Boggs Lab

Overview:

Given an amount of food acquired by an organism, how does it allocate that food to reproduction, maintenance, storage and acquiring more food? How do allocation patterns differ under restricted food availability? What then are the effects on fitness components? And finally, do results from lab experiments translate to patterns seen in the field?

We use the Mormon fritillary, Speyeria mormonia, as our focal species. This species feeds on Viola spp. as larvae, and nectar from a variety of floral species as an adult. Found in montane western North America, S. mormonia over-winters as an unfed first instar larva. We have examined the effects of reduced food availability to both adults and larvae on fecundity, lifespan, flight and resting metabolic rates. We have also documented the incorporation of carbon into eggs from larval and adult sources under ad libitum feeding. In short, fecundity is reduced in proportion to a reduction in adult food intake; lifespan is unaffected. Adult-derived carbon constitutes 80% of the egg carbon at equilibrium. Quantitative restriction of larval food yields smaller butterflies with altered allometry among body parts, but fecundity is as expected based on female size. Lifespan was shortened in one experiment, but remained constant in another. The two experiments differed in whether adults were fed sugar-water or honey-water.

These lab results translate to the field. For example, population growth from generation t to t+1 correlates positively with a measure of flowers/butterfly in generation t. This indicates that adult food availability in the field affects likely affects population growth via an effect on fecundity.

Current work, led by doctoral student Chloé Keck, focuses on allocation of larval-derived food to wing pigments vs. oocytes vs. fat body during pupal development. Allocation to pigment should affect “orange-ness” of a female’s wings, which serve as a signal to males searching for mates. In fact, males in the field are more likely to approach decoys that are the color of well-fed females than of semi-starved females. Under restricted food availability, then, what is the trade-off among allocation to mate signaling vs. investment in making eggs vs. investment in storage for egg production and female survival, using nitrogen as a common currency? Is female fitness maximized at all levels of larval food acquisition?

In 1977, Cheri Holdren and Paul Ehrlich introduced 10,000 eggs and larvae from 39 female Euphydras gillettii to a site within the Rocky Mountain Biological Lab, 500km south of the butterfly’s native range. That introduced population persists today, with no addition of individuals over the past 46 years. The population size remained small in a 2ha patch through the 1980s and 1990s, but exhibited erratic dynamics beginning in 2002. A satellite population 1.75km from the original population, with independent dynamics, was naturally established likely in 2007. We monitor the population vital rates and collect samples of larvae for future genetic/genomic work annually.

Questions include: What are the abiotic and biotic drivers of the population dynamics? What caused the differences in dynamics among decades? As part of this question set, doctoral student Laurent Duverglas is also examining the role of intra-guild predation, occurring when ungulates unintentionally eat larvae while browsing the host plants.

The population genetic dynamics also interest us. Previous work showed that, by 2012, the population had lost 40% of the heterozygosity present in a population within the native range. Yet, we see no obvious fitness effect. A diversity of genetic/genomic questions await the interested post-doc!

Plants introduced into the range of a native herbivore lead to one of several outcomes. First, ovipositing females may ignore the new species, so no interaction ensues. Second, the native herbivore may incorporate the new species into the herbivore’s host plant repertoire, expanding its host plant range. Third, ovipositing females may lay eggs on the plant, but larvae cannot develop on it, leading to an evolutionary trap.

The mustard Thlaspi arvense arrived in our field site in Colorado sometime between 1850-1880. The native butterfly Pieris macdunnoughi lays eggs on the plant. However, larvae either won’t eat the plant or develop very slowly, such that no larvae survived to pupation in the field. We ask: After 140+ butterfly generations, why does this pattern still hold? What prevents adaptation, either of females’ ignoring the plant or larvae eating the plant?

Migration / selection balance, either for female oviposition or larval feeding, may explain the persistence of the maladaptive behavior. Thlaspi arvense is patchily distributed within the East River Valley. Earlier, we documented that female oviposition choice between the native Cardamine cordifolia and T. arvense is heritable and sex-linked. This result indicates that genetic variation exists for selection to act on. However, whether heritable choice is based on glucosinolate cues or some other plant characteristic is unclear. Lab alum Nitin Ravikanthachari directly tested the hypothesis by examining the population structure. He found that the Valley contains one inter-breeding population, yet transcriptomic differences stand out between areas with and without T. arvense. These relate to larval feeding physiology, rather than female oviposition. We hypothesize that larval feeding variation may be maintained in areas with T. arvense if drought (for example) alters the plant’s chemistry sufficiently to allow a few larvae to survive.

Mud puddling: Butterflies use mud, dung and carrion as salt licks. In most species, it’s young males that ‘puddle’. In one case, males transfer the sodium they gain from puddling to females during mating, providing much of the females’ sodium intake. This is a social behavior. Multiple species congregate at puddles, and puddling butterflies are unusually quiescent. Undergraduate projects in the lab continue to explore this behavior, including a phylogenetic analysis of preference for different sodium concentrations (various lab alums), an answer to the question of what attracts the first butterfly to a puddle (lab alum Hannah Walton), and tests of changes in adult sodium intake in response to different concentrations of sodium in the larval diet, e.g., from salt spray (Laura Littleton).

Biodiversity re-survey: During the 1980s to 1991, Michael Soule and Earthwatch volunteers conducted surveys every three days of a set of butterfly species along a 2 mile trail near the Rocky Mountain Biological Lab (RMBL). This area has seen substantial warming and shifts in snowpack, snow water-equivalent, and snow melt-out dates. Led by undergraduates Nimue Shive, Elena Renshaw and Michelle Sanders with participation from RMBL docents, we are conducting a re-survey of butterfly phenology, abundance and diversity. We follow Soule’s methods closely. Initial data suggest unexpected changes in phenology in several butterfly species, and extinction of two species. We cannot detect species additions, since the original survey followed only a subset of the species that we know were present in the 1980s.

A variant on the Geometric Framework of Nutritional Ecology: Butterfly labs around the world variously feed their adults sugar-water solutions, honey-water solutions, gatorade solutions, or various formulations of artificial nectar. We know from work in other animals that the ratio of acquired nutrients (protein, carbohydrate, fat, etc) affects life history traits, including fecundity and lifespan. Does what the subjects are fed bias the results of butterfly lab studies? Led by undergraduate Michelle Sanders, we are addressing this question.

Comparative studies of Drosophila & butterfly fat body and ovarian cell signaling: In collaboration with Alissa Armstrong‘s lab, we ask whether what is known about reproductive cell signaling in response to variation in nutrition in Drosophila generalizes to other insects. Drosophila have a nutritionally complete diet in both the larval and adult stage; most butterflies do not. Heliconius, which feeds on pollen and nectar, is a useful exception. Drosophila have stem cells in the ovaries; most butterflies do not, although Heliconius is again an exception. Drosophila fat body lines the body cavity; butterfly fat body is dispersed throughout the body. Can we predict cell signaling responses in butterflies, based on what’s known in Drosophila? We have developed tools that will allow such a comparative study.