Analysis of airborne pollen fall in Sakarya, Turkey

In this study, pollen grains were identified using Durham sampler in the atmosphere of Sakarya in 2000 and 2001. During these two years, a total of 10 805 pollen grains were recorded. A total of 5 386 pollen grains per cm² were recorded in 2000 and a total of 5 419 pollen grains per cm² in 2001. Pollen fall in the years 2000–2001 comprised grains belonging to 40 taxa and some unidentified pollen grains. Of these taxa, 22 belonged to arboreal and 18 taxa to non arboreal plants. Total pollen grains consisted of 69.45% grains from arboreal plants, 28.11% grains from non-arboreal plants and 2.44% unidentified pollen grains. In the region investigated, Gramineae, Pinus sp., Quercus sp., Cupressaceae/Taxaceae, Salix sp., Platanus sp., Populus sp., Carpinus sp., Fagus sp., Chenopodiaceae/Amaranthaceae, Xanthium sp., Moraceae, Corylus sp., Fraxinus sp., and Urticaceae released the greatest amount of pollen. The season of maximum pollen fall was from March to May, with a prevalence of arboreal pollen in the first months, and of pollen from non-arboreal plants in the last months of the year.     

Precocious pollen germination in Arabidopsis plants with altered callose deposition during microsporogenesis

Pollination is essential for seed reproduction and for exchanges of genetic information between individual plants. In angiosperms, mature pollen grains released from dehisced anthers are transferred to the stigma where they become hydrated and begin to germinate. Pollen grains of wild-type Arabidopsis thaliana do not germinate inside the anther under normal growth conditions. We report two Arabidopsis lines that produced pollen grains able to in situ precociously germinate inside the anther. One of them was a callose synthase 9 (cs9) knockout mutant with a T-DNA insertion in the Callose Synthase 9 gene (CalS9). Male gametophytes carrying a cs9 mutant allele were defective and no homozygous progeny could be produced. Heterozygous mutant plants (cs9/+) produced approximately 50% defective pollen grains with an altered male germ unit (MGU) and aberrant callose deposition in bicellular pollen. Bicellular pollen grains germinated precociously inside the anther. Another line, a transgenic plant expressing callose synthase 5 (CalS5) under the CaMV 35S promoter, also contained abnormal callose deposition during microsporogenesis and displaced MGUs in pollen grains. We also observed that precocious pollen germination could be induced in wild-type plants by incubation with medium containing sucrose and calcium ion and by wounding in the anther. These results demonstrate that precocious pollen germination in Arabidopsis could be triggered by a genetic alteration and a physiological condition.     

Cytopathology of Anthers and Pollen From Lettuce Plants Infected by Lettuce Mosaic Virus

Thin sections of mature anthers and pollen grains from three lettuce (Lactuca sativa) plants infected with lettuce mosaic potyvirus (LMV) were studied by immunogold labelling. Labelled LMV particles were present externally on the exine of pollen grains from all plants, but were observed internally in the pollen grains from only one plant. Within mature pollen grains the virus particles were associated with the cytoplasmic bundle inclusions typical of infection by potyviruses. The tapetal plasmodium and the epidermal and endothecial layers of mature anthers from all infected plants also contained labelled virus particles, together with pinwheel and bundle inclusions.     

Pollen size in Carex: The effect of different chemical treatments and mounting media

Pollen size and pollen aperture size for ten species of the genus Carex L., native to Estonia, have been measured using light microscopy. The species selected represent different sections of the genus, a range of habitats and different chromosome numbers. The effects of two basic chemical treatments, two mounting media and the effect of chemically induced dehydration with tertiary butyl alcohol (TBA) on the size of pollen grains were then recorded.

In general pollen size and pollen aperture size of the species examined is highly variable at both intraspecific and interspecific levels. Carex hirta has notably larger pollen grains than any of the other species investigated and, although correlations between size and chromosome number in the species examined are limited, it also has the highest chromosome number. Statistically significant size differences resulted from variations in chemical treatment, mounting media and tertiary butyl alcohol (TBA) induced dehydration. Acetolysed pollen grains are larger than potassium hydroxide (KOH) treated pollen grains. Pollen grains dehydrated after chemical treatment with TBA are larger than pollen grains not dehydrated. Pollen grains mounted in silicon oil are smaller than grains mounted in glycerine. But considering the great size variation of Carex pollen grains, the size changes caused by preparation procedures fall within the size variation range of the species examined.

All the samples contained a high number of deformed pollen grains and pollen grains with hardly distinguishable or no lateral apertures.     

Anther development, microsporogenesis and microgametogenesis in Heliconia (Heliconiaceae, Zingiberales)

Anther development, microsporogenesis and microgametogenesis in several species of Heliconia were investigated as part of a complementary embryological study of the Heliconiaceae. All studied Heliconia species present bithecate and tetrasporangiate anthers with fertile pollen grains; only H. rivularis, a natural hybrid, presented sterile pollen grains of variable size and no content. The anther wall has an uniseriate epidermis and endothecium, the latter with helicoidal thickenings, although some cells of the middle layers also showed thickenings; the biseriate tapetum is of amoeboid non-syncytial type, since the tapetum cells did not fuse together forming a true plasmodium. The microsporogenesis is successive leading to isobilateral tetrads. The inaperturate pollen grains had a very reduced exine consisting of a thin, more or less continuous layer with small spines upon; the pollen grain shape is variable among the species, all of them presenting heteropolar pollen, except H. angusta with isopolar ones. Most of these characteristics were shared with other studied Zingiberales, although more studies need to be done.     

Pollen in packrat middens

Fossil packrat middens are used to reconstruct late Quaternary environments in the arid Western U.S.A. Airborne pollen is contributed to middens both directly from the air, and through adherence to plant macrofossils. I examined the filtration of atmospheric pollen by Larrea tridentata leaves (resinous, with sparse recumbent hairs), Cercidium microphyllum twigs (glabrous with recumbent hairs) and Sphaeralcea ambigua leaves (covered with stellate hairs) and evaluated the potential biases of scavenged pollen on the interpretation of pollen records from middens. Pollen grains collected from the surfaces of the plants were most numerous during the spring when (airborne) pollen concentrations were high. Of the species investigated, the Sphaeralcea leaves filtered the greatest number of spring pollen grains from the air (3419 grains cm^?2); captured pollen was very low during other seasons. Sticky Larrea leaves continuously captured airborne pollen ( = 246 grains cm^?2). Pollen captured by smooth Cercidium was consistently low ( = 31 grains cm^?2). Only the spring pollen was filtered in sufficient numbers to obtain minimum pollen counts of 200 grains by all 3 taxa. The plants frequently captured greater proportions of their own pollen. Pollen content of packrat middens will reflect the season of plant macrofossil collection and the species composition of the plants incorporated.     

Pollen exine ornamentation in the F2 generation of an interspecific hybrid of Chorisia (Bombacoideae,Malvaceae) in relation to inheritance pattern

Pollen morphology in ten plants of F₂ progeny of an interspecific tree hybrid, Chorisia insignis H. B.& K.×C. speciosa St. Hil (Bombacoideae, Malvaceae) has been studied with a view to have an insight in to the effect of hybridization on the pollen morphological features in F₂ generation. The study is a sequel to an earlier study on the pollen morphology of Chorisia species and their F₁ hybrid, in which case the hybrid pollen uniformly exhibited the apertural features of the male parent and exine features of the female parent. In the F₂ progeny the pollen grains display the apertural feature of the male parent in all the plants. However, with regard to exine ornamentation, variability has been observed. Of the ten plants, two plants exhibit the exine features of the male parent showing empty lumina (without bacula), five plants have their pollen exactly like that of the F₁ plant (showing prominent columellar heads in the lumina of the apocolpium region) and the remaining three plants showing columellae in lumina of both apocolpium and mesocolpium region being different from the above types. It has been inferred that the variability in the exine pattern in F₂ pollen indicates that the pollen exine pattern in hybrids, perhaps, is not unequivocally controlled by sporophytic or gametophytic genomes.     

An aeropalynological study of metropolitan Toronto

This study uses 6 years of atmospheric pollen data to examine temporal variability of airborne pollen concentrations at various scales. Airborne pollen was collected from 1985 to 1990 with a Burkard trap, located 18 m above ground at Scarborough College, Toronto, Canada. Pollen season parameters are defined and summarized for all taxa in preparation for developing forecasting models. Annual totals of pollen concentration show great interannual variability. The highest coefficient of variation occurs inTsuga, Fraxinus, Betula andFagus, while the lowest inQuercus andAmbrosia. Some taxa show periodic cycles consistent with mast reproductive behaviour. In many studies, the start of the pollen season is defined as an arbitrary percentage of the annual sum. As a result, the start of the season cannot be identified until the season has passed. As well, due to large fluctuations in annual sum, start dates are more variable. This is not practical for the purposes of forecasting. In this study, the start of the pollen season is defined by a critical concentration threshold which signals the onset of the main pollen season in all years. These critical levels ranged from 2 to 60 grains/m³ for the abundant taxa. Interannual variation in the start of the season is approximately 20 days for tree taxa, 5 days for Poaceae, and 2 days forAmbrosia. For many plants, dehiscence is triggered at a critical level of accumulated degree-days. Since annual rates of temperature increase show great variation, there is also interannual variability in the onset of pollen release. Multi-year average pollen curves incorporate these differences in onset and may give an inaccurate representation of the pollen season in a typical year. This paper presents a method of aligning yearly pollen curves to reduce seasonal variation and more accurately represent both the average timing and magnitude of the pollen season. For some types, such asBetula and Poaceae, the resulting curves are positively skewed. Tree taxa, in general, exhibit a more symmetric pollen concentration curve. Aligned average pollen concentration curves are presented for Toronto in the form of a pollen calendar. In addition, phenological data for all common taxa are summarized.     

Genetic modification of the waxy character in barley under the action of exogenous DNA of the wild variety

Genetic transformation was studied on spring barley mutants carrying the recessive mutant allele of waxy locus.Analysis of the pollen grains of 38 control plants that were not subjected to any treatment showed that in the whole sample there was no mutant grain that stained black with iodine. It is also indicative of the genotypical purity of the original waxy plants.After mechanical damage (puncture in the top and in the side of a grain) there were 5 pollen grains fo wild type among 124000 mutant grains. Injection of grains with 2 μl distilled water led to the emergence of pollen grains that stained black at a frequency similar to that after puncture. The overall frequency of wild barley pollen grains for all control variants was 2.2·10⁻⁵.The frequencies of wild-type pollen grains were practically the same after injections of DNA from E. coli, extensively deproteinized highly polymeric DNA from barley leaves or slightly deproteinized low polymeric DNA from barley endospermal material.There was no marked increase of the wild pollen frequency after the injection of slightly deproteinized low polymeric DNA from the endospernal material (2.2·10⁻⁵ in the control versus 3.7·10⁻⁵ in the experiment).The analysis of the material for the amount of altered pollen grains in invidual plants also unequivocally demonstrated significant differences between the control and the experiment. In the first four variants there was no plant having over these altered pollen grains among the 2500 mutant grains examined. In the variants with injections of barley DNA differing in the extent of deproteinization and in polymerisation only in one case (of low polymeric slightly deproteinized DNA) there were no plants with many altered pollen grains. In all the other variants there were plants having much pollen of wild type.The largest number of plants with a great many affected pollen grains occurred in the variant with highly polymeric DNA from the endospermal material not subjected to deproteinization with chloroform and isoamyl alcohol.     

Infrageneric variability of pollen morphology in Palaua (Malveae,Malvaceae) and the taxonomic utility of quantitative pollen characters

Palaua (Malvaceae) comprises 15 species endemic to the coastal deserts of Chile and Peru. Previous molecular phylogenetic analyses have shown that this genus is monophyletic and can be subdivided into three clades. In the present study, pollen morphology of all species of Palaua was examined using light and scanning electron microscopy to determine whether it provides additional data in support of the proposed infrageneric groups. The pollen grains are spheroidal, medium to large in diameter, spinose and pantocolporate – the ectocolpi are very short. The tectum is perforate and characterised by granula. In all species the nexine is of a similar thickness in all areas of the pollen grain, while the sexine thickness varies, being notably thicker in the areas where the broad spine bases are sited (‘spine cushions’). Many of the quantitative characters have conspicuous variability, but the variability shows considerable overlap between species. Nonetheless, P. guentheri, P. inconspicua, P. malvifolia and P. modesta are united as a group by having the smallest pollen grains with the smallest apertures, the shortest spines and the shortest interspinal distance. This grouping reflects only partially the suggested infrageneric clades, although it does tend to unite the species with the smallest flowers. A possible link between reproductive resources and pollen size is also considered, as well as the influence of polyploidy. However, the taxonomic utility of these quantitative characters is weakened by species with more than one cytotype.     

Apoptosis-like degradation of pollen grains in siberian squill related to the lack of low temperatures during their development

Low temperature is necessary for completing the cycle of anther development in early-spring ephemeroids. Pollen grains were investigated in Scilla sibirica L. growing under natural conditions in a park and taken to a greenhouse for the cold season. In control plants grown in the open ground, the development of microspores was normal. The plants transferred to the greenhouse sharply differed from control material. By the middle of November, the aggregations of condensed chromatin were formed near the nucleus envelope of their generative cells. In some mitochondria, the system of cristae was reduced. In December, all the mitochondria showed a reduced system of cristae. In the matrix of some mitochondria with ruptured envelopes, rather large vesicles were detected. In the vegetative and generative cells, autophagic vacuoles were formed. They also occurred in the nucleus itself; as a result, the cells of pollen grains became much more vacuolated. In January, only the products of protoplast lysis were detected in all the pollen grains. It was concluded that the lack of low temperatures induced apoptosis-like degradation of the cells of Siberian squill pollen grains.     

In vitro allelopathic effects of pollen from three Hieracium species (Asteraceae) and pollen transfer to sympatric Fabaceae

Pollen allelopathy occurs when pollen toxins inhibit sexual reproduction in heterospecifics. To quantify pollen allelopathic effects in vitro, pollen of Hieracium aurantiacum, H. floribundum, and H. pratense was extracted with double distilled water into concentrations of 0 (control), 0.1, 1, 3, 5, 10, and 25 pollen grains/μl. A subset of these extracts was partitioned by ion-exchange chromatography into acidic, basic, and neutral fractions. Pollen from six sympatric species of Fabaceae (Lotus corniculatus, Medicago sativa, Trifolium hybridum, T. pratense, T. repens, Vicia cracca) and the species of Hieracium was germinated on agar media containing different extract concentrations. As extract concentrations from intact pollen or from acidic fractions increased, mean percent germination of pollen of all Fabaceae decreased nonlinearly. Extract concentrations of 0.1 grains/μl and above inhibited pollen germination in the Fabaceae. In no case was any Hieracium species affected, i.e., there was no autotoxicity. Similar results were obtained using plants from nine populations of H. floribundum and H. pratense across eastern Canada. Observation of floral visitors indicated that there were relatively few movements between Hieracium and the Fabaceae (about 15% of total visits). Hence there is limited opportunity for heterospecific pollen transfer. This was consistent with censuses of stigma, i.e., few pollen grains of Hieracium were found on stigma of the Fabaceae. Thus, pollen allelopathy in Hieracium is unlikely to affect reproductive success in these six species of Fabaceae.     

Identification of genes required for pollen-stigma recognition in Arabidopsis thaliana

In higher plants, cell-cell recognition reactions taking place following pollination allow the selective restriction of self-pollination and/or interspecific pollination. Many of these systems function by regulating the process of water transfer from the cells found at the stigmatic surface to the individual pollen grain. Interspecific pollination studies on the cruciferous weed Arabidopsis thaliana revealed only a broad specificity of pollen recognition such that pollen from all tested members of the crucifer family were recognized, whereas pollen from almost all other species failed to hydrate. Genetic analysis of A. thaliana has identified three genes that are essential for this recognition process. Recessive mutations in any of these genes result in male sterility due to the production of pollen grains that fail to hydrate when placed on the stigma, but that are capable of hydrating and growing a pollen tube in vitro. Results from mixed pollination experiments suggest that the mutant pollen grains specifically lack a functional pollen-stigma recognition system. All three mutations described also result in a defect in the wax layer normally found on stems and leaves, similar to previously described eceriferum (cer) mutations. Genetic complementation and mapping experiments demonstrated that the newly identified mutants are allelic to the previously identified genes cer1, cer3 and cer6. TEM analysis of the ultrastructure of the pollen coating revealed that all of the mutant pollen grains bear coatings of normal thickness and that tryphine lipid droplets are missing in cer1-147, are reduced in size in cer6-2654 and appear normal in cer3-2186. Temperature shift experiments revealed that the block in the recognition step of the mutant pollen grains can be suppressed by pollination at lower temperatures but not by reduced temperatures during pollen development. These results suggest that the lipids which are altered in the cer mutations may be important in regulating some biophysical property of the pollen coating.     

Early in vitro flowering and seed production in culture in <Emphasis Type="Italic">Dendrobium</Emphasis> Chao Praya Smile (Orchidaceae)

Plantlets of Dendrobium Chao Praya Smile maintained in vitro were induced to flower, which produced viable seeds within about 11 months. A two-layer (Gelrite-solidified layer topped with a layer of liquid medium of the same volume and composition) culture system containing benzyladenine (BA) at 11.1 μM induced the highest percent of flowering (45%) in plantlets within 6 months from germination. The percentage of inflorescence induction was increased to 72% by pre-selecting morphologically normal seedlings prior to two-layer culture. Plantlets in culture produced both complete (developmentally normal but smaller than flowers of field grown plants) and incomplete flowers. Pollen and female reproductive organs of in vitro-developed complete flowers were morphologically and anatomically similar to flowers of field grown plants. In addition, 65% of the pollen grains derived from in vitro-developed flower were tetrad suggesting that regular meiosis occurred during microsporogenesis. The percentage of germination of pollen grains derived from in vitro-developed flowers and flowers of field grown plants, incubated on modified Knops’ medium for 8 days, were 18.2 and 52.8%, respectively. Despite a lower percentage of germination of the pollen grains derived from in vitro-developed flowers, flowers induced in culture could be self-pollinated and developed seedpods with viable seeds. Nearly 90% of these seeds developed into protocorms on germination in vitro. These seedlings were grown in culture and induced to flower in vitro again using the same procedure.     

Dose effect of UV-B irradiation on pollen tube growth and seed-specific promoter activities in irradiated pollen grains of Nicotiana plumbaginifolia

Low doses of UV-B irradiation applied to mature Nicotiana plumbaginifolia pollen grains stimulated pollen tube growth. The most pronounced effect was achieved after 1.5 min of irradiation. Using transgenic N. plumbaginifolia plants expressing the GFP reporter gene under the control of the seed-specific promoters USP (unknown seed protein) or LegB4 (legumin B4) genes, it was shown that these promoters are also inducible by UV-B irradiation of the pollen grains. The improvement of pollen viability and germination by UV-light is discussed with respect to effects on plant flowering and reproduction. Received: 10 November 1999 / Revision accepted: 14 February 2000     

Induction of male sterility in transgenic chrysanthemums (Chrysanthemum morifolium Ramat.) by expression of a mutated ethylene receptor gene, Cm-ETR1/H69A, and the stability of this sterility at varying growth temperatures

Chrysanthemum (Chrysanthemum morifolium Ramat.) is one of the most popular ornamental flowers in the world, and many agronomic traits have recently been introduced to chrysanthemum cultivars by gene transformation. Concerns have been raised, however, regarding transgene flow from transgenic plants to wild plants. In early studies, ethylene receptor genes have been used for genetic modification in plants, such as flower longevity and fruit ripening. Recently, overexpression of ethylene receptor genes from melon (CmETR1/H69A) caused delayed tapetum degradation of the anther sac and a reduction in pollen grains. We therefore introduced the ethylene receptor gene into chrysanthemums to induce male sterility and prevent transgene flow via pollen. The chrysanthemum cultivar Yamate shiro was transformed using a disarmed strain of Agrobacterium tumefaciens, EHA105, carrying the binary vector pBIK102H69A, which contains the CmETR1/H69A gene. A total of 335 shoots were regenerated from 1,282 leaf discs on regeneration medium (26.1%). The presence of the Cm-ETR1/H69A gene was confirmed in all of the regenerated plantlets by Southern blot analysis. These genetically modified (GM) plants and their non-GM counterparts were grown in a closed greenhouse and flowered at temperatures between 10 and 35°C. In 15 of the 335 GM chrysanthemum lines, the number of mature pollen grains was significantly reduced, particularly in three of the lines (Nos. 91, 191 and 324). In these three lines, pollen grains were not observed at temperatures between 20 and 35°C but were observed at 10 and 15°C, and mature pollen grains were formed only at 15°C. In northern blot analyses, expression of the CmETR1/H69A gene was suppressed at low temperatures. This phenomenon was observed as a result of both the suppression of CmETR1/H69A expression at low temperatures and the optimal growth temperature of chrysanthemums (15–20°C). Furthermore, the female fertility of these three GM lines was significantly lower than that of the non-GM plants. Thus, the mutated ethylene receptor is able to reduce both male and female fertility significantly in transgenic chrysanthemums, although the stability of male and/or female sterility at varying growth temperatures is a matter of concern for its practical use.     

An investigation of the role of the anther tapetum during microspore development using genetic cell ablation

The effects on anther development of a fusion of the Arabidopsis anther-specific apg gene promoter to a ribonuclease (barnase) in transgenic tobacco plants were examined. Contrary to expectations, viable pollen grains were produced by these plants despite the demonstration that ribonuclease expression in the microspores and tapetum caused targeted cell ablation. Transformed plants were reduced in male fertility due to ablation of a proportion of pollen dependent on apg-barnase locus number. Plants were otherwise phenotypically normal and fully female fertile, confirming the anther-specific nature of the apg promoter. In microspores inheriting an apg-barnase locus following meiosis, loss of cell viability, as judged by fluorescein diacetate staining, occurred during mid to late microspore development. Microspores not inheriting a transgene went on to mature into viable pollen grains. Premature degeneration of the tapetum was also observed as a result of apg-barnase expression, but this did not appear to disrupt the subsequent microspore and pollen developmental programmes. This was substantiated by observations of microspore development in plants in which the tapetum was rescued from ablation by crossing in a second transgene encoding a tapetum-specific inhibitor of the ribonuclease. It was determined that tapetum cell disruption occurs at the early to mid uninucleate microspore stage in apg-barnase transformants. The data presented show that after this point in microspore development the tapetum is no longer essential for the production of viable pollen in tobacco.     

Ectopic expression of <Emphasis Type="Italic">MNX</Emphasis> gene from <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> involved in auxin biosynthesis confers male sterility in transgenic cotton (<Emphasis Type="Italic">Gossypium hirsutum</Emphasis> L.) plants

A transgenic male sterile line of upland cotton was generated by the ectopic expression of the monooxygenase (MNX) gene from Arabidopsis thaliana via Agrobacterium-mediated transformation. The bacterium harbored a plasmid pBinplus carrying a 1.25-kb MNX coding sequence together with a GUS reporter gene; the former was driven by the MS2 promoter of a male sterility gene in Arabidopsis, and the latter was under the control of CaMV 35S promoter. Twenty-seven putative transgenic plants (T₀) were obtained, all of which showed GUS activity and positive signals of NPTII and MNX genes by PCR analysis, and also showed male sterility to some extent. It was further confirmed by Southern blotting that one copy of the NPTII and MNX gene was integrated in the genome of the plants which expressed male sterility to a higher degree. Northern blotting assay also demonstrated that the transgenes stably transcribed in the genome of the transgenic plants in F₄ generation. The male sterile plants usually display lower plant height, shortened internodes, shrunken anthers without pollen grains or with some abortive pollen grains, and unusual leaves with deeper multi-lobes. Microscope observations on the meiosis processes of pollen mother cells (PMCs) showed that the abortion of pollen grains mainly resulted from abnormalities of meiosis such as direct degeneration of PMCs, degenerations of dyad and tetrads, amitosis, lagging chromosomes, and the multi-polar segregations of chromosomes and so on. This study indicates a method of developing novel cotton male sterile materials for potential application in agriculture and for engineering of male sterility in other important crops.     

Specialist pollinators deplete pollen in the spring ephemeral wildflower Claytonia virginica

Pollinators that collect pollen – and specifically, pollen‐specialist bees – are often considered to be the best pollinators of a (host) plant. Although pollen collectors and pollen specialists often benefit host plants, especially in the pollen that they deliver (their pollination “effectiveness”), they can also exact substantial costs because they are motivated to collect as much pollen as possible, reducing the proportion of pollen removed that is subsequently delivered to stigmas (their pollination “efficiency”). From the plant perspective, pollen grains that do not pollinate conspecific stigmas are “wasted”, and potentially costly. We measured costs and benefits of nectar‐collecting, pollen‐collecting, and pollen‐specialist pollinator visitation to the spring ephemeral Claytonia virginica. Visits by the pollen‐specialist bee Andrena erigeniae depleted pollen quickly and thoroughly. Although all pollinators delivered roughly the same number of grains, the pollen specialist contributed most to C. virginica pollen delivery because of high visitation rates. However, the pollen specialist also removed a large number of grains; this removal may be especially costly because it resulted in the depletion of pollen grains in C. virginica populations. While C. virginica appears to rely on pollen transfer by the pollen specialist in these populations, nectar‐collecting visitors could provide the same benefit at a lower cost if their visitation rates increased. Pollen depletion affects a pollinator's value to plants, but is frequently overlooked. If they lower the effectiveness of future floral visitors, visits by A. erigeniae females to C. virginica may be more detrimental than beneficial compared to other pollinators and may, in some circumstances, reduce plant fitness rather than increase it. Therefore, A. erigeniae and C. virginica may vary in their degree of mutualism depending on the ecological context.