Plant evolution can mediate negative effects from honey bees on wild pollinators

Pollinators are introduced to agroecosystems to provide pollination services. Introductions of managed pollinators often promote ecosystem services, but it remains largely unknown whether they also affect evolutionary mutualisms between wild pollinators and plants.Here, we developed a model to assess effects of managed honey bees on mutualisms between plants and wild pollinators. Our model tracked how interactions among wild pollinators and honey bees affected pollinator and plant populations.We show that when managed honey bees have a competitive advantage over wild pollinators, or a greater carrying capacity, the honey bees displace the wild pollinator. This leads to reduced plant density because plants benefit less by visits from honey bees than wild pollinators that coevolved with the plants.As wild pollinators are displaced, plants evolve by increasing investment in traits that are attractive for honey bees but not wild pollinators. This evolutionary switch promotes wild pollinator displacement. However, higher mutualism investment costs by the plant to the honey bee can promote pollinator coexistence.Our results show plant evolution can promote displacement of wild pollinators by managed honey bees, while limited plant evolution may lead to pollinator coexistence. More broadly, effects of honey bees on wild pollinators in agroecosystems, and effects on ecosystem services, may depend on the capacity of plant populations to evolve.

Pollen limitation in a single year is not compensated by future reproduction

Seed production is critical to the persistence of most flowering plant populations, but may be strongly pollen limited. To what extent long-lived plants can compensate pollen limitation by increasing future reproduction is poorly understood. We tested for compensation in two Dactylorhiza species that differ in reproductive investment by experimentally reducing and increasing pollination in two independent annual cohorts and monitoring demographic responses in the subsequent 2 years for the 2014 cohort and in 1 year for the 2015 cohort. Demographic rates in the second year were significantly affected by pollination treatment in both species, but specific responses differed both between species and years. There was no effect of pollination treatment on demographic responses in the third year. In sum, effects were too weak to make up for the lost reproduction; total fruit production across all 3 years was by far highest in the increased pollination treatment in both species. These results show that long-lived plants do not necessarily compensate for pollen limitation by increasing future reproduction. It further suggests that even periodic declines in pollination rates may have severe demographic consequences, particularly in populations where germination is not density dependent. This has implications for predicting plant population viability in response to changes in pollination intensity.

Progressive deterioration of pollination service detected in a 17-year study vanishes in a 26-year study

Widespread reports of declining populations of pollinators have raised concerns that plant populations may be incurring increasing shortfalls in pollination, but few studies have measured pollination deficits over enough seasons to detect such changes. I have conducted pollen-supplementation experiments in a wild population of the glacier lily (Erythronium grandiflorum, Liliaceae) from 1993 to 2018. Pollination deficits were estimated by comparing the fruit set of hand-pollinated, single-flowered plants to that of open-pollinated controls. For a subset of years, seed set data were also available. A previous publication reported a significant deterioration of pollination in this population from 1993 to 2009, and suggested phenological dislocation as a possible cause. That deterioration is no longer evident in the longer-term data set. Very long time series may be necessary to detect temporal trends in pollination service. This population consistently experiences stronger pollination deficits before its flowering peak than after. This heterogeneity suggests caution in characterizing a population as pollination-limited or not, even within a single season.

Mating systems and avoidance of inbreeding depression as evolutionary drivers of pollen limitation in animal-pollinated self-compatible plants

BACKGROUND AND AIMS

Most theory addressing the evolution of pollen limitation in flowering plants focuses on stochasticity in the relative abundance of plant and pollinator populations affecting trade-offs in resource allocation to ovule production or pollinator attraction vs. seed maturation. Mating system evolution is an underappreciated but potentially widespread additional mechanism for the evolutionary emergence of pollen limitation in animal-pollinated self-compatible plants.

METHODS

We model individual plant flowering phenologies influencing both pollinator attraction and geitonogamous self-fertilization caused by pollinator movements among flowers within plants, incorporating demographic but not environmental stochasticity. Plant phenology and the resulting pollen limitation are analysed at evolutionarily stable equilibria (ESS). Pollen limitation is measured by two quantities: the proportion of unpollinated flowers and the reduction in maternal fitness caused by inbreeding depression in selfed seeds.

KEY RESULTS

When pollinators visit multiple flowers per plant, pollen limitation is never minimized at an ESS and results from the evolution of flowering phenologies balancing the amount and genetic composition (outbred vs. inbred) of pollen receipt.

CONCLUSIONS

Results are consistent with previous theory demonstrating that pollen limitation can be an evolved property, not just a constraint; they complement existing models by showing that plant avoidance of inbreeding depression constitutes a genetic mechanism contributing to evolution of pollen limitation, in addition to ecological mechanisms previously studied.

Evolution of development of pollen performance

With the origin of pollination in ancient seed plants, the male gametophyte ("pollen") began to evolve a new and unique life history stage, the progamic phase, a post-pollination period in which pollen sexual maturation occurs in interaction with sporophyte-derived tissues. Pollen performance traits mediate the timing of the fertilization process, often in competition with other pollen, via the speed of pollen germination, sperm development, and pollen tube growth. Studies of pollen development rarely address the issue of performance or its evolution, which involves linking variation in developmental rates to relative fitness within populations or to adaptations on a macroevolutionary scale. Modifications to the pollen tube pathway and changes in the intensity of pollen competition affect the direction and strength of selection on pollen performance. Hence, pollen developmental evolution is always contextual-it involves both the population biology of pollen reaching stigmas and the co-evolution of sporophytic traits, such as the pollen tube pathway and mating system. For most species, performance evolution generally reflects a wandering history of periods of directional selection and relaxed selection, channeled by developmental limitations, a pattern that favors the accumulation of diversity and redundancy in developmental mechanisms and the genetic machinery. Developmental biologists are focused on finding universal mechanisms that underlie pollen function, and these are largely mechanisms that have evolved through their effects on performance. Here, we suggest ways in which studies of pollen performance or function could progress by cross-fertilization between the "evo" and "devo" fields.

GloPL, a global data base on pollen limitation of plant reproduction

Plant reproduction relies on transfer of pollen from anthers to stigmas, and the majority of flowering plants depend on biotic or abiotic agents for this transfer. A key metric for characterizing if pollen receipt is insufficient for reproduction is pollen limitation, which is assessed by pollen supplementation experiments. In a pollen supplementation experiment, fruit or seed production by flowers exposed to natural pollination is compared to that following hand pollination either by pollen supplementation (i.e. manual outcross pollen addition without bagging) or manual outcrossing of bagged flowers, which excludes natural pollination. The GloPL database brings together data from 2969 unique pollen supplementation experiments reported in 927 publications published from 1981 to 2015, allowing assessment of the strength and variability of pollen limitation in 1265 wild plant species across all biomes and geographic regions globally. The GloPL database will be updated and curated with the aim of enabling the continued study of pollen limitation in natural ecosystems and highlighting significant gaps in our understanding of pollen limitation.

Functional role of long-lived flowers in preventing pollen limitation in a high elevation outcrossing species

Low pollinator visitation in harsh environments may lead to pollen limitation which can threaten population persistence. Consequently, avoidance of pollen limitation is expected in outcrossing species subjected to habitually low pollinator service. The elevational decline in visitation rates on many high mountains provides an outstanding opportunity for addressing this question. According to a recent meta-analysis, levels of pollen limitation in alpine and lowland species do not differ. If parallel trends are manifested among populations of alpine species with wide elevational ranges, how do their uppermost populations contend with lower visitation? We investigated visitation rates and pollen limitation in high Andean Rhodolirium montanum. We test the hypothesis that lower visitation rates at high elevations are compensated for by the possession of long-lived flowers. Visitation rates decreased markedly over elevation as temperature decreased. Pollen limitation was absent at the low elevation site but did occur at the high elevation site. While initiation of stigmatic pollen deposition at high elevations was not delayed, rates of pollen arrival were lower, and cessation of pollination, as reflected by realized flower longevity, occurred later in the flower lifespan. Comparison of the elevational visitation decline and levels of pollen limitation indicates that flower longevity partially compensates for the lower visitation rates at high elevation. The functional role of flower longevity, however, was strongly masked by qualitative pollen limitation arising from higher abortion levels attributable to transference of genetically low-quality pollen in large clones. Stronger clonal growth at high elevations could counterbalance the negative fitness consequences of residual pollen limitation due to low visitation rates and/or difficult establishment under colder conditions. Visitation rates on the lower part of the elevational range greatly exceeded community rates recorded several decades ago when the planet was cooler. Current pollen limitation for some species in some habitats might underestimate historical levels.

Does an 'oversupply' of ovules cause pollen limitation?

Lifetime seed production can be constrained by shortfalls of pollen receipt ('pollen limitation'). The ovule oversupply hypothesis states that, in response to unpredictable pollen availability, plants evolve to produce more ovules than they expect to be fertilized, and that this results in pollen limitation of seed production. Here, we present a cartoon model and a model of optimal plant reproductive allocations under stochastic pollen receipt to evaluate the hypothesis that an oversupply of ovules leads to increased pollen limitation. We show that an oversupply of ovules has two opposing influences on pollen limitation of whole-plant seed production. First, ovule oversupply increases the likelihood that pollen receipt limits the number of ovules that can be fertilized ('prezygotic pollen limitation'). Second, ovule oversupply increases the proportion of pollen grains received that are used to fertilize ovules ('pollen use efficiency'). As a result of these opposing influences, ovule oversupply has only a modest effect on the degree to which lifetime seed production is constrained by pollen receipt, producing a small decrease in the incidence of pollen limitation. Ovule oversupply is not the cause of the pollen limitation problem, but rather is part of the evolutionary solution to that problem.

Pollen--tiny and ephemeral but not forgotten: New ideas on their ecology and evolution

Ecologists and evolutionary biologists have been interested in the functional biology of pollen since the discovery in the 1800s that pollen grains encompass tiny plants (male gametophytes) that develop and produce sperm cells. After the discovery of double fertilization in flowering plants, botanists in the early 1900s were quick to explore the effects of temperature and maternal nutrients on pollen performance, while evolutionary biologists began studying the nature of haploid selection and pollen competition. A series of technical and theoretic developments have subsequently, but usually separately, expanded our knowledge of the nature of pollen performance and how it evolves. Today, there is a tremendous diversity of interests that touch on pollen performance, ranging from the ecological setting on the stigma, structural and physiological aspects of pollen germination and tube growth, the form of pollen competition and its role in sexual selection in plants, virus transmission, mating system evolution, and inbreeding depression. Given the explosion of technical knowledge of pollen cell biology, computer modeling, and new methods to deal with diversity in a phylogenetic context, we are now more than ever poised for a new era of research that includes complex functional traits that limit or enhance the evolution of these deceptively simple organisms.

Pollen limitation is not the rule in nival plants: A study from the European Central Alps

PREMISE OF THE STUDY

Seed output of high-mountain plants in the uppermost life zones is highly variable. One possible reason might be pollen limitation due to inadequate pollinator services.

METHODS

We tested this hypothesis for the insect-pollinated species Cerastium uniflorum, Ranunculus glacialis, and Saxifraga bryoides, which have their distribution center in the subnival and nival zone of the European Central Alps. We recorded insect visitors and determined their impact as pollinators. By analyzing pollination success and seed set following natural and saturating hand pollination, the magnitude of quantitative and qualitative pollen limitation was assessed.

KEY RESULTS

Anthomyiid and muscid flies had the highest pollination impact for R. glacialis and S. bryoides and syrphids for C. uniflorum. Natural stigma pollen load was highly variable in individual flowers of all species, but in most cases the number of conspecific pollen grains clearly exceeded the number of ovules to be fertilized. There was also a surplus in germinated pollen grains, whereas the pollen tube to ovule ratio was only sufficient in R. glacialis (2.6 on average) and S. bryoides (1.3), but not in C. uniflorum (0.6). Accordingly, seed to ovule ratio was around 0.8 in R. glacialis, 0.7 in S. bryoides but 0.4 in C. uniflorum. In C. uniflorum, saturating pollination slightly increased seed set. Regression analyses revealed that natural pollination success was more frequently limited by quality than by quantity.

CONCLUSIONS

Our results do not support the idea of chronic, widespread pollen limitation in the subnival but rather fit into the concept of parental optimism by overinvesting in the number of ovules as an adaptation to variable resource availability.

Modest Pollen Limitation of Lifetime Seed Production Is in Good Agreement with Modest Uncertainty in Whole-Plant Pollen Receipt

We recently introduced a model that predicts the degree to which a plant's lifetime seed production may be constrained by unpredictable shortfalls of pollen receipt ("pollen limitation"). Burd's comment in this issue criticized our analysis, first by arguing that the empirical literature documents much higher levels of pollen limitation than our model predicts and then suggesting that the apparent discrepancy stemmed from our (1) underestimating the costs of securing a fertilized ovule and (2) assuming too little unpredictability in whole-plant pollen receipt. We reply as follows. First, the empirical literature must be consulted carefully. Burd relies on pollen supplementation experiments performed on parts of plants or on whole plants but during only one reproductive season for polycarpic perennials; in both cases, resource reallocation often leads to gross overestimates of pollen limitation. We comprehensively review pollen limitation estimates that are free of these estimation problems and find strong agreement with our model predictions. Second, although cost estimates for different components of seed production are imprecise, errors are likely to be small relative to the >1,000-fold differences observed across plant species, the primary focus of our article. Finally, contrary to Burd's argument, pollen receipt by entire plants is much more predictable than that by individual flowers because the flower-to-flower variation "averages out" when summed across many flowers. Our model uses parameter values that are in broad agreement with the empirical record of modest plant-to-plant variation in pollen receipt and thus predicts the generally modest pollen limitation that is observed in nature.

Pollen limitation and flower abortion in a wind-pollinated, masting tree

Pollen limitation is a key assumption of theories that explain mast seeding, which is common among wind-pollinated and woody plants. In particular, the pollen coupling hypothesis and pollination Moran effect hypothesis assume pollen limitation as a factor that synchronizes seed crops across individuals. The existence of pollen limitation has not, however, been unambiguously demonstrated in wind-pollinated, masting trees. We conducted a two-year pollen supplementation experiment on a masting oak species, Quercus lobata. Supplemental pollen increased acorn set in one year but not in the other, supporting the importance of pollen coupling and pollination Moran effect models of mast seeding. We also tracked the fate of female flowers over five years and found that the vast majority of flowers were aborted for reasons unrelated to pollination, even in the presence of excess pollen. Pollen limitation can reduce annual seed set in a wind-pollinated tree, but factors other than pollen limitation cause the majority of flower abortion.