Trade-off mitigation: a conceptual framework for understanding floral adaptation in multispecies interactions

What maintains variation in flower accessibility to pollinators in plant communities? A simulation study

BACKGROUND

Flowers in natural plant communities come in many shapes. Flowers with restrictive morphologies are considered complex, because only a subset of pollinators are able to learn how to access their nectar and pollen. Other flowers are easily accessible to diverse pollinating insects, and are regarded as simple. How and why do the two types of flowers coexist in natural plant communities? We developed a spatially explicit evolutionary simulation framework to explore this question. We modeled the dynamics of two types of flowers ('complex' and 'simple') that differ in accessibility to their simulated pollinators and in food rewards. The flowers are visited by a population of pollinators, which initially possess heritable variation in their ability to learn to forage on the complex flowers. We manipulated the pollinators' flying distances and the flowers' overall density, spatial distribution, and starting proportion of simple flowers. We recorded the resulting dynamics of the two flower types in the community, and of the pollinators' learning rates, over 100 generations.

RESULTS

Complex and simple flowers coexisted under all simulated conditions. The steady-state community always contained more simple flowers than complex ones. Complex flowers attained higher frequencies when flowers were highly aggregated than when flower aggregation was low. Long-distance fliers evolved higher learning abilities than short-distance fliers. Pollinator learning abilities, in turn, were positively correlated with the frequency of complex flowers.

CONCLUSIONS

Frequencies of complex flowers vary among natural plant communities. Our model predicts that this variation is shaped by the plants' spatial distribution as well as by the cognitive abilities of their pollinators. The model generates novel and testable hypotheses for understanding how diversity in flower shapes is maintained in natural plant communities.

A test of the Grant-Stebbins pollinator-shift model of floral evolution

Pollinators are thought to play a key role in driving incipient speciation within the angiosperms. However, the mechanisms underlying floral divergence in plants with generalist pollination systems, remains understudied. Brunsvigia gregaria displays significant geographical variation in floral traits and are visited by diverse pollinator communities. Because pollinators are often shared between populations, we investigated whether specific pollinators are responsible for driving floral divergence between them. Three distinct ecotypes were identified, each dominated by three different pollinators: bees, swallowtail butterflies, and long-proboscid flies. Across seven populations, we found a pattern of association between style length and the morphology of pollinators that visit the flowers most frequently and contact the reproductive parts most often. Furthermore, we found significant linear, quadratic and correlational selection on flower number, tepal length and style length within the butterfly- and bee-dominated populations. We also found partial evidence for divergent selection on these traits between experimental sites. Our findings suggest that a handful of key pollinators that vary in their importance have the potential to drive population-level divergence in floral traits, which may lead to pollination ecotype formation.

Interannual differences in pollinator contributions to pollen transfer are mainly driven by changes in pollinator abundance

With the rising threat to insect pollinators and the upcoming pollinator crisis, it is important to know how pollinators contribute to pollen transfer. The contributions of individual pollinator taxa to pollen transfer depend both on their abundance and on how much pollen each individual can carry, with overall importance being a multiplication of these two values. Here, we quantified pollen load across a diverse spectrum of insect pollinator taxa and variation in their abundance over 11 years. We found that, while variation in pollen load was relatively small among pollinator taxa (compared to relatively high variability among individuals within each insect taxon), the visitation levels changed significantly over the years, resulting in a high degree of variation in pollinator contributions to pollen transfer of each insect taxon at the community level. Thus, we conclude that the overall importance of pollinator taxa for pollen transfer is determined further by their abundances than by their taxon-specific capability for carrying various pollen loads. As the insect abundances vary over time and may change dramatically from year to year, our results highlight the importance of diverse and species-rich pollinator communities, as the population decline of one pollinator can be buffered by an increase in another pollinator taxa.

Pollination ecotypes and the origin of plant species

Ecological niche shifts are a key driver of phenotypic divergence and contribute to isolating barriers among lineages. For many groups of organisms, the history of these shifts and associated trait-environment correlations are well-documented at the macroevolutionary level. However, the processes that generate these patterns are initiated below the species level, often by the formation of ecotypes in contrasting environments. Here, I review the evidence in plants for 'pollination ecotypes' as microevolutionary responses to environmental gradients in pollinator availability. Pollinators are critical for population establishment and persistence in most species, thereby forming part of their fundamental niche. Novel floral trait combinations allow species to exploit particular pollination opportunities in local habitats and evolve primarily through sexual selection due to their effects on mating success. I examine selected case studies on the evolution of pollination ecotypes, including self-pollinating forms, and use these to illustrate challenging practical and conceptual issues. These issues include the paucity of reliable natural history data, the problem of implementing and interpreting reciprocal translocation experiments, and establishing criteria for when allopatric ecotypes should be considered species.

Linking divergence in phenotypic selection on floral traits to divergence in local pollinator assemblages in a pollination-generalized plant

Divergent patterns of phenotypic selection on floral traits can arise in response to interactions with functionally distinct pollinators. However, there are a limited number of studies that relate patterns of phenotypic selection on floral traits to variation in local pollinator assemblages in pollination-generalized plant species. We studied phenotypic selection on floral traits of Viscaria vulgaris, a plant that interacts with a broad range of diurnal and nocturnal pollinators, and related divergence in phenotypic selection on floral traits to the expected level of divergence in local pollinator assemblages. We detected phenotypic selection on floral traits involved in the attraction of pollinators and the mechanics of pollen removal and deposition, and demonstrated that floral traits are subject to spatiotemporal variation in the strength and direction of phenotypic selection. We revealed that diurnal and nocturnal pollinators, when considered in isolation, mediated divergent patterns of phenotypic selection on floral traits. Consistent with the Grant-Stebbins model, we observed that divergence in phenotypic selection on floral traits increased with the expected level of divergence in local pollinator assemblages. Thus, generalized plant-pollinator interactions can mediate phenotypic selection on floral traits, and distinct local pollinator assemblages can generate a geographic mosaic of divergent patterns of phenotypic selection. We underscore that these outcomes are not exclusive to specialized plant-pollinator interactions and can emerge at a local geographic scale.

Adaptive generalization in pollination systems: Hawkmoths increase fitness to long-tubed flowers, but secondary pollinators remain important

Selection on floral traits by animal pollinators is important in the evolution of flowering plants, yet whether floral divergence requires specialized pollination remains uncertain. Longer floral tubes, a trait associated with long-tongued pollinators, can also exclude other pollinators from accessing rewards, a potential mechanism for specialization. Across most of its range, Castilleja sessiliflora displays much longer corollas than most Castilleja species, though tube length varies geographically and correlates partially with hawkmoth visitation. To assess whether long corolla tubes reflect adaptation to hawkmoth pollinators, we performed a day/night pollinator exclusion experiment in nine natural populations that varied in corolla length across the range of C. sessiliflora and short-tubed members of the parapatric C. purpurea complex. We compared the fitness contributions of nocturnal and diurnal visitors, revealing that long-tubed populations visited predominantly by hawkmoths experienced greater fruit set at night, in contrast with short-tubed populations or those visited mainly by diurnal pollinators. Next, leveraging a range-wide multiyear dataset of pollinator visitation to these species, we identify that hawkmoth visitation is associated with increased fitness in long-tubed populations overall, and that long tubes are associated with less diverse visitor assemblages. Thus, long corollas represent an adaptation to hawkmoth pollination at the exclusion of diverse pollinators. Nonetheless, while hawkmoths were scarce in the northern range, secondary diurnal pollinators contributed to fruit set across the range, providing reproductive assurance despite possible trait mismatch. This study illustrates adaptive generalization in pollination systems and that floral divergence may proceed along a continuum of generalized and specialized pollinator interactions.

Adaptation to pollination by fungus gnats underlies the evolution of pollination syndrome in the genus Euonymus

BACKGROUND AND AIMS

Dipteran insects are known pollinators of many angiosperms, but knowledge on how flies affect floral evolution is relatively scarce. Some plants pollinated by fungus gnats share a unique set of floral characters (dark red display, flat shape and short stamens), which differs from any known pollination syndromes. We tested whether this set of floral characters is a pollination syndrome associated with pollination by fungus gnats, using the genus Euonymus as a model.

METHODS

The pollinator and floral colour, morphology and scent profile were investigated for ten Euonymus species and Tripterygium regelii as an outgroup. The flower colour was evaluated using bee and fly colour vision models. The evolutionary association between fungus gnat pollination and each plant character was tested using a phylogenetically independent contrast. The ancestral state reconstruction was performed on flower colour, which is associated with fungus gnat pollination, to infer the evolution of pollination in the genus Euonymus.

KEY RESULTS

The red-flowered Euonymus species were pollinated predominantly by fungus gnats, whereas the white-flowered species were pollinated by bees, beetles and brachyceran flies. The colour vision analysis suggested that red and white flowers are perceived as different colours by both bees and flies. The floral scents of the fungus gnat-pollinated species were characterized by acetoin, which made up >90 % of the total scent in three species. Phylogenetically independent contrast showed that the evolution of fungus gnat pollination is associated with acquisition of red flowers, short stamens and acetoin emission.

CONCLUSIONS

Our results suggest that the observed combination of floral characters is a pollination syndrome associated with the parallel evolution of pollination by fungus gnats. Although the role of the red floral display and acetoin in pollinator attraction remains to be elucidated, our finding underscores the importance of fungus gnats as potential contributors to floral diversification.

Putative Signals of Generalist Plant Species Adaptation to Local Pollinator Communities and Abiotic Factors

The reproductive success of flowering plants with generalized pollination systems is influenced by interactions with a diverse pollinator community and abiotic factors. However, knowledge about the adaptative potential of plants to complex ecological networks and the underlying genetic mechanisms is still limited. Based on a pool-sequencing approach of 21 natural populations of Brassica incana in Southern Italy, we combined a genome-environmental association analysis with a genome scan for signals of population genomic differentiation to discover genetic variants associated with the ecological variation. We identified genomic regions putatively involved in the adaptation of B. incana to the identity of local pollinator functional categories and pollinator community composition. Interestingly, we observed several shared candidate genes associated with long-tongue bees, soil texture, and temperature variation. We established a genomic map of potential generalist flowering plant local adaptation to complex biotic interactions, and the importance of considering multiple environmental factors to describe the adaptive landscape of plant populations.

Evolutionarily inspired solutions to the crop pollination crisis

The global decline in insect diversity threatens pollination services, potentially impacting crop production and food security. Here, we argue that this looming pollination crisis is generally approached from an ecological standpoint, and that consideration of evolutionary principles offers a novel perspective. First, we outline that wild plant species have overcome 'pollination crises' throughout evolutionary history, and show how associated principles can be applied to crop pollination. We then highlight technological advances that can be used to adapt crop flowers for optimal pollination by local wild pollinators, especially by increasing generalization in pollination systems. Thus, synergies among fundamental evolutionary research, genetic engineering, and agro-ecological science provide a promising template for addressing a potential pollination crisis, complementing much-needed strategies focused on pollinator conservation.

A generalized bird pollination system in Schotia brachypetala (Fabaceae)

Bird pollination systems are diverse, ranging from narrow-tubed flowers pollinated by specialist nectarivores such as hummingbirds and sunbirds, to relatively open flowers pollinated by opportunistic (i.e. generalist) nectarivores. The role of opportunistic avian nectarivores as pollinators has historically been under-appreciated. A key aspect to understanding the importance of opportunistic birds as pollinators is to investigate how efficiently they transfer pollen among flowers. Here, we document the pollination and breeding systems of Schotia brachypetala, a southern African tree known as the 'weeping boer-bean' on account of its prolific production of dilute hexose-dominated nectar. The cup-shaped flowers of this tree attract a large number of bird species, including both opportunistic and specialist nectarivores. We identified floral visitors using observations and camera traps and quantified the floral traits responsible for animal attraction. We documented the breeding system, used selective pollinator exclusion to test the contribution of birds to fecundity, and performed supplemental pollination to test for pollen limitation. Single-visit pollen deposition trials were undertaken to determine the efficacy of bird pollinators. Controlled hand-pollination experiments showed that S. brachypetala is genetically self-incompatible and therefore dependent on pollinators for seed production. Supplemental hand-pollination experiments showed that natural fecundity is limited by either the amount and/or the quality of pollen on stigmas. Flowers from which birds but not insects were experimentally excluded set fewer seeds than open control flowers. Opportunistic birds deposited more pollen per visit than did specialist sunbirds. We conclude that S. brachypetala has a generalized bird pollination system that mainly involves opportunistic nectarivores.

A Comparative Approach to Understanding Floral Adaptation to Climate and Pollinators During Diversification in European and Mediterranean Silene

Pollinator selection on floral traits is a well-studied phenomenon, but less is known about the influence of climate on this species interaction. Floral trait evolution could be a result of both adaptation to climate and pollinator-mediated selection. In addition, climate may also determine pollinator communities, leading to an indirect influence of climate on floral traits. In this study, we present evidence of both direct and indirect effects of climate on plant morphology through a phylogenetic comparative analysis of the relationships between climate, pollinators, and morphology in 89 European and Mediterranean Silene species. Climate directly influences vegetative morphology, where both leaf size and internode length were found to be smaller in habitats that are warmer in the driest quarter of the year and that have more precipitation in the coldest quarter of the year. Similarly, flower size was directly influenced by climate, where smaller calyxes were also associated with habitats that are warmer in the driest quarter of the year. These results suggest that reduced leaf and flower size promote water conservation in species that occupy arid climates. Floral traits also evolved in response to pollinators, with elongated calyxes associated with nocturnal pollination, though we also found evidence that climate influences pollinator distribution. Nocturnal pollinators of Silene are found in habitats that have more temperature evenness across seasons than diurnal pollinators. Correspondingly, nocturnally-pollinated Silene are more likely to occur in habitats that have lower daily temperature fluctuation and more temperature evenness across seasons. Altogether these results show that climate can directly influence vegetative and floral morphology, but it can also affect pollinator distribution, which in turn drives floral adaptation. Our study therefore suggests that climate mediates the influence of species interactions on trait evolution by imposing direct selective demands on floral phenotypes and by determining the pollinator community that imposes its own selective demands on flowers.

Eco-evolutionary feedbacks among pollinators, herbivores, and their plant resources

Eco-evolutionary feedbacks among multiple species occur when one species affects another species' evolution via its effects on the abundance and traits of a shared partner species. What happens if those two species enact opposing effects on their shared partner's population growth? Furthermore, what if those two kinds of interactions involve separate traits? For example, many plants produce distinct suites of traits that attract pollinators (mutualists) and deter herbivores (antagonists). Here, we develop a model to explore how pollinators and herbivores may influence each other's interactions with a shared plant species via evolutionary effects on the plant's nectar and toxin traits. The model results predict that herbivores indirectly select for the evolution of increased nectar production by suppressing plant population growth. The model also predicts that pollinators indirectly select for the evolution of increased toxin production by plants and increased counterdefenses by herbivores via their positive effects on plant population growth. Unless toxins directly affect pollinator foraging, plants always evolve increases in attraction and defense traits when they interact with both kinds of foragers. This work highlights the value of incorporating ecological dynamics to understand the entangled evolution of mutualisms and antagonisms in natural communities.

Exceptional evolutionary lability of flower-like inflorescences (pseudanthia) in Apiaceae subfamily Apioideae

PREMISE

Pseudanthia are widespread and have long been postulated to be a key innovation responsible for some of the angiosperm radiations. The aim of our study was to analyze macroevolutionary patterns of these flower-like inflorescences and their potential correlation with diversification rates in Apiaceae subfamily Apioideae. In particular, we were interested to investigate evolvability of pseudanthia and evaluate their potential association with changes in the size of floral display.

METHODS

The framework for our analyses consisted of a time-calibrated phylogeny of 1734 representatives of Apioideae and a morphological matrix of inflorescence traits encoded for 847 species. Macroevolutionary patterns in pseudanthia were inferred using Markov models of discrete character evolution and stochastic character mapping, and a principal component analysis was used to visualize correlations in inflorescence architecture. The interdependence between net diversification rates and the occurrence of pseudocorollas was analyzed with trait-independent and trait-dependent approaches.

RESULTS

Pseudanthia evolved in 10 major clades of Apioideae with at least 36 independent origins and 46 reversals. The morphospace analysis recovered differences in color and compactness between floral and hyperfloral pseudanthia. A correlation between pseudocorollas and size of inflorescence was also strongly supported. Contrary to our predictions, pseudanthia are not responsible for variation in diversification rates identified in this subfamily.

CONCLUSIONS

Our results suggest that pseudocorollas evolve as an answer to the trade-off between enlargement of floral display and costs associated with production of additional flowers. The high evolvability and architectural differences in apioid pseudanthia may be explained on the basis of adaptive wandering and evolutionary developmental biology.