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

Pollinator shift ensures reproductive success in a camouflaged alpine plant

BACKGROUND AND AIMS

There are intrinsic conflicts between signalling to mutualists and concealing (camouflaging) from antagonists. Like animals, plants also use camouflage as a defence against herbivores. However, this can potentially reduce their attractiveness to pollinators.

METHODS

Using Fritillaria delavayi, an alpine camouflaged plant with inter-population floral colour divergence, we tested the influence of floral trait differences on reproduction. We conducted pollination experiments, measured floral morphological characteristics, estimated floral colours perceived by pollinators, analysed floral scent and investigated reproductive success in five populations.

KEY RESULTS

We found that the reproduction of F. delavayi depends on pollinators. Under natural conditions, a flower-camouflaged population had 100 % fruit set and similar seed set to three out of four yellow-flowered populations. Bumblebees are important pollinators in the visually conspicuous yellow-flowered populations, whereas flies are the only pollinator in the flower-camouflaged population, visiting flowers more frequently than bumblebees. The camouflaged flowers cannot be discriminated from the rock background as perceived by pollinators, but may be located by flies through olfactory cues.

CONCLUSIONS

Collectively, our results demonstrate that the flower-camouflaged population has different reproductive traits from the visually conspicuous yellow-flowered populations. A pollinator shift from bumblebees to flies, combined with high visitation frequency, compensates for the attractiveness disadvantage in camouflaged plants.

Effective population size mediates the impact of pollination services on pollen limitation

Inadequate pollen receipt limits flowering plant reproduction worldwide. Ecological causes of pollen limitation (PL), like pollinator scarcity and low plant abundance, have been a primary research focus. The genetic diversity of plant populations could impact both quantity and quality components of PL in concert with ecological factors, yet empirical examples are lacking. We evaluated joint effects of ecological factors (flower abundance, pollinator visitation) and genetic effective population size (NE) on PL across 13 populations of the common herb Argentina anserina. We used a histological approach with 5504 styles from 1137 flowers to separate quantity and quality components of PL, and link these to reproductive output. NE and pollinator visitation interacted to shape PL, but NE had stronger direct effects. Effectively smaller populations experienced stronger quantity PL, and controlled crosses in a pollinator-free environment revealed that stigmatic pollen quantity was an intrinsic population-level attribute that increased with NE. Pollinator visitation enhanced pollen quality, but only in effectively larger populations. Quantity and quality PL negatively impacted fruit and seed set, respectively. Results highlight that PL is dictated by plant population genetic diversity in addition to commonly evaluated ecological factors. Efforts to support pollinators will more strongly enhance plant reproduction in genetically diverse populations.

Concepts in Alpine Plant Ecology

The alpine life zone is perhaps the only biome that occurs globally where mountains are high enough. At latitudinally varying elevation, the alpine belt hosts small stature plants that vary greatly in morphology, anatomy and physiology. In this contribution, I summarize a number of principles that govern life in what is often considered a cold and hostile environment. The 12 conceptual frameworks depicted include the key role of aerodynamic decoupling from free atmospheric climatic conditions, the problematic concepts of limitation and stress in an evolutionary context, and the role of developmental flexibility and functional diversity. With its topography driven habitat diversity, alpine plant diversity is buffered against environmental change, and the multitude of microclimatic gradients offers 'experiments by nature', the power of which awaits multidisciplinary exploration.

Disentangling the components of pollen limitation in a widespread herb with gametophytic self-incompatibility

PREMISE

Seed production is frequently limited by the receipt of insufficient or low-quality pollen, collectively termed "pollen limitation" (PL). In taxa with gametophytic self-incompatibility (GSI), incompatible pollen can germinate on stigmas but pollen tubes are arrested in styles. This allows for estimates of pollen performance before, during, and after self-recognition, as well as insight into the factors underlying pollen quality limitation in GSI taxa.

METHODS

We scored pollen performance following self and outcross pollinations in Argentina anserina to identify the location of self-recognition and establish the relationship between pollen tubes and seed production. We then estimated quantity and quality components of PL from >3300 field-collected styles. We combined our results with other studies to test the prediction that low pollen quality, but not quantity, drives higher PL in self-incompatible (SI) taxa than in self-compatible taxa (SC).

RESULTS

Self and outcross pollen germinated readily on stigmas, but 96% of germinated self-pollen was arrested during early tube elongation. Reproduction in the field was more limited by pollen quality than by quantity, and pollen failure near the location of self-recognition was a stronger barrier to fertilization than pollen germination. Across 26 taxa, SI species experienced stronger pollen quality, but not quantity, limitation than SC species.

CONCLUSIONS

Evaluating pollen performance at multiple points within pistils can elucidate potential causes of pollen quality limitation. The receipt of incompatible pollen inhibits fertilization success more than insufficient pollen receipt or poor pollen germination in A. anserina. Likewise, pollen quality limitation drives high overall PL in other SI taxa.

The Early Season Community of Flower-Visiting Arthropods in a High-Altitude Alpine Environment

In mountain ecosystems, climate change can cause spatiotemporal shifts, impacting the composition of communities and altering fundamental biotic interactions, such as those involving flower-visiting arthropods. On of the main problems in assessing the effects of climate change on arthropods in these environments is the lack of baseline data. In particular, the arthropod communities on early flowering high-altitude plants are poorly investigated, although the early season is a critical moment for possible mismatches. In this study, we characterised the flower-visiting arthropod community on the early flowering high-altitude Alpine plant, Androsace brevis (Primulaceae). In addition, we tested the effect of abiotic factors (temperature and wind speed) and other variables (time, i.e., hour of the day, and number of flowers per plant) on the occurrence, abundance, and diversity of this community. A. brevis is a vulnerable endemic species growing in the Central Alps above 2000 m asl and flowering for a very short period immediately after snowmelt, thus representing a possible focal plant for arthropods in this particular moment of the season. Diptera and Hymenoptera were the main flower visitors, and three major features of the community emerged: an evident predominance of anthomyiid flies among Diptera, a rare presence of bees, and a relevant share of parasitoid wasps. Temperature and time (hour of the day), but not wind speed and number of flowers per plant, affected the flower visitors' activity. Our study contributes to (1) defining the composition of high-altitude Alpine flower-visiting arthropod communities in the early season, (2) establishing how these communities are affected by environmental variables, and (3) setting the stage for future evaluation of climate change effects on flower-visiting arthropods in high-altitude environments in the early season.

Niche Variation in Endemic Lilium pomponium on a Wide Altitudinal Gradient in the Maritime Alps

The relationship between altitudinal and ecological gradients has long been a dominant theme in plant ecology; moreover, how species respond to climate change has renewed this interest. Mediterranean mountains are often hotspots of endemism, and some endemic species have local distributions that span different climatic belts; hence, local variations in topography and fine-scaled niche conditions may play crucial roles in their persistence along such gradients. Studies of the fine-scaled niche are, however, very rare; most studies involve broad-scale variations in climatic parameters. The Turban lily, Lilium pomponium L. is endemic to the Maritime and Ligurian Alps, where it occurs across a wide altitudinal gradient. Previous work has shown no link between climatic marginality and geographic range limits on morphological traits and genetic variability; however, possible variations of local topographic and ecological parameters have not yet been examined. The objective of this paper is to characterise local ecological niche conditions of L. pomponium populations in the different bioclimatic zones it occupies along the altitudinal gradient. The species occurs in four main types of microecological niches. One of these niche types, with a high mineral cover, is the most abundant—type 2: it was detected in 39% of sampled quadrats and occurs across the whole bioclimatic gradient. Other niche types are more limited to subsections of the gradient: type 3 (in 19% of sampled quadrats) is restricted to high-altitude sites (>1070 m.a.s.l.) and is characterised by high vegetation and litter cover; type 4 (26%) corresponds to more forested habitats on substrates with low water retention capacities, in more inland zones close to the centre of L. pomponium distribution and across a range of altitudes; and type 1 (16% of quadrat) only occurs in the Mediterranean part of the gradient, close to distribution limits in pockets of soil among large blocks of rocks, mainly found at mid-altitudes. Despite heterogeneity in the spatial locations of niche types, there is no correspondence between ecological gradients and the distribution limits of this species. Knowledge of the fine-scaled ecological conditions that determine niche types is thus essential for conservation management of the habitats of this species and for the exploration of its possible response to ongoing climate change.

Flowering Phenology Adjustment and Flower Longevity in a South American Alpine Species

Delayed flowering due to later snowmelt and colder temperatures at higher elevations in the alpine are expected to lead to flowering phenological adjustment to prevent decoupling of peak flowering from the warmest time of the year, thereby favoring pollination. However, even if flowering is brought forward in the season at higher elevations, an elevational temperature gap is likely to remain between the high- and low-elevation populations of a species at the time these reach peak flowering on account of the atmospheric reduction in temperature with increasing elevation. The negative effect of this temperature gap on pollination could be compensated by plastically-prolonged flower life spans at higher elevations, increasing the probability of pollination. In a tightly temperature-controlled study, the flowering phenology adjustment and flower longevity compensation hypotheses were investigated in an alpine species in the Andes of central Chile. The snow free period varied from 7 to 8.2 months over 810 m elevation. Temperatures were suitable for growth on 82-98% of the snow free days. Flowering onset was temporally displaced at the rate of 4.6 d per 100 m increase in elevation and flowering was more synchronous at higher elevations. Flowering phenology was adjusted over elevation. The latter was manifest in thermal sums tending to decrease with elevation for population flowering onset, 50% flowering, and peak flowering when the lower thermal limit for growth (T_BASE) was held constant over elevation. For T_BASE graded over elevation so as to reflect the growing season temperature decline, thermal sums did not vary with elevation, opening the door to a possible elevational decline in the thermal temperature threshold for growth. Potential flower longevity was reduced by passive warming and was more prolonged in natural populations when temperatures were lower, indicating a plastic trait. Pollination rates, as evaluated with the Relative Pollination Rate index (RPR), when weighted for differences in floral abundance over the flowering season, declined with elevation as did fruit set. Contrary to expectation, the life-spans of flowers at higher elevations were not more prolonged and failed to compensate for the elevational decrease in pollination rates. Although strong evidence for phenological adjustment was forthcoming, flower longevity compensation did not occur over Oxalis squamata´s elevational range. Thus, flower longevity compensation is not applicable in all alpine species. Comparison with work conducted several decades ago on the same species in the same area provides valuable clues regarding the effects of climate change on flowering phenology and fitness in the central Chilean alpine where temperatures have been increasing and winter snow accumulation has been declining.

Manual Sampling and Video Observations: An Integrated Approach to Studying Flower-Visiting Arthropods in High-Mountain Environments

Despite the rising interest in biotic interactions in mountain ecosystems, little is known about high-altitude flower-visiting arthropods. In particular, since the research in these environment can be limited or undermined by harsh conditions and logistical difficulties, it is mandatory to develop effective approaches that maximize possibilities to gather high-quality data. Here we compared two different methods, manual sampling and video observations, to investigate the interactions between the high-mountain arthropod community and flowers of Androsace brevis (Primulaceae), a vulnerable endemic alpine species with a short flowering period occurring in early season. We manually sampled flower-visiting arthropods according to the timed-observations method and recorded their activity on video. We assessed differences and effectiveness of the two approaches to estimate flower-visiting arthropod diversity and to identify potential taxa involved in A. brevis pollination. Both methods proved to be effective and comparable in describing the diversity of flower visitors at a high taxonomic level. However, with manual sampling we were able to obtain a fine taxonomic resolution for sampled arthropods and to evaluate which taxa actually carry A. brevis pollen, while video observations were less invasive and allowed us to assess arthropod behavior and to spot rare taxa. By combining the data obtained with these two approaches we could accurately identify flower-visiting arthropods, characterize their behavior, and hypothesize a role of Hymenoptera Apoidea and Diptera Brachycera in A. brevis pollination. Therefore, we propose integrating the two approaches as a powerful instrument to unravel interactions between flowering plants and associated fauna that can provide crucial information for the conservation of vulnerable environments such as high-mountain ecosystems.

Genetic rescue by distant trees mitigates qualitative pollen limitation imposed by fine-scale spatial genetic structure

Restricted seed dispersal frequently leads to fine-scale spatial genetic structure (i.e., FSGS) within plant populations. Depending on its spatial extent and the mobility of pollinators, this inflated kinship at the immediate neighbourhood can critically impoverish pollen quality. Despite the common occurrence of positive FSGS within plant populations, our knowledge regarding the role of long-distance pollination preventing reproductive failure is still limited. Using microsatellite markers, we examined the existence of positive FSGS in two low-density populations of the tree Pyrus bourgaeana. We also designed controlled crosses among trees differing in their kinship to investigate the effects of increased local kinship on plant reproduction. We used six pollination treatments and fully monitored fruit production, fruit and seed weight, proportion of mature seeds per fruit, and seed germination. Our results revealed positive FSGS in both study populations and lower fruit initiation in flowers pollinated with pollen from highly-genetically related individuals within the neighbourhood, with this trend intensifying as the fruit development progressed. Besides, open-pollinated flowers exhibited lower performance compared to those pollinated by distant pollen donors, suggesting intense qualitative pollen limitation in natural populations. We found positive fine-scale spatial genetic structure is translated into impoverished pollen quality from nearby pollen donors which negatively impacts the reproductive success of trees in low-density populations. Under this scenario of intrapopulation genetic rescue by distant pollen donors, the relevance of highly-mobile pollinators for connecting spatially and genetically distant patches of trees may be crucial to safeguarding population recruitment.

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.

An Unexplored Side of Regeneration Niche: Seed Quantity and Quality Are Determined by the Effect of Temperature on Pollen Performance

In 1977, Peter Grubb introduced the regeneration niche concept, which assumes that a plant species cannot persist if the environmental conditions are only suitable for adult plant growth and survival, but not for seed production, dispersal, germination, and seedling establishment. During the last decade, this concept has received considerable research attention as it helps to better understand community assembly, population dynamics, and plant responses to environmental changes. Yet, in its present form, it focuses too much on the post-fertilization stages of plant sexual reproduction, neglecting the fact that the environment can operate as a constraint at many points in the chain of processes necessary for successful regeneration. In this review, we draw the attention of the plant ecology research community to the pre-fertilization stages of plant sexual reproduction, an almost ignored but important aspect of the regeneration niche, and their potential consequences for successful seed production. Particularly, we focus on how temperature affects pollen performance and determines plant reproduction success by playing an important role in the temporal and spatial variations in seed quality and quantity. We also review the pollen adaptations to temperature stresses at different levels of plant organization and discuss the plasticity of the performance of pollen under changing temperature conditions. The reviewed literature demonstrates that pre-fertilization stages of seed production, particularly the extreme sensitivity of male gametophyte performance to temperature, are the key determinants of a species' regeneration niche. Thus, we suggest that previous views stating that the regeneration niche begins with the production of seeds should be modified to include the preceding stages. Lastly, we identify several gaps in pollen-related studies revealing a framework of opportunities for future research, particularly how these findings could be used in the field of plant biology and ecology.

Effects of cold treatments on fitness and mode of reproduction in the diploid and polyploid alpine plant Ranunculus kuepferi (Ranunculaceae)

Background and Aims

Alpine plants grow in harsh environments and are thought to face occasional frost during the sensitive reproductive phase. Apomixis (asexual reproduction via seed) can be advantageous when sexual reproduction is disturbed by cold stress. Apomictic polyploids tend to grow in colder climates than their sexual diploid relatives. Whether cold temperatures actually induce apomixis was unknown to date.

Methods

We tested experimentally in climate cabinets for effects of low temperatures and repeated frost on phenology, fitness and mode of reproduction in diploid and tetraploid cytotypes of the alpine species Ranunculus kuepferi. The reproduction mode was determined via flow cytometric seed screening (FCSS).

Key Results

Diploids produced the first flowers earlier than the tetraploids in all treatments. Cold treatments significantly reduced the fitness of both cytotypes regarding seed set, and increased the frequency of apomictic seed formation in diploids, but not in tetraploids. Over consecutive years, the degree of facultative apomixis showed individual phenotypic plasticity.

Conclusions

Cold stress is correlated to expression of apomixis in warm-adapted, diploid R. kuepferi, while temperature-tolerant tetraploids just maintain facultative apomixis as a possible adaptation to colder climates. However, expression of apomixis may not depend on polyploidy, but rather on failure of the sexual pathway.

Altitudinal, temporal and trophic partitioning of flower-visitors in Alpine communities

The cross-pollination of most alpine plants depends on insects, whose altitudinal distribution is limited by temperature. However, although global warming is causing shifts in temporal and spatial species distribution, we are still largely unaware of how plant-pollinator interactions change with elevation and time along altitudinal gradients. This makes the detection of endangered interactions and species challenging. In this study, we aimed at providing such a reference, and tested if and how the major flower-visiting insect orders and families segregated by altitude, phenology and foraging preferences along an elevational gradient from 970 m to 2700 m in the Alps. Flies were the main potential pollinators from 1500 m, as bees and beetles decreased rapidly above that limit. Diptera, Coleoptera and Hymenoptera differed significantly in the angiosperm assemblages visited. Within Diptera, the predominant group, major families segregated by both phenology and foraging preferences along the gradient. Empidids, muscids and anthomyiids, whose role in pollination has never been investigated, dominated the upper part of the gradient. Our results thus suggest that flies and the peculiar plants they visit might be particularly at risk under global warming, and highlight the blatant lack of studies about critical components of these rich, yet fragile mountain ecosystems.

How does climate change affect regeneration of Mediterranean high-mountain plants? An integration and synthesis of current knowledge

Mediterranean mountains are extraordinarily diverse and hold a high proportion of endemic plants, but they are particularly vulnerable to climate change, and most species distribution models project drastic changes in community composition. Retrospective studies and long-term monitoring also highlight that Mediterranean high-mountain plants are suffering severe range contractions. The aim of this work is to review the current knowledge of climate change impacts on the process of plant regeneration by seed in Mediterranean high-mountain plants, by combining available information from observational and experimental studies. We also discuss some processes that may provide resilience against changing environmental conditions and suggest some research priorities for the future. With some exceptions, there is still little evidence of the direct effects of climate change on pollination and reproductive success of Mediterranean high-mountain plants, and most works are observational and/or centred only in the post-dispersal stages (germination and establishment). The great majority of studies agree that the characteristic summer drought and the extreme heatwaves, which are projected to be more intense in the future, are the most limiting factors for the regeneration process. However, there is an urgent need for studies combining elevational gradient approaches with experimental manipulations of temperature and drought to confirm the magnitude and variability of species' responses. There is also limited knowledge about the ability of Mediterranean high-mountain plants to cope with climate change through phenotypic plasticity and local adaptation processes. This could be achieved by performing common garden and reciprocal translocation experiments with species differing in life history traits.

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.

A linkage between flowering phenology and fruit-set success of alpine plant communities with reference to the seasonality and pollination effectiveness of bees and flies

To clarify the linkage between flowering phenology and pollination success in alpine plant communities, we quantified the seasonality of flower visitors, the temporal transition of floral resources, and the variation in pollination success of alpine plants in northern Japan. Bumble bees, syrphid flies, and non-syrphid flies were the predominant flower visitors. Foraging activity of bumble bees increased toward the late flowering period reflecting the life cycle of colony development. The activity of syrphid flies was sensitive to ambient temperature, while that of non-syrphid flies remained high throughout the season. Flower production of bee-pollinated plants fluctuated significantly between years with a bimodal pattern peaking in the early and late periods, while flower production of fly-pollinated plants was less variable between years. Fruit-set success of bee-pollinated plants increased considerably from the early to the late flowering period, while the trend for fly-pollinated plants was less marked. Three times more visits of dipteran insects are necessary for fly-pollinated plants to achieve fruiting success comparable to bee-pollinated plants. Bumble bees are potentially excellent pollinators, but the visitation frequency is low early in the season. Lower pollination ability of dipteran insects may be compensated for by abundant flower visits. The relationships between flowering phenology and fruit-set success of alpine plant communities highly depend on the type of pollinators.

Plastic Responses Contribute to Explaining Altitudinal and Temporal Variation in Potential Flower Longevity in High Andean Rhodolirion montanum

The tendency for flower longevity to increase with altitude is believed by many alpine ecologists to play an important role in compensating for low pollination rates at high altitudes due to cold and variable weather conditions. However, current studies documenting an altitudinal increase in flower longevity in the alpine habitat derive principally from studies on open-pollinated flowers where lower pollinator visitation rates at higher altitudes will tend to lead to flower senescence later in the life-span of a flower in comparison with lower altitudes, and thus could confound the real altitudinal pattern in a species´ potential flower longevity. In a two-year study we tested the hypothesis that a plastic effect of temperature on flower longevity could contribute to an altitudinal increase in potential flower longevity measured in pollinator-excluded flowers in high Andean Rhodolirium montanum Phil. (Amaryllidaceae). Using supplemental warming we investigated whether temperature around flowers plastically affects potential flower longevity. We determined tightly temperature-controlled potential flower longevity and flower height for natural populations on three alpine sites spread over an altitudinal transect from 2350 and 3075 m a.s.l. An experimental increase of 3.1°C around flowers significantly decreased flower longevity indicating a plastic response of flowers to temperature. Flower height in natural populations decreased significantly with altitude. Although temperature negatively affects flower longevity under experimental conditions, we found no evidence that temperature around flowers explains site variation in flower longevity over the altitudinal gradient. In a wetter year, despite a 3.5°C temperature difference around flowers at the extremes of the altitudinal range, flower longevity showed no increase with altitude. However, in a drier year, flower longevity increased significantly with altitude. The emerging picture suggests an increase in flower longevity along the altitudinal gradient is less common for potential flower longevity than for open-pollination flower longevity. Independently of any selection that may occur on potential longevity, plastic responses of flowers to environmental conditions are likely to contribute to altitudinal variation in flower longevity, especially in dry alpine areas. Such plastic responses could push flowers of alpine species towards shorter life-lengths under climate change, with uncertain consequences for successful pollination and plant fitness in a warming world.

One fly to rule them all-muscid flies are the key pollinators in the Arctic

Global change is causing drastic changes in the pollinator communities of the Arctic. While arctic flowers are visited by a wide range of insects, flies in family Muscidae have been proposed as a pollinator group of particular importance. To understand the functional outcome of current changes in pollinator community composition, we examined the role of muscids in the pollination of a key plant species, the mountain avens (Dryas). We monitored the seed set of Dryas across 15 sites at Zackenberg, northeast Greenland, and used sticky flower mimics and DNA barcoding to describe the flower-visiting community at each site. To evaluate the consequences of shifts in pollinator phenology under climate change, we compared the flower visitors between the early and the late season. Our approach revealed a diverse community of insects visiting Dryas, including two-thirds of all insect species known from the area. Even against this diverse background, the abundance of muscid flies emerged as a key predictor for seed set in Dryas, whereas overall insect abundance and species richness had little or no effect. With muscid flies as the main drivers of the pollinating function in the High Arctic, a recently observed decline in their abundances offers cause for concern.

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.