to liquid chromatography equipment, insect incubators and plant growth chambers, greenhouse space, facilities at the Koffler Scientific Reserve, and an outstanding community of scholars in the Department of Ecology and Evolutionary Biology. 

Specialization in plant-insect interactions


Of the more than 1 million described insect species, over half are herbivores. It is estimated that more than 75% of herbivorous insect species are dietary specialists that feed from a single host plant family (and in many cases, from a single host plant genus or species). One hypothesis for the prevalence of specialization among herbivorous insects is the vast array of toxic secondary metabolites produced across the plant kingdom; toxins specific to particular plant lineages have selected for metabolic and behavioural adaptations in herbivores, which may impose tradeoffs that limit the ability to feed on alternative plants. Our lab is interested in characterizing the metabolic, sensory, and ecological basis of host plant specialization in insects. Examples of research in this area include:


In previous work, we found that monarch butterfly populations around the world exposed to different assemblages of milkweed host plants show evidence for local adaptation to these hosts. However, this pattern was not driven by evolution of increased performance ability on novel hosts in the monarch's introduced range. Instead, our results were more consistent with newly established monarch populations showing reduced performance on their ancestral North American milkweed hosts. These findings highlight that neutral evolutionary processes (e.g., relaxed selection for performance on ancestral hosts) may contribute to evolution of local adaptation in plant-insect interactions. We are interested in expanding this line of research to include other specialized plant-herbivore systems, such as milkweed beetles (Tetraopes spp.) and Arctiid moths. Read more here: Freedman et al. (2020), Evolution


Dietary specialization is an important axis of niche breadth. Species with broad dietary breadth must be flexible in their ability to recognize and process disparate host plants. By contrast, dietary specialists often have elaborate but narrowly tuned adaptations for finding and processing particular hosts. In ongoing research, we have found that North American monarch butterflies (which have been documented feeding on more than 40 species of milkweed throughout their migratory range) show greater gene expression plasticity in tissues associated with digestion and toxin metabolism than Puerto Rican monarch butterflies, which are associated with just two species of milkweed. 


Red milkweed beetle (Tetraopes tetrophthalmus) on A. syriaca in Chicago, IL. Milkweed beetles feed on milkweed as both larvae and adults and share many adaptations with monarchs (e.g., aposematic coloration and target site insensitivity to milkweed toxins).

Toxin sequestration


Toxin sequestration--a phenomenon in which organisms process, store, and deploy diet-derived toxins for defence against natural enemies--is widespread across the tree of life. Monarch butterflies are a textbook example of this process and actively sequester cardiac glycoside toxins produced by their milkweed host plants (cardenolides). This behaviour affords protection against monarch predators and parasitoids, as exemplified in Lincoln Brower's classic image of a barfing blue jay. Our lab is interested in studying cardenolide sequestration in monarchs and other milkweed-feeding insects, both to learn more about the physiology of this process and also to further develop "cardenolide fingerprinting" as a technique for studying monarch ecology. Some examples of past and future work include:


We measured patterns of cardenolide sequestration in six global populations of monarchs reared on six associated species of milkweed. Overall patterns of toxin sequestration were not population-specific, with one exception: monarchs from Puerto Rico sequester considerably lower levels of cardenolides from the primary North American milkweed host, A. syriaca. This result could suggest either recent loss of the ability to sequester from A. syriaca or strong selection for sequestration ability on this host after the Puerto Rican population diverged from its North American ancestor. Read more here: Freedman et al. (2022), BioRxiv.


In ongoing research, we have used RNA-sequencing in monarch caterpillars to study differences in patterns of sequestration across monarch populations and milkweed host species. We are also using association mapping to find genomic regions associated with the difference in cardenolide sequestration between North American and Puerto Rican monarchs. 


Beginning in the 1980s, researchers showed that different milkweed species impart unique cardenolide fingerprints onto developing monarchs. Along with stable isotope analysis, this chemical fingerprinting can provide a powerful tool for inferring natal origins of migratory monarchs. We are currently working with colleagues from UC Davis and Colorado State University on a U.S. Fish and Wildlife Service funded project to understand geographic origins and natal host plants of western North American monarchs. Our lab is also working on developing a comprehensive reference library of cardenolide fingerprints from nearly 50 milkweed host species, which will be used for comparison with contemporary and historical monarch specimens to infer patterns of host plant usage in space and time.



Representative chromatograms showing profiles of sequestered cardenolides on A. syriaca in North American (green) and Puerto Rican (orange) butterflies. Differences in sequestration likely reflect divergent natural selection for sequestration ability from this common North American milkweed host.

Monarch caterpillar from Puerto Rico, feeding on an introduced Calotropis procera host plant. Puerto Rican monarchs effectively sequester milkweed cardenolides from a local host (A. curassavica) but struggle to sequester from the most abundant North American milkweed host (A. syriaca).

Monarch butterfly migration


Monarchs are well-known for their spectacular seasonal migration in North America. Each year, millions of monarchs from eastern North America migrate up to 4,000 kilometers to high elevation fir forests in Central Mexico, where they will spend the winter; a separate migration also occurs in western North America, with monarchs overwintering at hundreds of locations along the coast of California. Our lab uses diverse approaches to learn about the physiological, morphological, and behavioural adaptations associated with long-distance migration, and we apply these findings to make predictions about how climate change may impact monarch migration. Specific areas of emphasis include:


In preparation for their autumn migration, monarchs undergo a series of physiological, behavioural, and metabolic changes that are mediated by seasonal cues. Our lab is working to understand how monarchs weigh seasonal signals (e.g., declining photoperiod, fluctuating temperature, and host plant senescence) and the developmental windows that define the shift from summer breeding to autumn migration. Ultimately, this research will help to determine how monarchs may shift (or not) the timing of their migration in a warming climate.


Over recent evolutionary history, monarchs have expanded their range globally and can now be found in locations as disparate as Guam, the Galapagos, and Portugal. It is largely unknown whether these recently-established populations retain the ability to sense and integrate seasonal cues associated with migration initiation, though early evidence suggests that they do. Our lab will expand this research to understand how history of migration loss is associated with sensitivity to seasonal cues in monarch populations around the world.


Monarch populations with divergent histories of migration loss and divergent phenotypes can be crossed to map the genes underlying migration-associated traits. With collaborators, we are exploring the genetic basis of wing morphological variation between monarch populations (e.g., see here). We also hope to expand this research to include other migration-associated traits such as lipid metabolism and diapause initiation. For more on this topic, see this recent review paper.


We are working with colleagues to understand reasons behind the precipitous decline of western North American monarchs, which culminated in fewer than 2,000 butterflies at California overwintering sites in 2020. Our lab is developing cardenolide fingerprinting as a tool to understand the prevalence of winter-breeding in association with cultivated milkweed species and to determine whether landscape-scale changes in agricultural practice are detectable in historical fingerprint data. 


Monarch butterfly at an overwintering site in San Luis Obispo County, California, in January 2023. Western North American monarchs declined more than 99% between 1996-2020 before unexpectedly rebounding in 2021 and 2022.

Other miscellaneous projects


Animated map of larval monarch occurrence records in western North America, built using a database compiled from iNaturalist records. Each panel corresponds to one of the primary milkweed host plants, and each dot corresponds to a individual occurrence record.