DIFFERENTIAL SUCCESS OF POLLEN DONORS IN A SELF-COMPATIBLE LILY

If pollen donors are equally effective at siring seeds, the presence of equal proportions of pollen from two pollen donors on a stigma will lead to equal proportions of seeds sired by each pollen donor. Variation in germination rates, pollen-tube growth, and embryo viability may cause one donor to sire more seed than another. We looked for differential donor success in the field by simultaneously applying equal amounts of pollen from two pollen donors. We simultaneously applied equal amounts of self and outcross pollen to receptive stigmas and simultaneously applied pollen from two donors at different physical distances from the recipient. Following simultaneous application of self and outcross pollen, significantly more of the seeds were sired by outcross pollen donors. Seed set following simultaneous application of two outcross donors was also nonrandom. Pollen donors from 100 m were more likely to sire seeds when competing with pollen from plants nearby (1 m). To determine whether pollen-tube growth rates were responsible for these patterns of paternity, we varied the timing of deposition of outcross pollen allowing self pollen tubes a head start on the stigma. Outcross pollen was applied 3 or 24 h after self pollen. In spite of this time delay, the majority of the seeds were again sired by outcross pollen. There was no significant difference in the amount of seeds sired by self pollen between the two delay treatments. This result suggests that mechanisms operating after ovule fertilization may contribute to the discordance between the proportions of the pollen present and the proportions of seeds sired.     

VARIATION IN POLLEN SIZE,FERTILIZATION ABILITY,AND POSTFERTILIZATION SIRING ABILITY IN ERYTHRONIUM GRANDIFLORUM

The mean volume of pollen grains and total pollen production varied both within and among plants of Erythronium grandiflorum. The second flowers of two-flowered plants tended to produce smaller and fewer grains than first flowers, but there was no overall relationship between mean pollen grain size and production per flower. I evaluated the effects of pollen size differences within and among plants on two components of male reproductive success: pollen tube growth and postfertilization siring ability. Pollen tubes grown in media were longer for second flowers, but were not correlated with the mean size of pollen grains, suggesting that (1) internal resource content of pollen (i.e., carbohydrates plus lipids) was not associated with the hydrated size of pollen, and that (2) pollen from second flowers contained more resources. I analyzed the growth rate and the fertilization ability of pollen growing in styles. Growth rate differed among donors and recipients, but no effects of pollen or donor characters (i.e., pollen production, grain size, and flower position) were detected. In single donor pollinations, pollen size was negatively correlated with fertilization ability across donors, and positively correlated with postfertilization siring ability of donors. A second experiment used pairs of donors; within-plant differences in pollen size and flower position had effects similar to the single donor experiment on fertilization ability, but among-plant differences were not significant. The results corroborate earlier experiments that suggest that the growth of pollen tubes in the style is probably controlled by the recipient, since donor characters had minimal effects on pollen fertilization ability. Postfertilization siring ability was not affected by within-plant differences in mean grain size and production. For among-donor differences, the number of seeds set for each donor was positively correlated with the mean grain volume, and when a donor producing large pollen fertilized ovules in an ovary, there was increased seed abortion for seeds in the same ovary sired by a second donor. In addition, the total number of seeds produced by a fruit was decreased when both donors had large pollen, apparently due to increased postfertilization abortion. Postfertilization processes appear to be influenced by paternal differences that are expressed through competition among developing seeds for maternal resources.     

POLLEN TUBE DISTRIBUTIONS IN NICOTIANA GLAUCA: EVIDENCE FOR DENSITY DEPENDENT GROWTH

I hand sectioned styles of Nicotiana glauca at intervals along their length and counted the number of pollen tubes in each section using fluorescence microscopy. Evidence of density dependent growth was found for three stages of pollen growth. Pollen germination on the stigma increased with increasing pollen population size. Pollen tube penetration in the stigma was unaffected by increasing density from low to moderate levels but was reduced at high densities. Pollen tube penetration in the style was enhanced by increasing density. This enhanced growth in the style was apparently confounded by interference among pollen tubes growing at high densities. In particular, the area of tissue able to support pollen tube growth decreases from the stigma into the lower style, which could cause overcrowding of pollen tubes growing at high densities. Enhanced pollen tube penetration with increasing density combined with interference among pollen tubes growing at high densities resulted in greater mean pollen tube lengths for populations with moderate densities. The shift from density independent growth in the stigma to positively density dependent growth in the style may represent a shift from autotrophic to heterotrophic growth stages of pollen.     

POLLEN TUBE ATTRITION IN ERYTHRONIUM GRANDIFLORUM

Seed set after selfing in E. grandiflorum is often reduced relative to seed set after crossing; however, the compatibility patterns seen are not due to genes of major effect (i.e., S alleles). There is quantitative variation in the proportion of pollen tubes reaching the base of the style after both self- and cross-pollinations. Pollen tubes require between 24 and 72 hr to reach the ovary, but pollen tube growth ceases after 72 hr. When styles were removed from the ovaries 5 days after pollination, between 10 and 80% of the pollen tubes in the stigma had not reached the base of the style. The number of pollen tubes at the base of the style is a much better predictor of seed set than is the number of pollen tubes in the stigma. Pollen tube attrition is not affected by the age of the recipient flower or by the number of pollen donors contributing to the stigmatic pollen pool. The number of pollen tubes reaching the base of the style is dependent upon the source of the pollen and appears to be a decelerating function of the number of pollen tubes present in the stigma.     

Environmental Effects on Components of Pollen Performance in Faramea occidentalis (L.) A. Rich. (Rubiaceae)1

Studies focusing on gametophytic competition have focused on differences in pollen performance (e.g., pollen germination rate and pollen tube growth rate) among donors in order to examine genetic sources of variation in siring success among them. Donors that produce the fastest growing pollen tubes are expected to fertilize more ovules relative to donors with slow growing pollen tubes under conditions of gametophytic competition. However, the performance of pollen in the field is known to be influenced by environmental conditions in addition to the genotype of donor plants. This field study of Faramea occidentalis was conducted to: (1) determine the effect of environmental conditions during pollination on pollen performance; (2) measure differences among pollen donors in pollen performance; (3) determine if the pollen of different donors responds differently to a variety of environmental conditions surrounding pollinated flowers; and (4) measure differences among pollen recipients in pollen performance. Single-donor crosses were made between four pollen donors and four pollen recipients under a variety of environmental conditions. Pollen performance was then quantified as the growth rate of the fastest pollen tube, the mean pollen tube growth rate, and by a pollen germination index. Pollination environment (the environment surrounding a pollinated flower) and recipient significantly affected all three measures of pollen performance. Pollen donors did not differ overall in pollen performance. However, there was significant among-donor variation in two of the five pollination environment conditions. Future studies of variation in relative siring success may benefit by considering pollination environment in addition to donor and recipient identity.     

EFFECT OF CROSSING DISTANCE AND MALE PARENT ON IN VIVO POLLEN TUBE GROWTH IN CHAMAECRISTA FASCICULATA

Gametophytic competition among pollen grains has been proposed as an important mechanism of sexual selection in plants. The purpose of this paper is to examine the contribution of pollen source on in vivo pollen tube growth in Chamaecrista fasciculata. We addressed two questions: 1) Is pollen tube growth affected by the genetic relatedness between the pollen source and the pollen recipient? 2) Is there significant phenotypic variation among pollen donors for pollen tube growth? We compared pollen tube growth by measuring the number of pollen tubes which germinated, reached quarter length of style, and reached the ovary resulting from self- and outcross-pollinations. The outcross pollinations included three interplant distance classes: near (within genetic neighborhood, ca. 1 m), far (between far neighborhoods and within subpopulation, ca. 20 m), and distant (between neighborhoods and adjacent subpopulations, ca. 50–100 m). Our results show that pollen tube growth was not affected by genetic relatedness, by differences between self and outcross, nor by differences due to phenotypic variation among pollen donors. In contrast, maternal environment had a strong impact on pollen tube growth. These results suggest a lack of gametophytic competition and indicate little opportunity for sexual selection on pollen tube growth in C. fasciculata.     

Dynamics of pollen tube growth under different competition regimes

Pollen tube dynamics following different competition regimes were studied in sweet cherry (Prunus avium L.). In the process from pollination to fertilization, a constant reduction in the number of pollen tubes that travel along the style is observed. There could be two main causes of this reduction. One is a physical or physiological constraint consisting of the progressive decrease in the reserves and space available for pollen tube growth along the transmitting tissue of the style, and the other is genetic interaction both among the male gametophytes and between the male gametophytes and the female tissues of the flower. To evaluate the roles that these two forces play in reducing the number of pollen tubes that travel along the style, pistils were subjected to various pollen competition regimes by applying different mixtures of live and dead pollen onto the stigmata. The results obtained were similar when the experiment was repeated with different genotypes over 2 years, both in the laboratory and in the field. The role of stylar constriction is important, but it is not the only cause of pollen tube attrition because with low pollen loads fewer pollen tubes reach the different parts of the style than could fit therein. The fact that under different pollen competition regimes the number of pollen tubes is reduced by the same proportion in each stylar level indicates that genetic interactions play an important role in the control of pollen tube attrition.     

Post-pollination mechanisms in Nicotiana longiflora and N. plumbaginifolia: pollen tube growth rate, offspring paternity and hybridization

In natural populations where interfertile species coexist, conspecific and heterospecific pollen can be delivered to the stigmas. Post-pollination mechanisms might determine the seed siring success of different pollen donors within species as well as the chances for hybridization between species. Nicotiana longiflora and N. plumbaginifolia occur in sympatry in Northwest Argentina, where they have overlapping flowering seasons and share floral visitors. We explored (1) pollen tube growth rates for outcross versus self pollen in single-donor pollinations; (2) siring success of self versus outcross pollen donors in competitive pollinations, and (3) possibilities for hybridization by performing two- (outcross conspecific vs. heterospecific) and three-pollen donor (self vs. outcross vs. heterospecific) crosses. In N. longiflora, both pollen tube growth rate and siring success favored outcross pollen over self pollen and strong rejection of heterospecific pollen. In N. plumbaginifolia, pollen tube growth rate was similar for self and outcross pollen, self pollen sired similar numbers of offspring than outcross pollen and heterospecific pollen sired roughly the same number of progeny than self pollen. Results suggest that in natural sympatric populations, interspecific crosses would likely lead to unidirectional hybridization with N. plumbaginifolia as the seed parent.     

Effects of crossing distance and genetic relatedness on pollen performance in Alstroemeria aurea (Alstroemeriaceae)

Prezygotic barriers may represent effective mechanisms to avoid the deleterious effects of inbreeding. This study reports the existence of distance-dependent prezygotic barriers in self-compatible Alstroemeria aurea, a clonal herb native to temperate forests of the southern Andes. We analyzed pollen germination and tube growth as indicators of donor-recipient affinity using crossing distances of 1, 10, and 100 m. We used allozyme electrophoresis to determine the actual genetic relatedness between donor and recipient ramets. Pollen germination was not affected by distance between mates, but the number of pollen tubes reaching the base of the style increased strongly with distance between donor and recipient. This pattern was related to an increase in genetic dissimilarity with distance between mates. In contrast, pollen tube-style interactions did not change with distance when we restricted analysis to individuals at different distances that appeared to be genetically identical. This test implied genetic dissimilarity as the critical factor affecting pollen performance. We propose that the existence of prezygotic barriers might contribute to the high degree of genetic mixing exhibited by some clonal species.     

梅花个体内花柱长度的变异及其对繁殖成功的影响

植物个体内花型的变异影响繁殖成功, 雌性繁殖性状的变异可能影响雌性的繁殖成功, 也可能作为花粉的受体影响雄性的繁殖成功。然而, 植物个体内不同花柱长度的花产生的花粉是否影响植物的繁殖成功却少有研究。梅(Armeniaca mume)是原产我国的重要木本花卉和经济果树, 我们的野外观察发现在同一植株内, 不同花的花柱长度有变异, 存在长柱型、短柱型和雄花型(雌蕊败育)三种花型, 是比较雌性繁殖性状的变异对两性繁殖成功的影响的理想材料。本文主要测量了不同花型的花部特征, 统计花期, 并开展体外花粉萌发以及人工控制授粉实验。结果表明: 长柱型的花冠直径、雌蕊长、单花花粉数、花粉体积显著大于短柱型和雄花型。长柱型的单花期以及雌花期显著长于短柱型。长柱型、短柱型以及雄花型花粉在活体柱头上的萌发率没有显著性差异, 雄花型的花粉管长度显著高于长柱型和短柱型。长柱型为母本的花粉萌发率以及花粉管的长度要显著高于短柱型。长柱型、短柱型、雄花型花粉授粉与自然对照处理的坐果率没有显著性差异, 而长柱型为母本的坐果率要显著高于短柱型为母本的坐果率。这些结果表明野生梅花的长柱型为母本有利于花粉的萌发和花粉管的伸长, 有高的坐果率; 但其作为花粉供体的雄性功能与其他花型没有差异。     

Interspecific crosses in the genus Tulipa L.: identification of pre-fertilization barriers

 Pollen tube growth in the pistil and pollen tube penetration of ovules have both been studied in crosses between cultivars from Tulipa gesneriana L. and 12 tulip species from all eight sections of the genus Tulipa to identify pre-fertilization barriers. Depending on the cross, pollen tubes grew as far as the stigma or the style or continued growing down into the ovary. Pollen tubes penetrated none or only a few percent of the ovules of some crosses, despite the presence of many pollen tubes in the ovary. In other crosses, from which no or only a few hybrids have been obtained after seed maturation on the plant, pollen tube penetration was found in up to 79% of the ovules. Apparently, various kinds of barriers preventing fertilization or normal embryogenesis occur in interspecific tulip crosses. Received: 26 July 1996 / Revision accepted: 31 January 1997     

Expression of stylar incompatibility in the Andean clonal tuber crop oca (Oxalis tuberosa Mol., Oxalidaceae)

Long-, mid-, and short-styled clonal accessions of oca (Oxalis tuberosa) were intercrossed in a complete diallelic design. Pollen tube growth in styles was monitored in all diallelic crosses. Pollen fertility was estimated by two tests: staining of pollen grains with aceto-carmine and detection of β-galactosidase activity by the substrate X-Gal. The two methods of pollen fertility estimation were equally useful to detect fertility levels. Pollen originating from short stamens had the highest fertility (85%) and pollen from long stamens had the lowest fertility (70%). Pollen fertility was high throughout, but its degree varied with the stylar morph on which the pollen was formed. Long-styled accessions had the highest rates of fertile pollen. Differences in pollen fertility at different anther levels in the same style morph were also apparent. Pollen grain diameter of the six morph-anther level combinations was inversely correlated with pollen fertility. Pollen grains from long stamens were the largest and pollen grains from short stamens were the smallest. Neither pollen fertility nor pollen grain size had an influence on pollen tube growth in the style or on fruit and seed set. Pollen tubes growing within the styles were inhibited at a different level for each of the 18 cross combinations in the diallel. Although legitimate crosses had greatest pollen tube growth, some of the illegitimate inter- and intramorph crosses had equally high scores. Of all illegitimate crosses, mid-styled seed parents had the lowest level of stylar incompatibility. Fruit and seed set were highly correlated with the extent of pollen tube growth in the style. The number of pollen tubes entering ovules in a flower was in good agreement with the number of seeds produced per fruit. Therefore, it is concluded that stylar incompatibility is the major determinant of limited seed formation in oca even in the most successful legitimate cross combinations. Received: 1 September 1999 / Revision accepted: 4 April 2000     

CONTRIBUTION OF CRYPTIC INCOMPATIBILITY TO THE MATING SYSTEM OF EICHHORNIA PANICULA TA (PONTEDERIACEAE)

Tristylous populations of the annual aquatic Eichhornia paniculata have high levels of outcrossing and intermorph mating despite being fully self- and intramorph compatible. Experimental studies of pollen germination, ???pollen-tube growth, and pollinations with mixtures of genetically marked pollen were used to determine whether postpollination processes contribute to the observed mating patterns. Differences in pollen germination were small and did not contribute to differences in pollen siring ability. The fraction of pollen tubes first entering the ovary, however, was greater for legitimate outcross pollen than for either of the other two pollen types (self or outcross illegitimate pollen) in all recipient morphs. Moreover, legitimate pollen had higher siring success when in competition with illegitimate pollen types (self or outcross) in each recipient style morph. The ranking of pollen performance for different pollen-style combinations was the same for both the pollen-tube growth and marker-gene experiments indicating that differences in pollen-tube growth rate are the principal cause of differences in pollen siring ability. Cryptic incompatibility in E. paniculata may represent a weak heteromorphic incompatibility system because the observed patterns of pollen-tube growth parallel pollen-tube growth and seed-set patterns that occur in related species with strong trimorphic incompatibility. The ability to produce mostly outcrossed progeny when pollinators are abundant, but to reliably produce seed under a variety of environmental and demographic conditions may be favored in E. paniculata because of its colonizing life history and occurrence in ephemeral habitats. Cryptic incompatibility may be more likely to occur in species subject to wide fluctuations in population size and levels of pollinator service.     

Nectarless flowers with deep corolla tubes in Pedicularis: does long pistil length provide an arena for male competition?

Pollinator‐mediated selection does not seem to have a direct influence on the evolution of a long corolla tube in a nectarless flower. We hypothesized that the long pistil length of the nectarless flower with a deep corolla tube provided an opportunity for male competition. Pedicularis siphonantha, a nectarless and partially self‐incompatible lousewort with substantial variation in corolla tube length, was used to test the hypothesis. We tested whether and how corolla tube length affected seed production per capsule and seed germination rate with different pollination treatments. Flowers were hand‐pollinated with pollen from one self donor and one outcross donor and mixed pollen grains consisting of equal amounts from the two donor types, respectively. Additionally, seeds from open‐pollinated flowers with different corolla tube lengths were collected separately for measurement of germination rate. Pollination treatment and corolla tube length significantly affected number of seeds per capsule. Moreover, a significant positive relationship between seeds per capsule and corolla tube length was found when mixed hand pollination was conducted. Seeds of self hand‐pollinated flowers had a lower germination rate than those from outcross‐pollinated flowers. Under open pollination, seeds from flowers with longer corolla tubes tended to have higher germination rate. In P. siphonantha, outcross pollen may have a higher probability of contributing to the next generation when transferred to flowers with longer corolla tubes. The pistil length, therefore, should be seen as a female choice mechanism, which provides an arena for male‐to‐male competition. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179 , 526–532.     

Effects of pollen competitive environment on pollen performance in Mirabilis jalapa (Nyctaginaceae)

 We examined the influence of pollen competitive environment on pollen performance in Mirabilis jalapa. We used the number of pollen grains and the number of pollen tubes per pistil as measures of pollen competition. Pollen germination, pollen tube penetration into the style, and pollen tube growth rates were used as measures of pollen performance. All three measures of pollen performance were affected by the competitive environment. Pollen germination was greatest at intermediate pollen load sizes. The percentage of germinated pollen grains that penetrated the stigma and grew into the style decreased with pollen load size. Pollen tube growth rate in the style was greater and more variable with larger numbers of pollen tubes in the style. Controlling for the degree of selection at the stigma indicated that pollen-pollen or pollen-style interactions were the likely causes of increased growth rates. Received: 28 October 1996 / Revision accepted: 24 January 1997     

Effects of pre-dispersal selection on offspring growth and survival in Erythronium grandiflorum

Effects of pre-seed-dispersal processes on offspring vigor were examined in Erythronium grandiflorum using manipulations of the number of pollen donors contributing to the pollen pool and comparisons of means and variances in offspring growth measurements. There were no effects of the number of donors on measures of pollen-tube growth, ovule abortion, seed set, mean seed weight, or seedling germination. Seeds from pollinations with only one donor produced corms that averaged 5% lighter after one season of growth and had lower overall survival after three years compared to corms from pollinations with either three or ten donors. Patterns of within- and among-family variance estimates for the different treatments were consistent with the hypothesis that less-vigorous offspring were eliminated prior to seed dispersal in the multiple-donor treatments. The difference in the growth of offspring from different treatments was apparently not due to pollen competition because pre-zygotic attrition of pollen tubes led to incomplete fertilization of ovules. Results from this study suggest that post-fertilization abortion of less-vigorous progeny, perhaps as a consequence of early-acting inbreeding depression, is responsible for the increase in the average vigor of offspring from multiply-sired fruits.     

Postfertilization causes of differential success of pollen donors in Erythronium grandiflorum (Liliaceae): nonrandom ovule abortion

Seed paternity in Erythronium grandiflorum does not fully reflect the proportion of pollen on the stigma. When two types of pollen are simultaneously applied to the stigma, outbred seeds are produced over inbred, and seeds from more distant donors are produced over seeds from donors nearby. I looked for postfertilization causes of these previously reported patterns of differential success of pollen donors. I simultaneously pollinated stigmas with pollen from two donors and observed ovule development through a window sliced in the ovary. Pollen donor pairs were self and cross, donors 1 and 100 m from the recipient, and two donors each 100 m from the recipient. Since one donor was always the alternate homozygote from the recipient at the malate dehydrogenase locus, I could determine the paternity of developing seeds. When it appeared that ovules were aborting, I removed them and determined their paternity using starch gel electrophoresis. Ovules fertilized by self pollen were more likely to abort than ovules fertilized by cross pollen, and ovules fertilized by nearby donors were more likely to abort than ovules fertilized by distant donors. Ovules fertilized by donors 100 m from the recipient were equally likely to abort. There was not a significant relationship between the proportion of ovules fertilized by a pollen donor and the probability of those fertilized ovules developing into seeds. There was no relationship between ovule position within a fruit and ovule abortion. I manipulated available resources by removing leaves and by permitting only one fruit to develop per plant. Decreasing the amount of resources increased the proportion of aborted ovules. Abortion of ovules of lesser quality appears to release resources that can then be used to develop other offspring.     

Effects of Nutrient Level on Maternal Choice and Siring Success in <Emphasis Type="Italic">Cucumis sativus</Emphasis> (Cucurbitaceae)

Plants have evolved many mechanisms to increase the chance of gene dispersal mainly through pollen and environmental factors play an important role. Understanding the mechanism behind gene dispersal is therefore crucial in the correct evaluation of the use of genetically modified crops for cultivation. In this paper we address the question of weather nutrient availability for the female affects the outcome of pollen competition between two pollen donor cultivars of Cucumis sativus. We do this by carrying out controlled crosses of female plants grown at three different nutrient levels. We separated the effect of a specific donor from the effect of pollen tube growth rate by using reversed crosses of fast and slow pollen. Our results show that female effects on siring ability vary with nutrient level. Pollen with a high pollen tube growth rate was more successful when nutrient availability for the female was high. This could be the result of selection on the female to adjust preference according to environmental circumstances. Pollen tube growth rate was measured under nutrient rich circumstances, thus high performers possessed traits adapted to a nutrient rich situation. Due to trade-off effects, these traits might not be advantageous in poor environments. Instead, individuals adapted to low nutrient circumstances will have a higher pollen tube growth rate. If siring ability varies with the environment of the recipient plant, this means that assessments of gene flow must account for this variation and include both pollen donors and recipient plants subjected to a range of environmental circumstances. In risk assessments of transgenic plants, plants are often kept under experimental, homogenous conditions. If our results also apply to other species, estimates of gene flow under constant conditions may be misleading. Selection on siring ability and female preference have fundamental effects on gene flow and need to be considered in risk assessments of transgenic plants.Co-ordinating editor: I. Olivieri     

Analysis of pollen-style interactions in Petunia hybrida; the determination of variance in male reproductive success

Summary A factorial cross design was used to evaluate the influence of differences among pollen donors and recipients on variation in stylar attrition of pollen tubes in self-fertile plants of Petunia hybrida. Pollinations to flower buds were used to reduce the inhibitory ability of the style and these crosses were compared with flower pollinations to assess the degree of stylar influence on pollen fertilization ability. There was less pollen tube attrition after bud pollinations than after flower pollinations, indicating that styles of buds were less able to inhibit pollen tubes. The variance component for plants acting as pollen donors tended to be greater after flower pollinations than after bud pollinations. The lower variance in male success after bud pollinations indicates that differences among pollen donors after flower pollinations were due to stylar inhibition of pollen rather than differences in pollen vigor. Since the level of variation in pollen growth after pollination to flowers was greater among clones than among ramets within clones, the differences in pollen fertilization ability are probably genetically based.     

Tests of two hypotheses concerning pollen competition in a self-compatible,long-styled species (Lobelia cardinalis: Lobeliaceae)

Using the long-styled, self-compatible species Lobelia cardinalis, I examined the relative abilities of self pollen and two outcross pollen sources to fertilize ovules when these three kinds of pollen were applied simultaneously to stigmas. Paternity was determined electrophoretically for 712 progeny of 25 seed parents. Two hypotheses were tested. First, it was hypothesized that self pollen would be less effective than outcross pollen at fertilizing ovules, as a means of reducing the selfing rate. Outcross pollen outperformed self pollen in eight of the 25 seed parents, self outperformed outcross in five, and they were competitively indistinguishable in 12. The mean proportion of offspring produced by self pollen was not significantly different from the value expected under random tube growth and fertilization. The rate of self-fertilization is therefore determined by prepollination events. Second, to test one stage of the sexual-selection process, it was hypothesized that the pollen of some individuals would outcompete that of others, across a range of maternal plants. The 25 maternal parents were divided into five groups of approximately five individuals. Each member of a group received pollen from the same two outcross pollen sources (as well as from itself). With offspring pooled across seed parents, one outcross pollen parent outcompeted the other in two of the five groups. In two other groups, pollen sources were competitively superior or inferior depending on the maternal parent. Overall, pollen donors were competitively indistinguishable in 13 of 25 maternal parents. There is thus no evidence for strong postpollination sexual selection. If such selection is present, it is weak, and its importance to the evolution of style length in Lobelia cardinalis requires other kinds of study.