Pollen carryover, pollinator movement, and spatial context impact the delivery of pollination services in apple orchards

Assessing the relative contributions of different pollinator taxa to pollination services is a central task in both basic eco-evolutionary research and applied conservation and agriculture. To that end, many studies have quantified single-visit pollen deposition and visitation frequency, which together determine a pollinator species' rate of conspecific pollen delivery. However, for plant species that require or benefit from outcrossing, pollination service quality further depends upon the ratio of outcross to self-pollen deposited, which is determined by two additional pollinator traits: pollen carryover and movement patterns among genetically compatible plant individuals. Here, we compare the pollination capacities of managed honey bees, native bumble bees, and native mining bees in apple-a varietally self-incompatible commercial crop-when pollen carryover and pollinator movement patterns are considered. We constructed simulation models of outcross pollen deposition parameterized using empirically measured single-visit pollen deposition, visitation frequency, and probabilities of intertree movement exhibited by each pollinator type, as well as pollen carryover patterns simulated based on parameters reported in the literature. In these models, we also explicitly specified the spatial relationships among cross-compatible trees based on field-realistic orchard layout schemes. We found that estimated pollination service delivery was considerably reduced for all pollinator types when pollen carryover and pollinator movement patterns were considered, as compared to when only single-visit pollen deposition and visitation frequency were considered. We also found that the performance of different pollinator types varied greatly across simulated orchard layout schemes and pollen carryover scenarios, including one instance where bumble and mining bees reversed their relative rankings. In all simulations, native bumble and mining bees outperformed managed honey bees in terms of both outcross pollen delivery per unit time and per flower visited, with disparities being greatest under scenarios of low pollen carryover. We demonstrate the degree to which pollination studies may reach inaccurate conclusions regarding pollination service delivery when pollen carryover and pollinator movement patterns are ignored. Our finding of the strong context dependence of pollination efficiency, even within a single plant-pollinator taxon pair, cautions that future studies in both basic and applied pollination biology should explicitly consider the ecological context in which pollination interactions take place.

Pollinator visitation closely tracks diurnal patterns in pollen release

PREMISE

Animal-pollinated plants face a high risk of pollen loss during its transfer. To limit the negative effect of pollen losses by pollen consumption and heterospecific transfer, plant species may adjust and stratify their pollen availability during the day (i.e., "schedule" their pollen presentation) and attract pollinators in specific time frames.

METHODS

We investigated diurnal patterns of pollen availability and pollinator visitation in three coflowering plant species: Succisa pratensis with open flowers and accessible pollen, pollinated mainly by pollen-feeding hoverflies; Centaurea jacea with open flowers and less accessible pollen, pollinated mainly by pollen-collecting bees; and Trifolium hybridum with closed flowers and pollen accessible only after the active opening of the flower, pollinated exclusively by bees.

RESULTS

The three plant species differed in the peak pollen availability, tracked by the visitation activity of their pollinators. Succisa pratensis released all pollen in the morning, while pollinator activity was still low and peaked with a slight delay. In contrast, C. jacea and T. hybridum had distinct pollen presentation schedules, peaking in the early afternoon. The pollinator visitation to both of these species closely matched their pollen availability.

CONCLUSIONS

Stratifying pollen availability to pollinators during the day may be one of several mechanisms that allow coflowering plants to share their pollinators and decrease the probability of heterospecific pollen transfer.

Habitat effects on reproductive phenotype, pollinator behavior, fecundity, and mating outcomes of a bumble bee-pollinated herb

PREMISE

Fecundity and mating outcomes commonly differ among plant populations occupying contrasting environments. If self-pollination occurs primarily among flowers within plants, contrasting reproductive outcomes among populations must reflect environmental effects on plant-pollinator interactions. Specifically, local conditions could affect features of plant phenotypes that influence pollinator behavior, in turn modifying plant reproductive outcomes.

METHODS

We compared phenotypes, pollinator abundance and behavior, and female fecundity and mating in two meadow populations and two forest populations of Aconitum kusnezoffii within 3 km of each other. Mating outcomes were assessed using microsatellites.

RESULTS

Meadow plants generally produced more, shorter ramets with more, larger flowers, but less nectar per flower than forest plants. These differences likely largely represent phenotypic plasticity. Individual bumble bees visited more flowers on forest plants, likely because the more abundant bees in the meadows depleted nectar availability, as indicated by briefer visits to individual flowers. Despite similar fruit set in both habitats, forest plants set more seeds per fruit. Nevertheless, meadow plants produced more seeds overall, owing to sevenfold greater flower production. Consistent with individual bees visiting fewer flowers on meadow plants, more of their seeds were outcrossed. However, the outcrossed seeds of forest plants included more male mates.

CONCLUSIONS

Reproductive outcomes can vary among populations of animal-pollinated plants as a result of differences in the availability of effective pollinators and environmental effects on plant phenotypes, and their functional consequences for pollinator behavior that governs pollen dispersal.

The function of floral traits and phenotypic selection in Aconitum gymnandrum (Ranunculaceae)

Floral evolution in angiosperms is thought to be driven by pollinator-mediated selection. Understanding flower integration and adaptation requires resolving the additive and nonadditive contributions of floral pollinator attraction and pollination efficiency traits to fitness components. In this study, a flower manipulation experiment with a factorial design was used to study the adaptive significance of galea height (a putative attraction trait) and entrance width (a putative efficiency trait) in Aconitum gymnandrum Maxim. flowers. Simultaneously, phenotypic selection analysis was conducted to examine selection by pollinators on galea height, entrance width, nectar production and plant height. Increased galea height increased the pollinator visitation rate, which confirmed its attractiveness function. Increasing floral entrance width by spreading the lower sepals increased the seed number per fruit without affecting pollinator visitation. This suggests a pollination efficiency role for the entrance width. The phenotypic selection analysis, however, did not provide evidence of pollinator-mediated selection for either of these traist, but it did for plant height. According to the manipulation treatment and correlational selection results, the combined variation in galea height and entrance width of A. gymnandrum flowers did not have nonadditive effects on female reproductive success. This study demonstrated the adaptive value of A. gymnandrum flowers through manipulation of an attractiveness trait and an efficiency trait. However, neither trait was associated with pollinator-mediated selection. A combination of manipulating traits and determining current phenotypic selection could help to elucidate the mechanism of selection on floral traits involved in different functions and flower integration.

On the possible role of nonreproductive traits for the evolution of unisexuality: Life-history variation among males, females, and hermaphrodites in Opuntia robusta (Cactaceae)

In angiosperms, dioecy has arisen in 871-5,000 independent events, distributed in approximately 43% of the flowering families. The reproductive superiority of unisexuals has been the favorite explanation for the evolution of separate sexes. However, in several instances, the observed reproductive performance of unisexuals, if any, does not seem to compensate for the loss of one of the sex functions. The involvement of fitness components not directly associated with reproduction is a plausible hypothesis that has received little attention. Life-history traits recently recognized as predictors of plant performance were compared among males, females, and hermaphrodites of a rare trioecious Opuntia robusta population in the field, using the cladode as the study unit. Cladode mortality by domestic herbivores was common and higher in females and hermaphrodites than in males. Males, females, or both displayed lower shrinkage and higher rates of survival, growth, and reproductive frequency than hermaphrodites. Unisexuals simultaneously outperformed hermaphrodites in demographic traits known to compete for common limiting resources, such as the acceleration of reproductive maturation (progenesis) and survival. A meta-analysis combining the outcomes of each of the analyzed life-history traits revealed a tendency of males (d++ = 1.03) and females (d++ = 0.93) to outperform hermaphrodites in presumably costly demographic options. Clonality is induced by human or domestic animal plant sectioning; and males and females highly exceeded hermaphrodites in their clonality potential by a factor of 8.3 and 5.3, respectively. The performances of unisexuals in the analyzed life-history traits may enhance their reproductive potential in the long run and their clonality potential and could explain the observed increase of unisexuality in the population. Life-history traits can be crucial for the evolution of unisexuality, but their impact appears to be habitat specific and may involve broad ontogenetic changes.

No evolutionary change in the mating system of Solanum rostratum (Solanaceae) during its invasion in China

The mating system of flowering plants plays a key role during the process of invasion. Evolution from outcrossing to selfing can allow rapid regeneration of a population after long-distance dispersal by providing reproductive assurance. Solanum rostratum is a self-compatible annual herb that exhibits a high level of outcrossing in its native populations. However, the mating system of invasive populations of S. rostratum has never been assessed. Here, we investigated the mating system based on 11 microsatellite loci and explored ecological factors that may influence the outcrossing rate among 10 invasive populations of S. rostratum in China. We found that the mean outcrossing rate was 0.69 ± 0.12 (range 0.49 to 0.83) with multiple paternity within progeny arrays (average effective number of sires = 7.86), which suggests a mixed mating system dominated by outcrossing. Combined with the uniformly high outcrossing rate (0.70 ± 0.03) previously reported in its native range, these results indicate that there has been no evolutionary shift in mating system during the invasion in China by S. rostratum. There were no relationships between outcrossing and population size, population density, altitude, latitude or longitude. Furthermore, high outcrossing of S. rostratum in China may be facilitated by enantiostyly and heteranthery.

When it pays to cheat: Examining how generalized food deception increases male and female fitness in a terrestrial orchid

Background

Experimental manipulations of floral nectar in food deceptive species can reveal insights into the evolutionary consequences of the deceptive strategy. When coupled to pollen tracking, the effects of the deceptive pollination syndrome on both male and female reproductive success may be quantified. Attraction of pollinators in deceit-pollinated species often relies on producing a conspicuous floral display which may increase visibility to pollinators, but in-turn may increase within plant selfing.

Methodology

To understand the role of deception in Orchidaceae reproduction we studied Cypripedium candidum. All species of the Cypripedium genus employ a generalized food deceptive pollination strategy and have been suggested as a model system for the study of pollinator deception. We conducted a nectar addition experiment that randomly assigned the four plants closest to a transect point to receive one of four histochemical dyes. Two individuals selected for nectar addition in each of altogether 25 blocks received 2μl of 25% sucrose solution in the labellum of each flower, while two others received no artificial nectar. Number of fruits produced, fruit mass and fruit abortion were scored at the end of the four-month experiment.

Results

Nectar addition increased (p<0.0001) self-pollination and pollen discounting by nearly 3x, while plants not receiving nectar had greater (p<0.0001) numbers of non-self pollinia deposited and lower rates of pollen discounting. There was a non-significant (p = 0.0645) trend for deceptive plants to set more fruit, while presence of nectar did not affect pollen export.

Conclusions

This study demonstrates the adaptive advantages of food deception by showing a concurrent reduction in particular male and female functions when a food reward is restored to a deceptive flower. We found generalized food deception to not only decrease inbreeding depression in the system, but concurrently have no effect on pollinator attraction and fruit set when compared with rewarding flowers.

Adaptive pattern of nectar volume within inflorescences: bumblebee foraging behavior and pollinator-mediated natural selection

Larger floral displays increase pollinator visitation as well as among-flower self-pollination (geitonogamy) in self-compatible species. Dichogamy (temporal separation of gender expression) can limit geitonogamy and increase outcrossing but this depends on pollinator behavior within inflorescences. Declining nectar volume from lower to upper flowers is a hypothesized adaptation to increase outcrossing and pollen export by encouraging the upward movment of pollinators from female to male flowers and by reducing the number of flowers probed per inflorescence, but supporting evidence has been equivocal. We tested this hypothesis in Aconitum gymnandrum by studying floral display and rewards, pollinator visitation, and pollinator-mediated selection on floral traits. We found that larger inflorescences of A. gymnandrum attracted more pollinators, but did not increase the number of flowers probed per visit. Nectar production declined with increasing flower height on average, but the opposite pattern was also common. Bumblebees responded strongly to the nectar pattern, moving from higher to lower nectar concentration. Finally, there was significant pollinator-mediated direct selection for this pattern of declining nectar volume after correcting for correlations with flower size, number, and mean nectar volume. Together, the results strongly suggest that declining nectar production in higher flowers is an adaptation to enhance outcrossing in A. gymnandrum.

Characterization of 19 microsatellite loci in the clonal monkshood Aconitum kusnezoffii (Ranunculaceae)

PREMISE OF THE STUDY

Microsatellite loci were isolated and characterized from Aconitum kusnezoffii (Ranunculaceae) to estimate male and female reproductive success and evaluate the effects of clonal growth on sexual reproduction.

METHODS AND RESULTS

A genomic enrichment approach was used to develop microsatellite markers. In three investigated A. kusnezoffii populations, a total of 19 microsatellite loci were successfully amplified, and 13 of these loci were polymorphic. Most of the primer pairs designed for the identified loci also amplified corresponding microsatellite loci in A. barbatum var. puberulum and A. alboviolaceum.

CONCLUSIONS

The identified microsatellite loci will be useful for quantifying male and female fitness in A. kusnezoffii and evaluating the effects of clonal growth on sexual reproduction.

Experimental analysis of mating patterns in a clonal plant reveals contrasting modes of self-pollination

Hermaphrodite plants commonly practice self-fertilization (selfing), but the mechanisms responsible vary depending on the mode of self-pollination, pollinator behavior, and degree of clonality. Whether selfing occurs within (autogamy) or between flowers (geitonogamy) is of evolutionary significance because their fitness consequences differ. We used floral manipulations and genetic markers to determine the relative contribution of autogamy and within- versus between-ramet geitonogamy to the selfing rate of the bumblebee-pollinated, clonal herb Aconitum kusnezoffii. Data on flowering phenology and bumblebee foraging were also collected to determine opportunities for different modes of self-pollination. Autogamy accounted for only 12% of the selfing rate with the remainder resulting from geitonogamy. Whole-ramet emasculation of clones with multiple ramets reduced selfing by 78%, indicating that within-ramet geitonogamy contributed significantly (68%) to total selfing. Selfing of single-ramet plants was 44% less than multiple-ramet plants, indicating that the contribution of between-ramet geitonogamy was substantially less (20%) than within-ramet geitonogamy, probably because of bumblebee foraging behavior. Our results demonstrate for the first time in a clonal plant that within-ramet geitonogamy is substantially greater than between-ramet geitonogamy and highlight the importance of considering the influence of clonal architecture and pollinator foraging on modes of self-pollination.

Influences of clonality on plant sexual reproduction

Flowering plants possess an unrivaled diversity of mechanisms for achieving sexual and asexual reproduction, often simultaneously. The commonest type of asexual reproduction is clonal growth (vegetative propagation) in which parental genotypes (genets) produce vegetative modules (ramets) that are capable of independent growth, reproduction, and often dispersal. Clonal growth leads to an expansion in the size of genets and increased fitness because large floral displays increase fertility and opportunities for outcrossing. Moreover, the clonal dispersal of vegetative propagules can assist "mate finding," particularly in aquatic plants. However, there are ecological circumstances in which functional antagonism between sexual and asexual reproductive modes can negatively affect the fitness of clonal plants. Populations of heterostylous and dioecious species have a small number of mating groups (two or three), which should occur at equal frequency in equilibrium populations. Extensive clonal growth and vegetative dispersal can disrupt the functioning of these sexual polymorphisms, resulting in biased morph ratios and populations with a single mating group, with consequences for fertility and mating. In populations in which clonal propagation predominates, mutations reducing fertility may lead to sexual dysfunction and even the loss of sex. Recent evidence suggests that somatic mutations can play a significant role in influencing fitness in clonal plants and may also help explain the occurrence of genetic diversity in sterile clonal populations. Highly polymorphic genetic markers offer outstanding opportunities for gaining novel insights into functional interactions between sexual and clonal reproduction in flowering plants.

Consequences of clonality for sexual fitness: Clonal expansion enhances fitness under spatially restricted dispersal

Clonality is a pervasive feature of sessile organisms, but this form of asexual reproduction is thought to interfere with sexual fitness via the movement of gametes among the modules that comprise the clone. This within-clone movement of gametes is expected to reduce sexual fitness via mate limitation of male reproductive success and, in some cases, via the production of highly inbred (i.e., self-fertilized) offspring. However, clonality also results in the spatial expansion of the genetic individual (i.e., genet), and this should decrease distances gametes and sexually produced offspring must travel to avoid competing with other gametes and offspring from the same clone. The extent to which any negative effects of clonality on mating success might be offset by the positive effects of spatial expansion is poorly understood. Here, we develop spatially explicit models in which fitness was determined by the success of genets through their male and female sex functions. Our results indicate that clonality serves to increase sexual fitness when it is associated with the outward expansion of the genet. Our models further reveal that the main fitness benefit of clonal expansion might occur through the dispersal of offspring over a wider area compared with nonclonal phenotypes. We conclude that, instead of interfering with sexual reproduction, clonal expansion should often serve to enhance sexual fitness.