Effects of experimental warming on floral scent, display and rewards in two subalpine herbs

BACKGROUND AND AIMS

Floral volatiles, visual traits and rewards mediate attraction and defence in plant-pollinator and plant-herbivore interactions, but these floral traits might be altered by global warming through direct effects of temperature or longer-term impacts on plant resources. We examined the effect of warming on floral and leaf volatile emissions, floral morphology, plant height, nectar production, and oviposition by seed predators.

METHODS

We used open-top chambers that warmed plants in the field by +2-3 °C on average (+6-11 °C increase in daily maxima) for 2-4 weeks across 1-3 years at three sites in Colorado, USA. Volatiles were sampled from two closely related species of subalpine Ipomopsis with different pollinators: Ipomopsis aggregata ssp. aggregata, visited mainly by hummingbirds, and Ipomopsis tenuituba ssp. tenuituba, often visited by hawkmoths.

KEY RESULTS

Although warming had no detected effects on leaf volatiles, the daytime floral volatiles of both I. aggregata and I. tenuituba responded in subtle ways to warming, with impacts that depended on the species, site and year. In addition to the long-term effect of warming, temperature at the time of sampling independently affected the floral volatile emissions of I. aggregata during the day and I. tenuituba at night. Warming had little effect on floral morphology for either species and it had no effect on nectar concentration, maximum inflorescence height or flower redness in I. aggregata. However, warming increased nectar production in I. aggregata by 41 %, a response that would attract more hummingbird visits, and it reduced oviposition by fly seed predators by ≥72 %.

CONCLUSIONS

Our results suggest that floral traits can show different levels of plasticity to temperature changes in subalpine environments, with potential effects on animal behaviours that help or hinder plant reproduction. They also illustrate the need for more long-term field warming studies, as shown by responses of floral volatiles in different ways to weeks of warming vs. temperature at the time of sampling.

Natural selection on floral volatiles and other traits can change with snowmelt timing and summer precipitation

Climate change is disrupting floral traits that mediate mutualistic and antagonistic species interactions. Plastic responses of these traits to multiple shifting conditions may be adaptive, depending on natural selection in new environments. We manipulated snowmelt date over three seasons (3-11 d earlier) in factorial combination with growing-season precipitation (normal, halved, or doubled) to measure plastic responses of volatile emissions and other floral traits in Ipomopsis aggregata. We quantified how precipitation and early snowmelt affected selection on traits by seed predators and pollinators. Within years, floral emissions did not respond to precipitation treatments but shifted with snowmelt treatment depending on the year. Across 3 yr, emissions correlated with both precipitation and snowmelt date. These effects were driven by changes in soil moisture. Selection on several traits changed with earlier snowmelt or reduced precipitation, in some cases driven by predispersal seed predation. Floral trait plasticity was not generally adaptive. Floral volatile emissions shifted in the face of two effects of climate change, and the new environments modulated selection imposed by interacting species. The complexity of the responses underscores the need for more studies of how climate change will affect floral volatiles and other floral traits.

The Functional Dilemma of Nectar Mimic Staminodes in Parnassia wightiana (Celastraceae): Attracting Pollinators and Florivorous Beetles

While floral signaling plays a central role in the reproductive success of all animal-pollinated plants, it may also attract herbivores eager to feed on flowers. False nectaries with glossy surfaces reflecting incident light may produce signals that attract floral visitors guiding their movements to and within the flower. Whether false nectaries also attract herbivores that lower the reproductive success of natural populations requires attention. In this study, we focus on Parnassia wightiana, a subalpine species with a whorl of staminodes that act as false nectaries attracting bees, flies, and herbivorous beetles. We tested the functions of staminodes using controlled manipulative experiments under field and lab conditions. We found a significant decrease in pollinator visits, and subsequent seed set, in flowers in which we removed staminodes or staminode apices confirming the function of these organs. In our natural populations, we found that a beetle, Nonarthra variabilis (Alticinae; Chrysomelidae), chews first on staminode apices, then it eats the entire staminodes and other flower parts, but rarely feeds on ovaries. Additional experiments suggested these beetles preferred staminodes to ovaries. Our results suggest this is a case of selective florivory, in which staminodes play a dual role, attracting pollinators and herbivores at the same time causing the attractive dilemma. Although selective florivory by beetles did not directly damage fruits, it influenced plant-pollinator interactions, decreasing reproductive success in plant populations. Our study highlights the importance of plant-pollinator-herbivore interactions in selecting floral traits.

The Hierarchical Coevolutionary Units of Ecological Networks

In ecological networks, cohesive groups of species may shape the evolution of interactions, serving as coevolutionary units. Ranging across network scales, from motifs to isolated components, elucidating which cohesive groups are more determinant for coevolution remains a challenge in ecology. We address this challenge by integrating 376 empirical mutualistic and antagonistic networks and coevolutionary models. We identified cohesive groups at four network scales containing a significant proportion of potential direct coevolutionary effects. Cohesive groups displayed hierarchical organisation, and potential coevolutionary effects overflowing lower-scale groups were contained by higher-scale groups, underscoring the hierarchy's impact. However, indirect coevolutionary effects blurred group boundaries and hierarchy, particularly under strong selection from ecological interactions. Thus, under strong selection, indirect effects render networks themselves, and not cohesive groups, as the likely coevolutionary units of ecological systems. We hypothesise hierarchical cohesive groups to also shape how other forms of direct and indirect effects propagate in ecological systems.

The potential for floral evolution in response to competing selection pressures following the loss of hawkmoth pollination in Ruellia humilis

PREMISE

In the absence of hawkmoth pollinators, chasmogamous (CH) flowers of Ruellia humilis self-pollinate by two secondary mechanisms. Other floral visitors might exert selection on CH floral traits to restore outcrossing, but at the same time preferential predation of CH seeds generates selection to increase the allocation of resources to cleistogamous (CL) flowers.

METHODS

To assess the potential for an evolutionary response to these competing selection pressures, we estimated additive genetic variances ( σ A 2 ${\sigma }_{{\rm{A}}}^{2}$ ) and covariances for 14 reproductive traits and three fitness components in a Missouri population lacking hawkmoth pollinators.

RESULTS

We found significant σ A 2 ${\sigma }_{{\rm{A}}}^{2}$ for all 11 floral traits and two measures of resource allocation to CL flowers, indicating the potential for a short-term response to selection on most reproductive traits. Selection generated by seed predators is predicted to increase the percentage of CL flowers by 0.24% per generation, and mean stigma-anther separation is predicted to decrease as a correlated response, increasing the fraction of plants that engage in prior selfing. However, the initial response to this selection is opposed by strong directional dominance.

CONCLUSIONS

The predicted evolutionary decrease in the number of CH flowers available for potential outcrossing, combined with the apparent preclusion of potential diurnal pollinators by the pollen-harvesting activities of sweat bees, suggest that 100% cleistogamy is the likely outcome of evolution in the absence of hawkmoths. However, rare mutations with large effects, such as delaying budbreak until after sunrise, could provide pathways for the restoration of outcrossing that are not reachable by gradual quantitative-genetic evolution.

Genetic and spatial variation in vegetative and floral traits across a hybrid zone

PREMISE

Genetic variation influences the potential for evolution to rescue populations from impacts of environmental change. Most studies of genetic variation in fitness-related traits focus on either vegetative or floral traits, with few on floral scent. How vegetative and floral traits compare in potential for adaptive evolution is poorly understood.

METHODS

We measured variation across source populations, planting sites, and genetic families for vegetative and floral traits in a hybrid zone. Seeds from families of Ipomopsis aggregata, I. tenuituba, and F₁ and F₂ hybrids of the two species were planted into three common gardens. Measured traits included specific leaf area (SLA), trichomes, water-use efficiency (WUE), floral morphology, petal color, nectar, and floral volatiles.

RESULTS

Vegetative traits SLA and WUE varied greatly among planting sites, while showing weak or no genetic variation among source populations. Specific leaf area and trichomes responded plastically to snowmelt date, and SLA exhibited within-population genetic variation. All aspects of floral morphology varied genetically among source populations, and corolla length, corolla width, and sepal width varied genetically within populations. Heritability was not detected for volatiles due to high environmental variation, although one terpene had high evolvability, and high emission of two terpenes, a class of compounds emitted more strongly from the calyx than the corolla, correlated genetically with sepal width. Environmental variation across sites was weak for floral morphology and stronger for volatiles and vegetative traits. The inheritance of three of four volatiles departed from additive.

CONCLUSIONS

Results indicate stronger genetic potential for evolutionary responses to selection in floral morphology compared with scent and vegetative traits and suggest potentially adaptive plasticity in some vegetative traits.

The Boechera model system for evolutionary ecology

Model systems in biology expand the research capacity of individuals and the community. Closely related to Arabidopsis, the genus Boechera has emerged as an important ecological model owing to the ability to integrate across molecular, functional, and eco-evolutionary approaches. Boechera species are broadly distributed in relatively undisturbed habitats predominantly in western North America and provide one of the few experimental systems for identification of ecologically important genes through genome-wide association studies and investigations of selection with plants in their native habitats. The ecologically, evolutionarily, and agriculturally important trait of apomixis (asexual reproduction via seeds) is common in the genus, and field experiments suggest that abiotic and biotic environments shape the evolution of sex. To date, population genetic studies have focused on the widespread species B. stricta, detailing population divergence and demographic history. Molecular and ecological studies show that balancing selection maintains genetic variation in ~10% of the genome, and ecological trade-offs contribute to complex trait variation for herbivore resistance, flowering phenology, and drought tolerance. Microbiome analyses have shown that host genotypes influence leaf and root microbiome composition, and the soil microbiome influences flowering phenology and natural selection. Furthermore, Boechera offers numerous opportunities for investigating biological responses to global change. In B. stricta, climate change has induced a shift of >2 weeks in the timing of first flowering since the 1970s, altered patterns of natural selection, generated maladaptation in previously locally-adapted populations, and disrupted life history trade-offs. Here we review resources and results for this eco-evolutionary model system and discuss future research directions.

An analytical pipeline to support robust research on the ecology, evolution, and function of floral volatiles

Research on floral volatiles has grown substantially in the last 20 years, which has generated insights into their diversity and prevalence. These studies have paved the way for new research that explores the evolutionary origins and ecological consequences of different types of variation in floral scent, including community-level, functional, and environmentally induced variation. However, to address these types of questions, novel approaches are needed that can handle large sample sizes, provide quality control measures, and make volatile research more transparent and accessible, particularly for scientists without prior experience in this field. Drawing upon a literature review and our own experiences, we present a set of best practices for next-generation research in floral scent. We outline methods for data collection (experimental designs, methods for conducting field collections, analytical chemistry, compound identification) and data analysis (statistical analysis, database integration) that will facilitate the generation and interpretation of quality data. For the intermediate step of data processing, we created the R package bouquet, which provides a data analysis pipeline. The package contains functions that enable users to convert chromatographic peak integrations to a filtered data table that can be used in subsequent statistical analyses. This package includes default settings for filtering out non-floral compounds, including background contamination, based on our best-practice guidelines, but functions and workflows can be easily customized as necessary. Next-generation research into the ecology and evolution of floral scent has the potential to generate broadly relevant insights into how complex traits evolve, their genomic architecture, and their consequences for ecological interactions. In order to fulfill this potential, the methodology of floral scent studies needs to become more transparent and reproducible. By outlining best practices throughout the lifecycle of a project, from experimental design to statistical analysis, and providing an R package that standardizes the data processing pipeline, we provide a resource for new and seasoned researchers in this field and in adjacent fields, where high-throughput and multi-dimensional datasets are common.