Pollinator Preference Research Articles

DOES SELECTION ON FLORAL ODOR PROMOTE DIFFERENTIATION AMONG POPULATIONS AND SPECIES OF THE SEXUALLY DECEPTIVE ORCHID GENUS OPHRYS?

Sexually deceptive orchids from the genus Ophrys attract their pollinators primarily through the chemical mimicry of female hymenopteran sex pheromones, thereby deceiving males into attempted matings with the orchid labellum. Floral odor traits are crucial for the reproductive success of these pollinator-limited orchids, as well as for maintaining reproductive isolation through the attraction of specific pollinators. We tested for the signature of pollinator-mediated selection on floral odor by comparing intra and interspecific differentiation in odor compounds with that found at microsatellite markers among natural populations. Three regions from southern Italy were sampled. We found strong floral odor differentiation among allopatric populations within species, among allopatric species and among sympatric species. Population differences in odor were also reflected in significant variation in the attractivity of floral extracts to the pollinator, Colletes cunicularius. Odor compounds that are electrophysiologically active in C. cunicularius males, especially alkenes, were more strongly differentiated among conspecific populations than nonactive compounds in the floral odor. In marked contrast to these odor patterns, there was limited population or species level differentiation in microsatellites (FST range 0.005 to 0.127, mean FST 0.075). We propose that the strong odor differentiation and lack of genetic differentiation among sympatric taxa indicates selection imposed by the distinct odor preferences of different pollinating species. Within species, low FST values are suggestive of large effective population sizes and indicate that divergent selection rather than genetic drift accounts for the strong population differentiation in odor. The higher differentiation in active versus non-active odor compounds suggests that divergent selection among orchid populations may be driven by local pollinator preferences for those particular compounds critical for pollinator attraction.

Preferences of Pollinators and Herbivores in Gynodioecious Geranium sylvaticum

For the maintenance of gynodioecy (i.e. the coexistence of female and hermaphroditic plants), females need to compensate for the lack of pollen production through higher seed production or better progeny quality compared to hermaphrodites. In Geranium sylvaticum, females produce more seeds per flower than hermaphrodites. This difference in seed production might be modified by biological interactions with pollinators and herbivores that may favour one sex and thus affect the maintenance of gynodioecy. Sexual dimorphism in flower size and flowering phenology, and in attractiveness to pollinators, pre-dispersal seed predators and floral herbivores were examined in natural populations of G. sylvaticum. Pollinators preferred hermaphrodites 25 % more often than females in two of the three study populations, and floral herbivores attacked hermaphrodites 15 % more often than females in two of the six study populations. These preferences might be explained by the larger flower size of hermaphrodites. In contrast, seed predators did not prefer either sex. The data suggest that pollinator preference does not benefit females, whereas the higher floral herbivory of hermaphrodites might enhance the maintenance of females in G. sylvaticum. Thus, although the data support the view that ecological factors may contribute to the maintenance of gynodioecy, they also suggest that these contributions may vary across populations and that they may function in opposite directions.

DOES SELECTION ON FLORAL ODOR PROMOTE DIFFERENTIATION AMONG POPULATIONS AND SPECIES OF THE SEXUALLY DECEPTIVE ORCHID GENUS OPHRYS?

Sexually deceptive orchids from the genus Ophrys attract their pollinators primarily through the chemical mimicry of female hymenopteran sex pheromones, thereby deceiving males into attempted matings with the orchid labellum. Floral odor traits are crucial for the reproductive success of these pollinator-limited orchids, as well as for maintaining reproductive isolation through the attraction of specific pollinators. We tested for the signature of pollinator-mediated selection on floral odor by comparing intra and interspecific differentiation in odor compounds with that found at microsatellite markers among natural populations. Three regions from southern Italy were sampled. We found strong floral odor differentiation among allopatric populations within species, among allopatric species and among sympatric species. Population differences in odor were also reflected in significant variation in the attractivity of floral extracts to the pollinator, Colletes cunicularius. Odor compounds that are electrophysiologically active in C. cunicularius males, especially alkenes, were more strongly differentiated among conspecific populations than non-active compounds in the floral odor. In marked contrast to these odor patterns, there was limited population or species level differentiation in microsatellites (FST range 0.005 to 0.127, mean FST 0.075). We propose that the strong odor differentiation and lack of genetic differentiation among sympatric taxa indicates selection imposed by the distinct odor preferences of different pollinating species. Within species, low FST values are suggestive of large effective population sizes and indicate that divergent selection rather than genetic drift accounts for the strong population differentiation in odor. The higher differentiation in active versus non-active odor compounds suggests that divergent selection among orchid populations may be driven by local pollinator preferences for those particular compounds critical for pollinator attraction.

POLLINATOR PREFERENCES FOR NICOTIANA ALATA, N. FORGETIANA, AND THEIR F1HYBRIDS

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed-species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.

OPPOSING NATURAL SELECTION FROM HERBIVORES AND PATHOGENS MAY MAINTAIN FLORAL-COLOR VARIATION IN CLAYTONIA VIRGINICA (PORTULACACEAE)

The maintenance of floral-color variation within natural populations is enigmatic because directional selection through pollinator preferences combined with random genetic drift should lead to the rapid loss of such variation. Fluctuating, balancing, and negative frequency-dependent selection mediated through pollinators have been identified as factors that may contribute to the maintenance of floral-color variation, and recently it has been suggested that indirect responses to selection on correlated characters through agents of selection other than pollinators may substantially shape the evolution of floral traits. Here, I provide empirical support for this latter view in Claytonia virginica (Portulacaceae) through a multiseason field study, a pollen supplementation study, and an artificial herbivory experiment. These studies show that most individuals fall into one of four discrete color classes, and suggest pollinator-mediated selection for increased floral redness in concurrent years. Floral color is also an indirect target of opposing directional selection via herbivores and pathogens that fluctuates through time. Taken together, these data suggest a novel mechanism by which floral-color variation may be maintained, and illustrate the importance of an inclusive, pluralistic view of selection when investigating the evolution of complex phenotypes.

Factors of divergence in co-occurring varieties of Dactylorhiza incarnata (Orchidaceae)

Due to often large variation in traits within and between populations, taxonomy of orchid species is problematic. In this study, we aim to assess the probability of sympatric speciation in mixed populations of nectarless Dactylorhiza incarnata varieties (incarnata, ochroleuca and cruenta). We conducted morphological, phenological and ecological measurements in five populations in Hiiumaa, western Estonia. In most populations, we did not find any differences to reveal genetic divergence between the varieties. In one population, however, differences in shoot and flower traits were found. This population also differed at the community level from the other four study populations. Composition of pollinator fauna on one hand, and the presence of rewarding companion species on the other, may cause selection to act towards different directions in different populations of nectarless species. In contrast to earlier studies with another nectarless orchid species, we found pollination success to be positively frequency-dependent, which may be caused by a stable pollinator preference for one of the varieties.

Nectar guide fluctuating asymmetry does not relate to female fitness in Mimulus luteus

Most studies assessing the importance of developmental instability of floral characters for pollinator visits and plant fitness have focused on the fluctuating asymmetry (FA) of the corolla phenotype. The importance of stability process for nectar guide characters that signal floral reward has not been considered in the literature. In principle, flowers with symmetrical guides should be more successful at attracting pollinators, therefore increasing their reproductive success in comparison to asymmetrical flowers. In this paper we test this hypothesis in a population of 171 individuals of the Andean monkey flower, Mimulus luteus in northern Chile. This species shows a conspicuous red spot in the landing yellow petal, which permits assessment of the functional relationship between nectar guide FA and female fitness. Our results did not reveal a significant linear nor nonlinear relationship between nectar guide FA and fitness. This result was consistent after controlling the level of FA by guide and corolla size. Because the corolla of M. luteus did not show evidence of UV wavelength reflectance, our negative result could not be attributable to a confounding effect of UV guides. Even though we can not rule out that nectar guide FA correlates better with male than female fitness, the low fraction of the variance in female fitness accounted for nectar guide FA, suggests that other components of the floral phenotype as well as environmental factors may be more important to predict pollinator preference and reproductive success in this species.

Exploitation of floral signals by crab spiders (Thomisus spectabilis, Thomisidae)

Flowers exhibit characteristics through which they exploit the sensory biases of pollinating insects, and both signaler and receiver benefit from this interaction, either through reproductive service or food reward. However, the preferences of pollinators for certain flower traits such as color or odor might be exploited by predators that target pollinating insects. Crab spiders, Thomisus spectabilis, position themselves on flowers to prey on pollinators such as honeybees, Apis mellifera. We gave both honeybees and crab spiders the choice between two randomly chosen white Chrysanthemum frutescens, including olfactory signals in one experiment and excluding odor in a second experiment. When olfactory signals were included, crab spiders and honeybees clearly preferred the same flower out of a pair. However, agreement level was at chance in the absence of olfactory signals. We also analyzed the visual flower characteristics that might influence the decision of the animals. Neither the size of flowers (diameter of flower and diameter of reproductive flower center) nor the reflectance properties (receptor excitation values in ultraviolet, blue, and green; overall brightness) influenced the choices of crab spiders and honeybees. Therefore, odor seems to be the floral signal that bees use to identify high-quality flowers and that crab spiders exploit to encounter honeybees. Key words: Apis mellifera, communication, crab spider, floral signal, honeybee, signal exploitation. [Behav Ecol 15:321–326 (2004)]

POLLINATOR PREFERENCES FOR NICOTIANA ALATA, N. FORGETIANA, AND THEIR F1 HYBRIDS

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed-species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.

OPPOSING NATURAL SELECTION FROM HERBIVORES AND PATHOGENS MAY MAINTAIN FLORAL-COLOR VARIATION IN CLAYTONIA VIRGINICA (PORTULACACEAE)

The maintenance of floral-color variation within natural populations is enigmatic because directional selection through pollinator preferences combined with random genetic drift should lead to the rapid loss of such variation. Fluctuating, balancing, and negative frequency-dependent selection mediated through pollinators have been identified as factors that may contribute to the maintenance of floral-color variation, and recently it has been suggested that indirect responses to selection on correlated characters through agents of selection other than pollinators may substantially shape the evolution of floral traits. Here, I provide empirical support for this latter view in Claytonia virginica (Portulacaceae) through a multiseason field study, a pollen supplementation study, and an artificial herbivory experiment. These studies show that most individuals fall into one of four discrete color classes, and suggest pollinator-mediated selection for increased floral redness in concurrent years. Floral color is also an indirect target of opposing directional selection via herbivores and pathogens that fluctuates through time. Taken together, these data suggest a novel mechanism by which floral-color variation may be maintained, and illustrate the importance of an inclusive, pluralistic view of selection when investigating the evolution of complex phenotypes.

Natural selection inIpomopsishybrid zones: implications for ecological speciation

SummaryThe fitness of hybrids relative to parental species plays an important role in models of speciation. In ecological speciation, reproductive isolation evolves owing to divergent natural selection, which implies reduced fitness of hybrid phenotypes. The source of the divergent selection in flowering plants may be animal pollinators, or environmental features of habitats that lead to physiological adaptations. Reciprocal transplants, combined with examination of pollination and other fitness components, allow exploration of these mechanisms of speciation. Much of this information is available from hybrid zones betweenIpomopsis aggregataandIpomopsis tenuituba. Pollination studies reveal some disruptive selection on corolla width, but also geographical variation in pollinator preference, and hybrids do not generally suffer lower pollination. Survival of hybrids depends on both genotype and environment. One genotypic class of hybrids is as fit or more fit than the parents, while another type suffers reduced fitness in parental environments. The dynamics of this hybrid zone involve a complex mixture of selection mediated by pollinators and other sources, and this combination of selection may have contributed to the original speciation.

Allele substitution at a flower colour locus produces a pollinator shift in monkeyflowers.

The role of major mutations in adaptive evolution has been debated for more than a century. The classical view is that adaptive mutations are nearly infinite in number with infinitesimally small phenotypic effect, but recent theory suggests otherwise. To provide empirical estimates of the magnitude of adaptive mutations in wild plants, we conducted field studies to determine the adaptive value of alternative alleles at a single locus, YELLOW UPPER (YUP). YUP controls the presence or absence of yellow carotenoid pigments in the petals of pink-flowered Mimulus lewisii, which is pollinated by bumblebees, and its red-flowered sister species M. cardinalis, which is pollinated by hummingbirds. We bred near-isogenic lines (NILs) in which the YUP allele from each species was substituted into the other. M. cardinalis NILs with the M. lewisii YUP allele had dark pink flowers and received 74-fold more bee visits than the wild type, whereas M. lewisii NILs with the M. cardinalis yup allele had yellow-orange flowers and received 68-fold more hummingbird visits than the wild type. These results indicate that an adaptive shift in pollinator preference may be initiated by a single major mutation.

THE ROLE OF HERBIVORES IN THE MAINTENANCE OF A FLOWER COLOR POLYMORPHISM IN WILD RADISH

Plant species exhibiting polymorphisms with respect to flower color are widespread. Our understanding of the selection pressures that may maintain these color polymorphisms has primarily been confined to one set of organisms—pollinators. Yet, selection on flower color may also be driven by other agents, such as herbivores, especially in cases where pollinators and herbivores are using the same or correlated traits to select plants. A wealth of studies have documented pollinator preference for anthocyanin-recessive color morphs (A−; yellow and white flowers) of wild radish, Raphanus sativus, over anthocyanin-dominant morphs (A+; pink and bronze flowers); yet, differences in pollination alone do not explain the maintenance of the flower color polymorphism. Here, we ask whether variation in flower color in R. sativus influences the preference and performance of herbivores for A− flower color morphs vs. A+ flower color morphs in four types of herbivores (generalist and specialist Lepidoptera, slugs, aphids, and thrips). We found that all herbivores except for aphids and thrips preferred flowering A− color morphs compared to A+ morphs of R. sativus. Furthermore, all herbivores except larvae of specialist and generalist Lepidoptera performed better on A− color morphs. Differences in plant secondary chemistry may play a role in differential herbivore preference and performance on the anthocyanin flower color morphs. Chemical analyses of leaf secondary compounds (indole glucosinolates) revealed that A+ color morphs produced higher concentrations of indole glucosinolates than A− morphs in the presence of herbivore damage. Therefore, herbivores may exhibit lower preference for A+ color morphs, and these morphs may support lower herbivore performance because they are heavily defended once damaged. This is the first study, to our knowledge, to document differential preference and performance of herbivores for different flower color morphs. Previous studies have shown that increased herbivore damage can have profound negative direct and indirect effects on the reproduction of R. sativus. The data presented here suggest that differential preference and performance of herbivores for R. sativus color morphs may counter selection on flower color exerted by pollinators.

Outcrossing rates in the gynomonoecious‐gynodioecious species Dianthus sylvestris (Caryophyllaceae)

Some species described as gynodioecious are truly gynomonoecious-gynodioecious. Three distinct phenotypes may be found in their natural populations-female and hermaphrodite pure-sexed plants bearing either only pistillate or perfect flowers, respectively, and mixed plants bearing both types of flowers. In one such species, Dianthus sylvestris, we investigated mating system parameters using allozyme data. Outcrossing rates and correlations of outcrossed paternity were calculated for the three types of plants and separately for pistillate and perfect flowers on mixed plants. The mean outcrossing rate for the population was t(m) ± SD = 0.885 ± 0.032. Females were more outcrossed than hermaphrodites (0.987 ± 0.112 and 0.790 ± 0.076, respectively), whereas mixed plants were not significantly more or less outcrossed than hermaphrodites (0.840 ± 0.060). Within mixed plants, perfect flowers showed an intermediate outcrossing rate (0.898 ± 0.057), whereas pistillate flowers were as selfed as perfect flowers on hermaphrodite plants (0.782 ± 0.111). Family estimates of outcrossing rates were highly variable. Globally, no biparental inbreeding was detected in this species, and there was a mean of 61.5 ± 19.9% of full-sibs within families. Floral dimorphism between small pistillate and large perfect flowers together with pollinator preference for larger flowers could explain the observed patterns for both mating parameters. The advantages of gynomonoecy-gynodioecy are discussed. We conclude that mixed plants do not reduce selfing for all flowers on a plant, but perfect flowers on these plants seem to have an outcrossing advantage.

Why does the flower stalk of Pulsatilla cernua (Ranunculaceae) bend during anthesis?

Flower stalks of Pulsatilla cernua, an early spring herb in north temperate Asia, changed position from erect to pendulous and back to erect during 6-10 d anthesis. We tested three possible explanations for this movement. Our results showed that (1) this movement is unlikely to be a mechanism to attract pollinators or enhance pollen output, because no pollinator preference was observed between erect and pendulous flowers and we found no buzz-pollination in this species; (2) hand self-pollination yielded higher seed set than open pollination in the field, but spontaneous selfing rarely occurred. Among open-pollinated flowers, seed set was depressed by emasculation, indicating that in the presence of insects, self-pollen provided reproductive assurance in this protogynous and self-compatible species. However, the change in flower orientation cannot be explained as reproductive assurance in that even self-pollination largely depended on pollinator visits rather than gravity. (3) A pollen germination experiment indicated that pollen damage by water is serious in this species. We deduced that the bending of the flower stalk during anthesis was to avoid rain damage to pollen grains in this species. During the 3-6 d period of pollen presentation, the petals elongated and were covered with unwettable hairs. Together with flower stalk movement, this was enough to protect the organs inside the flower from rain. This movement of the flower stalk seems to be important to maintain pollen viability in a rainy habitat with a scarcity of pollinators.

HOST EFFECTS ON HERBIVORY AND POLLINATION IN A HEMIPARASITIC PLANT

The indirect effects of hosts on interactions between parasites and other species are not well understood, and it may be difficult to predict the outcome of host species effects on parasite performance due to the complexity of potential direct and indirect effects. For example, parasitic plants obtain defensive compounds as well as nutrients from their hosts, and thus many attributes of parasitic plants are dependent on the quality of their host species. Here I measure the effect of a lupine host species ( Lupinus argenteus) compared to other host species on herbivory, pollination, and female plant fitness in the hemiparasite Indian paintbrush (Castilleja miniata) using a series of field experiments. Association with lupine was determined in the field by assaying Indian paintbrush leaves for lupine alkaloids. I found that Indian paintbrush plants parasitizing lupines experienced reduced herbivory from plume moth larvae, agromyzid fly larvae, and deer, relative to Indian paintbrush plants parasitizing other host species. However, there was no correlation between alkaloid content of inflorescences and plume moth performance. Host species did not affect pollinator preference for Indian paintbrush in the field. Indian paintbrush para- sitizing lupines produced twice as many seeds overall as Indian paintbrush parasitizing other host species. Correlations suggest that this benefit arises both from reduced herbivory and increased nitrogenous resources. The reduction of herbivory in Indian paintbrush plants parasitizing lupines indicates that host species can affect performance of hemiparasites via indirect pathways, and that the larger community of herbivores could alter the impact of a host on its plant parasites.

Flower Scent and Pollination in Selected Neotropical Palms

Abstract:The flower scents of 14 palm species were collected in the field in Ecuador and Puerto Rico by head‐space adsorption and analysed by gas chromatography‐mass spectrometry. Insect visitors were recorded in seven of the species in Ecuador. The floral scent of the different species was dominated by a variety of compounds, e.g., the fatty‐acid derived 3‐pentanone and the hydrocarbon series dodecane to pentadecane, the benzenoid compound 1,4‐dimethoxybenzene, the isoprenoids (E)‐ocimene, myrcene, linalool, and (E)‐α‐farnesene and the nitrogen‐containing compound 2‐methoxy‐sec‐butylpyrazine. Rather than mirroring the systematics of the studied palm species, the chemical composition of the floral scent reflected the pollination mode. The scent of beetle‐pollinated species was characterized by large amounts of one or a few dominant compounds, whereas fly‐ and bee‐pollinated species contained a mixture of several compounds in smaller total amounts. We suggest that specific scent compounds, as found in the beetle‐pollinated species, have evolved as a response to pollinator preferences. The importance of olfactory cues in relation to visual cues is higher in beetle‐pollinated species than in species pollinated by flies and bees.

Tritrophic interactions in a pollination system: impacts of species composition and size of flower patches on the hunting success of a flower-dwelling spider.

Size and species composition of flower patches were manipulated to determine whether these factors exerted indirect effects on the hunting success of flower-dwelling spiders via direct effects on insect pollinators. Multivariate analysis of variance (MANOVA) of two pollinator response variables (visitation rate and visitor size) revealed a significant pollinator preference for Bidens aristosa (tickseed sunflower) over Solidago juncea (goldenrod) and for large patches of a given plant species over small patches. Bidens received significantly more pollinator visits per floral unit per unit time and attracted significantly larger insects than Solidago. The significant patch size effect seen in the MANOVA was driven by the strong impact of patch size on the size of insects visiting the experimental patches: the size (mass) of insects visiting large patches was significantly greater than the size of insects visiting small patches of a given plant species, but visitation rates to large and small patches of a given plant species were similar. MANOVA indicated that hunting success of a flower-dwelling crab spider, Misumenoides formosipes, was also significantly affected by species composition and size of flower patches. Three measures of spider hunting success (rate of mass gain and its components, total prey mass captured per spider per day, and number of prey captured per spider per day) were evaluated, and the experimental treatments exerted similar effects on all three measures. Spiders occupying Bidens patches experienced greater hunting success than spiders occupying Solidago patches, and in patches of a given plant species, spiders occupying large patches experienced greater hunting success than spiders occupying small patches. The pattern of spider hunting success most closely paralleled the pattern described by the size of insects visiting the patches (BL>BS=SL>SS), suggesting that the size of visiting pollinators, rather than frequency of visitation, exerted a greater influence on spider hunting success. Taxonomic composition of a spider's diet varied with the plant species occupied. The size of insects captured by spiders was significantly greater than average size of insects visiting the patches, indicating that spiders selectively captured larger pollinator taxa. Spider movements among patches revealed a pattern of migration from Solidago to Bidens.

Negative frequency-dependent selection maintains a dramatic flower color polymorphism in the rewardless orchid Dactylorhiza sambucina (L.) Soo.

The orchid Dactylorhiza sambucina shows a stable and dramatic flower-color polymorphism, with both yellow- and purple-flowered individuals present in natural populations throughout the range of the species in Europe. The evolutionary significance of flower-color polymorphisms found in many rewardless orchid species has been discussed at length, but the mechanisms responsible for their maintenance remain unclear. Laboratory experiments have suggested that behavioral responses by pollinators to lack of reward availability might result in a reproductive advantage for rare-color morphs. Consequently, we performed an experiment varying the relative frequency of the two color morphs of D. sambucina to test whether rare morph advantage acted in the natural habitat of the species. We show here clear evidence from this manipulative experiment that rare-color morphs have reproductive advantage through male and female components. This is the first demonstration, to our knowledge, that negative frequency-dependent selection through pollinator preference for rare morphs can cause the maintenance of a flower-color polymorphism.

Understanding the basis of pollinator selectivity in sexually dimorphic Fragaria virginiana

Pollinators of gender dimorphic (e.g., gynodioecious or dioecious) species often show a preference for hermaphrodite or male flowers over conspecific females, and such pollinator selectivity can have implications for the evolution of sexual dimorphism. However, because the sex morphs differ in several floral features, it is not readily apparent which specific traits generate differential pollinator service. In this study, we sought to understand the basis of pollinator preferences for hermaphrodites to females of gynodioecious Fragaria virginiana. We extend a previous study that revealed that lack of pollen‐filled anthers and shorter petals were not solely responsible for small bee and fly discrimination against females, by investigating the roles of other morphological features, reward characters, and floral context in bringing about pollinator discrimination. Using floral manipulations we revealed that small bees preferentially visit flowers with wide petals, more nectar, intact stamens, as well as flowers located in hermaphrodite‐dominated patches. Specifically, bees were able to remotely detect nectar presence. In addition, bees were attracted to both the anther sac and the filament regions of the stamen, indicating that not only anthers, but also filaments may function as visual or olfactory cues to pollen reward. Furthermore, pollinator preference for flowers in hermaphrodite‐dominated patches serves to intensify their discrimination against female flowers which occur predominantly in female‐dominated patches. For the most part, preferences of flies mirrored those of the bees with the exception that flies also showed a significant preference for flowers held high over those held closer to the ground. These results indicate that a suite of character differences contributes to pollinators’ preferences for hermaphrodite to female flowers. Furthermore, they suggest pollinators could mediate selection on both floral morphology and reward in this species.