Pollen Transfer Research Articles

Secondary pollen presentation: More than to increase pollen transfer precision

AbstractPollination precision and efficiency have been deemed to be important driving forces in floral evolution. Herkogamy reduction is a main mechanism to increase pollination precision. Secondary pollen presentation (SPP), by which pollen is presented on other floral organs especially pistils, has been widely accepted as a special mechanism to increase pollen transfer precision through spatial reduction of the anther–stigma distance, that is, minimized herkogamy. This overlooks a potential driving force, that is expanding the pollination niche through converting pollen thieves and nectar robbers into effective pollinators. We selected two species as study models with typical pistillate SPP, Pavetta hongkongensis Bremek. (Rubiaceae) and Scaevola taccada (Gaertn.) Roxb. (Goodeniaceae). In both species, two distinct pollinator functional groups were recognized. Short‐tongued bees and flies fed on pollen on stigmas but also stole pollen from anthers and robbed nectar, whereas long‐tongued hawkmoths and butterflies only collected nectar. Emasculation had no influence on long‐tongued pollinators, but significantly decreased the visitation frequency of short‐tongued visitors and fruit set, compared to intact flowers, demonstrating short‐tongued visitors did not effectively pollinate and acted merely as pollen thieves or nectar robbers when SPP was absent. Data from the two plant species clearly indicated pistillate SPP has additional adaptive advantages of converting ineffective visitors into pollinators and consequently widening the pollination niche, which could help plants overcome environmental stochasticity. Our results suggest that multiple selective forces drive the evolution of SPP and the minimization of herkogamy.

Optimal pollen stickiness to pollinators for maximizing paternal fitness: Increased number of recipient flowers or increased pollen deposition on recipient flowers?

A plant can sire more seeds by increasing the number of pollen recipient flowers or the amount of pollen deposited on recipient flowers. We theoretically analyzed how pollen stickiness contributes to paternal fitness through changing the pattern of pollen dispersal including both the number of recipient flowers and overall pollen deposition (the overall amount of pollen deposited on recipient flowers) in animal-pollinated plants. We developed a numerical model in which pollen stickiness to pollinators increases with production of expensive materials on pollen surfaces, and a high level of stickiness diminishes the proportions of pollen lost from a pollinator body during a flight and pollen deposited on a stigma during a visit. We found that the number of recipient flowers monotonically increased with increasing pollen stickiness allocation while overall pollen deposition was maximized at a certain amount of stickiness allocation. We demonstrated that evolutionarily stable pollen stickiness attained many recipient flowers at the expense of overall pollen deposition in most cases while it merely favored maximization of overall pollen deposition in all other cases. Sticky pollen evolved if pollinators were highly likely to drop pollen during flights and did not diffuse well. In this situation, the evolutionarily stable pattern of pollen dispersal was acquisition of many pollen recipient flowers rather than maximization of overall pollen deposition. Sticky pollen also evolved if additional sticking elements were moderately effective in increasing the force of adhesion to pollinators. Pollen stickiness has a significant effect on the pattern of pollen dispersal via the extent of pollen carryover, and our results suggest that plants maximize paternal fitness by giving pollen the optimal stickiness, which varies with pollinating partners.

Pollinator sharing among co-flowering plants mediates patterns of pollen transfer

Co-flowering plant species often share pollinators, which could result in interspecific pollination. Despite some evidence suggesting plant species with overlapping pollinators influence each other’s pollination, the relationship between pollinator sharing and heterospecific pollen transfer (HPT) is not well explained at the community level. Here, we sampled a plant-pollinator visitation network to calculate the degree of pollinator sharing, and we identified pollen deposition on stigmas of co-flowering species to construct a HPT network in a sub-alpine meadow in southwest China. We also analyzed floral colour using reflectance spectra to measure the flower reflectance dissimilarity between each pair species. We found that there was no pollen transfer between most species pairs with shared pollinators. A higher proportion of HPT links between plant species pairs with shared pollinators was observed than between pairs with distinct pollinators. More pollen grains were transferred from donor to recipient species with greater pollinator sharing. Flowers with more dissimilar flower colours exchanged less heterospecific pollen. Our study evaluated the effect of pollinator sharing in shaping pollen transfer patterns among co-flowering species, which may shed light on the ecological implications of plant–pollinator interactions, mediated by pollinator sharing.

A case study of insect visitors and pollen vectors in sponge gourd (Luffa cylindrica (Linn.)) during summer and rainy seasons in India

Insect crop visitations do not necessarily always translate to transfer or carriage of pollen. To determine the potential of the various insects visiting summer and rainy seasons sponge gourd (Luffa cylindrica (Linn.)) flowers to facilitate the pollen transfer, the present study examines the diversity of visiting insects to sponge gourd seed crops in relation to the time of day and season, diversity and abundance to its flowers, fruits and seed yield. These visiting insects taxonomically belonged to 8 different insect orders viz. Coleoptera, Diptera, Hymenoptera, Hemiptera, Neuroptera, Lepidoptera, Odonata and Orthoptera/Dictyoptera. Rainy season sponge gourd crops had higher numbers of insect visitors and higher diversity (0.80 of Simpson’s Diversity Index and 1.71 of Shannon–Weaver Index of diversity as compared to summer season crop (0.72 and 1.51, respectively). The summer crop had the highest number of visiting insects belonged to the order Coleoptera followed by Hemiptera and Hymenoptera and their reciprocal form of Berger Parker’s dominance indices were 2.21, 5.05 and 6.17, respectively. In summer crop, honeybees were the main pollinators comprising 57.69% of total pollinators in sponge gourds. All the four honeybee species viz., Apis cerana indica, A. dorsata, A. florea and A. mellifera were found to visit in sponge gourd. However, the dominant species was A. florea with 39% of total honeybee share. In rainy season crop, in contrary, honeybees comprised only 16.32% share on total visiting pollinators in sponge gourd. Rice skipper, Pelopidas mathias (46.84% share) was found almost thorough out the day with higher visitation during evening hours. Other pollinators were stingless bees (11.58% share), butterflies and moths. Consequently, we suggest optimizing pollinators’ management including honeybees and thereby improving the attraction of sponge gourd lines to these pollinators to improve pollination rates for sponge gourd seed crops.

Increased variation in sex organ positions can increase reciprocity and pollination success in heterostylous plant populations

AbstractThe reciprocal position of sexual organs in complementary floral morphs is central to our understanding of heterostyly. Reciprocity indices are used to quantify the spatial match between complementary sex organs, but previous indices fail to appropriately account for intra‐population variation in sex organ positions. In this study, we examine how an increase in intra‐population variation in sex organ heights affects reciprocity and consequently reproductive success. We formulated a reciprocity index that incorporates this variation and asked if estimates of reciprocity can predict reproductive success in naturally occurring heterostylous populations. We developed a reciprocity index that assumed pollen transfer success equalled one for a perfectly matched stigma–anther pair, and decreased to zero with increasing mismatch. We examined the relationship between intra‐population variation in organ position and reciprocity, compared previously proposed indices using simulated populations and empirical data from natural populations, and tested the ability of the indices to predict reproductive success. We observed that when differences between mean complementary sex‐organ heights are small, increasing intra‐population variation in heights resulted in a decrease in reciprocity. However, when this difference is larger, reciprocity increased, reached a peak, and then decreased with increasing variation. Previous indices failed to capture this behavior. Seed set was positively related to reciprocity for our index. These results challenge the current understanding that increasing variation in sex‐organ heights will always decrease reciprocity in heterostylous populations. This may help explain why heterostylous systems exhibit and tolerate high amounts of intra‐population variation in sex organ heights.

Unexpectedly low paternal diversity is associated with infrequent pollinator visitation for a bird-pollinated plant.

The behaviour of pollinators has important consequences for plant mating. Nectar-feeding birds often display behaviour that results in more pollen carryover than insect pollinators, which is predicted to result in frequent outcrossing and high paternal diversity for bird-pollinated plants. We tested this prediction by quantifying mating system parameters and bird visitation in three populations of an understory bird-pollinated herb, Anigozanthos humilis (Haemodoraceae). Microsatellite markers were used to genotype 131 adult plants, and 211 seeds from 23 maternal plants, from three populations. While outcrossing rates were high, estimates of paternal diversity were surprisingly low compared with other bird-pollinated plants. Despite nectar-feeding birds being common at the study sites, visits to A. humilis flowers were infrequent (62 visits over 21,552 recording hours from motion-triggered cameras, or equivalent to one visit per flower every 10days), and the majority (76%) were by a single species, the western spinebill Acanthorhynchus superciliosus (Meliphagidae). Pollen counts from 30 captured honeyeaters revealed that A. humilis comprised just 0.3% of the total pollen load. For 10 western spinebills, A. humilis pollen comprised only 4.1% of the pollen load, which equated to an average of 3.9 A. humilis pollen grains per bird. Taken together, our findings suggest that low visitation rates and low pollen loads of floral visitors have led to the low paternal diversity observed in this understory bird-pollinated herb. As such, we shed new light on the conditions that can lead to departures from high paternal diversity for plants competing for the pollination services of generalist nectar-feeding birds.

Managed honeybees and South American bumblebees exhibit complementary foraging patterns in highbush blueberry

Despite Apis mellifera being the most widely managed pollinator to enhance crop production, they are not the most suitable species for highbush blueberries, which possess restrictive floral morphology and require buzz-pollination. Thus, the South American bumblebee Bombus pauloensis is increasingly managed as an alternative species in this crop alongside honeybees. Herein, we evaluated the foraging patterns of the two species, concerning the potential pollen transfer between two blueberry co-blooming cultivars grown under open high tunnels during two seasons considering different colony densities. Both managed pollinators showed different foraging patterns, influenced by the cultivar identity which varied in their floral morphology and nectar production. Our results demonstrate that both species are efficient foragers on highbush blueberry and further suggest that they contribute positively to its pollination in complementary ways: while bumblebees were more effective at the individual level (visited more flowers and carried more pollen), the greater densities of honeybee foragers overcame the difficulties imposed by the flower morphology, irrespective of the stocking rate. This study supports the addition of managed native bumblebees alongside honeybees to enhance pollination services and emphasizes the importance of examining behavioural aspects to optimize management practices in pollinator-dependent crops.

Diversity and composition of pollen loads carried by pollinators are primarily driven by insect traits, not floral community characteristics.

Flowering plants require conspecific pollen to reproduce but they often also receive heterospecific pollen, suggesting that pollinators carry mixed pollen loads. However, little is known about drivers of abundance, diversity or composition of pollen carried by pollinators. Are insect-carried pollen loads shaped by pollinator traits, or do they reflect available floral resources? We quantified pollen on 251 individual bees and 95 flies in a florally diverse community. We scored taxonomic order, sex, body size, hairiness and ecological specialization of pollinators, and recorded composition of available flowers. We used phylogenetically controlled model selection to compare relative influences of pollinator traits and floral resources on abundance, diversity and composition of insect-carried pollen. We tested congruence between composition of pollen loads and available flowers. Pollinator size, specialization and type (female bee, male bee, or fly) described pollen abundance, diversity and composition better than floral diversity. Pollen loads varied widely among insects (10-80,000,000 grains, 1-16 species). Pollen loads of male bees were smaller, but vastly more diverse than those of female bees, and equivalent in size but modestly more diverse than those of flies. Pollen load size and diversity were positively correlated with body size but negatively correlated with insect ecological specialization. These traits also drove variation in taxonomic and phylogenetic composition of insect-carried pollen loads, but composition was only weakly congruent with available floral resources. Qualities of pollinators best predict abundance and diversity of carried pollen indicating that functional composition of pollinator communities may be important to structuring heterospecific pollen transfer among plants.

Significant habitat loss of the black vanilla orchid (Nigritella nigra s.l., Orchidaceae) and shifts in its pollinators availability as results of global warming

The black vanilla orchid (Nigritella nigra s.l.) is a perennial plant found in the main European mountain ranges. It occurs in large numbers in the Alps, but it has become a rare and endangered species in Scandinavia. Two subspecies of N. nigra s.l are recognized by most authors – the nominal subspecies which is restricted in its distribution to Scandinavia and N. nigra subsp. austriaca which is found in the alpine region. Some taxonomists postulated that these two taxa should be considered as separated species. The N. nigra s.l. is pollinated mostly by Lepidoptera. As a mainly montane species it is especially fragile to global warming, however, some previously published reports indicated that the climate changes will not significantly affect northern populations of this species. The aim of this study was to evaluate the differences in the climatic niche preferences between Scandinavian and central European populations of the black vanilla orchid and to estimate the impact of global warming on the distribution of this species using (ENM). Additionally, the predicted future location and extent of suitable climatic niches of insects pollinating studied orchid was modeled in order to estimate the availability of pollinators for this montane plant. This approach was applied as the persistence of an orchid population depends on pollination and subsequent seed production. Our analyses showed that the northern and southern populations differ in bioclimatic preferences and geographical distribution of these orchids together with variations in their chromosomes number and morphological discrepancies, suggest that N. nigra in not homogenous taxon as postulated by some authors and this approach should be also applied in assessing conservation priorities in Europe. Both, northern and southern population of N. nigra, will face significant habitat loss as a result of the global warming. In the best case scenario (rcp2.6) only 31.7% and 60.5% of the currently suitable niches will be still available for N. nigra respectively. The most damaging rcp8.5 scenario will cause the extreme decline of the coverage of orchid and just 2.3% of the habitats will be still suitable for N. nigra in the North and 18% for N. nigra in the South. Within the future potential range of N. nigra and N. nigra subsp. austriaca some pollinators will be still available for orchids, mostly for the latter taxon. Considering that the flowering time of the orchid seems to overlap with the activity of pollinators for the last 70 years we not expect significant desynchronization of these events which could prevent pollen transfer. Noteworthy as the Scandinavian populations N. nigra is characterized by lower genetic diversity than in the alpine region of central Europe, species seems to be more endangered in the North (Moen and Øien, 2002) and the future loss of habitats predicted in our study together with limited availability of pollinators can led to loss of the unique gene pool of this orchid. In our opinion this fact should be considered in the establishment of nature protection projects and the conservation efforts should focus on areas which in the future will be still suitable for occurrence of this species and its pollen vectors.

Asymmetrical reproductive barriers in sympatric jewelflowers: are floral isolation, genetic incompatibilities and floral trait displacement connected?

Abstract Floral visitors influence reproductive interactions among sympatric plant species, either by facilitating assortative mating and contributing to reproductive isolation, or by promoting heterospecific pollen transfer, potentially leading to reproductive interference or hybridization. We assessed preference and constancy of floral visitors on two co-occurring jewelflowers [Streptanthus breweri and Streptanthus hesperidis (Brassicaceae)] using field arrays, and quantified two floral rewards potentially important to foraging choice – pollen production and nectar sugar concentration – in a greenhouse common garden. Floral visitors made an abundance of conspecific transitions between S. breweri individuals, which thus experienced minimal opportunities for heterospecific pollen transfer from S. hesperidis. In contrast, behavioural isolation for S. hesperidis was essentially absent due to pollinator inconstancy. This pattern emerged across multiple biotic environments and was unrelated to local density dependence. S. breweri populations that were sympatric with S. hesperidis had higher nectar sugar concentrations than their sympatric congeners, as well as allopatric conspecifics. Previous work shows that S. breweri suffers a greater cost to hybridization than S. hesperidis, and here we find that it also shows asymmetrical floral isolation and floral trait displacement in sympatry. These findings suggest that trait divergence may reduce negative reproductive interactions between sympatric but genetically incompatible relatives.

Using RNA-seq to characterize pollen\u2013stigma interactions for pollination studies

Insects are essential for the reproduction of pollinator-dependent crops and contribute to the pollination of 87% of wild plants and 75% of the world’s food crops. Understanding pollen flow dynamics between plants and pollinators is thus essential to manage and conserve wild plants and ensure yields are maximized in food crops. However, the determination of pollen transfer in the field is complex and laborious. We developed a field experiment in a pollinator-dependent crop and used high throughput RNA sequencing (RNA-seq) to quantify pollen flow by measuring changes in gene expression between pollination treatments across different apple (Malus domestica Borkh.) cultivars. We tested three potential molecular indicators of successful pollination and validated these results with field data by observing single and multiple visits by honey bees (Apis mellifera) to apple flowers and measured fruit set in a commercial apple orchard. The first indicator of successful outcrossing was revealed via differential gene expression in the cross-pollination treatments after 6 h. The second indicator of successful outcrossing was revealed by the expression of specific genes related to pollen tube formation and defense response at three different time intervals in the stigma and the style following cross-pollination (i.e. after 6, 24, and 48 h). Finally, genotyping variants specific to donor pollen could be detected in cross-pollination treatments, providing a third indicator of successful outcrossing. Field data indicated that one or five flower visits by honey bees were insufficient and at least 10 honey bee flower visits were required to achieve a 25% probability of fruit set under orchard conditions. By combining the genotyping data, the differential expression analysis, and the traditional fruit set field experiments, it was possible to evaluate the pollination effectiveness of honey bee visits under orchards conditions. This is the first time that pollen-stigma-style mRNA expression analysis has been conducted after a pollinator visit (honey bee) to a plant (in vivo apple flowers). This study provides evidence that mRNA sequencing can be used to address complex questions related to stigma–pollen interactions over time in pollination ecology.

Frequency of flower visitors and achene production increase with rising population size in the self-incompatible herb Centaurea scabiosa (Asteraceae)

Self-incompatible, non-clonal plant species are especially dependent on the activity of flower visitors for seed production. Therefore, populations of these plants are vulnerable to a reduced flower visitation rates, but also to increased isolation by extinction of local populations. To study how local populations of Centaurea scabiosa, a self-incompatible, bee-pollinated herb species changed over time in the region of Bonn, we collected historical population records in the area and investigated their current status. We found that more than half of the subpopulations mentioned in the literature between 1950 and 2012 (38 of 65) have since disappeared. Small populations were most vulnerable, whereas medium to large populations increased in size. In a second step, we studied visitation frequencies and achene production and weight across 14 extant populations. We found that both flower visitation frequency and achene production were positively related to population size. Achene weight was neither related to the frequency of flower visitors nor to achene production. These results clearly indicate that reduced pollen transfer and lower pollination rates may contribute to local extinction in small populations of Centaurea scabiosa. Overall our results call for an intensified monitoring of the populations of self-incompatible plant species.

Floral and pollination characteristics of Eriocaulon heleocharioides, an extinct species in the wild, for evidence-based conservation management.

Generally, floral characteristics and pollination are important factors enhancing the quality and quantity of reproductive output for regeneration in plant conservation. However, lack of evidence-based management could decrease fitness under ex-situ conservation. We investigated the capitulum and pollination characteristics of Eriocaulon heleocharioides Satake (Eriocaulaceae), which is extinct in the wild, to develop an evidence-based conservation management plan incorporating previously ignored reproductive characteristics. To evaluate the functional characteristics of capitula, pollen-ovule ratio, and reproductive status (maximum pollination success/florivory damage) were investigated along six flowering sequences of capitulum. To evaluate the effect of plant density on pollen transfer, high- and low-density plots were established. Total deposited pollen on stigma, insect visitation, and visit duration per capitulum were observed. A significantly lower pollen-ovule ratio was observed in the first of six capitula, reflecting higher female functionality. The highest pollination success was found in the second-fourth capitula, whereas florivory increased along the terminal capitula position. High plant density affected the pollen deposited on stigmas via insect visitation and low pollinator visit duration. Different capitula in E. heleocharioides could have different effects: different sexual functionality, enhancement of reproductive output both in quality and quantity through active pollen transfer, and escaping from florivores. High plant density could facilitate outcross-pollen transfer in E. heleocharioides. Multiple perspectives are important for determining potential reproductive success in ex-situ conservation. Thus, density management reflecting capitulum characteristics could improve the efficiency of conservation efforts for E. heleocharioides.

Reproductive biology of Mimosa microphylla and Albizia julibrissin (Fabaceae: Caesalpinioideae) with a new explanation for the evolution of polyads, restricted stigmas, and polycarpelly in the mimosoid clade1

Although it is yet to be conclusively established, floral features of many legumes of the mimosoid clade suggest the existence of a highly wasteful late-acting form of genetic self-incompatibility. We evaluated the role this may have played in the evolution of aggregated pollen, restricted stigmas, and polycarpelly in these plants by first investigating the reproductive biology of two species from the southeastern United States: Mimosa microphylla and Albizia julibrissin. Insights gained from detailed study of their reproductive biology informed a wider, critical review of reproductive characters in other members of the clade. Descriptions of the flowers and inflorescences of the two southeastern U.S. species fail to define clearly the functional situation, including that both species are andromonoecious. Pollen of M. microphylla is released as tetrads, whereas polyads of 16 pollen grains characterize A. julibrissin. Diameters of the cup-like stigmas are constrained so that typically only one or a few tetrads or one polyad will fit. These features are consistent with a syndrome involving, in its fullest manifestation, low fruit set, restriction of stigmatic surfaces, pollen aggregation, reduction of ovule number, and polycarpelly. We interpret all of these unusual features as adaptations to circumvent a highly wasteful (of pollen and ovules) form of late-acting self-incompatibility in which rejection of self-pollen occurs in the ovary. These adaptations reduce the likelihood of a mixed load of self- and cross-pollen being delivered. Enhanced efficiency of pollen transfer may also contribute to the evolution of aggregated pollen, but this explanation fails to account for the other unusual features of this syndrome.

Evolutionary and Ecological Considerations on Nectar-Mediated Tripartite Interactions in Angiosperms and Their Relevance in the Mediterranean Basin.

The Mediterranean basin hosts a high diversity of plants and bees, and it is considered one of the world’s biodiversity hotspots. Insect pollination, i.e., pollen transfer from male reproductive structures to conspecific female ones, was classically thought to be a mutualistic relationship that links these two groups of organisms, giving rise to an admirable and complex network of interactions. Although nectar is often involved in mediating these interactions, relatively little is known about modifications in its chemical traits during the evolution of plants. Here, we examine how the current sucrose-dominated floral nectar of most Mediterranean plants could have arisen in the course of evolution of angiosperms. The transition from hexose-rich to sucrose-rich nectar secretion was probably triggered by increasing temperature and aridity during the Cretaceous period, when most angiosperms were radiating. This transition may have opened new ecological niches for new groups of insects that were co-diversifying with angiosperms and for specific nectar-dwelling yeasts that originated later (i.e., Metschnikowiaceae). Our hypothesis embeds recent discoveries in nectar biology, such as the involvement of nectar microbiota and nectar secondary metabolites in shaping interactions with pollinators, and it suggests a complex, multifaceted ecological and evolutionary scenario that we are just beginning to discover.

OBSERVATIONS OF ORANGE-CROWNED WARBLERS IN VINE MAPLE

We conducted a pilot study assessing the role that Orange-crowned Warblers (Oreothlypis celata) may play in the pollination of Vine Maple (Acer circinatum). We observed an Orange-crowned Warbler probing its head into different inflorescences between 2 focal trees. We also incidentally observed O. celata individuals foraging in Big-leaf Maple (Acer macrophyllum) inflorescences and a Wilson's Warbler (Cardellina pusilla) feeding in the flowers of a Vine Maple. Our observations and results from the pilot experiment excluding large-bodied pollinators from flowers lead us to suggest that warblers may forage at Vine Maple flowers in a manner that could contribute to pollination; however, further study to confirm that warblers transfer pollen from anthers to conspecific stigmas is warranted.

Investigating pollination strategies in disturbed habitats: the case of the narrow-endemic toadflax Linaria tonzigii (Plantaginaceae) on mountain screes

Plant mating systems may reflect an adaptation to a habitat type, with self-pollination being potentially common in unstable and disturbed conditions. We investigated the reproductive ecology of an Alpine, narrow-range toadflax, Linaria tonzigii Lona (Plantaginaceae), occurring in steep and dynamic mountain screes. We explored self-compatibility and spontaneous autogamy, seed viability, daily nectar production, pollinator behaviour and pollen transfer in wild populations, using hand pollination treatments, quantification of nectar volume, viability Tetrazolium assay, active pollinator sampling and video recordings, and UV-bright dust for pollen substitution. After ex novo sequencing of several genetic regions of L. tonzigii, we performed a multi-marker phylogenetic analysis of 140 Linaria species and tracked the occurrence of the self-compatibility trait. Our results showed that this species is self-compatible, pollinated mostly via spontaneous autogamy and pollinator-mediated geitonogamy, and self-pollinated seeds are as viable as cross-pollinated ones. Selfing could be due to pollinator rarity because, despite the studied species providing a high nectar volume, wild bees, moths and small beetles infrequently visited its flowers in the sparsely vegetated scree slopes. In addition, a preliminary survey showed a low genetic haplotype diversity in the study plant. Moreover, the phylogeny shows that self-compatibility is scattered in the tree, suggesting the adaptive nature of this reproductive trait in the genus Linaria. This study supports for theories that high prevalence of selfing is an adaptation to environments unfavourable for cross-pollination, and particularly where pollinators are rare, such as in perturbed, poorly vegetated high-elevation habitats.

Teaching Through Trade Books: Pollen Transfer and Pea Plants

Teaching Through Trade Books: Pollen Transfer and Pea Plants

Nectar addition changes pollinator behavior but not plant reproduction in pollen-rewarding Lupinus argenteus.

In addition to its role as the male gamete, pollen is often used as a food reward for pollinators. Roughly 20,000 species of angiosperms are strictly pollen-rewarding, providing no other rewards to their pollinators. However, the influence of this strategy on pollinator behavior and plant reproduction is poorly understood, especially relative to the nectar-reward strategy. We performed a field experiment using the strictly pollen-rewarding Lupinus argenteus to explore how the absence of nectar influences pollinator behavior and plant reproduction. We added artificial nectar to Lupinus argenteus individuals to simulate a phenotype that would reward pollinators with both nectar and pollen. We compared bee pollinator behavior, via direct observation, and female reproduction between nectar-added and nectarless control plants. Bees exhibited behavioral responses to the novel reward, collecting nectar as well as pollen and spending 27% longer per flower. Pollen transfer increased with flower visit duration. However, plants in the study population were not pollen-limited; consequently, the observed changes in pollinator behavior did not result in changes in female components of plant reproduction. The addition of nectar to pollen-rewarding plants resulted in modest increases in per-flower pollinator visit duration and pollen transfer, but had no effect on reproduction because, at the place and time the experiment was conducted, plants were not pollen-limited. These results suggest that a pollen-only reward strategy may allow plants that are visited by pollen foragers to minimize some costs of reproduction by eliminating investment in other rewards, such as nectar, without compromising female plant fitness.

Flower evolution in the presence of heterospecific gene flow and its contribution to lineage divergence.

The success of species depends on their ability to exploit ecological resources in order to optimize their reproduction. However, species are not usually found within single-species ecosystems but in complex communities. Because of their genetic relatedness, closely related lineages tend to cluster within the same ecosystem, rely on the same resources, and be phenotypically similar. In sympatry, they will therefore compete for the same resources and, in the case of flowering plants, exchange their genes through heterospecific pollen transfer. These interactions, nevertheless, pose significant challenges to species co-existence because they can lead to resource limitation and reproductive interference. In such cases, divergent selective pressures on floral traits will favour genotypes that isolate or desynchronize the reproduction of sympatric lineages. The resulting displacement of reproductive characters will, in turn, lead to pre-mating isolation and promote intraspecific divergence, thus initiating or reinforcing the speciation process. In this review, we discuss the current theoretical and empirical knowledge on the influence of heterospecific pollen transfer on flower evolution, highlighting its potential to uncover the ecological and genomic constraints shaping the speciation process.