Conspecific Pollen Research Articles

Patterns of among- and within-species variation in heterospecific pollen receipt: The importance of ecological generalization.

Coflowering plants are at risk for receiving pollen from heterospecifics as well as conspecifics, yet evidence shows wide variation in the degree that heterospecific pollen transfer occurs. Evaluation of patterns and correlates of among- and within-species variation in heterospecific pollen (HP) receipt is key to understanding its importance for floral evolution and species coexistence; however, the rarity of deeply sampled multispecies comparisons has precluded such an evaluation. We evaluated patterns of among- and within-species variation in HP load size and diversity in 19 species across three distinct plant communities. We assessed the importance of phenotypic specialization (floral phenotype), ecological specialization (contemporary visitor assemblage), and conspecific flower density as determinants of among-species variation. We present hypotheses for different accrual patterns of HP within species based on the evenness and quality of floral visitors and evaluated these by characterizing the relationship between conspecific pollen (CP) and HP receipt. We found that within-species variation in HP receipt was greater than among-species and among-communities variation. Among species, ecological generalization emerged as the strongest driver of variation in HP receipt irrespective of phenotypic specialization. Within-species variation in HP load size and diversity was predicted most often from two CP-HP relationships (linear or exponentially decreasing), suggesting that two distinct types of plant-pollinator interactions prevail. Our results give important insights into the potential drivers of among- and within-species variation in HP receipt. They also highlight the value of explorations of patterns at the intraspecific level, which can ultimately shed light on plant-pollinator-mediated selection in diverse plant communities.

Can plants evolve tolerance mechanisms to heterospecific pollen effects? An experimental test of the adaptive potential inClarkiaspecies

Flowering plants do not occur alone and often grow in mixed‐species communities where pollinator sharing is high and interactions via pollinators can occur at pre‐ and post‐pollination stages. While the causes and consequences of pre‐pollination interactions have been well studied little is known about post‐pollination interactions via heterospecific pollen (HP) receipt, and even less about the evolutionary implications of these interactions. In particular, the degree to which plants can evolve tolerance mechanisms to the negative effects of HP receipt has received little attention. Here, we aim to fill this gap in our understanding of post‐pollination interactions by experimentally testing whether two co‐floweringClarkiaspecies can evolve HP tolerance, and whether tolerance to specific HP ‘genotypes’ (fine‐scale local adaptation to HP) occurs. We find thatClarkiaspecies vary in their tolerance to HP effects. Furthermore, conspecific pollen performance and the magnitude of HP effects were related to the recipient's history of exposure to HP inC. xantianabut not inC. speciosa. Specifically, better conspecific pollen performance and smaller HP effects were observed in populations ofC. xantianaplants with previous exposure to HP compared to populations without such exposure. These results suggest that plants may have the potential to evolve tolerance mechanisms to HP effects but that these may occur not from the female (stigma, style) but from the male (pollen) perspective, a possibility that is often overlooked. We find no evidence for fine‐scale local adaptation to HP receipt. Studies that evaluate the adaptive potential of plants to the negative effects of HP receipt are an important first step in understanding the evolutionary consequences of plant–plant post‐pollination interactions. Such knowledge is in turn crucial for deciphering the role of plant–pollinator interactions in driving floral evolution and the composition of co‐flowering communities.

Conspecific Pollen Loads on Insects fromPrunus fasciculata(Rosaceae) Female Flowers in Southern Nevada

Desert almond, Prunus fasciculata (Rosaceae), is a Mojave Desert shrub that primarily produces male and female flowers on different plants. I investigated the plant's pollination by examining pollen loads on insects aspirated from female flowers on 3 dioecious shrubs, growing near male-flowering shrubs, at one locality in southern Nevada during March 2014. Pollen loads were analyzed on 9 species of Diptera in Bombyliidae, Syrphidae, Calliphoridae, and Tachinidae and 4 species of Hymenoptera in Andrenidae (all female Andrena). All but 4 of the 65 flies and 38 bees aspirated carried P. fasciculata pollen grains, recognized by their tricolpate shape in polar view and oblate shape in equatorial view. Two species of syrphid flies in Copestylum were frequently aspirated and carried high P. fasciculata pollen loads and moderate proportions of conspecific pollen. High conspecific pollen loads on the tachinid Chaetogaedia also indicated potential for pollinating P. fasciculata. Three of the 4 species of Andrena bees carried large amounts of P. fasciculata pollen. Conspecific pollen also comprised most of the pollen load on Andrena bees, suggesting high flower constancies to the plant. Pollinators of P. fasciculata would vary over the plant's range, and likely between years, because of the localized populations or narrow larval diets of many of the insects collected from flowers. The female desert almond shrubs examined in southern Nevada during 2014 appeared to be pollinated by a variety of native flies and bees, especially syrphid flies in Copestylum and andrenid bees in Andrena.

Negative effects of heterospecific pollen receipt vary with abiotic conditions: ecological and evolutionary implications

Studies that have evaluated the effects of heterospecific pollen (HP) receipt on plant reproductive success have generally overlooked the variability of the natural abiotic environment in which plants grow. Variability in abiotic conditions, such as light and water availability, has the potential to affect pollen-stigma interactions (i.e. conspecific pollen germination and performance), which will probably influence the effects of HP receipt. Thus, a more complete understanding of the extent, strength and consequences of plant-plant interactions via HP transfer requires better consideration of the range of abiotic conditions in which these interactions occur. This study addresses this issue by evaluating the effects of two HP donors (Tamonea curassavica and Angelonia angustifolia) on the reproductive success of Cuphea gaumeri, an endemic species of the Yucatan Peninsula. Mixed (conspecific pollen and HP) and pure (conspecific pollen only) hand-pollinations were conducted under varying conditions of water and light availability in a full factorial design. Reproductive success was measured as the number of pollen tubes that reached the bottom of the style. Only one of the two HP donors had a significant effect on C. gaumeri reproductive success, but this effect was dependent on water and light availability. Specifically, HP receipt caused a decrease in pollen tube growth, but only when the availability of water, light or both was low, and not when the availability of both resources was high. The results show that the outcome of interspecific post-pollination interactions via HP transfer can be context-dependent and vary with abiotic conditions, thus suggesting that abiotic effects in natural populations may be under-estimated. Such context-dependency could lead to spatial and temporal mosaics in the ecological and evolutionary consequences of post-pollination interactions.

Effects of non-native Melilotus albus on pollination and reproduction in two boreal shrubs.

The establishment of abundantly flowered, highly rewarding non-native plant species is expected to have strong consequences for native plants through altered pollination services, particularly in boreal forest where the flowering season is short and the pollinator pool is small. In 18 boreal forest sites, we added flowering Melilotus albus to some sites and left some sites as controls in 2 different years to test if the invasive plant influences the pollination and reproductive success of two co-flowering ericaceous species: Vaccinium vitis-idaea and Rhododendron groenlandicum. We found that M. albus increased the pollinator diversity and tended to increase visitation rates to the focal native plant species compared to control sites. Melilotus albus facilitated greater seed production per berry in V. vitis-idaea when we added 120 plants compared to when we added 40 plants or in control sites. In R. groenlandicum, increasing numbers of M. albus inflorescences lowered conspecific pollen loads and percentage of flowers pollinated; however, no differences in fruit set were detected. The number of M. albus inflorescences had greater importance in explaining R. groenlandicum pollination compared to other environmental variables such as weather and number of native flowers, and had greater importance in lower quality black spruce sites than in mixed deciduous and white spruce sites for explaining the percentage of V. vitis-idaea flowers pollinated. Our data suggest that the identity of new pollinators attracted to the invaded sites, degree of shared pollinators between invasive and native species, and variation in resource limitation among sites are likely determining factors in the reproductive responses of boreal native plants in the presence of an invasive.

Competition for Pollination and the Evolution of Flowering Time.

Competition for Pollination and the Evolution of Flowering Time.

Interspecific pollen transfer between two coflowering species was minimized by bumblebee fidelity and differential pollen placement on the bumblebee body

Aims When sympatric flowering plant species in a natural community share pollinators, study of plant–plant interactions via interspecific pollen transfer (iPT) is essential for understanding species coexistence. However, little is known about the extent of iPT between interactive species and its causes. Methods To explore how sympatric flowering plants sharing pollinators minimize deleterious effects of iPT, we investigated the pollination ecology of two endemic species, Salvia przewalskii and Delphinium yuanum, in an alpine meadow in the Hengduan mountains, southwest China. We quantified conspecific and interspecific visits by shared bumblebee pollinators, amounts of pollen placed on differ ent body sites of the pollinators and stigmatic pollen loads on openpollinated flowers. To examine whether iPT affects female fitness, we measured pollen germination and seed production in the two species in an artificial pollination experiment. Important Findings one bumblebee species, Bombus trifasciatus, was found to be the sole effective pollinator for the two coflowering species. Pollination experiments indicated that deposition of heterospecific pollen could significantly decrease seed set in both species. Experiments showed that S. przewalskii pollen could germinate well on stigmas of D. yuanum, inhibiting conspecific pollen germination in D. yuanum. However, seed set was not lower under open pollination than under cross-pollination within species, suggesting that no female fitness loss was caused by iPT. in foraging bouts with pollinator switches, switches from S. przewalskii to D. yuanum were relatively more frequent (8.27%) than the converse (1.72%). However, iPT from S. prze walskii to D. yuanum accounted for only 1.82% of total stigmatic pollen loads while the reverse iPT to S. przewalskii was 8.70%, indicating that more switches of bumblebees to D. yuanum did not result in higher iPT. by contrast, selection for reduced iPT to S. przewalskii would limit pollinator switches from D. yuanum. We found that a bumblebee generally carried pollen grains from both species but the two species differed in the position of pollen placement on the bumblebee’s body; S. przewalskii’ s pollen was concentrated on the dorsal thorax while D. yuanum’ s pollen was concentrated ventrally on the head. This differential pollen placement along with pollinator fidelity largely reduced iPT between the two species with a shared pollinator.

The showy invasive plant Ranunculus ficaria facilitates pollinator activity, pollen deposition, but not always seed production for two native spring ephemeral plants

Showy invasive plants can reduce pollinator visitation to native plants, resulting in a reduction of seed-set in insect-pollinated species. Seed set in native plants may also be reduced due to the presence of foreign pollen. Alternatively, additional floral resources provided by invasive plants may increase pollinator activity, and facilitate pollination of native species. Ranunculus ficaria is a perennial herb invading deciduous forests of the eastern United States. It produces many showy flowers that may influence pollinator activity in heavily invaded habitats. We compared pollinator visitation rates, stigma pollen loads, and seed production of two sympatrically flowering, entomophilous native species (Claytonia virginica and Cardamine concatenata) between habitats invaded by R. ficaria and uninvaded habitats. We found significantly higher pollinator activity (visits to plots) in invaded plots. We also found significantly higher per flower visitation rates to C. virginica in invaded plots. Claytonia virginica stigmas from invaded habitats had more conspecific pollen resulting in more seeds per capsule in invaded areas. There was more conspecific pollen on C. concatenata stigmas in invaded compared to uninvaded habitats, but there were significantly more seeds per silique in uninvaded habitats. Our results indicated that this invasive species increases pollinator activity and conspecific pollen deposition on sympatrically flowering native ephemerals, but may have differential effects on native seed production. Management options should consider the facilitative effects of this species on pollination services, as well as the possible negative competitive effects.

Feedingbehaviour of the dawn bat (<i>Eonycteris spelaea</i>) promotes cross pollination of economically important plants in Southeast Asia

Eonycteris spelaea is recognized as the principal pollinator of most chiropterophilous plants in SE Asia. The present study describes its feeding behaviour and clarifies its role in cross pollinating these highly self-incompatible plants. Ten individuals of E. spelaea were radio-tracked during the flowering period of durian (Durio zibethinus) and petai (Parkia speciosa) in an agricultural mosaic in southern Thailand. Eonycteris spelaea makes a mean of seven visits per hour to these trees and 80-86% of each feeding bout involves visits to multiple conspecific trees. During each visit, 93% of D. zibethinus stigmas and 50% of P. speciosa stigmas were loaded with conspecific pollen. Eonycteris spelaea was the most common bat visitor to the trees. High visitation frequency and conspecific pollen deposition by E. spelaea to D. zibethinus and P. speciosa indicates that this nectarivorous bat is an effective pollinator. Mixed planting of chiropterophilous trees in fruit orchards is recommended to ensure regular visits of E. spelaea. Protecting natural roost caves of E. spelaea is also essential in order to maintain the vital ecosystem service provided by these bats.

Breeding system of <i>Linum rigidum</i> and effect of heterospecific pollen from introduced <i>Euphorbia esula</i>

This study investigates the reproductive biology of, and effect of heterospecific pollen from introduced Euphorbia esula on, Linum rigidum, an annual plant native to western and central North America. Breeding-system studies revealed that L. rigidum is self-compatible, with similar pollination success, fruit set and seed set for flowers pollinated with self or outcrossed pollen. Untreated flowers not exposed to pollinators set seed, albeit at a lower rate than hand-pollinated flowers, indicating that L. rigidum can autonomously self pollinate. Experiments investigating whether heterospecific pollen transfer from E. esula interferes with pollination of L. rigidum indicated that large amounts of heterospecific pollen receipt 2 - 4 hours prior to conspecifc pollen receipt reduced fruit and seed production, but that small amounts of heterospecific pollen or larger amounts received immediately prior to conspecific pollen did not affect reproduction. Pollen of E. esula was observed to germinate on and penetrate into L. rigidum stigmas and styles. Nevertheless, Euphorbia esula is unlikely to interfere with L. rigidum’s reproduction because L. rigidum is self-compatible, capable of autonomous self-pollination, and unaffected by receipt of small amounts of Euphorbia pollen.

Pollinator sharing and low pollen–ovule ratio diminish reproductive success in two sympatric species of Portulaca (Portulacaceae)

We studied the breeding systems, phenology, pollination, and reproductive success of Portulaca grandiflora and P. hirsutissima (Portulacaceae) at Parque Nacional do Catimbau in the Brazilian Caatinga. The species flowered synchronously, exhibited similar floral morphology, offered pollen as the sole floral resource and shared the same bee species as pollinators. They visited flowers of both Portulaca species at similar frequencies, and their visitation peaks occurred at the same time of day. The pollinators made no distinction between the species during foraging. Both plant species are self-compatible, but produce fewer fruits and seeds after self-pollination than after cross-pollination. Comparing the reproductive success in areas where both species co-occur to that of areas with only one of them, seed set of P. grandiflora decreased by 25% and of P. hirsutissima by 36%. This is likely due to diminished flow of conspecific pollen grains, as the pollen–ovule ratio was extraordinarily small.

Pollinator-mediated competition influences selection for flower-color displacement in sympatric monkeyflowers.

When coflowering plant species share pollinators, pollinator-mediated competition may favor divergent floral characters associated with pollinator attraction. One potential outcome of this process is that sympatric populations will display increased divergence in floral traits compared with allopatric populations. We developed a new system to study the pattern and process of character displacement. In the central Sierra Nevada of California, USA, Mimulus bicolor is a spring wildflower with two flower-color morphs, one of which resembles coflowering M. guttatus. We documented a fine-scale geographic pattern of character displacement in sympatric and allopatric patches and, using experimental arrays, measured seed set in M. bicolor color morphs in the presence versus absence of M. guttatus. In sympatric arrays yellow, guttatus-like M. bicolor morphs had lower relative fitness (0.35 ± 0.05) and reduced conspecific pollen deposition compared with the distinct alternative morph, whereas in allopatric arrays yellow, guttatus-like morphs were occasionally strongly favored. Pollinator-mediated competition with M. guttatus is consistent with ecological character displacement in M. bicolor and likely contributes to a geographic pattern of character displacement.

Pollen competition between two sympatric Orchis species (Orchidaceae): the overtaking of conspecific of heterospecific pollen as a reproductive barrier.

The frequency of hybrid formation in angiosperms depends on how and when heterospecific pollen is transferred to the stigma, and on the success of that heterospecific pollen at fertilising ovules. We applied pollen mixtures to stigmas to determine how pollen interactions affect siring success and the frequency of hybrid formation between two species of Mediterranean deceptive orchid. Plants of Orchis italica and O.anthropophora were pollinated with conspecific and heterospecific pollen (first conspecific pollen then heterospecific pollen and vice versa) and molecular analysis was used to check the paternity of the seeds produced. In this pair of Mediterranean orchids, competition between conspecific and heterospecific pollen functions as a post-pollination pre-zygotic barrier limiting the frequency of the formation of hybrids in nature. Flowers pollinated with heterospecific pollen can remain receptive for the arrival of conspecific pollen for a long time. There is always an advantage of conspecific pollen for fruit formation, whether it comes before or after heterospecific pollen, because it overtakes the heterospecific pollen. The conspecific pollen advantage exhibited in O.italica and O.anthropophora is likely to result from the reduced germination of heterospecific pollen or retarded growth of heterospecific pollen tubes in the stigma and ovary. Overall, the results indicate that our hybrid zone represents a phenomenon of little evolutionary consequence, and the conspecific pollen advantage maintains the genetic integrity of the parental species.

Patterns of pollen quantity and quality limitation of pre‐zygotic reproduction in Mimulus guttatus vary with co‐flowering community context

Pollen quantity limitation has been widely recognized as one of the main causes of plant reproductive failure in nature. However, its negative effects on fruit and seed production have been often confounded with those of low pollen quality (i.e. low conspecific pollen viability and/or slow pollen tube growth rate). The lack of differentiation between these two aspects of pollen limitation has resulted not only in a potential overestimation of the incidence of pollen quantity limitation but has led to a poor understanding of the factors and mechanisms that affect pollen quality limitation. Knowledge of the relative importance and underlying causes of both aspects of pollen limitation (quantity and quality) is required to fully understand the ecological and evolutionary consequences of pollen limitation in natural populations. Co‐flowering community context (e.g. species richness and conspecific density), in particular, can be an important driver of overall pollen limitation. However, how pollen quantity and quality limitation vary separately with the community context has not previously been examined, even though they may vary differently as they arise from different mechanisms, e.g. insect visitation rate versus inbreeding depression. Here we evaluate the effect of co‐flowering diversity and conspecific density on the relative importance of pollen quantity and quality limitation for Mimulus guttatus pre‐zygotic reproduction (i.e. pollen tube success) at serpentine seeps in California over two years. We found overall pollen limitation of pre‐zygotic reproductive success at all seeps regardless of the co‐flowering context. However, plants in high‐diversity/low‐ conspecific density communities were mostly limited by pollen quantity, whereas plants in low‐diversity/high‐conspecific density ones experienced stronger quality limitation – a pattern that was consistent across years. These results not only highlight the importance of conducting comprehensive studies on pollen limitation that evaluate both, its quantity and quality aspects, but are the first to show how their relative contribution to overall pollen limitation can vary with extrinsic factors such as the co‐flowering community context.

Generalist Bees Pollinate Red-flowered Penstemon eatonii: Duality in the Hummingbird Pollination Syndrome

Abstract The red tubular flowers of Penstemon eatonii (Plantaginaceae) typify the classic pollination syndrome for hummingbirds. Bees are thought to diminish its seed siring potential, but we found that foraging female generalist bees (Apis, Anthophora) deposited substantial amounts of conspecific pollen on P. eatonii stigmas. In the absence of hummingbirds, bee pollination of cultivated P. eatonii annually generated massive seed yields from a 1.5 ha field. Most penstemons with red tubular flowers like P. eatonii present a symmetrically flared floral opening that facilitates landing by foraging bees. Derived floral traits that attract and position nectar-foraging hummingbirds for efficient pollen export (red deeply tubular flowers secreting abundant dilute nectar) need not compromise pollination and seed production that result from visitation by generalist bees seeking pollen and/or nectar.

Disentangling multiple drivers of pollination in a landscape-scale experiment

Animal pollination is essential for the reproductive success of many wild and crop plants. Loss and isolation of (semi-)natural habitats in agricultural landscapes can cause declines of plants and pollinators and endanger pollination services. We investigated the independent effects of these drivers on pollination of young cherry trees in a landscape-scale experiment. We included (i) isolation of study trees from other cherry trees (up to 350 m), (ii) the amount of cherry trees in the landscape, (iii) the isolation from other woody habitats (up to 200 m) and (iv) the amount of woody habitats providing nesting and floral resources for pollinators. At the local scale, we considered effects of (v) cherry flower density and (vi) heterospecific flower density. Pollinators visited flowers more often in landscapes with high amount of woody habitat and at sites with lower isolation from the next cherry tree. Fruit set was reduced by isolation from the next cherry tree and by a high local density of heterospecific flowers but did not directly depend on pollinator visitation. These results reveal the importance of considering the plant's need for conspecific pollen and its pollen competition with co-flowering species rather than focusing only on pollinators' habitat requirements and flower visitation. It proved to be important to disentangle habitat isolation from habitat loss, local from landscape-scale effects, and direct effects of pollen availability on fruit set from indirect effects via pollinator visitation to understand the delivery of an agriculturally important ecosystem service.

Relative floral density of an invasive plant affects pollinator foraging behaviour on a native plant

Interactions between invasive and native plants for pollinators vary from competition to facilitation of pollination of native plants. Theory predicts that relative floral densities should account for some of this variation in outcomes, with facilitation at low floral densities and competition at high floral densities of the invader. We tested this prediction by quantifying pollination and female reproductive success of a native herb, Geranium maculatum, in three experimental arrays that varied in floral density of the invasive shrub Lonicera maackii: control (no L. maackii), low floral density of L. maackii, and high floral density of L. maackii. A low density of L. maackii flowers was associated with an increase in pollinator visitation rate to G. maculatum flowers and an increase in conspecific pollen deposition compared to controls and high density arrays. Increased visitation rates were not associated with an increase in the number of visitors to low density arrays, suggesting instead that a behavioural switch in visitation within the array accounted for increased pollen deposition. In contrast, the only evidence of competition in high density arrays was a shorter duration of visits to G. maculatum flowers relative to the other treatments. The number of seeds per flower did not vary among treatments, although trends in seeds per flower were consistent with patterns of pollinator foraging behaviour. Given increased pollinator visits and pollen deposition at a low density of the invader, our study indicates that complete eradication of invasives as a management or restoration technique may have unintended negative consequences for pollination of native plants.

Selection of trait combinations through bee and fly visitation to flowers of Polemonium foliosissimum

Pollinators are known to exert natural selection on floral traits, but the extent to which combinations of floral traits are subject to correlational selection (nonadditive effects of two traits on fitness) is not well understood. Over two years, we used phenotypic manipulations of plant traits to test for effects of flower colour, flower shape and their interaction on rates of pollinator visitation to Polemonium foliosissimum. We also tested for correlational selection based on weighting visitation by the amount of conspecific pollen delivered per visit by each category of insect visitor. Although bumblebees were the presumed pollinators, solitary bees and flies contributed substantially (42%) to pollination. In manipulations of one trait at a time, insects visited flowers presenting the natural colour and shape over flowers manipulated to present artificial mutants with either paler colour or a more open or more tubular flower. When both colour and shape were manipulated in combination, selection on both traits arose, with bumblebees responding mainly to colour and flies responding mainly to shape. Despite selection on both floral traits, in a year with many bumblebees, we saw no evidence for correlational selection of these traits. In a year when flies predominated, fly visitation showed a pattern of correlational selection, but not favouring the natural phenotype, and correlational selection was still not detected for expected pollen receipt. These results show that flower colour and shape are subject to pollinator-mediated selection and that correlational selection can be generated based on pollinator visitation alone, but provide no evidence for correlational selection specifically for the current phenotype.

Active pollination favours sexual dimorphism in floral scent

Zoophilous flowers often transmit olfactory signals to attract pollinators. In plants with unisexual flowers, such signals are usually similar between the sexes because attraction of the same animal to both male and female flowers is essential for conspecific pollen transfer. Here, we present a remarkable example of sexual dimorphism in floral signal observed in reproductively highly specialized clades of the tribe Phyllantheae (Phyllanthaceae). These plants are pollinated by species-specific, seed-parasitic Epicephala moths (Gracillariidae) that actively collect pollen from male flowers and pollinate the female flowers in which they oviposit; by doing so, they ensure seeds for their offspring. We found that Epicephala-pollinated Phyllanthaceae plants consistently exhibit major qualitative differences in scent between male and female flowers, often involving compounds derived from different biosynthetic pathways. In a choice test, mated female Epicephala moths preferred the scent of male flowers over that of female flowers, suggesting that male floral scent elicits pollen-collecting behaviour. Epicephala pollination evolved multiple times in Phyllantheae, at least thrice accompanied by transition from sexual monomorphism to dimorphism in floral scent. This is the first example in which sexually dimorphic floral scent has evolved to signal an alternative reward provided by each sex, provoking the pollinator's legitimate altruistic behaviour.

Selection on plant male function genes identifies candidates for reproductive isolation of yellow monkeyflowers.

Understanding the genetic basis of reproductive isolation promises insight into speciation and the origins of biological diversity. While progress has been made in identifying genes underlying barriers to reproduction that function after fertilization (post-zygotic isolation), we know much less about earlier acting pre-zygotic barriers. Of particular interest are barriers involved in mating and fertilization that can evolve extremely rapidly under sexual selection, suggesting they may play a prominent role in the initial stages of reproductive isolation. A significant challenge to the field of speciation genetics is developing new approaches for identification of candidate genes underlying these barriers, particularly among non-traditional model systems. We employ powerful proteomic and genomic strategies to study the genetic basis of conspecific pollen precedence, an important component of pre-zygotic reproductive isolation among yellow monkeyflowers (Mimulus spp.) resulting from male pollen competition. We use isotopic labeling in combination with shotgun proteomics to identify more than 2,000 male function (pollen tube) proteins within maternal reproductive structures (styles) of M. guttatus flowers where pollen competition occurs. We then sequence array-captured pollen tube exomes from a large outcrossing population of M. guttatus, and identify those genes with evidence of selective sweeps or balancing selection consistent with their role in pollen competition. We also test for evidence of positive selection on these genes more broadly across yellow monkeyflowers, because a signal of adaptive divergence is a common feature of genes causing reproductive isolation. Together the molecular evolution studies identify 159 pollen tube proteins that are candidate genes for conspecific pollen precedence. Our work demonstrates how powerful proteomic and genomic tools can be readily adapted to non-traditional model systems, allowing for genome-wide screens towards the goal of identifying the molecular basis of genetically complex traits.