Project Number: 2080-21000-017-04-R
Project Type: Reimbursable Cooperative Agreement
Start Date: Apr 15, 2015
End Date: Mar 31, 2019
Body size influences nearly every aspect of an organism's biology, from metabolism and population growth to thermoregulation and flight energetics, so geographic variation in body size is of fundamental interest. Increased body size can facilitate thermoregulation at cooler temperatures, and bees should thus exhibit similar clines across both latitude and altitude. However, high elevation flight is challenging, and can be facilitated by reducing body weight relative to wing area (wing loading), suggesting that bees should be smaller at higher altitudes, or that wings should be larger. Replicated sampling across species, latitudinal, and altitudinal gradients will be used to test the hypothesis that contrasting abiotic challenges (temperature and air density) drive shifts in body and wing morphology, potentially explaining conflicting clinal patterns observed in volant insects. Investigate the role of plasticity in determining Bombus trait variation Morphological and physiological differentiation can occur as a result genetic adaptation and plastic responses to conditions during development. Spring queens will be sampled from a set of low/high elevation/latitude populations and raise colonies from these queens in a common rearing environment to test the hypothesis that morphological and physiological differences observed in different environments will be retained in common-garden laboratory reared colonies and thus signify potentially adaptive differences. Apply a landscape genomic approach to distinguish genetic differentiation driven by natural selection or demography across environmental gradients Detecting local adaptation relies on the theory that genome regions linked to the targets of selection can be distinguished by exhibiting greater differentiation (e.g., FST) than neutral loci governed by migration-drift equilibrium. Testing for correlations between allele frequencies at these “FST outliers” and environmental variables can then identify regions involved in adaptation. Our study design makes it possible to test for parallel adaptation within and among species (e.g., testing for similar outliers), or for unique selection pressures (evident as an excess of unique outlier loci) across sampled environmental gradients. Grounded in strong preliminary data, for each species the research will test the related hypotheses that 1) populations at different latitudes and altitudes exhibit consistently elevated genetic differentiation due to isolation imposed by distance and landscape structure; 2) adaptation will be revealed by the presence of markers that show excess frequency differentiation, non-random associations with site-specific environmental variables, and repeated patterns in replicate transects and species; and 3) temperature-related outlier loci will be replicated for latitudinal and elevation gradients, but altitude specific adaptations will be evident as a high proportion of unique outliers along elevation transects.
Target species—Bombus vosnesenskii and Bombus bifarius are both targets for domestication by bumble bee producers. Both are abundant western US species, and have long been studied with respect to its role in agriculture and is a key member of the western bumble bee. In addition to broad north-south distributions, both species also occur across substantial altitudinal gradients ranging from sea level to >3000m. Sampling Design—To address adaptation and population genetic structure in bumble bees distributed across a climatically and topographically complex landscape, a hierarchical sampling design will nest replicated elevation transects within a broad latitudinal transect along the Sierra-Cascades and ensures sampling at relevant scales. PIs have identified six suitable sampling regions along this transect: Inyo/Sequoia N.F.; Yosemite, CA; Mt. Shasta, CA; Mt. Ashland, OR; Mt. Hood, OR; Snoqualmie N.F., WA. Each species will be sampled to represent the complete latitudinal spread. Nested within each region, bees will be sampled at two pairs of high and low elevation populations (>1000m difference). Specific Activity 1: Quantifying adaptive morphological and physiological variation across latitudinal and altitudinal gradients (performed by all three PI’s) Morphological traits will be measured in field-caught bees. During summer field seasons 20 workers per site per species will be collected, along with GPS data and local weather conditions. In the field, mass of foragers and pollen loads will be estimated. In the lab, body size, wing morphology and pile length will be. Wing loading will be estimated as the body weight /total wing area. Linear mixed effects models will be applied with morphological traits as response variables, and with latitude, altitude and body size as covariates, and population of origin as random effects, to identify geographic variation and plasticity in key traits among field-collected and common garden bees of each species. Specific Activity 2: Investigate the role of plasticity in determining trait variation (performed by USDA-ARS and U. Wyo) Comparisons of field-collected and common garden bees will allow examination of trait plasticity, with the null hypothesis that traits for bees reared from different environments will converge in a common garden. Two elevation transects at latitudinal extremes will be visited in spring to collect queens for initiating laboratory colonies. To initiate common garden populations, at least 40 spring queens will be collected from each of four sites representing latitudinal and altitudinal extremes of the sampling transect. Nests will be established at the USDA-ARS-PIRU in Logan, UT by Co-PD Strange using established protocols. Nest establishment of 4-8 colonies per location will represent the F1 generation. Specific Activity 3: Landscape genomics (performed at U. Alabama) Developing a de novo genomic marker set—SNPs for ~15 individuals per species per sampled population, 24 populations per species will be sequenced (760 total specimens in 8 sequencing plates Genome wide sequencing will involve RADseq, digesting genomic DNA from each specimen. Results will be analyzed by PD Lozier.