Divergence in pollen performance between Clarkia sister species with contrasting mating systems supports predictions of sexual selection

Animal taxa that differ in the intensity of sperm competition often differ in sperm production or swimming speed, arguably due to sexual selection on postcopulatory male traits affecting siring success. In plants, closely related self‐ and cross‐pollinated taxa similarly differ in the opportunity for sexual selection among male gametophytes after pollination, so traits such as the proportion of pollen on the stigma that rapidly enters the style and mean pollen tube growth rate (PTGR) are predicted to diverge between them. To date, no studies have tested this prediction in multiple plant populations under uniform conditions. We tested for differences in pollen performance in greenhouse‐raised populations of two Clarkia sister species: the predominantly outcrossing C. unguiculata and the facultatively self‐pollinating C. exilis. Within populations of each taxon, groups of individuals were reciprocally pollinated (n = 1153 pollinations) and their styles examined four hours later. We tested for the effects of species, population, pollen type (self vs. outcross), the number of competing pollen grains, and temperature on pollen performance. Clarkia unguiculata exhibited higher mean PTGR than C. exilis; pollen type had no effect on performance in either taxon. The difference between these species in PTGR is consistent with predictions of sexual selection theory.     

Relative impact of mate versus pollinator availability on pollen limitation and outcrossing rates in a mass‐flowering species

Plant mating systems are driven by several pre‐pollination factors, including pollinator availability, mate availability and reproductive traits. We investigated the relative contributions of these factors to pollination and to realized outcrossing rates in the patchily distributed mass‐flowering shrub Rhododendron ferrugineum. We jointly monitored pollen limitation (comparing seed set from intact and pollen‐supplemented flowers), reproductive traits (herkogamy, flower size and autofertility) and mating patterns (progeny array analysis) in 28 natural patches varying in the level of pollinator availability (flower visitation rates) and of mate availability (patch floral display estimated as the total number of inflorescences per patch). Our results showed that patch floral display was the strongest determinant of pollination and of the realized outcrossing rates in this mass‐flowering species. We found an increase in pollen limitation and in outcrossing rates with increasing patch floral display. Reproductive traits were not significantly related to patch floral display, while autofertility was negatively correlated to outcrossing rates. These findings suggest that mate limitation, arising from high flower visitation rates in small plant patches, resulted in low pollen limitation and high selfing rates, while pollinator limitation, arising from low flower visitation rates in large plant patches, resulted in higher pollen limitation and outcrossing rates. Pollinator‐mediated selfing and geitonogamy likely alleviates pollen limitation in the case of reduced mate availability, while reduced pollinator availability (intraspecific competition for pollinator services) may result in the maintenance of high outcrossing rates despite reduced seed production.     

Maladaptation beyond a geographic range limit driven by antagonistic and mutualistic biotic interactions across an abiotic gradient

Species’ geographic range limits often result from maladaptation to the novel environments beyond the range margin. However, we rarely know which aspects of the n‐dimensional environment are driving this maladaptation. Especially of interest is the influence of abiotic versus biotic factors in delimiting species’ distributions. We conducted a 2‐year reciprocal transplant experiment involving manipulations of the biotic environment to explore how spatiotemporal gradients in precipitation, fatal mammalian herbivory, and pollination affected lifetime fitness within and beyond the range of the California annual plant, Clarkia xantiana ssp. xantiana. In the first, drier year of the experiment, fitness outside the range edge was limited mainly by low precipitation, and there was some evidence for local adaptation within the range. In the second, wetter year, we did not observe abiotic limitations to plant fitness outside the range; instead biotic interactions, especially herbivory, limited fitness outside the range. Together, protection from herbivory and supplementation of pollen resulted in three‐ to sevenfold increases in lifetime fitness outside the range margin in the abiotically benign year. Overall, our work demonstrates the importance of biotic interactions, particularly as they interact with the abiotic environment, in determining fitness beyond geographic range boundaries.     

Pollination ecology of Isoglossa woodii,a long‐lived,synchronously monocarpic herb from coastal forests in South Africa

Synchronous monocarpy in long‐lived plants is often associated with pollination by wind, in part because infrequent mass flowering may satiate pollinators. Selfing in synchronous monocarps may provide reproductive assurance but conflict with the benefits of outcrossing, a key evolutionary driver of synchrony. We predicted that animal‐pollinated species with synchronous flowering would have unspecialised flowers and attract abundant generalised pollinators, but predictions for selfing and outcrossing frequencies were not obvious. We examined the pollination biology of Isoglossa woodii (Acanthaceae), an insect‐pollinated, monocarpic herb that flowers synchronously at 4–7‐year intervals. The most frequent visitor to I. woodii flowers was the African honeybee, Apis mellifera adansonii. Hand‐pollination failed to enhance seed production, indicating that the pollinators were not saturated. No seed was set in the absence of pollinators. Seed set was similar among selfed and outcrossed flowers, demonstrating a geitonogamous mixed‐mating strategy with no direct evidence of preferential outcrossing. Flowers contained four ovules, but most fruits only developed one seed, raising the possibility that preferential outcrossing occurs by post‐pollination processes. We argue that a number of the theoretical concerns about geitonogamous selfing as a form of reproductive assurance do not apply to a long‐lived synchronous monocarp such as I. woodii.     

Experimental studies of adaptation in Clarkia xantiana. III. Phenotypic selection across a subspecies border

Sister taxa with distinct phenotypes often occupy contrasting environments in parapatric ranges, yet we generally do not know whether trait divergence reflects spatially varying selection. We conducted a reciprocal transplant experiment to test whether selection favors “native phenotypes” in two subspecies of Clarkia xantiana (Onagraceae), an annual plant in California. For four quantitative traits that differ between subspecies, we estimated phenotypic selection in subspecies’ exclusive ranges and their contact zone in two consecutive years. We predicted that in the arid, pollinator‐scarce eastern region, selection favors phenotypes of the native subspecies parviflora: small leaves, slow leaf growth, early flowering, and diminutive flowers. In the wetter, pollinator‐rich, western range of subspecies xantiana, we expected selection for opposite phenotypes. We investigated pollinator contributions to selection by comparing naturally pollinated and pollen‐supplemented individuals. For reproductive traits and for subspecies xantiana, selection generally matched expectations. The contact zone sometimes showed distinctive selection, and in ssp. parviflora selection sometimes favored nonnative phenotypes. Pollinators influenced selection on flowering time but not on flower size. Little temporal variation in selection occurred, possibly because of plastic trait responses across years. Though there were exceptions and some causes of selection remain obscure, phenotypic differentiation between subspecies appears to reflect spatially variable selection.     

FLORAL BIOLOGY AND REGULATION OF SEED SET AND SEED SIZE IN THE LILY,CLINTONIA BOREALIS

We studied the reproductive ecology of Clintonia borealis, a clonal understory species, in eastern Ontario. Flowers are protogynous and require insect pollination for outcrossing and maximum seed set. Most pollination is done by nectar-gathering bumble bees. We recorded pollen deposition and flowering patterns in 1983. Flowering lasted for about 10 days in mid-June. Stigmatic pollen loads were lowest at the onset of blooming, reflecting the scarcity of male-phase flowers. Pollination increased significantly by peak bloom and subsequently remained high. However, since bees fly mostly between neighboring stems, much of the pollen transferred may have been geitonogamous. In 1983, seed set per flower was not increased by supplementing pollination or by reducing the number of fruits competing for resources per stem. Rather, flowers set more seeds (14%) only if both treatments were performed simultaneously. Seed size was increased by 25% when competitive fruits were removed and by 5% more when pollen was added under the removal treatment. Further work in 1984 showed that pollination effects may be related to changes in pollen source. Selfed flowers set fewer and smaller seeds than outcrossed ones when screened from insects and hand pollinated. Outcrossing distance had little effect on seed set or seed size. Thus, in C. borealis seed number and seed size are limited by a balance between maternal resource availability and the amount of outcrossing provided by pollinators.     

No evidence for simultaneous pollen and resource limitation in Aciphylla squarrosa: A long‐lived,masting herb

Abstract For successful reproduction animal pollinated plants must provide resources for both pollinator attraction and offspring production, and theory suggests that resources and pollen delivery limit reproduction simultaneously. We conducted a series of experiments involving supplemental pollination, flower removal, fertilizer addition and foliage removal to investigate the interaction of resources and pollen on fruit‐set of Aciphylla squarrosa, a long‐lived, dioecious, masting herb in Wellington, New Zealand. Reducing floral display decreased open‐pollinated fruit‐set, suggesting that display size is a reflection of an optimal investment between attraction and fecundity. In combination with supplemental pollination, resource reduction and fertilization addition did not alter fruit‐set, suggesting that changes in resource availability did not limit reproduction in the current year. In addition, supplemental pollination of non‐manipulated treatments did not increase fruit‐set, demonstrating that plants were not naturally pollen limited. While we found that simultaneous pollen and resource limitation did not occur within a season, this is possibly mitigated by life history patterns including mast flowering and a storage taproot. Multiple year studies are required to further examine simultaneous resource and pollen limitation.     

Bumblebee visitation and seedset in Melampyrum pratense and Viscaria vulgaris: heterospecific pollen and pollen limitation

Summary Fruiting and seed set in two bumblebee-pollinated herbs, Melampyrum pratense L. (annual, Scrophulariaceae) and Viscaria vulgaris Bernh. (perennial, Caryophyllaceae) were studied on a dry meadow in southwestern Sweden in June 1986 and 1988. Both species produced seeds by self-fertilization. In Melampyrum (homogamous) fruiting and seed set by selfing were much lower than by natural pollination; in Viscaria (protandrous) fruiting by selfing and by natural polination were similar, but seed set per flower was lower by selfing than by natural pollination. Sequential hand pollinations increased seed set in Melampyrum, but not in Viscaria. Thus, the number of pollinations is important for high seed set in Melampyrum, and number of pollen grains deposited one pollination is important for high seed set in Viscaria. Late flowering resulted in the production of fewer seeds in both species, although the visitation rate in pure Viscaria stands was sufficient, because of limited resources. Pollen was the limiting resource in Viscaria, because hand pollination increased natural seed set. In Melampyrum pollen was limiting in 1988 but so were consumable resources, because the seedset decreased with time despite hand pollination. Pure stands of Viscaria had sced set similar to plants in mixed stands (with Melampyrum and Rhinanthus), although plants in mixed stands received fewer visits. Many seeds produced late in the season are the result of self pollination; emasculated Viscaria flowers had a very low seedset late in the season. Pollen loads containing approximately 50% heterospecific grains did not affect seed set in either species. Application of heterospecific (Lupinus) pollen to receptive Viscaria styles 6 h before conspecific pollen did not affect seed set.     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

Floral display size and spatial distribution of potential mates affect pollen deposition and female reproductive success in distylous Pulmonaria officinalis (Boraginaceae)

In animal‐pollinated plants, both the spatial distribution of flowering individuals and the number of flowers that an individual displays affect pollen deposition rates and female reproductive success. Heterostylous species are likely to be particularly sensitive to the contingencies of spatial distribution, as they are reproductively subdivided into distinct mating groups, which usually exhibit self‐ and intra‐morph incompatibility and differ in floral morphology. In this paper, we explore the joint effects of both spatial distribution of potential mates and floral display size on morph‐specific pollen deposition rates and seed set patterns in two natural populations of Pulmonaria officinalis, a distylous species with a weak self‐incompatibility system. Both total stigmatic pollen load and the proportion of legitimate pollen decreased with increasing spatial isolation. Legitimate (intermorph) pollen transfer was, however, asymmetric and decreased more rapidly with decreasing proximity to a compatible legitimate mating partner in the S‐morph than in the L‐morph. Total stigmatic pollen loads per flower increased with increasing floral display size, indicating that large plants are disproportionately more visited than smaller individuals. However, because legitimate pollen deposition decreased with increasing floral display size, these results also suggest that larger numbers of flowers increase the degree of geitonogamous pollination. In both the L‐ and S‐morph, seed set significantly decreased with increasing isolation from a legitimate mating partner, but in the L‐morph seed set was less dependent on the spatial distribution of the S‐morph. In addition, seed set significantly increased with floral display size in the L‐morph, but not in the S‐morph. These findings indicate that the spatial distribution of potential mates and variation in floral display size may cause morph‐specific differences in pollen deposition rates and female reproductive success.     

Pollination ecology of Penstemon roseus (Plantaginaceae), an endemic perennial shifted toward hummingbird specialization?

Summary Pink-flowered tubular Penstemon roseus (Plantaginaceae), which has shifted partially to hummingbird pollination, blooms on high-elevation slopes in the mountains in Tlaxcala, Mexico. We studied the interactions between pollinator visitation rates to flowers, pollen removal and deposition, flower size, and nectar removal frequency on seed production in P. roseus. We combine observational and experimental studies in two contrasting natural populations. Our manual pollinations revealed that P. roseus is fully self-compatible. Autonomous self- and manual self-pollinated flowers matured as many seeds as when outcrossed, but outcrossing seems to become better than selfing as the flowering season progressed. Early in the season flowers that were bagged and hand-selfed, hand-outcrossed, or autonomously selfed, or unbagged and naturally pollinated had equal seed set in all four treatments. But later in the season, outcross pollen gave approximately twice as much seed set as the two self-treatments. Low levels of pollen receipt and pollen removal were consistent with the long time elapsed for a given plant to be visited by hummingbirds, which suggests pollen shortage in both sites. Despite differences in pollinator visitation rates to flowers, probability of flower visitation, removal and deposition of pollen, and nectar production rates between populations, we found that total nectar production had no effect on seed production at either site. The daily nectar secretion rate of 0.3–0.65 mg sugar per flower per 1–3 days was low relative to other hummingbird-adapted Penstemon species (typical range: 1.5–5 mg sugar per flower), and it might be intermediate between hummingbird- and bee-adapted Penstemon flowers. Our results support the hypothesis about a shift toward hummingbird pollination, and provide an example of a ‘despecialized’ Penstemon species, which attracts high-energy pollinators (hummingbirds) and profits from outcrossing, but retains bee-syndrome floral traits and low sugar production rates.     

Pollinator preference and pollen viability mediated by flower color synergistically determine seed set in an Alpine annual herb

Gentiana leucomelaena manifests dramatic flower color polymorphism, with both blue‐ and white‐flowered individuals (pollinated by flies and bees) both within a population and on an individual plant. Previous studies of this species have shown that pollinator preference and flower temperature change as a function of flower color throughout the flowering season. However, few if any studies have explored the effects of flower color on both pollen viability (mediated by anther temperature) and pollinator preference on reproductive success (seed set) in a population or on individual plants over the course of the entire flowering season. Based on prior observations, we hypothesized that flower color affects both pollen viability (as a function of anther temperature) and pollen deposition (as a function of pollinator preference) to synergistically determine reproductive success during the peak of the flowering season. This hypothesis was tested by field observations and hand pollination experiments in a Tibetan alpine meadow. Generalized linear model and path analyses showed that pollen viability was determined by flower color, flowering season, and anther temperature. Anther temperature correlated positively with pollen viability during the peak of the early flowering season, but negatively affected pollen viability during the peak of the mid‐ to late flowering season. Pollen deposition was determined by flower color, flowering season (early, or mid‐ to late season), and pollen viability. Pollen viability and pollen deposition were affected by flower color that in turn affected seed set across the peak of the flowering season (i.e., when the greatest number of flowers were being pollinated). Hand pollination experiments showed that pollen viability and pollen deposition directly influenced seed set. These data collectively indicate that the preference of pollinators for flower color and pollen viability changed during the flowering season in a manner that optimizes successful reproduction in G. leucomelaena. This study is one of a few that have simultaneously considered the effects of both pollen viability and pollen deposition on reproductive success in the same population and on individual plants.     

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

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

Temporal variation in the pollen:ovule ratios of Clarkia (Onagraceae) taxa with contrasting mating systems: field populations

Among plants, pairs of selfing vs. outcrossing sister taxa provide interesting systems in which to test predictions concerning the magnitude and direction of temporal changes in sex allocation. Although resource availability typically declines towards the end of the growing season for annual taxa, temporal changes in mating opportunities depend on mating system and should change less in selfing taxa. Consequently, given that the pollen:ovule (P:O) ratio of flowers reflects the investment in (and potential fitness pay-off due to) male vs. female function, we predicted that the P:O ratio should also be less variable among and within selfers than in closely related outcrossers. To test these predictions, we measured temporal changes in sex allocation in multiple field populations of two pairs of sister taxa in the annual flowering plant genus Clarkia (Onagraceae). In the outcrossing Clarkia unguiculata and the selfing Clarkia exilis, ovule production declined similarly from early to late buds, whereas pollen production remained constant or increased in the outcrosser but remained constant or decreased in the selfer. Consequently, the P:O ratio increased within unguiculata populations but marginally increased or stayed constant in exilis populations. In all populations of the selfing Clarkia xantiana spp. parviflora and the outcrossing C. x. spp. xantiana, both ovule and pollen production per flower declined over time. The effects of these declines on the P:O ratio, however, differed between subspecies. In each xantiana population, the mean P:O ratio did not differ between early and late flowers, although individuals varied greatly in the direction and magnitude of phenotypic change. By contrast, parviflora populations differed in the mean direction of temporal change in the P:O ratio. We found little evidence to support our initial predictions that the P:O ratio of the selfing taxa will consistently vary less than in outcrossing taxa.     

Are mismatches the norm? Timing of flowering,fruiting, dispersal and germination and their fitness effects in Frangula alnus (Rhamnaceae)

The close morphological and temporal links between phases of plant growth and reproduction call for integrated studies incorporating several reproductive phases from flowering to recruitment, and associated plant‐animal interactions. Phenological strategies, as well as plastic phenological response to climate change, incorporate complex interactions between developmental constraints, pollination and seed dispersal. Relationships between reproductive phenology and components of fitness were studied for two years in the north‐temperate, self‐incompatible, insect‐pollinated, and bird‐dispersed shrub Frangula alnus (Rhamnaceae). Fruit set, dispersal, germination and juvenile survival, as well as seed mass and juvenile size were measured in relation to flowering, fruiting and germination time. The results suggest that effects of flowering and fruiting time prevailed in subsequent phases, to some extent as far as to the juvenile phase, but effects of timing were complex and had partly opposing effects on different fitness components. Early flowers had higher fruit‐set and experiments indicated that synchronous peak flowering increased fruit‐set, but later flowers had higher seed mass. Peak fruiting was not associated with peak dispersal. Late fruits derived from late flowers promoted dispersal. Juvenile recruitment was enhanced by increasing seed size. We conclude that the phenology of flowering and fruiting in F. alnus comprises several features, each with different and sometimes counteracting effects on fitness components. From a general perspective, this result implies that we should not expect to find finely tuned matches in timing specifically between flowering and pollinators, and fruiting and seed dispersing birds.     

How shrub encroachment under climate change could threaten pollination services for alpine wildflowers: A case study using the alpine skypilot,Polemonium viscosum

Under climate change, shrubs encroaching into high altitude plant communities disrupt ecosystem processes. Yet effects of encroachment on pollination mutualisms are poorly understood. Here, we probe potential fitness impacts of interference from encroaching Salix (willows) on pollination quality of the alpine skypilot, Polemonium viscosum. Overlap in flowering time of Salix and Polemonium is a precondition for interference and was surveyed in four extant and 25 historic contact zones. Pollinator sharing was ascertained from observations of willow pollen on bumble bees visiting Polemonium flowers and on Polemonium pistils. We probed fitness effects of pollinator sharing by measuring the correlation between Salix pollen contamination and seed set in naturally pollinated Polemonium. To ascertain whether Salix interference occurred during or after pollination, we compared seed set under natural pollination, conspecific pollen addition, and Salix pollen addition. In current and past contact zones Polemonium and Salix overlapped in flowering time. After accounting for variance in flowering date due to latitude, Salix and Polemonium showed similar advances in flowering under warmer summers. This trend supports the idea that sensitivity to temperature promotes reproductive synchrony in both species. Salix pollen is carried by bumble bees when visiting Polemonium flowers and accounts for up to 25% of the grains on Polemonium pistils. Salix contamination correlates with reduced seed set in nature and when applied experimentally. Postpollination processes likely mediate these deleterious effects as seed set in nature was not limited by pollen delivery. Synthesis: As willows move higher with climate change, we predict that they will drive postpollination interference, reducing the fitness benefits of pollinator visitation for Polemonium and selecting for traits that reduce pollinator sharing.     

Variation of pollen limitation in the early flowering Mediterranean geophyte Narcissus assoanus (Amaryllidaceae)

A recent literature review indicates that pollen limitation of female fertility is a common feature of flowering plants. Despite the ecological and evolutionary significance of pollen limitation, most studies have only examined fertility in a single population at one time. Here we investigate pollen limitation of fruit and seed set in five populations of Narcissus assoanus, a self- sterile, insect-pollinated geophyte, over 2–3 years in southern France. In common with many early spring flowering plants, pollinator visitation to N. assoanus is often infrequent. Supplemental hand-pollination of flowers with outcross pollen significantly increased overall fruit and seed set by 11% and 19%, respectively. Four of the five populations experienced some pollen limitation during the study. For a given year, there was significant variation in pollen limitation among populations. Two of the populations were pollen limited in one year but not in other years in which they were studied. Seed:ovule ratios for open- and hand-pollinated flowers averaged 0.29 and 0.33, respectively. While hand pollination significantly increased the seed:ovule ratio, the low value obtained indicates that the majority of ovules in flowers do not mature seeds despite hand pollination. The role of genetic and environmental factors governing low seed:ovule ratios in N. assoanus is discussed. Received: 28 December 1999 / Accepted: 6 April 2000     

Geographic variation of reproductive traits and competition for pollinators in a bird‐pollinated plant

Geographic variation in the reproductive traits of animal‐pollinated plants can be shaped by spatially variable selection imposed by differences in the local pollination environment. We investigated this process in Babiana ringens (Iridaceae), an enigmatic species from the Western Cape region of South Africa. B. ringens has evolved a specialized perch facilitating cross‐pollination by sunbirds and displays striking geographic variation in perch size and floral traits. Here, we investigate whether this variation can be explained by geographic differences in the pollinator communities. We measured floral and inflorescence traits, and abiotic variables (N, P, C, and rainfall) and made observations of sunbirds in populations spanning the range of B. ringens. In each population, we recorded sunbird species identity and measured visitation rates, interfloral pollen transfer, and whether the seed set of flowers was pollen limited. To evaluate whether competition from co‐occurring sunbird‐pollinated species might reduce visitation, we quantified nectar rewards in B. ringens and of other co‐flowering bird‐pollinated species in local communities in which populations occurred. Variation in abiotic variables was not associated with geographical variation of traits in B. ringens. Malachite sunbirds were the dominant visitor (97% of visits) and populations with larger‐sized traits exhibited higher visitation rates, more between‐flower pollen transfer and set more seed. No sunbirds were observed in four populations, all with smaller‐sized traits. Sunbird visitation to B. ringens was not associated with local sunbird activity in communities, but sunbird visitation was negatively associated with the amount of B. ringens sugar relative to the availability of alternative nectar sources. Our study provides evidence that B. ringens populations with larger floral traits are visited more frequently by sunbirds, and we propose that visitation rates to B. ringens may be influenced, in part, by competition with other sunbird‐pollinated species.     

Pollen and Resource Limitation in Veratrum nigrum L. （Liliaceae）, an Andromonoecious Herb

Pollen limitation and resource limitation were invoked to account for the pattern that flowering plants produce more flowers and ovules than fruits and seeds. This study aimed to determine their relative importance in Veratrum nigrum, a self-compatible, perennial, andromonoecious herb. In order to determine whether female production was limited by pollen grains on stigmas or by available resources, we performed supplemental hand pollination in three populations, male-flower-bud removal in three other populations, and emasculation of hermaphroditic flowers in still another population, resulting in a total of seven populations experimentally manipulated. Across the three populations, supplemental hand pollination did not significantly increase fruit set, seed number per fruit, and total seed production per individual, nor did emasculation of hermaphroditic flowers. Taken together, our results suggest that pollen grains deposited on stigmas were abundant enough to fertilize all the ovules. Male-flower-bud removal significantly increased the mean size of hermaphroditic flowers in all three populations. Female reproductive success was increased in one population, but not in the other two populations possibly due to heavy flower/seed predation. We concluded that the female reproductive success of V. nigrum was not limited by pollen grains but by available resources, which is consistent with Bateman＇s principle. Furthermore, the female reproduction increase of male-flower-bud removal individuals might suggest a trade-off between male and female sexual functions.     

Genetic rescue by distant trees mitigates qualitative pollen limitation imposed by fine‐scale spatial genetic structure

Restricted seed dispersal frequently leads to fine‐scale spatial genetic structure (i.e., FSGS) within plant populations. Depending on its spatial extent and the mobility of pollinators, this inflated kinship at the immediate neighbourhood can critically impoverish pollen quality. Despite the common occurrence of positive FSGS within plant populations, our knowledge regarding the role of long‐distance pollination preventing reproductive failure is still limited. Using microsatellite markers, we examined the existence of positive FSGS in two low‐density populations of the tree Pyrus bourgaeana. We also designed controlled crosses among trees differing in their kinship to investigate the effects of increased local kinship on plant reproduction. We used six pollination treatments and fully monitored fruit production, fruit and seed weight, proportion of mature seeds per fruit, and seed germination. Our results revealed positive FSGS in both study populations and lower fruit initiation in flowers pollinated with pollen from highly‐genetically related individuals within the neighbourhood, with this trend intensifying as the fruit development progressed. Besides, open‐pollinated flowers exhibited lower performance compared to those pollinated by distant pollen donors, suggesting intense qualitative pollen limitation in natural populations. We found positive fine‐scale spatial genetic structure is translated into impoverished pollen quality from nearby pollen donors which negatively impacts the reproductive success of trees in low‐density populations. Under this scenario of intrapopulation genetic rescue by distant pollen donors, the relevance of highly‐mobile pollinators for connecting spatially and genetically distant patches of trees may be crucial to safeguarding population recruitment.