Plant evolution in urban environments

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Collecting clover in Pittsburgh

Urbanization is one of the most widespread anthropogenic changes on earth and a leading cause of habitat alteration and species extinction. Most research has focused on how ecological communities and interactions are altered by cities, but little is known about whether plants and animals are adapting to urbanized environments. Furthermore, cities are spatially replicated across the planet and likely exert similar selection pressures on inhabiting organisms. Thus, urbanization allows us to address questions about the predictability and repeatability of evolution on a massive scale. Using white clover (Trifolium repens) as a model system, I am looking at the change in frequency of a defensive phenotype — the production of hydrogen cyanide from damaged leaves — across urban-rural gradients in 16 cities along the east coast of North America stretching from Tampa, Fl, to Montréal, Qc. This project builds off of work by Ken Thompson, a previous master’s student in the lab, and together we hope to address the following questions: (1) How common are urban-rural clines in cyanogenesis? and (2) What best predicts the presence and/or strength of urban-rural phenotypic clines in cyanogenesis across cities? I am also conducting a series of spatially-explicit simulations to better understand how adaptive (e.g. natural selection) and non-adaptive (e.g. gene flow, genetic drift) processes interact to generate clines in cyanogenesis, thereby increasing our understanding of how stochastic and deterministic processes shape evolution. You can read more about this work here.

Antiherbivore defences and herbivore- and pollinator-mediated selection on plant floral traits

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A halictid bee pollinating a clover flower in our 2015 summer field experiment.

Herbivory is one of the most ubiquitous and important interactions in terrestrial ecosystems and has driven the evolution of a wide array of plant defences. In addition to herbivores, many plants interact with  pollinators, which are key drivers of floral evolution and plant diversification in some flowering plant clades. While the roles of herbivores and pollinators in exerting selection on plant floral traits and defence have traditionally been studied in isolation, it is becoming increasingly clear that their effects are co-dependent and thus a better understanding of plant trait evolution is had by studying their interactive effects. Furthermore, the expression of plant defences themselves can alter patterns of selection on plant traits, either by changing patterns of plant resource allocation or by altering interactions with community members (i.e. herbivores and pollinators).  By planting genotypes of white clover that vary in their production of hydrogen cyanide, a potent antiherbivore defence, and manipulating the herbivory and pollination environment, I hope to address the following questions: (1) Do herbivores and/or pollinators exert natural selection on floral traits in Trifolium repens? (2) Are herbivore- and/or pollinator-mediated selection altered by the expression of plant defences?

The effects of fungal endosymbiont on plant ecology and evolution

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Increase in the frequency of vertically transmitted endophytes with increasing time protected from herbivorous mammals.

Plant-fungal interactions are widespread in nature and can have dramatic effects on the ecology and evolution of plants. Working as an undergraduate under Nash Turley and Marc Johnson, we set out to test the defensive mutualism hypothesis, which posits that, in exchange for shelter and sugars provided by their host plants, vertically transmitted fungal endophytes — fungal symbionts living inside of plants — defend their hosts by producing herbivore-deterring alkaloids. We tested this hypothesis by examining the change in endophyte frequency in Festuca rubra hosts using replicate herbivore exclosures from Silwood Park, UK, ranging from 4 months to 21 years in age. If the defensive mutualism hypothesis is supported, we would expect the frequency of fungal symbionts to decrease the longer their hosts have been protected from damage since the plants bear the cost of the association while receiving no benefit. Surprisingly, we found the exact opposite pattern (see figure). You can read more about this work here.

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Festuca rubra plants growing in pots during our experiment testing the ecological roles of horizontally transmitted fungal endophytes.

In addition to the vertically transmitted endophytes described above, plants also interact with a diverse array of horizontally transmitted fungal symbionts whose ecological roles are comparatively less well understood. In collaboration with Peter Kotanen, I examined endophyte-mediated growth and survival of Festuca rubra hosts collected from Akimiski Island, ON, where plants grow in salty intertidal habitats and are frequently grazed by geese. We conducted an experiment manipulating endophyte status (with or without endophytes), salt concentration, and simulated grazing (grazed or ungrazed) and asked the following questions: (1) Which fungal endophytes are present in this plant community? (2) Do fungal endophytes influence plant survival? (3) Do endophytes influence plant growth and/ or tolerance to simulated herbivory? (4) Do the costs and benefits of endophytes vary with soil salinity? We found that endophytes increased plant survival in the absence of salt or simulated grazing, but reduced plants’ ability to regrow tissue following damage. These results identify a potential endophyte-mediated trade-off in host growth and survival and suggests that the ecological roles of horizontally transmitted endophytes in nature may be under-appreciated. You can read more about this work here.