Global invasion history of the agricultural pest butterfly revealed with genomics and citizen science.

The small cabbage white butterfly, , is a major agricultural pest of cruciferous crops and has been introduced to every continent except South America and Antarctica as a result of human activities. In an effort to reconstruct the near-global invasion history of , we developed a citizen science project, the "Pieris Project," and successfully amassed thousands of specimens from 32 countries worldwide. We then generated and analyzed nuclear (double-digest restriction site-associated DNA fragment procedure [ddRAD]) and mitochondrial DNA sequence data for these samples to reconstruct and compare different global invasion history scenarios. Our results bolster historical accounts of the global spread and timing of introductions. We provide molecular evidence supporting the hypothesis that the ongoing divergence of the European and Asian subspecies of (∼1,200 y B.P.) coincides with the diversification of brassicaceous crops and the development of human trade routes such as the Silk Route (Silk Road). The further spread of over the last ∼160 y was facilitated by human movement and trade, resulting in an almost linear series of at least 4 founding events, with each introduced population going through a severe bottleneck and serving as the source for the next introduction. Management efforts of this agricultural pest may need to consider the current existence of multiple genetically distinct populations. Finally, the international success of the Pieris Project demonstrates the power of the public to aid scientists in collections-based research addressing important questions in invasion biology, and in ecology and evolutionary biology more broadly

Butterfly colors and global change: anthropogenic influence on a sexually selected wing trait in the cabbage white butterfly

Habitat changes (e.g. deforestation, urbanization, etc.), the introduction of exotic species, human harvesting (e.g. fisheries), climate change, and pollution are creating novel environments at an unprecedented rate, affecting ecological and evolutionary processes in a countless number of ways. One massive shift that has occurred in the United States over the last few centuries has been land-use change primarily into agricultural and urban areas. These shifts in land types have affected the nitrogen cycle through fertilizer and atmospheric nitrogen deposition. There is evidence that these nutritional shifts are changing nitrogen availability which is having ecological effects as well as an evolutionary effect on animal communication. However, condition-dependent signals, or honest signals that relate information about mate quality, have been less studied in terms of responses to increased nitrogen pools. Using a combination of field-based observations and lab-based experiments, I have investigated how land use change is affecting the sexually selected phenotypic characteristics of the cabbage white butterfly, Pieris rapae. We know this butterfly is nitrogen limited, and the white color of the cabbage white butterfly comes from nitrogen-rich pterin pigments. Coloration was quantified using reflectance measurements as a proxy for pigment concentration. Using these methods, I was able to perform the investigations described in this dissertation. First, I start by introducing land-type changes in the Anthropocene, honest signals, my focal species (the cabbage white butterfly), and brief methods used for wing coloration metrics. In chapter one I looked at color differences in male and female cabbage white butterflies across multiple land types across the United States. Chapter two investigated the effects of varying nitrogen availability (through manipulated artificial diets) on the sequestration and allocation of nitrogen to cabbage white head, thorax, abdomen, and wing tissue in addition to coloration. Finally, in chapter three, I looked at how four populations, with differing levels of surrounding agricultural land, varied in wing coloration, investment in reproductive tissue, and mating behavior. Taken together, these three chapters provide evidence that wing coloration is affected by land use and increased nitrogen availability that has a direct relationship with reproductive tissues, however not in the way that had been predicted at the start of my investigations. I conclude with a summary of my findings, the importance of research into anthropogenically induced effects on honest signals, and future directions

All data for "Anthropogenic increases in nutrients alter sexual selection dynamics: a case study in butterflies"

Each tab corresponds to a separate dataset for the study. In general, "population" refers to either Minnesota (MN) or North Dakota (ND, agricultural) populations. "Diet" refers to common garden rearing on either cabbage or artificial diet (low or high nitrogen manipulation within each diet type). "WT" refers to wild type individuals collected in the field (versus common garden reared individuals). Three different "Pterin measures" are described in the methods. For several measures (pterins, butterfly nitrogen content), there are at least two technical replicates for each biological replicate (each listed on a separate line within that file). Additional details can be found in the manuscript methods section

Data from: Anthropogenic increases in nutrients alter sexual selection dynamics: a case study in butterflies

Anthropogenic increases in nutrient availability offer opportunities to study evolutionary shifts in sexual selection dynamics in real time. A rapid increase in nutrient availability may reduce the utility of condition-dependent ornaments as signals of quality and lessen any nutritional benefits to females from re-mating. We explored these ideas using cabbage white butterflies, focusing on nitrogen as a nutrient, as it is important to this species in not only in ornamentation but also as a key macronutrient transferred in spermatophores. We compared a non-agricultural population to an agricultural population, which has seen an increase in nitrogen availability over the last 40 years due in part to fertilizer application. When reared in a common garden, both male and female butterflies from the agricultural population allocate significantly more to nitrogen-rich wing pigments used in mate choice. However, this increase in allocation is not correlated with an increase in the ability to assimilate nitrogen from their diet. Spermatophore counts from wild females show that females in the agricultural population rarely mate more than once, while those in the non-agricultural population are much more likely to be polyandrous. Similarly, the structure in the female reproductive tract that processes spermatophores has evolved lower tooth density in the agricultural population, consistent with the idea that these females benefit less from high throughput of spermatophores. These results suggest that anthropogenic increases in nutrient availability have altered sexual selection dynamics and the evolution of sexual traits: investment in ornamentation has increased, and there has been a decline in the nutritional benefits of re-mating in females

Data from: Anthropogenic changes in sodium affect neural and muscle development in butterflies

The development of organisms is changing drastically because of anthropogenic changes in once-limited nutrients. Although the importance of changing macronutrients, such as nitrogen and phosphorus, is well-established, it is less clear how anthropogenic changes in micronutrients will affect organismal development, potentially changing dynamics of selection. We use butterflies as a study system to test whether changes in sodium availability due to road salt runoff have significant effects on the development of sodium-limited traits, such as neural and muscle tissue. We first document how road salt runoff can elevate sodium concentrations in the tissue of some plant groups by 1.5–30 times. Using monarch butterflies reared on roadside- and prairie-collected milkweed, we then show that road salt runoff can result in increased muscle mass (in males) and neural investment (in females). Finally, we use an artificial diet manipulation in cabbage white butterflies to show that variation in sodium chloride per se positively affects male flight muscle and female brain size. Variation in sodium not only has different effects depending on sex, but also can have opposing effects on the same tissue: across both species, males increase investment in flight muscle with increasing sodium, whereas females show the opposite pattern. Taken together, our results show that anthropogenic changes in sodium availability can affect the development of traits in roadside-feeding herbivores. This research suggests that changing micronutrient availability could alter selection on foraging behavior for some roadside-developing invertebrates

Species with more volatile population dynamics are differentially impacted by weather.

Climatic variation has been invoked as an explanation of population dynamics for a variety of taxa. Much work investigating the link between climatic forcings and population fluctuation uses single-taxon case studies. Here, we conduct comparative analyses of a multi-decadal dataset describing population dynamics of 50 co-occurring butterfly species at 10 sites in Northern California. Specifically, we explore the potential commonality of response to weather among species that encompass a gradient of population dynamics via a hierarchical Bayesian modelling framework. Results of this analysis demonstrate that certain weather conditions impact volatile, or irruptive, species differently as compared with relatively stable species. Notably, precipitation-related variables, including indices of the El Niño Southern Oscillation, have a more pronounced impact on the most volatile species. We hypothesize that these variables influence vegetation resource availability, and thus indirectly influence population dynamics of volatile taxa. As one of the first studies to show a common influence of weather among taxa with similar population dynamics, the results presented here suggest new lines of research in the field of biotic-abiotic interactions

Species with more volatile population dynamics are differentially impacted by weather.

Climatic variation has been invoked as an explanation of population dynamics for a variety of taxa. Much work investigating the link between climatic forcings and population fluctuation uses single-taxon case studies. Here, we conduct comparative analyses of a multi-decadal dataset describing population dynamics of 50 co-occurring butterfly species at 10 sites in Northern California. Specifically, we explore the potential commonality of response to weather among species that encompass a gradient of population dynamics via a hierarchical Bayesian modelling framework. Results of this analysis demonstrate that certain weather conditions impact volatile, or irruptive, species differently as compared with relatively stable species. Notably, precipitation-related variables, including indices of the El Niño Southern Oscillation, have a more pronounced impact on the most volatile species. We hypothesize that these variables influence vegetation resource availability, and thus indirectly influence population dynamics of volatile taxa. As one of the first studies to show a common influence of weather among taxa with similar population dynamics, the results presented here suggest new lines of research in the field of biotic-abiotic interactions

Butterfly and plant raw data

This excel file contains three tabs. Tab 1 includes data for the monarch rearing experiment including sex (males, females), rearing treatment (roadside collected milkweed or prairie collected milkweed), wing length (the measure of body size), thoracic protein, dry thoracic weight and the width of each eye (note the final analyses used log transformed values of several metrics as described in the methods). Tab 2 includes data for the cabbage white butterfly rearing experiment, including sex (males, females), rearing treatment (low sodium or medium sodium artificial diet), wing length (the measure of body size), thoracic protein, dry thoracic weight and brain volume of a subset of specimens (note that the final analysis used log transformed values of several metrics as described in the methods). Tab 3 includes element measurements of leaf tissue of plants collected from along roadsides and in control sites >100 m from the road. Our analyses focused on sodium and nitrogen (note that sodium values were log transformed for normality in statistical analyses). Here we also include values of other elements that are measured in a standard ICP-AES analysis; N values came from a separate analysis of the same samples (using the Dumas method)

Data from: Nutrition shapes life-history evolution across species

Nutrition is a key component of life-history theory, yet we know little about how diet quality shapes life-history evolution across species. Here, we test whether quantitative measures of nutrition are linked to life-history evolution across 96 species of butterflies representing over 50 independent diet shifts. We find that butterflies feeding on high nitrogen host plants as larvae are more fecund, but their eggs are smaller relative to their body size. Nitrogen and sodium content of host plants are also both positively related to eye size. Some of these relationships show pronounced lineage-specific effects. Testis size is not related to nutrition. Additionally, the evolutionary timing of diet shifts is not important, suggesting that nutrition affects life histories regardless of the length of time a species has been adapting to its diet. Our results suggest that, at least for some lineages, species with higher nutrient diets can invest in a range of fitness-related traits like fecundity and eye size while allocating less to each egg as offspring have access to a richer diet. These results have important implications for the evolution of life histories in the face of anthropogenic changes in nutrient availability