Elevated Temperature May Affect Nectar Microbes, Nectar Sugars, and Bumble Bee Foraging Preference

Floral nectar, an important resource for pollinators, is inhabited by microbes such as yeasts and bacteria, which have been shown to influence pollinator preference. Dynamic and complex plant-pollinator-microbe interactions are likely to be affected by a rapidly changing climate, as each player has their own optimal growth temperatures and phenological responses to environmental triggers, such as temperature. To understand how warming due to climate change is influencing nectar microbial communities, we incubated a natural nectar microbial community at different temperatures and assessed the subsequent nectar chemistry and preference of the common eastern bumble bee, Bombus impatiens. The microbial community in floral nectar is often species-poor, and the cultured Brassica rapa nectar community was dominated by the bacterium Fructobacillus. Temperature increased the abundance of bacteria in the warmer treatment. Bumble bees preferred nectar inoculated with microbes, but only at the lower, ambient temperature. Warming therefore induced an increase in bacterial abundance which altered nectar sugars and led to significant differences in pollinator preference.

Rational Design of a Novel Hawkmoth Pollinator Interaction in Mimulus Section Erythranthe

Diversification of the ca. 275,000 extant flowering plant species has been driven in large part by coevolution with animal pollinators. A recurring pattern of pollinator shifts from hummingbird to hawkmoth pollination has characterized plant speciation in many western North American plant taxa, but in the genus Mimulus (monkeyflowers) section Erythranthe the evolution of hawkmoth pollination from hummingbird-pollinated ancestors has not occurred. We manipulated two flower color loci and tested the attractiveness of the resulting four color phenotypes (red, yellow, pink, and white) to naïve hawkmoths (Manduca sexta). Hawkmoths strongly prefer derived colors (yellow, pink, white) over the ancestral red when choosing an initial flower to visit, and generally preferred derived colors when total visits and total visit time were considered, with no hawkmoth preferring ancestral red over derived colors. The simple flower color genetics underlying this innate pollinator preference suggests a potential path for speciation into an unfilled hawkmoth-pollinated niche in Mimulus section Erythranthe, and the deliberate design of a hawkmoth-pollinated flower demonstrates a new, predictive method for studying pollination syndrome evolution.

Limited phenological and pollinator-mediated isolation among selfing and outcrossing Arabidopsis lyrata populations

Transitions from outcrossing to selfing have been a frequent evolutionary shift in plants and clearly play a role in species divergence. However, many questions remain about the initial mechanistic basis of reproductive isolation during the evolution of selfing. For instance, how important are pre-zygotic pre-pollination mechanisms (e.g. changes in phenology and pollinator visitation) in maintaining reproductive isolation between newly arisen selfing populations and their outcrossing ancestors? To test whether changes in phenology and pollinator visitation isolate selfing populations of Arabidopsis lyrata from outcrossing populations, we conducted a common garden experiment with plants from selfing and outcrossing populations as well as their between-population hybrids. Specifically, we asked whether there was isolation between outcrossing and selfing plants and their between-population hybrids through differences in (1) the timing or intensity of flowering; and/or (2) pollinator visitation. We found that phenology largely overlapped between plants from outcrossing and selfing populations. There were also no differences in pollinator preference related to mating system. Additionally, pollinators preferred to visit flowers on the same plant rather than exploring nearby plants, creating a large opportunity for self-fertilization. Overall, this suggests that pre-zygotic pre-pollination mechanisms do not strongly reproductively isolate plants from selfing and outcrossing populations of Arabidopsis lyrata .

When to be a male? Role of resource-limitation and pollinators in determining floral sex in an andromonoecious spiderwort

The evolution and maintenance of sexual systems in plants is often driven by resource allocation and pollinator preferences, and very little is known about their role in determining floral sex expression in plants. In annual, entomophilous plants three major constraints can be identified towards optimal reproduction: 1) nutrient resources available from the environment, 2) nutrient resources allocated towards reproduction, i.e., fruits vs. flowers, and 3) pollinator visitations.Andromonoecy is a sexual system where plants bear both staminate and hermaphrodite flowers on the same inflorescence. The optimal resource allocation hypothesis suggests that under nutrient constraints, plants will produce more male flowers since they are energetically cheaper to produce over the more expensive hermaphrodite flowers. We test this hypothesis in the andromonoecious Murdannia simplex (Commelinaceae) by quantifying male and hermaphrodite flowers in a natural population and contrasting the distribution of the two sexes in plants from two resource conditions (stream population vs. plateau population). We next carried out choice experiments to test pollinator preference towards a specific sex.We found that in M. simplex, production of hermaphrodite flowers is resource-dependent and under resource constraints fewer numbers of flowers were produced and most of them were males. We failed to observe pollinator preference towards either sex but Amegilla spp. and Apis cerana showed higher visitation towards the most abundant sex within a trial, suggesting frequency-dependent visitation. Thus, we conclude that environmentally driven resource constraints play a bigger role in driving floral sex expression in Murdannia over direct pollinator-driven constraints.

Inbreeding and competition, but not abiotic stresses, increase fluctuating asymmetry of Mimulus guttatus flowers

Genetic and environmental disturbances are expected to increase developmental instability, which may result in higher fluctuating asymmetry (FA), i.e. small random deviations from symmetry. Plant leaves often do not show this pattern, possibly due to high phenotypic plasticity of leaf shape and low adaptive significance of leaf symmetry. In contrast, symmetry in many animal traits but also in flower shape is considered to be under selection, and FA in such traits may better reflect developmental instability. Using geometric morphometrics, I analysed the symmetry of flowers of inbred and outbred Mimulus guttatus (Phrymaceae) plants grown under five stress treatments with and without grass competition. Flower FA was not increased by abiotic stress, but by inbreeding and competition. As inbreeding and competition affected different principal components of flower FA, different mechanisms may be involved in their effects on FA. FA decreased with individual biomass particularly in selfed offspring, which suggests that inbreeding increased FA particularly when growth was limited by environmental or genetic constraints. Increased flower FA of inbred offspring may explain increased flower handling time and reduced pollinator preference for inbred plants in other M. guttatus studies, and could thus have important consequences for plant demography and plant–pollinator interactions.

Limited phenological and pollinator-mediated isolation among selfing and outcrossing Arabidopsis lyrata populations

Transitions from outcrossing to selfing have been a frequent evolutionary shift in plants and clearly play a role in species divergence. However, many questions remain about the initial mechanistic basis of reproductive isolation during the evolution of selfing. For instance, how important are prezygotic pre-pollination mechanisms (e.g. changes in phenology and pollinator visitation) in maintaining reproductive isolation between newly arisen selfing populations and their outcrossing ancestors? To test whether changes in phenology and pollinator visitation isolate selfing populations of Arabidopsis lyrata from outcrossing populations, we conducted a common garden experiment with plants from selfing and outcrossing populations as well as their F1 hybrids. Specifically, we asked whether there was isolation between outcrossing and selfing plants and their F1 hybrids through differences in 1) the timing or intensity of flowering; and/or 2) pollinator visitation. We found that phenology largely overlapped between plants from outcrossing and selfing populations. There were also no differences in pollinator preference related to mating system. Additionally, pollinators preferred to visit flowers on the same plant rather than exploring nearby plants, creating a large opportunity for self-fertilization. Overall, this suggests that prezygotic pre-pollination mechanisms do not strongly reproductively isolate plants from selfing and outcrossing populations of Arabidopsis lyrata.

How are pollinators guided by colourful floral structures? A commentary on: ‘The role of pollinator preference in the maintenance of pollen colour variation’

This article comments on: Jennifer L. Ison, Elizabeth S. L. Tuan, Matthew H. Koski, Jack S. Whalen and Laura F. Galloway. 2019. The role of pollinator preference in the maintenance of pollen colour variation. Annals of Botany 123(6): 951–960.

Associative learning of flowers by generalist bumble bees can be mediated by microbes on the petals

Communication is often vital to the maintenance of mutualisms. In plant-pollinator mutualisms, plants signal pollinators via floral displays, composed of olfactory, visual, and other plant-derived cues. While plants are understood to be associated with microbes, only recently has the role of microbial (yeast and bacteria) inhabitants of flowers as intermediaries of plant-pollinator communication been recognized. Animals frequently use microbial cues to find resources, yet no study has examined whether microbes directly mediate learned and innate pollinator responses. Here, we asked whether microbes on the flower surface, independent of their modification of floral rewards, can mediate these key components of pollinator preference. In the field, we characterized flower and bumble bee microbial abundance, and in laboratory assays we tested whether bumble bees (Bombus impatiens) discriminated flowers on the basis of an experimental floral microbial community on the petals and whether microbe-derived chemicals were effective cues. Learning of microbial community cues was associative and reward context-dependent and mediated by microbial chemicals. Deconstructing the experimental microbial community showed bees innately avoided flowers with bacteria, but were undeterred by yeast. Microbial cues thus potentially facilitate dynamic communication between plants and pollinators such as bumble bees, especially as pollinator visitation can change flower microbiota. We suggest that the study of communication in mutualism generally would benefit by considering not only the multicellular eukaryote partners, but their microbial associates.