Patterns Of Pollen Flow Research Articles

Patterns of pollen flow in monomorphic enantiostylous species: the importance of floral morphology and pollinators’ size

Enantiostyly is a floral polymorphism consisting in the presence of two floral morphs that differ in the deflection of the style to the right or left in the populations. Monomorphic enantiostyly consists in plants showing both morphs within an individual and is thought to promote pollen transference between morphs, reduce levels of self-pollination and geitonogamy and avoid damage to floral parts during pollinator’s visit. In this study, we examined the function of this floral polymorphism in a group of plants with non-typical enantiostylous characteristics, the family Vochysiaceae. In particular, we evaluated how floral morphology (both sexual organ separation and the match between complementary sexual organs in the morphs) is related to pollen deposition on the pollinators’ body and to patterns of pollen flow among individuals. For this, we used floral morphometrics, insect captures and fluorescent powder dyes. We confirmed that monomorphic enantiostyly increased pollen transference between morphs compared to non-enantiostyly. Moreover, pollen flow occurred only between flowers of opposite morph. We also observed that pollen deposition on the insect’s body depended on their body shape rather than floral morphology. In fact, an intertegular distance slightly larger than the anther–stigma distance in the flowers increased pollen transference because anthers filaments and stigmas are flexible, which facilitated pollen brushing on insects’ body when approaching the flowers. Since the species are self-incompatible, we further discuss the role of mirror flowers in the reduction of geitonogamy and inbreeding depression.

Pollen mediated gene flow in a small experimental population of Moringa oleifera Lam. (Moringaceae)

Large-scale plantations of Moringa oleifera Lam. are carrying out in the dry-hot valley of Red River in Yunnan Province to make it the core cultivation area in China. Nevertheless, there is a deficient understanding of detailed pollen flow patterns, which leads to uncertainty in designing and managing seed orchards of M. oleifera to fit the breeding demand. In order to facilitate the design of breeding strategies for inbreeding lines, paternity analysis by SSR (Simple Sequence Repeat) was used to estimate the outcrossing rate and pollen mediated gene flow of M. oleifera. The analysis was carried out in an experimental population domesticated in the dry-hot valley area of Red River with two germplasms (India and Myanmar) of 60 trees, including 12 maternal trees and 60 pollen donors (bisexual flowers, maternal trees were also possible paternal trees in case of selfing). One hundred fifteen seeds out of 288 were confirmed pollen donors by 8 microsatellite loci at 95% strict confidence level. The multilocus outcrossing rate is tm = 0.797 ± 0.096, and single-locus outcrossing rate is ts = 0.742 ± 0.058. Pollen dispersal distance varied from 0 m (self-pollination) to 24.7 m, with an average of 11.9 m, and 95% of effective pollen was dispersed within 20 m, indicating that the effective pollen mediated gene flow is restricted to a limited distance. 50.3% of effective pollen was donated by 9 trees. Moreover, there was no apparent correlation between pollen dispersal distance and pollen contribution rate, as well as between germplasm and reproducibility. The maximum pollen dispersal distance between two trees estimated in this experimental stand is highly localized at current 3 × 3 m planting density, indicating that M. oleifera seed orchards and clones could be planted nearer to each other than previously thought (as far as kilometers).

Physiological effects of climate warming on flowering plants and insect pollinators and potential consequences for their interactions

Growing concern about the influence of climate change on flowering plants, pollinators, and the mutualistic interactions between them has led to a recent surge in research. Much of this research has addressed the consequences of warming for phenological and distributional shifts. In contrast, relatively little is known about the physiological responses of plants and insect pollinators to climate warming and, in particular, how these responses might affect plant-pollinator interactions. Here, we summarize the direct physiological effects of temperature on flowering plants and pollinating insects to highlight ways in which plant and pollinator responses could affect floral resources for pollinators, and pollination success for plants, respectively. We also consider the overall effects of these responses on plant-pollinator interaction networks. Plant responses to warming, which include altered flower, nectar, and pollen production, could modify floral resource availability and reproductive output of pollinating insects. Similarly, pollinator responses, such as altered foraging activity, body size, and life span, could affect patterns of pollen flow and pollination success of flowering plants. As a result, network structure could be altered as interactions are gained and lost, weakened and strengthened, even without the gain or loss of species or temporal overlap. Future research that addresses not only how plant and pollinator physiology are affected by warming but also how responses scale up to affect interactions and networks should allow us to better understand and predict the effects of climate change on this important ecosystem service.

Pollen flow and effects of population structure on selfing rates and female and male reproductive success in fragmented Magnolia stellata populations

BackgroundFragmentation of plant populations may affect mating patterns and female and male reproductive success. To improve understanding of fragmentation effects on plant reproduction, we investigated the pollen flow patterns in six adjacent local populations of Magnolia stellata, an insect-pollinated, threatened tree species in Japan, and assessed effects of maternal plant (genet) size, local genet density, population size and neighboring population size on female reproductive success (seed production rates), and effects of mating distance, paternal genet size, population size and separation of populations on male reproductive success.ResultsThe seed production rate, i.e. the proportion of ovules that successfully turned into seeds, varied between 1.0 and 6.5%, and increased with increasing population size and neighboring population size, and with decreasing maternal genet size and local genet density. The selfing rate varied between 3.6 and 28.9%, and increased with increasing maternal genet size and with declining local genet density. Male reproductive success increased with increasing paternal genet size, and decreased with increasing mating distance and separation of population. Pollen flow between the populations was low (6.1%) and highly leptocurtic.ConclusionsOur results indicate that habitat fragmentation, separation and reduced size of populations, affected mating patterns and reproductive success of M. stellata. Local competition for pollinators and plant display size were likely to alter the reproductive success.

Marked point pattern analysis on genetic paternity data for uncertainty assessment of pollen dispersal kernels

Summary 1. Obtaining accurate estimates of the pollen dispersal kernel is central to a wide range of ecological studies. Assessing their statistical uncertainty is as important, but rarely considered. 2. We developed a new method of marked point processes for nonparametric estimation of dispersal kernels based on data of genetic paternity analysis that does not require assumptions about the shape of the dispersal distribution. This allows for construction of Monte Carlo simulation envelopes of a given null model, such as random mating, and for uncertainty assessment of the observed dispersal kernel. 3. We applied our method to characterize spatial patterns of pollen flow in an isolated population of Populus nigra in Central Germany and to assess the associated statistical uncertainty in estimates of within-population dispersal kernels. We compared our nonparametric within-population kernel estimate with that of established methods of parametric kernel fitting including: (i) a general mating model, (ii) a simplified mating model using categorical paternity data and (iii) least-squares regression of the nonparametric kernel estimate. 4. Our analysis showed a significant departure from the random mating null model. We found a highly significant excess of mating events at short distances ( 2000 m), indicating large uncertainty on the detailed shape of the kernel’s tail. 5. Results of the point pattern analysis were consistent with kernel fits using published maximum-likelihood mating models. Model selection revealed that two-component pollen dispersal kernels were the most parsimonious functions. 6. Synthesis. Our approach of nonparametric kernel estimation could be widely applied for dispersal data from genetic paternity analysis and complements traditional kernel estimation by providing a nonparametric kernel estimate and effective methods for an uncertainty assessment in kernel estimation. Our results indicate that statistical model fitting may substantially underestimate the uncertainty in kernel estimation, especially at larger distances.

Mating system and pollen flow between remnant populations of the endangered tropical tree, Guaiacum sanctum (Zygophyllaceae)

Tropical trees are generally long-lived making it difficult to assess the long-term effects of habitat fragmentation on genetic diversity. Maintenance of genetic diversity in fragmented landscapes is largely dependent on the species’ mating system and the degree of genetic connectivity (seed and pollen flow) among fragments. Currently, these parameters are largely unknown for many endangered tropical tree species. Additionally, landscape fragmentation may isolate tropical tree populations from larger, more continuous populations. The role of isolated individuals in pollen transfer within and between remnant populations is not clear. In this study, we estimate the mating system and pollen flow patterns in continuous and remnant populations of the endangered tropical tree Guaiacum sanctum (Zygophyllaceae). Fractional paternity analyses were used to estimate average gene flow distances between fragmented remnant populations and the siring success of an intermediately located, but isolated individual. In these populations, G. sanctum is a mixed-mating species (tm = 0.72 − 0.95) whose pollen is transported over large distances (>4 km). An isolated tree may have functioned as a stepping-stone between two clusters of individuals, assisting long-distance pollen movement. This individual also sired a disproportionately high number of seeds (13.9%), and is thus an important component of the reproductive success of these populations, thus rejecting Janzen’s “living-dead” hypothesis. The high levels of genetic diversity maintained as a consequence of long-distance pollen-flow suggest that this endangered species may have the potential for future adaptation and population expansion if suitable habitats become available.

Spontaneous Interspecific Hybridization and Patterns of Pollen Dispersal in Ex Situ Populations of a Tree Species (Sinojackia xylocarpa) that is Extinct in the Wild

For endangered plants interspecific hybridization occurring in ex situ collections may lead to failure of reintroduction actions. We used Sinojackia xylocarpa, a well documented Chinese endemic species that is extinct in the wild, as a model case to address this concern. We used paternity analyses to assess the spontaneous hybridization and patterns of pollen flow between S. xylocarpa and its congener species, S. rehderiana, in conserved populations in Wuhan Botanic Garden. Interspecific hybridization events were detected in seven out of eight maternal trees of S. xylocarpa, and an average of 32.7% seeds collected from maternal trees of S. xylocarpa were hybrids. The paternity of 93 out of 249 seedlings from S. xylocarpa assigned to S. rehderiana provided convincing evidence that spontaneous interspecific hybridization occurred extensively in the living garden collection we studied. Different patterns of pollen dispersal (predominantly short-distance vs. long-distance pollination) were observed between intra- and interspecific hybridization events in the garden. Pollen dispersal within the ex situ populations was not restricted by distance, as evidenced by a lack of significant correlations between the average effective pollen dispersal distance (delta) and the geographic distances (d1 and d2) between maternal and paternal trees. The interspecific pollen-dispersal distance ranged from 10 to 620.1 m (mean 294.4 m). Such extensive hybridization in ex situ collections could jeopardize the genetic integrity of endangered species and irrevocably contaminate the gene pool if such hybrids are used for reintroduction and restoration. We recommend strongly that measures be taken to minimize the genetic risks of this kind of hybridization, including establishing buffer zones in ex situ collections, manipulating flowering phenology, testing seed lots before use in reintroduction programs, and controlling pollination for seed purity.

On gene dispersal studies in complex landscapes: a reply to the comment on García et al. (2005, 2007)

On gene dispersal studies in complex landscapes: a reply to the comment on García <i>et al.</i> (2005, 2007)

Mating patterns and contemporary gene flow by pollen in a large continuous and a small isolated population of the scattered forest tree Sorbus torminalis

The influence of population size and spatial isolation on contemporary gene flow by pollen and mating patterns in temperate forest trees are not well documented, although they are crucial factors in the life history of plant species. We analysed a small, isolated population and a large, continuous population of the insect-pollinated tree species Sorbus torminalis in two consecutive years. The species recently experienced increased habitat fragmentation due to altered forest management leading to forests with closed canopies. We estimated individual plant size, percentage of flowering trees, intensity of flowering, degree of fruiting and seed set per fruit, and we determined mating patterns, pollen flow distances and external gene flow in a genetic paternity analysis based on microsatellite markers. We found clear effects of small population size and spatial isolation in S. torminalis. Compared with the large, continuous population, the small and isolated population harboured a lower percentage of flowering trees, showed less intense flowering, lower fruiting, less developed seeds per fruit, increased selfing and received less immigrant pollen. However, the negative inbreeding coefficients (F(IS)) of offspring indicated that this did not result in inbred seed at the population level. We also show that flowering, fruiting and pollen flow patterns varied among years, the latter being affected by the size of individuals. Though our study was unreplicated at the factor level (i.e. isolated vs non-isolated populations), it shows that small and spatially isolated populations of S. torminalis may also be genetically isolated, but that their progeny is not necessarily more inbred.

Pollen flow in Eucalyptus grandis determined by paternity analysis using microsatellite markers

Gene flow by pollen dispersal from forestry plantations containing introduced species, provenances or selected elite breeding material may impact on local native forest by changing the genetic diversity, introducing new genes or gene combinations, or causing the extinction of rare genotypes in adjacent native forest areas. Patterns of pollen flow can be used to assess the risk of genetic pollution of native forest areas from nearby plantations. Pollen flow in an artificial population of Eucalyptus grandis was estimated using molecular markers and paternity analysis. Microsatellite genotyping was used to identify pollen parents of progeny arrays from six mother trees. Of 329 progeny analysed, 178 (54%) were assigned to pollen parents within the population. Pollen parents located within the population were between 0–192 m from the respective mother trees, with an average pollination distance of 57.96 m. Pollination of mother trees was outcrossed, not by nearest neighbours, and displayed a preference for inter-provenance matings within the population. Progeny that could not be assigned pollen parents within the population (46%) were assumed to have resulted from pollen immigration from external sources. These pollen flow parameters provide useful information about the dynamics of pollen movement within E. grandis populations and may be used in risk assessment of gene flow from plantations to adjacent areas of native forest.

How do floral display size and the density of surrounding flowers influence the likelihood of bumble bee revisitation to a plant?

Summary 1Most pollination biologists have used the collective pollinator visits to a plant as the measure of its pollinator attraction. However, we know very little about how many returns by the same individuals compose these visits, and how far each visitor travels after leaving the plant. Such behavioural aspects of individual pollinators are essential to understand the patterns of pollen flow among plants. 2We observed plant visits by tagged bumble bees Bombus diversus in a field population of Cirsium purpuratum. By dissecting the collective visitation data into visits made by individual foragers, we addressed how ‘visitor density’ (number of individuals that visited a plant per 2 h) and ‘individual visitation rate’ (number of visits made by each individual per 2 h) are related to floral display size (number of flowering heads on a plant) and local flower density (number of flowering heads on neighbouring plants). We also tracked individual bees to determine how display size and local flower density of a plant influences its relative position in a bee's foraging area. 3Plants attracted both regular visitors (bees that visited a plant more than three times per 2 h) and occasional visitors (bees that visited a plant fewer than four times per 2 h). Densities of both types of visitors increased with floral display size, whereas only occasional visitor's density increased with local flower density. 4Individual bees preferred to visit central plants within their own foraging areas, plants with larger displays, and plants with lower local flower density. However, these preferences were independent from one another. Plants with large displays were not necessarily chosen by a bee as the centre of its own foraging area. On the other hand, plants with high local flower density were often located near the centre of a bee's foraging area. 5The observed pollinator movements have implications for pollen flow in the plant population. Plants with larger displays probably experience greater mate diversity by attracting more occasional visitors, but they also assure matings with particular plants by increasing returns from regular visitors.

Real‐time patterns of pollen flow in the wild‐service tree, Sorbus torminalis (Rosaceae). III. Mating patterns and the ecological maternal neighborhood

Understanding the role of mother plants as pollen recipients in shaping mating patterns is essential for understanding the evolution of populations and in particular to predict the consequence of habitat fragmentation. Here, we investigated variation in mating patterns due to maternal phenotypic traits, phenological variance, and landscape features in Sorbus torminalis, a hermaphroditic, insect-pollinated and low-density, European temperate forest tree. The diversity and composition of pollen clouds received by maternal trees in S. torminalis were mainly determined by their conspecific neighborhood: isolated individuals sample more diversity through more even paternal contributions, low relatedness among paternal genes, and high rates of long-distance pollen dispersal within their progenies. Maternal phenotypic traits related to pollinator attractiveness also had an effect, but only when competition was strong: in this case, larger mother trees with more flowers sampled more diversity. The floral architecture of S. torminalis, with multiple-seeded fruit, strongly shaped mating patterns, with higher levels of correlated paternity among seeds belonging to the same fruit (30% full sibs) than among seeds belonging to different fruits (14% full sibs). Finally, flowering phenology affected the distribution of diversity among maternal pollen clouds, but the earliest and latest mother trees did not receive less diversity of pollen than the others.

Eight microsatellite markers for sympatric alpine shrubs, Phyllodoce aleutica and P. caerulea (Ericaceae)

Abstract Phyllodoce aleutica and Phyllodoce caerulea are sympatric alpine‐snowbed plants in northern Japan. They compete for pollinators (bumblebees) each other and the competitive situation varies depending on snow conditions. We isolated and characterized eight microsatellite loci in these species. Additionally, one of 13 primers developed for Rhododendron metternichii was available in these species. The expected heterozygosity of these nine markers ranged from 0.06 to 0.93 in P. aleutica and from 0.09 to 0.96 in P. caerulea. These markers may be useful to reveal the mating system evolution, patterns of pollen flow and the process of natural hybridization in these Phyllodoce species.

Patterns of Pollen Flow and Genetic Differentiation Among Pollen Pools in Quercus salicina in a Warm Temperate Old–growth Evergreen Broad-leaved Forest

Abstract Paternity analysis and analysis of molecular variance were used to determine patterns of pollen flow and genetic differentiation among pollen pools in Quercus salicina in an 11.56-ha plot in a temperate old-growth evergreen broad-leaved forest. The genotypes at seven microsatellite loci were determined for 111 adult trees and 276 seeds collected from under eight seed parents. The proportion of pollen flow from outside the plot (further than 100 m) was 52.2%, indicating that long-distance pollen flow occurred frequently in this species, as observed in other Quercus species. The pollen pools from inside and outside the plot differed genetically, and genetic structure was detected in the population of adult trees within the plot. Therefore, longdistance pollen flow from outside the plot may introduce new or low-frequent alleles, and increase genetic diversity in this population. However, the actual average distance of pollen flow within the plot was significantly shorter than the average potential distance, and negative exponential curves explained well the frequencies of matings as functions of the distance between parents, as found in other Quercus species. The genetic composition of pollen pools differed among the eight seed parents. The genetic differentiation of pollen pools between the seed parents showed significant positive correlation with the spatial distance between them, indicating that neighboring seed parents tend to accept similar pollen pools, probably because matings are frequently mediated by pollen transported over short distances

Microsatellite analysis of pollen flow in Rhododendron metternichii var. hondoense

Microsatellite analysis was used to characterize the patterns of pollen flow in a 150 m × 70 m quadrat containing 18 flowering trees of Rhododendron metternichii Sieb. et Zucc. var. hondoense Nakai. Using six microsatellite genetic markers and exclusion analysis, we determined the paternal trees of 216 seedlings germinated from five fruits produced by four adults. Each fruit was pollinated by a small number of largely adjacent trees, but 20–30% of pollen came from outside the quadrat. Adult trees that produced many flowers had a high self‐pollination rate. A directional flow of pollen from late‐blooming trees to early ones was observed.

Influence of local floral density and sex ratio on pollen receipt and seed output: empirical and experimental results in dichogamous Alstroemeria aurea (Alstroemeriaceae).

Local density and sexual composition are two aspects of floral neighborhoods thought to influence pollination and seed output of recipient plants. I characterized the floral neighborhood of 436 flowering ramets of Alstroemeria aurea, a southern Andean perennial, distributed among three sites. On each ramet, I measured total pollen receipt and seed output. The long-lived, bumblebee-pollinated flowers of A. aurea are synchronously protandrous with a given ramet being either all male or all female and thus incapable of self or geitonogamous pollination at the ramet level. Even though each ramet changes sex over time, A. aurea forms floral neighborhoods that remain stable with respect to density and sex ratio during the span of a focal ramet female phase. Contrary to expectation, under field conditions neither local density nor sexual identity explained significant amounts of variation in pollen receipt. Density of neighboring flowering ramets marginally affected pollen receipt in two of the three populations but in opposite directions. Despite the absence of strong effects of neighborhood sexual composition on pollen receipt, the sexual identity of neighbors affected seed output which suggests effects on the quality of pollination due to changes in patterns of pollen flow. I also compared pollen loads on the stigmas of artificially isolated ramets (6 m) with those on experimental focal ramets surrounded by six close neighbors (20 cm) that were either all male or all female. Here, pollen receipt by focal ramets in all-male neighborhoods was 1.3 times greater than in isolated ramets, and 3.8 times greater than in ramets in all-female neighborhoods. In these artificial neighborhoods, stigmatic pollen deposition increased significantly over time. In nature, rates of bumblebee visits were higher in female-biased (early-flowering) than in male-biased (late-flowering) co-occurring floral patches. Thus, spatio-temporal shifts in visitation frequencies associated with the sexual composition of floral neighborhoods might compensate for spatial variability in pollen availability within populations and explain the discrepancies between empirical and experimental results.

Foraging Behaviour of Marsupial Pollinators in a Population of Banksia spinulosa

Spool-and-line tracking was used to investigate the foraging behaviour of three marsupial species (Petaurus breviceps, Antechinus stuartii and Cercartetus nanus) which regularly visit inflorescences of the Australian native, Banksia spinulosa (Proteaceae). Although there were as many as 10 open inflorescences on a plant, 72% of movements by animals were to inflorescences on a different plant. Furthermore. nearly 70% of these movements were over 5 m in length, and movements ranged up to 59 m. Inter-plant movements were greater for P. breviceps than other species, particularly late in the flowering season when fewer inflorescences were available. This suggests that patterns of pollen flow in this population of B. spinulosa may vary spatially and temporally

Effect of spatially restricted pollen flow on spatial genetic structure of an animal-pollinated allogamous plant population

The consequences of population genetic structure under different degrees of restriction of pollen flow in an animal-pollinated allogamous plant species were investigated through computer simulations. A single locus with two alleles in a population of self-incompatible, bisexual diploid plants was considered. The formula f∝e−Kd was adopted to describe pollen flow patterns where f is the probability of mating, d is the distance from the pollen source, and K is a constant. Values of K estimated by fitting the formula to data in the literature for different plant species varied from 0.076 to 0.787 for gene flow through pollen and from 0.652 to 2.805 for pollinator movement. Starting with randomly distributed genotypes with equal allele frequencies, simulation runs up to the 200th generation were made for different values of K in the range from 0.06 to 3.00. Even with the highest empirically based value of K(=3.00), the decrease in heterozygosity and development of patch structure in our simulations were less than those of the ‘relaxed‘ model by Turner et al. (1982) and were similar to the ‘neighborhood’ model by Sokal & Wartenberg (1983) and Epperson (1990). Local differentiation was very sensitive to the degree of gene dispersal. We conclude that one cannot ascribe any local differentiation observed solely to restriction of gene flow unless one explicitly examines whether the degree of restriction is sufficiently severe to produce the differentiation.

The function and adaptive significance of tristyly in Pontederia cordata L. (Pontederiaceae)

Darwin proposed that the adaptive significance of tristyly is to promote insect-mediated pollination among floral morphs with anthers and stigmas at equivalent levels (legitimate pollination). Strong pollen trimorphism in Pontederia cordata enables an evaluation of this hypothesis. In an investigation of pollen flow patterns in different parts of the North American range of the species, legitimate pollination of some morphs was observed in all but one population investigated. However at only one site (Ft. McCoy, Florida) was Darwin's hypothesis confirmed for the three floral morphs. The long-styled morph most frequently exhibited legitimate pollination, the mid-styled morph was intermediate, and the short-styled morph rarely experienced legitimate pollination. Temporal variation in pollen flow was observed during the flowering season of P. cordata. Total stigmatic pollen loads decreased as the density of inflorescences and activity of pollinators declined. In spite of variable total loads the legitimate component was relatively constant in the long- and mid-styled forms, while in the short-styled form this measure increased as the season progressed. Little variation was detected between stigmatic samples collected at different times during a single day. Legitimate pollination may be inhibited by local foraging of pollinators and the spatial segregation of floral morphs. At Paugh Lake (Ontario) over 75% of all bumble bee flights are among the five nearest neighbours of a particular inflorescence. In the same population, there is a probability of over 70% that the nearest three neighbours of an inflorescence are of the same floral morph. Despite these influences, legitimate pollination occurs in P. cordata, suggesting substantial pollen carryover.

HETEROSTYLY, POPULATION COMPOSITION, AND POLLEN FLOW IN HEDYOTIS CAERULEA

Hedyotis caerulea is a perennial, spring‐flowering herb native to eastern North America with distylous flowers that differ in a number of morphological and physiological traits. However, the pin and thrum morphs produce the same numbers of buds, flowers, and fruits per plant, although it is possible that differences in these may occur in some populations at certain times of the flowering period. The two morphs are self‐incompatible and cross‐compatible. Most populations contained an excess of pins over thrums (anisoplethy); less commonly pins and thrums were equally represented (isoplethy). Populations change from anisoplethy to isoplethy and in the reverse direction. The spatial distribution of pin and thrum flowers in populations was random in some populations but non‐random in others. There is some indication that the two morphs in some populations have somewhat different flowering periods. Pollinators seem to be chiefly bombyliid flies and perhaps thrips, but insects were rarely observed visiting flowers. In some populations, the two morphs produced equal numbers of pollen grains per flower; in others they did not. The average pollen viability varied, but on the average there was a moderate level of pollen sterility. High numbers of pollen grains remained in dehisced anthers, probably as a result of low pollinator activity. However, between 5% and 9% of the pollen produced participated in pollination. Stigmas of most pin flowers received more pin pollen grains than thrum pollen grains; on stigmas of thrum flowers pin pollen grains outnumbered thrum pollen grains. Thus, compatible pollen flow from pins to thrums was greater than in the reverse direction. Anisoplethic and isoplethic populations had the same pollen flow patterns. A plant‐by‐plant examination of stigmas indicated that many stigmas bore few or no pollen grains. Seed production of the two morphs was equal. Despite the inequities in pollen flow patterns, the widespread and occasionally weedy nature of H. caerulea suggests that its breeding system must be viewed as a successful one.