Rearing the Cabbage White Butterfly (<em>Pieris rapae</em>) in Controlled Conditions: A Case Study with Heavy Metal Tolerance

The cabbage white butterfly (Pieris rapae) is an important system for applied pest control research and basic research in behavioral and nutritional ...

University of Minnesota

Sweet Briar College

Novel adaptations must originate and function within an already established genome [1]. As a result, the ability of a species to adapt to new environmental challenges is predicted to be highly contingent on the evolutionary history of its lineage [2-6]. Despite a growing appreciation of the importance of historical contingency in the adaptive evolution of single proteins [7-11], we know surprisingly little about its role in shaping complex adaptations that require evolutionary change in multiple genes. One such adaptation, extreme resistance to tetrodotoxin (TTX), has arisen in several species of snakes through coevolutionary arms races with toxic amphibian prey, which select for TTX-resistant voltage-gated sodium channels (Nav) [12-16]. Here, we show that the relatively recent origins of extreme toxin resistance, which involve the skeletal muscle channel Nav1.4, were facilitated by ancient evolutionary changes in two other members of the same gene family. A substitution conferring TTX resistance to Nav1.7, a channel found in small peripheral neurons, arose in lizards ∼170 million years ago (mya) and was present in the common ancestor of all snakes. A second channel found in larger myelinated neurons, Nav1.6, subsequently evolved resistance in four different snake lineages beginning ∼38 mya. Extreme TTX resistance has evolved at least five times within the past 12 million years via changes in Nav1.4, but only within lineages that previously evolved resistant Nav1.6 and Nav1.7. Our results show that adaptive protein evolution may be contingent upon enabling substitutions elsewhere in the genome, in this case, in paralogs of the same gene family.

Historical Contingency in a Multigene Family Facilitates Adaptive Evolution of Toxin Resistance.

While the effects of lead pollution have been well studied in vertebrates, it is unclear to what extent lead may negatively affect insect cognition. Lead pollution in soils can elevate lead in plant tissues, suggesting it could negatively affect neural development of insect herbivores. We used the cabbage white butterfly (Pieris rapae) as a model system to study the effect of lead pollution on insect cognitive processes, which play an important role in how insects locate and handle resources. Cabbage white butterfly larvae were reared on a 4-ppm lead diet, a concentration representative of vegetation in polluted sites; we measured eye size and performance on a foraging assay in adults. Relative to controls, lead-reared butterflies did not differ in time or ability to search for a food reward associated with a less preferred color. Indeed, lead-treated butterflies were more likely to participate in the behavioral assay itself. Lead exposure did not negatively affect survival or body size, and it actually sped up development time. The effects of lead on relative eye size varied with sex: lead tended to reduce eye size in males, but increase eye size in females. These results suggest that low levels of lead pollution may have mixed effects on butterfly vision, but only minimal impacts on performance in foraging tasks, although follow-up work is needed to test whether this result is specific to cabbage whites, which are often associated with disturbed areas.

Developmental lead exposure has mixed effects on butterfly cognitive processes.

Agricultural environments have long presented an opportunity to study evolution in action, and genomic approaches are opening doors for testing hypotheses about adaptation to crops, pesticides, and fertilizers. Here, we begin to develop the cabbage white butterfly (Pieris rapae) as a system to test questions about adaptation to novel, agricultural environments. We focus on a population in the north central United States as a unique case study: here, canola, a host plant, has been grown during the entire flight period of the butterfly over the last three decades.

Genomic adaptation to agricultural environments: cabbage white butterflies (Pieris rapae) as a case study.

Heavy metal pollution is a major problem in urban and industrial environments, and has a myriad of negative effects on animals. Quantifying the amount of population-level variation that exists for heavy metal tolerance and how plastic responses to heavy metals play out across generations are essential for understanding how animals respond to pollution. As an initial step toward studying transgenerational effects and population-level variation in concert, we brought cabbage white butterflies (Pieris rapae) from two populations-collected from St. Paul, MN, and Davis, CA-into common conditions and fed them a diet dosed with nickel. To measure transgenerational effects, we reared a second generation in a fully factorial design, within each population, to achieve all combinations of parent and offspring exposure to nickel or control diets. Across both generations, we quantified survival and other fitness-related traits, including development time, body size, and egg size and number. We found both population differences and complex transgenerational effects, including a positive effect of nickel on survival and development time in one of the populations. Overall, nickel exposure was stressful in one population, mainly after two generations of exposure, and had neutral or slightly positive effects on the other. We found no evidence for costs of mismatch between parental and offspring environments. While the reasons for the differences observed between the two populations are unclear, the variation in nickel tolerance observed in this species suggests that some organisms may be less affected by low levels of heavy metal pollution in urban and industrial areas than expected.

Nickel Exposure Has Complex Transgenerational Effects in a Butterfly.

On microevolutionary timescales, adaptive evolution depends upon both natural selection and the underlying genetic architecture of traits under selection, which may constrain evolutionary outcomes. Whether such genetic constraints shape phenotypic diversity over macroevolutionary timescales is more controversial, however. One key prediction is that genetic constraints should bias the early stages of species divergence along "genetic lines of least resistance" defined by the genetic (co)variance matrix, G. This bias is expected to erode over time as species means and G matrices diverge, allowing phenotypes to evolve away from the major axis of variation. We tested for evidence of this signal in West Indian  lizards, an iconic example of adaptive radiation. We found that the major axis of morphological evolution was well aligned with a major axis of genetic variance shared by all species despite separation times of 20-40 million years, suggesting that divergence occurred along a conserved genetic line of least resistance. Further, this signal persisted even as G itself evolved, apparently because the largest evolutionary changes in G were themselves aligned with the line of genetic least resistance. Our results demonstrate that the signature of genetic constraint may persist over much longer timescales than previously appreciated, even in the presence of evolving genetic architecture. This pattern may have arisen either because pervasive constraints have biased the course of adaptive evolution or because the G matrix itself has been shaped by selection to conform to the adaptive landscape.

Adaptive radiation along a deeply conserved genetic line of least resistance in  lizards.

Neonicotinoid pesticides harm nontarget insects, but their sublethal effects on butterflies are understudied. We exposed larvae of 3 butterfly species (Pieris rapae, Colias philodice, and Danaus plexippus) to low levels of the neonicotinoid imidacloprid in their host plants and followed individuals to adulthood. Imidacloprid altered adult body size, especially in female monarchs, but its effects varied across maternal families, highlighting the importance of considering genetic variation in ecotoxicological testing. Environ Toxicol Chem 2020;39:2228-2236. © 2020 SETAC.

Megan E. Kobiela

Deptartment of Ecology,
Evolution and Behavior,
Deptartment of Ecology, Evolution and Behavior

University of Minnesota

Rearing the Cabbage White Butterfly (Pieris rapae) in Controlled Conditions: A Case Study with Heavy Metal Tolerance

Megan E. Kobiela

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Deptartment of Ecology,Evolution and Behavior,Deptartment of Ecology, Evolution and Behavior

University of Minnesota

Rearing the Cabbage White Butterfly (Pieris rapae) in Controlled Conditions: A Case Study with Heavy Metal Tolerance

Deptartment of Ecology,
Evolution and Behavior,
Deptartment of Ecology, Evolution and Behavior