克隆生长对被子植物传粉过程的影响

克隆植物与其传粉者的相互作用是植物繁殖生态学的重要研究领域之一。植物克隆生长与有性繁殖通常相伴进行, 往往产生较大的花展示与复杂的克隆空间结构, 通过传粉过程对有性繁殖过程产生影响, 共同决定植物的适合度。本文回顾了克隆生长对被子植物传粉过程影响的国内外研究进展, 从植物克隆大小、花资源空间配置、克隆构型与种群遗传结构四个方面讨论了克隆生长对传粉过程的影响及其生态学与进化生物学意义。早期研究预期, 随着克隆增大, 同株异花授粉水平增加, 因而通过增大自交率或花粉阻塞效应降低植物的适合度。但是, 后来的一些模拟与野外实验研究发现, 传粉者在同一克隆内访问的花数量并不会随克隆增大而一直增加, 访花行为也主要发生在分株内; 而且分子标记的自交率组分分析也表明自交主要发生在分株内。另一方面, 人工模型模拟以及传粉者访问行为研究表明, 当花朵数量相同时, 与所有花集中生长在同一分株上相比, 将花朵分散在多个分株上的克隆生长方式不会增加, 反而降低了同株异花授粉的发生水平。如果花序内花雌雄同熟, 花朵同时提供与接收花粉, 克隆生长会使植物接收到更高比例的异交花粉, 在提高后代质量的同时不增加同株异花授粉概率。这是从传粉生物学角度对植物克隆生长习性进化的一个全新的解释。今后, 克隆植物传粉生物学研究需要针对传粉者与克隆生长之间的相互作用建立理论模型, 探究克隆大小、克隆构型、花资源空间配置模式对传粉者访问频率和行为、花粉散布、交配格局的影响。同时, 需要在自然种群中, 尤其是克隆与非克隆的近缘类群、同一物种克隆与非克隆种群开展比较研究, 利用更高效的分子标记来研究克隆生长的生态与进化意义。     

Pollen limitation and inbreeding depression in an 'old rare' bumblebee-pollinated grassland herb

Habitat fragmentation and reduction of population size have been found to negatively affect plant reproduction in 'new rare' species that were formerly common. This has been attributed primarily to effects of increased inbreeding but also to pollen limitation. In contrast, little is known about the reproduction of 'old rare' species that are naturally restricted to small and isolated habitats and thus may have developed strategies to cope with long-term isolation and small population size. Here we study the effects of pollen source and quantity on reproduction of the 'old rare' bumblebee-pollinated herb, Astragalus exscapus. In two populations of this species, we tested for pollen autodeposition, inbreeding depression and outbreeding depression. Caged plants were left unpollinated or were pollinated with pollen from the same plant, from the same population or from a distant population (50 km). Additionally, we tested for pollen limitation by pollen supplementation in four populations of different size and density. In the absence of pollinators, plants did not produce seed whereas self-pollinated plants did. This indicates a self-compatible breeding system dependent on insect pollination. Both self-pollination and, in one of the two populations, cross-pollination with pollen from plants from the distant population resulted in a lower number of seeds per flower than cross-pollination with pollen from plants from the resident population, indicating inbreeding and outbreeding depression. Pollen addition enhanced fruit set and number of seeds per flower in three of the four populations, indicating pollen limitation. The degree of pollen limitation was lowest in the smallest but densest population. Our results suggest that, similar to 'new rare' plant species, also 'old rare' species may be at risk of inbreeding depression and pollen limitation.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

How floral displays affect geitonogamy in an upward foraging bumblebee‐pollinated protandrous plant

Reducing geitonogamy (pollen transfer among flowers within the same plant) has been suggested as a major selective force for plants with multiple flowers. The occurrence of geitonogamy is generally different among flowers within inflorescences; however, no researchers have examined whether plants enlarge their display size without increasing the possibility of geitonogamy by presenting more flowers at positions where they are less likely to be geitonogamously pollinated. We observed that bumblebee pollinators foraged upward within the tower‐shaped inflorescences of protandrous Megacodon stylophorus (C. B. Clarke) Harry Sm. Because M. stylophorus did not strictly bloom bottom‐up, there were substantial frequencies of geitonogamous pollination resulting from upward and horizontal pollen transfer. Although there was a strong correlation between total numbers of flowers plants produced and numbers of flowers presented on single days, proportions of flowers possibly geitonogamously pollinated were weakly correlated with total numbers of flowers. This might have been because plants with more resources enlarged their display size by producing more flowers on lower floors where flowers had a low probability of being geitonogamously pollinated. This study shows that the tower‐shaped inflorescences of M. stylophorus enlarge their size without more cost of geitonogamous mating, suggesting that geitonogamy acts as an important selective agent in the evolution of inflorescence architectures.     

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.     

Experimental analysis of mating patterns in a clonal plant reveals contrasting modes of self‐pollination

Hermaphrodite plants commonly practice self‐fertilization (selfing), but the mechanisms responsible vary depending on the mode of self‐pollination, pollinator behavior, and degree of clonality. Whether selfing occurs within (autogamy) or between flowers (geitonogamy) is of evolutionary significance because their fitness consequences differ. We used floral manipulations and genetic markers to determine the relative contribution of autogamy and within‐ versus between‐ramet geitonogamy to the selfing rate of the bumblebee‐pollinated, clonal herb Aconitum kusnezoffii. Data on flowering phenology and bumblebee foraging were also collected to determine opportunities for different modes of self‐pollination. Autogamy accounted for only 12% of the selfing rate with the remainder resulting from geitonogamy. Whole‐ramet emasculation of clones with multiple ramets reduced selfing by 78%, indicating that within‐ramet geitonogamy contributed significantly (68%) to total selfing. Selfing of single‐ramet plants was 44% less than multiple‐ramet plants, indicating that the contribution of between‐ramet geitonogamy was substantially less (20%) than within‐ramet geitonogamy, probably because of bumblebee foraging behavior. Our results demonstrate for the first time in a clonal plant that within‐ramet geitonogamy is substantially greater than between‐ramet geitonogamy and highlight the importance of considering the influence of clonal architecture and pollinator foraging on modes of self‐pollination.     

Floral distribution,clonal structure,and their effects on pollination success in a self-incompatible <Emphasis Type="Italic">Convallaria keiskei</Emphasis> population in northern Japan

In plant species, when clonal growth produces a patchy structure and flowering ramets are clustered, the amount of pollen contributing to reproductive success is often regulated by pollinator efficiency and geitonogamy. The spatial population structure may influence reproductive success. We examined the clonal structure, the spatial ramet distribution, and their combined effects on fruit set in a natural population of the insect-pollinated, self-incompatible clonal herb, Convallaria keiskei, in northern Japan. The number of shoots, flowers, and fruits in 1-m² quadrats were counted at every 5 m grid point in an established 100 × 90-m study plot. From all the quadrats where shoots existed, leaf samples were collected for allozyme analysis. Using the two spatial parameters of flowering ramet densities and genotypes, we then constructed individual-based fruit-set models. A total of 236 quadrats contained shoots, and 135 contained flowering ramets, which indicated expanded distribution of this plant throughout the study plot, while shoots, flowers and fruits all showed clustering distributions. Allozyme analysis of 282 samples revealed 94 multilocus genotypes. The largest clone extended to more than 40 m, whereas 56 genotypes were detected in only one sample. Several large clones and many small clones were distributed close to each other. Fine-scale spatial modelling revealed that the neighbouring flower numbers of different genotypes, compared with local genet or flower diversity, more influenced fruit set, in which the range of the neighbour was 14.5 m. These findings indicate that the compatible pollen dispersed by insect pollinators has a significant effect on sexual reproduction, in this C. keiskei population. Consequently, the spatial structure, which includes both genet distribution and clonal expansion by ramets, had a significant effect on pollination success.     

The effects of nectar addition on pollen removal and geitonogamy in the non-rewarding orchid Anacamptis morio

It has been suggested that the absence of floral rewards in many orchid species causes pollinators to probe fewer flowers on a plant, and thus reduces geitonogamy, i.e. self-pollination between flowers, which may result in inbreeding depression and reduced pollen export. We examined the effects of nectar addition on pollinator visitation and pollen transfer by tracking the fate of colour-labelled pollen in Anacamptis morio, a non-rewarding orchid species pollinated primarily by queen bumble-bees. Addition of nectar to spurs of A. morio significantly increased the number of flowers probed by bumble-bees, the time spent on an inflorescence, pollinarium removal and the proportion of removed pollen involved in self-pollination through geitonogamy, but did not affect pollen carryover (the fraction of a pollinarium carried over from one flower to the next). Only visits that exceeded 18 s resulted in geitonogamy, as this is the time taken for removed pollinaria to bend into a position to strike the stigma. A mutation for nectar production in A. morio would result in an initial 3.8-fold increase in pollinarium removal per visit, but also increase geitonogamous self-pollination from less than 10% of pollen depositions to ca. 40%. Greater efficiency of pollen export will favour deceptive plants when pollinators are relatively common and most pollinaria are removed from flowers or when inbreeding depression is severe. These findings provide empirical support both for Darwin's contention that pollinarium bending is an anti-selfing mechanism in orchids and for the idea that floral deception serves to maximize the efficiency of pollen export.     

THE GENETIC CONSEQUENCES OF WORKER ANT POLLINATION IN A SELF-COMPATIBLE,CLONAL ORCHID

The self-compatible orchid Microtis parviflora is pollinated by the flightless worker caste of the ant Iridomyrmex gracilis. The orchid is clonal and forms small patches, usually less than 1 m², of disconnected individual ramets. Ant pollinators visited and revisited a limited proportion of available inflorescences, and 40% of all flower visits occurred within plants promoting self-pollination. Pollen labels indicated that self-pollination accounted for 51% of the pollen transfers, although pollen carryover extended beyond 16 flowers on 2 or 3 inflorescences. The distribution of ant movements between plants was leptokurtic with a mean of 12.4 ± 14.9 cm and a maximum of 89 cm, but a high proportion of movements were within clones accentuating the level of self-pollination. However, some pollen transfers between inflorescences of unlike genotypes contributed to a low incidence (max = 8%) of outcrossing. In 12 patches examined by electrophoresis, the density varied from 11 to 61 inflorescences per m² and a maximum of only 4 genotypes were detected. Electrophoretic analysis revealed populations were highly inbred: only 23% (N = 17) of the loci were polymorphic and the mean gene diversity h, was 2.7%. Heterozygotes were observed in only one population given a mean fixation index F, of 0.982. These results reflect the combined effects of restricted ant foraging and clonality. Nevertheless, while ant foraging was restricted, some outcrossing occurred and in the absence of clonality it is likely that ant foraging would have yielded a mixed mating system similar to those reported for a wide array of insect pollinators. Given the ability of ants to generate pollen flow, the reasons for the rarity of ant pollination appear to lie elsewhere.     

Delayed autonomous self-pollination in Hibiscus laevis (Malvaceae)

Delayed autonomous self-pollination allows outcrossing to occur while also ensuring that seeds are produced in the absence of pollen vectors. We investigated variation in the efficacy of this pollination mechanism in populations of Hibiscus laevis. Recurvature of stylar branches occurred after 1 d of anthesis, and in plants from Ohio, Illinois, Arkansas, and Oklahoma this behavior resulted in autonomous selfing (surprisingly, stylar movement was facultative in that it did not take place when the stigmas were already pollinated). In contrast to these more northern populations, the distance between anthers and stigmas was too great to allow autonomous selfing in plants from Texas, Mississippi, and Louisiana. Greenhouse studies of plants from Ohio demonstrated that autonomous selfing resulted in an average of 29.5 seeds per flower, as compared to 59.9 seeds per flower from hand-pollination of stigmas with self pollen. In an assessment of the possible significance of this selfing mode, emasculated flowers did not set significantly fewer seeds in a natural stand in Ohio, suggesting that few seeds resulted from autonomous selfing at that site. Modest inbreeding depression was detected at this population. Our results suggest that delayed autonomous selfing is more common in northern populations, where it may facilitate population establishment and persistence at times when pollinators are scarce.     

Pollen analogues are transported across greater distances in bee‐pollinated than in hummingbird‐pollinated species of Justicia (Acanthaceae)

Several hummingbird‐pollinated plant lineages have been demonstrated to show increased rates of diversification compared to related insect‐pollinated lineages. It has been argued that this pattern is produced by a higher degree of specialization on part of both hummingbirds and plants. We here test an alternative hypothesis: The often highly territorial hummingbirds may on average carry pollen over shorter distances than other pollinators and drive diversification by reducing gene flow distances. We present experimental data from pollen analogue tracking showing shorter dispersal distances in hummingbird‐pollinated than in bee‐pollinated species among ten Neotropical species of Justicia (Acanthaceae). Abstract in Spanish is available with online material.     

Insect and wind pollination of an alpine biennial Aconitum gymnandrum (Ranunculaceae)

Wind pollination can provide reproductive insurance for animal‐pollinated dioecious plants in the absence of available pollinators, but combinations of insect and wind pollination (ambophily) have rarely been studied in hermaphrodite herbs. We examined the stable occurrence of insect pollination and wind pollination over 4 years in a population of a biennial Aconitum species (A. gymnandrum) with actinomorphic and degenerate sepals. The total frequency of visits of two bumblebee species showed no distinct fluctuations in the studied population among the 4 years. However, seed production of netted flowers after emasculation indicated wind pollination had occurred. The seed number of bagged flowers with one visit by bumblebees was significantly less than that of netted flowers after one visit, or in control flowers. Both seed number and fruit set of netted flowers were significantly lower than in control flowers. These results suggest that wind pollination provides supplementary pollen to unvisited and/or once‐visited flowers, but accounts for only a small amount of seed production compared to bumblebee pollination in natural conditions. Such a combination of insect and wind pollination might play an important role in maintaining sexual reproduction of this biennial herb, allowing it to persist in arid habitats on the Qinghai‐Tibetan Plateau, especially during Quaternary glacial periods when pollinator populations oscillated extensively.     

Pollination Biology of Hosta sieboldiana (Lodd.) Engler and H. sieboldii (Paxton) J. Ingram (Liliaceae)

Abstract The pollination biology of Hosta sieboldiana and H. sieboldii is investigated comparatively in Central Japan. Both species have homogamous, one-day flowers pollinated by bumblebees. The abdomens of the bees touch the stigma on the extended style when they land on the anthers inside the herkogamous flower, and autogamy is effectively prevented. However, the flowers are fairly self-compatible, and geitonogamy may occur rather frequently because two or more flowers on a scape very often bloom at the same time and many ramets are contiguous. The pollen/ovule ratios suggest that these species are facultative outbreeders. The flower of H. sieboldii seems completely suited to bumblebee pollination. In H. sieboldiana the stigma of the flower, whose style strongly protrudes, is not always touched by bumblebees, but frequent visitation of bumblebees results in pollination of almost all the flowers. Both species have similar pollination systems but seem reproductively isolated by blooming times and habitats. Their common pollinators, however, may sometimes cause introgressive hybridization in contiguous populations.     

Spatially limited clonality and pollen and seed dispersal in a characteristic climber of Central African rain forests: Haumania danckelmaniana (Marantaceae)

Gene dispersal and clonality are important aspects of plant evolution affecting the spatial genetic structure (SGS) and the long‐term survival of species. In the tropics these parameters have mostly been investigated in trees and some herbs, but rarely in climbers which frequently: (1) show clonal growth leading to a patchy distribution pattern similar to that of understory herbs; and (2) flower in the canopy where they may have access to long‐distance dispersal like canopy trees. We thus hypothesize for climbers an intermediate genetic structure between herbs and trees. The study aims at assessing breeding system and spatial extent of clonality and gene dispersal in Haumania danckelmaniana (Marantaceae), a common climber in the tropical rain forests from western Central Africa. In eastern Cameroon, 330 ramets were sampled at three spatial scales and genotyped at seven microsatellite loci. Clonality was moderate (clonal extend: 15–25 m, clonal diversity 0.4–0.65) indicating the importance of recruitment from seeds at this locality. The low inbreeding (F_IS) suggested predominant outcrossing. The rate of decay of the relatedness between individuals with distance indicated limited gene dispersal distance (σ_g = 9–50 m, neighborhood sizes Nb = 23–67) in accordance with narrowly gravity dispersed seeds and restricted pollen transfer distance in densely flowering populations. The marked SGS (Sp = 0.011–0.026) was similar to that reported in tropical trees, but might increase with augmented clonality as in many herbs, especially under more severe disturbance regimes.     

Allee effects within small populations of Aconitum napellus ssp. lusitanicum, a protected subspecies in northern France

Plants growing at low density can suffer from Allee effects as a result of pollen limitation. Previous studies of Allee effects have focused on the effects of variation among populations in size or density on reproduction. Here, the effects of plant distribution within populations on fitness components are explored in a rare plant, Aconitum napellus ssp. lusitanicum, and ecological and genetic mechanisms underlying these effects are identified. To detect pollen limitation, seed production was compared under natural versus hand-supplemented pollinations on inflorescences of different sizes in natural patches differing both in flower density and in isolation from other patches. Germination rate and juvenile survival of seeds produced in low- and high-density patches were also compared. Pollen-supplemented flowers always produced more seeds than open-pollinated flowers, especially among small plants and plants growing at low density. Offspring produced in low-density patches exhibited lower fitness that those produced in high-density patches. This could have been caused by post-fertilization mechanisms, including inbreeding depression or differential maternal resource allocation. These results show that Allee effects on fitness components (ecological and genetic Allee effects) occur within A. napellus populations at different spatial scales. The spatial distribution of plants seems to be a crucial factor affecting reproductive output and fitness.     

Does pollen competition reduce the cost of inbreeding?

We hypothesize that floral features promoting pollen competition in angiosperms may have evolved, in some cases, in response to selection generated by the negative effects of inbreeding, at least in plants with mixed-mating systems. Screening of haploid genotypes through pollen competition may purge recessive (or additive) deleterious alleles that are expressed in haploid pollen and hence may reduce the fitness cost of self-pollination, geitonogamy, or biparental inbreeding. We tested one prediction of this hypothesis, that offspring produced by more intense competition among self-pollen have higher fitness than offspring produced by less intense competition. Dalechampia scandens (Euphorbiaceae) flowers were pollinated with pollen from other flowers on the same plant (geitonogamous self-fertilization). Those flowers experiencing more intense pollen competition as a result of low pollen dispersion (positional variance) on the stigma produced heavier seeds and seedlings with faster-growing radicles than flowers experiencing less intense pollen competition (high pollen dispersion), as predicted by our hypothesis.     

Seed production from the mixed mating system of Chesapeake Bay (USA) eelgrass (Zostera marina; Zosteraceae)

In monoecious plants, gametes can be exchanged in three ways: among unrelated genets (outbreeding), with close relatives (inbreeding), or within individuals (geitonogamous selfing). These different mating systems may have consequences for population demography and fitness. The experiment presented herein used artificial crosses to examine the mating system of Chesapeake Bay, Virginia, USA eelgrass (Zostera marina L; Zosteraceae), a bisexual submerged aquatic plant that can outbreed, inbreed, and self. Genetic data indicate severe heterozygosity deficiencies and patchy genotype distribution in these beds, suggesting that plants therein reproduce primarily by vegetative propagation, autogamy, or geitonogamy. To clarify eelgrass reproductive strategies, flowers from three genetically and geographically distinct beds were hand-pollinated in outbred, inbred, and selfed matings. Fertilization success and seed production, life history stages which contribute greatly to the numeric maintenance of populations, were monitored. We found no evidence that inbreeding had negative consequences for seed production. On the contrary, selfed matings produced seeds significantly more frequently than outcrossed matings and produced significantly larger numbers of seeds than either inbred or outbred matings. These results contrast with patterns for eelgrass in other regions but might be expected for similar populations in which pollen limitation or a short reproductive season renders selfing advantageous.     

Variation of breeding system, floral rewards, and reproductive success in clonal Calystegia species (Convolvulaceae)

The reproductive biology of four Japanese Calystegia species (Convolvulaceae) was studied to examine the effects of clonality and population structure on reproductive success. Calystegia soldanella, C. hederacea, and C. japonica are self-incompatible, while C. sepium is self-compatible but needs pollinator services for self-pollination. The showy, bisexual flowers of Calystegia offer pollen and nectar that attract many kinds of insects such as bees and syrphid flies. Clones of C. soldanella often formed mats just above the high tide line on beaches and produced a great number of seeds. Calystegia hederacea and C. japonica were distributed as patches of separate clones and often failed to transfer pollen grains among clones. Fruit and seed sets in C. hederacea and C. japonica were not limited by pollinators but by the number of compatible pollen grains. Although C. sepium clones were also distributed in patches, high and stable fecundity was achieved by self-compatibility and pollinator attraction. We suggest that self-compatibility in C. sepium has evolved under fertilization limitation caused by geitonogamy or facilitated selfing.     

Influence of clonal growth on selfing rate in Vaccinium myrtillus L

Clonal growth, which allows the multiplication of flowering shoots of the same genet, can lead to a large floral display and may thus increase the rate of selfing through geitonogamy as a consequence of an increase in the number of successively visited flowers. The aim of the present research was to analyse the combined effect of the diversity and intermingling of clones on the rate of selfing in Vaccinium myrtillus. Four mother plants were selected within patches characterised by contrasting clonal structure (low versus high number and intermingling of clones). The selfing rate was significantly lower for plants situated within patches characterised by a high number of intermingled clones (3%) than for plants situated in patches with a low number of clones (50%). Therefore, for this species suffering from inbreeding depression, an increase in the number or the intermingling of the clones could reduce the rate and the cost of geitonogamy and allow a large floral display to attract pollinators. We also found that the main pollinators, bumblebee queens, presented a foraging behaviour favouring geitonogamy, as their successive visits to flowers were quite short (89% of flights were 40 cm or less).     

GEITONOGAMOUS POLLINATION AND ITS CONSEQUENCES IN GERANIUM CAESPITOSUM

In many plants, the frequency of geitonogamous (within-plant) pollination is likely to increase as a function of the number of simultaneously blooming flowers. This increase in self-pollination often produces inbreeding depression. Thus, a dilemma may exist in that individuals cannot increase seed production without lowering the average fitness of each seed. Conditions necessary for the existence of the dilemma were confirmed in large individuals of the herbaceous perennial Geranium caespitosum. Manually geitonogamously pollinated flowers initiated as many fruits as manually outcrossed flowers, but showed a subsequently greater number of embryo abortions, matured fewer seeds, and had a lower average seed weight. Observations of pollinators and dye transfer showed that geitonogamous bee pollinations increased as the number of flowers per plant increased. A simple model predicted that detrimental effects from geitonogamy become likely when 55 flowers are simultaneously blooming. Plants with 55 or more flowers produced 30% of the flowers in populations. The effect, and possible circumvention, of the dilemma on Geranium, and angiosperm breeding systems in general, is discussed.