Patterns of color and nectar variation across an Ipomopsis (Polemoniaceae) hybrid zone

Hybridization may uncouple adaptive trait combinations that are present in parental species. I studied variation in flower color and reward quality across a hybrid zone of Ipomopsis aggregata and I. tenuituba. Individuals from hybrid populations showed considerable variation in flower color using corolla reflectance measurements. Flower spectra of such individuals were either intermediate or else resembled those flowers from the parental species. Ipomopsis aggregata populations had consistently higher nectar production rates and higher nectar standing crops than either I. tenuituba or hybrids. Ipomopsis aggregata flowers also produced more dilute nectar than those of hybrids and I. tenuituba, but the actual concentration values were quite variable among populations of the same type. Overall, the nectar quality of hybrid flowers most resembled that of I. tenuituba flowers. Based on the observed interpopulation patterns of color-reward associations in this hybrid zone, pollinators should be able to discriminate against I. tenuituba and hybrid populations and against most individuals within hybrid populations. However, they may visit those hybrids that resemble the most rewarding flower type (I. aggregata). The results emphasize the need for studies that address how hybridization affects subsequent plant fitness and the evolutionary dynamics of the species involved.     

ADAPTIVE SIGNIFICANCE OF FLOWER COLOR AND INTER-TRAIT CORRELATIONS IN AN IPOMOPSIS HYBRID ZONE

Flower color is often viewed as a trait that signals rewards to pollinators, such that the relationship between flower color and plant fitness might result from its association with another trait. We used experimental manipulations of flower color and nectar reward to dissociate the natural character correlations present in a hybrid zone between Ipomopsis aggregata and Ipomopsis tenuituba. Isozyme markers were used to follow the male and female reproductive success of these engineered phenotypes. One field experiment compared fitnesses of I. aggregata plants that varied only in flower color. Plants with flowers painted red received more hummingbird visits and sired more seeds than did plants with flowers painted pink or white to match those of hybrids and I. tenuituba. Our second field experiment compared fitnesses of I. aggregata, I. tenuituba, and hybrid plants in an unmanipulated array and in a second array where all flowers were painted red. In the unmanipulated array, I. aggregata received more hummingbird visits, set more seeds per flower, and sired more seeds per flower. These fitness differences largely disappeared when the color differences were eliminated. The higher male fitness of I. aggregata was due to its very high success at siring seeds on conspecific recipients. On both I. tenuituba and hybrid recipients, hybrid plants sired the most seeds, despite showing lower pollen fertility than I. aggregata in mixed donor pollinations in the greenhouse. Ipomopsis tenuituba had a fitness of only 13% relative to I. aggregata when traits varied naturally, compared to a fitness of 36% for white relative to red flowers when other traits were held constant.     

Ecophysiology of first and second generation hybrids in a natural plant hybrid zone

Hybrids between related species vary widely in relative fitness, and that fitness can depend upon the environment. We investigated aspects of physiology that might influence fitness patterns in a plant hybrid zone. Seeds of Ipomopsis aggregata, I. tenuituba, F1 hybrids, F2 hybrids, and offspring of crosses between natural hybrids were planted into the relatively mesic site of origin for I. aggregata and the drier site for natural hybrids. We measured rates of photosynthesis (A _max), transpiration (E), instantaneous (A/E) and long-term (δ¹³C) indices of water use efficiency (WUE), and leaf nitrogen and carbon. We also examined correlations of these traits with plant size. Photosynthetic rate and A/E were higher in vegetative than flowering plants. WUE varied between sites and years, but differences among genotypic classes were spatially and temporally consistent. Instantaneous WUE was higher for F1 hybrids than for the average of the parental species, thereby showing heterosis. There was no evidence of hybrid breakdown, as WUE was no different in the F2 than the average across the F1 and parental species. Nor did WUE depend on cross direction in producing F1 progeny. Carbon isotope discrimination revealed higher long-term water use efficiency in I. tenuituba than I. aggregata. Leaf nitrogen was higher in I. tenuituba than I. aggregata, and higher in offspring of natural hybrids than in the F2. Results indicate heterosis for water use efficiency, with no hybrid breakdown. Heterosis in WUE may help to explain the relatively high survival of both reciprocal F1 hybrids in dry sites within the natural hybrid zone.     

Floral scent in natural hybrids of Ipomopsis (Polemoniaceae) and their parental species

Background and Aims

Floral traits, such as floral volatiles, can contribute to pre-zygotic reproductive isolation by promoting species-specific pollinator foraging. When hybrid zones form, floral traits could also influence post-zygotic isolation. This study examined floral volatiles in parental species and natural hybrids in order to explore potential scent mediation of pre-zygotic and post-zygotic isolation.

Methods

Floral bouquets were analysed for the sister species Ipomopsis aggregata and I. tenuituba and their natural hybrids at two contact sites differing in both hybridization rate and temporal foraging pattern of hawkmoth pollinators. Floral volatiles were quantified in diurnal and nocturnal scent samples using gas chromatography–mass spectrometry.

Key Results

The bouquets of parental species and hybrids showed qualitative overlap. All flowers emitted similar sets of monoterpenoid, sesquiterpenoid, aliphatic and benzenoid compounds, but separated into groups defined by multivariate analysis of quantitative emissions. The parental species differed most strikingly in the nitrogenous compound indole, which was found almost exclusively in nocturnal bouquets of I. tenuituba. Natural hybrid bouquets were highly variable, and showed emission rates of several compounds that appeared transgressive. However, indole emission rates were intermediate in the hybrids compared with rates in the parents. Volatile bouquets at the contact site with lower hybridization did not show greater species specificity in overall scent emission, but I. tenuituba presented a stronger indole signal during peak hawkmoth activity at that site.

Conclusions

The two species of Ipomopsis differed in patterns of floral bouquets, with indole emitted in nocturnal I. tenuituba, but not in I. aggregata. Natural hybrid bouquets were not consistently intermediate between the parents, although hybrids were intermediate in indole emission. The indole signal could potentially serve as a hawkmoth attractant that mediates reproductive isolation both before and after hybrid formation.     

Silk filaments enhance the settlement of stream insect larvae

The recombination that follows natural hybridization may produce hybrid genotypes with traits that are intermediate or extreme relative to the parental species, and these traits may influence the relative fitness of the hybrids. Here we examined leaf ecophysiological traits that may influence fitness patterns in a natural plant hybrid zone. We compared the biochemical photosynthetic capacity of Ipomopsis aggregata, I. tenuituba, and early generation hybrids, as well as their photosynthetic responses to varying light and temperature, two abiotic factors found to differ among sites along the hybrid zone. In general, ecophysiological traits expressed in these plants were consistent with their natural habitat, even when grown under common greenhouse conditions. I. tenuituba reached higher photosynthetic rates (A) at higher light levels than I. aggregata, and also had a higher optimal temperature for photosynthesis (T _opt). This suite of traits may reflect adaptations to the more exposed, rocky sites where I. tenuituba is found, compared to the more vegetated, mesic I. aggregata site. Hybrids had characters that were largely intermediate or tenuituba-like, but particular individual hybrids were extreme for some traits, including light saturation level, light-saturated A, and T _opt. Many of these traits are consistent with adaptations reported for plants found in warm, dry sites, so they may put certain hybrids at an advantage at the relatively xeric center of the natural hybrid zone.     

INDIRECT SELECTION OF STIGMA POSITION IN IPOMOPSIS AGGREGATA VIA A GENETICALLY CORRELATED TRAIT

Experimental manipulation of a trait can be used to distinguish direct selection from selection of correlated traits and to identify mechanisms of selection. Here we use experiments to investigate phenotypic selection of stigma position in angiosperm flowers. In natural populations of the subalpine herb Ipomopsis aggregata, plants with more strongly exserted stigmas receive more pollen per flower, indicating selection favoring stigma exsertion during the pollination stage of the life cycle. We pose four hypotheses for this association, two involving direct selection on stigma position and two involving indirect selection of a correlated floral trait. The first three hypotheses were tested using hand pollinations that mimicked natural hummingbird visitation, and by presenting captive hummingbirds with a series of flowers that differed in stigma and anther positions, sex ratio, and presence of anthers. In these experiments, pollen deposition either was independent of stigma exsertion or was highest on inserted stigmas, suggesting direct selection against exserted stigmas. In natural populations, however, stigma exsertion is highly correlated with time spent by the protandrous flowers in the pistillate phase. When we manipulated the latter trait in the field, pollen deposition increased with duration of exposure to hummingbirds, indicating indirect selection for stigma exsertion. Stigma exsertion and time spent in the pistillate phase are genetically and phenotypically correlated, as shown by a quantitative genetic experiment conducted in the field with paternal half sibships. Our results suggest that the evolution of stigma position can be driven by selection of a genetically correlated trait.     

THE EFFECT OF DELPHINIUM NELSONII POLLEN ON SEED SET IN IPOMOPSIS AGGREGATA,A COMPETITOR FOR HUMMINGBIRD POLLINATION

Sympatric plant species can compete for pollination services in several ways. For example, pollinators may move between species and deposit heterospecific pollen on stigmas, which in turn may reduce the efficacy of conspecific pollen. We explored this possibility by determining the effect of Delphinium nelsonii pollen on seed set in Ipomopsis aggregata. These montane herbs are pollinated by hummingbirds, experience heterospecific pollen deposition in nature, and suffer reduced seed set in each other's presence. We hand-pollinated flowers of I. aggregata with either pure conspecific pollen or a mixture of pollen of the two species. Resulting pollen loads on stigmas ranged from 0–865 D. nelsonii grains and from 10–336 I. aggregata grains; mean seed set per flower was 11.3. There was no detectable effect of D. nelsonii pollen load on I. aggregata seed set. It is possible that seed set reductions seen in previous studies of competition for pollination between these species were caused by pollen wastage, pollen layering on the pollinator, or the temporal sequence of pollen arrival at the stigma.     

Experimental studies of pollen carryover: Hummingbirds and Ipomopsis aggregata

Summary We present results of experiments designed to identify floral characteristics that influence patterns of pollen carryover by hummingbirds visiting Ipomopsis aggregata flowers. We used fluorescent dye powders as pollen analogues. For all four experimental treatments considered, amounts of dye deposited on recipient stigmas declined linearly as a function of flower position in a visitation sequence. The decline was significantly steeper when recipient flowers had pollen-carrying anthers than when they did not; whereas degree of stigma clogging and presence or absence of empty anthers did not influence carryover. From this we conclude that presence of pollen on recipient flowers significantly reduces the average number of subsequent flowers reached by donor pollen. We discuss mechanisms for this effect and its significance for the evolution of floral structure.     

COMPONENTS OF PHENOTYPIC SELECTION: POLLEN EXPORT AND FLOWER COROLLA WIDTH IN IPOMOPSIS AGGREGATA

In the hummingbird-pollinated herb Ipomopsis aggregata, selection through male function during pollination favors wide corolla tubes. We explored the mechanisms behind this selection, using phenotypic selection analysis to compare effects of corolla width on two components of male pollination success, pollinator visit rate and pollen exported per visit. During single visits by captive hummingbirds, flowers with wider corollas exported more pollen, and more dye used as a pollen analogue, to stigmas of recipient flowers. Corolla width was less strongly related to visit rate in the field, and had no direct effect on visit rate after nectar production and corolla length were controlled for. Moreover, the phenotypic selection differential was 80% higher for the effect on pollen exported per visit, suggesting that this is the more important mechanism of selection.     

Pollinator‐mediated phenotypic selection does not always modulate flower size and number in the large‐flowered Mediterranean shrub Cistus ladanifer (Cistaceae)

Flower size and number usually evolve under pollinator‐mediated selection. However, hot, dry environments can also modulate display, counteracting pollinator attraction. Increased pollen deposition on larger flower displays may not involve higher female fitness. Consequently, stressful conditions may constrain flower size, favouring smaller‐sized flowers. The large‐flowered, self‐incompatible Mediterranean shrub Cistus ladanifer was used to test that: (1) this species suffers pollen limitation; (2) pollinators are spatially–temporally variable and differentially visit plants with more/larger flowers; (3) increased visits enhance reproduction under pollen limitation; (4) stressful conditions reduce female fitness of larger displays; and (5) phenotypic selection on floral display is not just pollinator‐mediated. We evaluated pollen limitation, related floral display to pollinator visits and fruit and seed production and estimated phenotypic selection. Flower size was 7.2–10.5 cm and varied spatially–temporally. Visitation rates (total visits/50 min) ranged from 0.26 to 0.43 and increased with flower size. Fruit set averaged 80% and seed number averaged 855, but only fruit set varied between populations and years. Selection towards larger flowers was detected under conditions of pollen limitation. Otherwise, we detected stabilizing selection on flower size and negative selection on flower number. Our results suggest that selection on floral display is not only pollinator‐dependent through female fitness in C. ladanifer. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 540–555.     

SELECTION ON POLEMONIUM BRANDEGEEI (POLEMONIACEAE) FLOWERS UNDER HUMMINGBIRD POLLINATION: IN OPPOSITION,PARALLEL, OR INDEPENDENT OF SELECTION BY HAWKMOTHS?

Particular floral phenotypes are often associated with specific groups of pollinators. However, flowering plants are often visited, and may be effectively pollinated by more than one type of animal. Therefore, a major outstanding question in floral biology asks: what is the nature of selection on floral traits when pollinators are diverse? This study examined how hummingbirds selected on the floral traits of Polemonium brandegeei, a species pollinated by both hummingbirds and hawkmoths. In array populations of P. brandegeei, we measured pollen movement, and female (seeds set) and male (seeds sired) fitness under hummingbird pollination. We then compared the patterns of selection by hummingbirds with our previous study examining selection by hawkmoths. We documented contrasting selection on sex organ positioning through female function, with hummingbirds selecting for stigmas exserted beyond the anthers and hawkmoths selecting for stigmas recessed below the anthers. Furthermore, hummingbirds selected for longer and wider corolla tubes, and hawkmoths selected for narrower corolla tubes. Therefore, contrasting selection by hawkmoths and hummingbirds may account for variation in sex organ arrangements and corolla dimensions in P. brandegeei. We documented how floral traits under selection by multiple pollinators can result in either an intermediate “compromise” between selective pressures (sex organs) or apparent specialization (corolla tube length) to one pollinator.     

Exploring the 'Most Effective Pollinator Principle' with Complex Flowers: Bumblebees and Ipomopsis aggregata

The ‘most effective pollinator principle’ impliesthat floral characteristics often reflect adaptation to thepollinator that transfers the most pollen, through a combinationof high rate of visitation to flowers and effective depositionof pollen during each visit. We looked for the expected positivecorrelation between quantity and quality of visits in Ipomopsisaggregata, whose red, tubular flowers are considered to be adaptedto hummingbirds. Hummingbirds were indeed the most common floralvisitors in 5 years of observation. However, long-tongued bumblebeesdeposited on average three-times as much outcross pollen pervisit to virgin flowers, and elicited four-times as much seedproduction, as did hummingbirds. Hence visitors that are relativelyinfrequent, and unexpected given the ‘pollination syndrome’of the plant, can be surprisingly good pollinators. One interpretationof this observation is that natural selection favours a specializedfloral morphology that excludes all but a single type of visitor,but that there are constraints on achieving this outcome. Analternative is that selection favours some degree of floralgeneralization, but that flowers can retain features that adaptthem to a particular type of pollinator in spite of this generalization.Copyright 2001 Annals of Botany Company Bumblebees, floral evolution, hummingbirds, Ipomopsis aggregata, pollination syndromes, pollinator effectiveness, scarlet gilia, seed set, specialization, temporal variation     

Evaluation of pollinator effectiveness based on pollen deposition and seed production in a gynodieocious alpine plant,Cyananthus delavayi

Examining variations in pollinator effectiveness can enhance our understanding of how pollinators and plants interact. Pollen deposition and seed production after a single visit by a pollinator are often used to estimate pollinator effectiveness. However, seed production is not always directly related to pollen deposition because not all pollen grains that are deposited on a stigma are compatible or conspecific. In the field, we tested pollinator effectiveness based on pollen deposition and the resulting seed production after single visits by different pollinator groups in a gynodieocious alpine plant Cyananthus delavayi (Campanulaceae). Our results showed that mean pollen deposition was generally inconsistent with mean seed production when comparisons were performed among different pollinator groups and sexes. In general, the correlations were not significant between pollen deposition and seed production in both perfect and female flowers after single visits by halictid bees, bumble bees, and hoverflies. We suggest seed set of virgin flowers after single visits is a more reliable indicator of pollinator effectiveness than pollen deposition and would be a better indicator of pollinator effectiveness for future studies.     

Is pollen removal or seed set favoured by flower longevity in a hummingbird-pollinated Salvia species?

Background and Aims

The period between the beginning of anthesis and flower senescence modulates the transport of pollen by pollinators among conspecific flowers, and its length may therefore influence reproductive success. This study evaluated whether floral longevity favours pollen removal from the anthers over fecundity (seed set) in an ornithophilous species that does not undergo pollen limitation.

Methods

Field investigations were conducted on floral longevity, nectar production, pollinator behaviour, and variations in fruit set (FS), mean number of seeds per fruit (MSF) and pollen removal by hummingbirds (PR) during the anthesis of Salvia sellowiana in south-east Brazil.

Key Results

Anthesis of flowers exposed to pollinators lasted 4 d, as well as on flowers with pollen removed from the anthers or deposited on the stigma. The longevity of bagged flowers was significantly higher (approx. 9 d). FS and PR reached 87·2 and 90 %, respectively, in natural conditions. PR increased gradually over the period of anthesis; however, FS and MSF reached their maxima in the first hours of anthesis. Nectar production was continuous, but the secretion rate was reduced after pollination. The removal of nectar from non-pollinated flowers stimulated its production.

Conclusions

The longevity of anthesis in S. sellowiana seems to be related to the mechanism of gradual dispensing of pollen, resulting in greater male reproductive success. This is in agreement with the pollen-donation hypothesis. The small number of ovules (four) of S. sellowiana and the high frequency and the foraging mode of its pollinators may favour the selection for floral longevity driven by male fitness in this system.     

The influence of nectar production on hawkmoth behavior,self pollination,and seed production in Mirabilis multiflora (Nyctaginaceae)

Nectar is the most common floral reward that plants produce to attract pollinators. To determine the effect of nectar production on hawkmoth behavior, pollen movement, and reproductive success in Mirabilis multiflora, I manipulated nectar volumes and observed the subsequent foraging behavior of the hawkmoth Hyles lineata and the resulting pollen movement patterns. Individual hawkmoths visited significantly more flowers on plants with more nectar. The increase in flower visits significantly increased pollen deposition on stigmas and pollen removal from anthers when nectar volume was raised to twice the highest level found in nature. As hawkmoths visited flowers consecutively on a plant, the proportion of self pollen deposited on stigmas increased significantly and rapidly. Based on simulated hawkmoth visits, seed set was significantly reduced for flowers later in a visit sequence. A simple model combining these results predicts that the form of selection on nectar production varies depending on pollinator abundance. Using a multiple regression analysis a nearly significant (P < 0.08) effect of stabilizing selection was detected during a single season as predicted by the model for the prevailing hawkmoth abundance. Although increased nectar production may indirectly affect plant fitness by reducing resources available for other plant functions, the direct effect of high nectar production on pollinator behavior and self pollination may generally limit floral nectar production.     

Absence of conspecific pollen advantage in the dynamics of an Ipomopsis (Polemoniaceae) hybrid zone

The frequency of hybrid formation in angiosperms depends on how often heterospecific pollen is transferred to the stigma and on the success of that heterospecific pollen at fertilizing ovules. Even if heterospecific pollen is capable of effecting fertilization it may perform poorly when conspecific pollen is also available on the stigma. We applied pollen mixtures to stigmas to determine how pollen interactions affect siring success and the frequency of hybrid formation between two species of Ipomopsis (Polemoniaceae) in Colorado. Plants of both parental species and natural hybrids were pollinated with I. aggregata and I. tenuituba pollen in ratios of 100:0, 80:20, 50:50, 20:80, and 0:100 by mass. Plants were homozygous for different alleles at an isozyme marker, allowing us to distinguish the type of pollen parent for 2166 viable seeds from 273 fruits. In contrast to studies of many other hybridizing taxa, there was no evidence of an advantage to conspecific pollen, nor did composition of the stigmatic pollen load affect seed set. Instead, the frequency of seeds sired by a given species was proportional to its representation in the pollen load. In this hybrid zone, both the frequency of first-generation hybrid formation and the relative male fitness of the two parental species should be predictable from the rates of pollen transfer to stigmas.     

Corolla wilting facilitates delayed autonomous self-pollination in Pedicularis dunniana (Orobanchaceae)

Structural changes associated with corolla wilting may serve as a mechanism for effecting self-pollination. Low pollinator visitation, high seed production and a corolla that persists after anthesis indicates that Pedicularis dunniana is autogamous. Delayed autonomous self-pollination is facilitated by corolla wilting. Wilting of the upper lip (galea) brought the pollen laden anthers into contact with the stigma resulting in the deposition of self pollen on the stigma. The seed set of flowers either emasculated, or with restrained galeae thus preventing anthers brushing against the stigma, was significantly lower than that of open-pollinated flowers. This demonstrates that autogamy occurs in this species through corolla wilting. Germination experiments indicated that outcross seedlings were more vigorous than selfed seedlings as a result of inbreeding depression. It is likely that autogamy provides reproductive assurance for P. dunniana under conditions of pollinator scarcity.     

Context-dependent reproductive isolation mediated by floral scent and color

Reproductive isolation due to pollinator behavior is considered a key mode of speciation in flowering plants. Although floral scent is thought to mediate pollinator behavior, little is known about its effects on pollinator attraction and floral visitation in the wild. We used field experiments with wild hawkmoths and laboratory experiments with naïve hawkmoths to investigate attraction to and probing of flowers in response to indole, a volatile emitted by Ipomopsis tenuituba but not its close relative I. aggregata, both alone and in combination with floral color differences. We demonstrated that indole attracts wild hawkmoths to flowers, but has little effect on the rate at which those attracted moths probe flowers. In contrast, white flower color did not influence hawkmoth attraction in the field, but caused more attracted moths to probe flowers. Thus, the moths require both scent and high visual contrast, in that order, to feed at flowers at dusk. Their preference for indole-scented flowers is innate, but species-specific preference is mitigated by previous experience and plant spatial patterning. This context-dependent behavior helps explain why these Ipomopsis species show geographical variation in the extent of hybridization and may potentially explain formation of hybrid bridges in other systems of hawkmoth-pollinated plants.     

POLLEN DISPERSAL DYNAMICS IN AN ALPINE WILDFLOWER,POLEMONIUM VISCOSUM

The alpine wildflower, Polemonium viscosum, depends on insect visitors for effective pollination. Here, I examine experimentally the effects of pollinator visitation on pollen removal, pollen dispersal success, paternity, and gene flow. Bumble bee pollinators visited donor individuals homozygous for marker alleles at an isozyme (GOT-2) encoding locus and then were presented with arrays of recipient plants lacking the marker alleles. Four aspects of male fitness were estimated for each donor: the number of pollen grains dispersed to flowers of the first recipient visited, the number of offspring sired on that recipient, the proportion of offspring sired in the full array, and the proportion of mates in the array bearing seeds of the donor. Pollen removal was strongly influenced by the number of bee visits to donor flowers. The amount of pollen removed in turn significantly affected the number of pollen grains reaching flowers of the first recipient. However, because seed production decelerates with stigma pollen load, the relationship between pollen export and paternal success at this proximate scale showed diminishing returns. The probability of reaching mates within the array also increased with pollen export. These findings show that floral characters enhancing pollinator visitation rate in P. viscosum have positive effects on paternity and gene flow.     

Does the morphological fit between flowers and pollinators affect pollen deposition? An experimental test in a buzz‐pollinated species with anther dimorphism

Some pollination systems, such as buzz‐pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the “fit” between flower and pollinator influences pollen transfer, particularly among buzz‐pollinated species. Here we use Solanum rostratum, a buzz‐pollinated plant with dimorphic anthers and mirror‐image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz‐pollinated species. We speculate that in buzz‐pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).