EFFECTS OF SOIL NITROGEN ON POLLEN PRODUCTION,POLLEN GRAIN SIZE,AND POLLEN PERFORMANCE IN CUCURBITA PEPO (CUCURBITACEAE)

To determine the effects of soil nitrogen on pollen production, pollen size, and pollen performance, two cultivars of zucchini (Cucurbita pepo) were grown under two nitrogen regimes in an experimental garden. The two cultivars were true breeding for alternative alleles for a one gene trait, ovary color. The soil nitrogen treatment had a significant effect on most measures of reproductive output through the female function. The nitrogen treatment did not affect the number of staminate flowers per plant but did have an effect on the number of pollen grains per staminate flower and the mean pollen grain size. A pollen mixture experiment revealed that pollen produced by plants in the high nitrogen treatment sired significantly more seeds than pollen from low nitrogen plants. Moreover, we found that the high nitrogen pollen sired even a greater percentage of seeds in the region of the fruit (ovary) previously shown to be fertilized by the fastest growing pollen tubes. Thus, the difference in the number of seeds sired by pollen from the two nitrogen treatments is due to differences in pollen performance. We conclude that spatial heterogeneity in soil nitrogen can influence the paternity of seeds in a plant population.     

POLLEN-TUBE COMPETITION AND MALE FITNESS IN HIBISCUS MOSCHEUTOS

The stigmas of animal-pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed-donor pollen loads are probably common. When this is the case, variation in average pollen-tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed-donor hand-pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen-tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants.     

Extensive long-distance pollen dispersal in a fragmented landscape maintains genetic diversity in white spruce

Conifers are among the most genetically diverse plants but show the lowest levels of genetic differentiation, even among geographically distant populations. High gene flow among populations may be one of the most important factors in maintaining these genetic patterns. Here, we provide empirical evidence for extensive pollen-mediated gene dispersal between natural stands of a widespread northern temperate/boreal conifer, Picea glauca. We used 6 polymorphic allozyme loci to quantify the proportion of seeds sired by pollen originating from different sources in a landscape fragmented by agriculture in North Central Ontario, Canada. In 7 stands, a small proportion of seeds were sired by self-pollen or neighboring trees but 87.1% (+/-1.7% standard error [SE]) of seeds were sired by pollen from at least 250 to 3000 m away. In 4 single isolated trees, self-fertilization rates were low and more than 96% (+/-1.3% SE) of seeds were sired by immigrant pollen. The average minimum pollen dispersal distance in outcrossed matings was 619 m. These results provide strong evidence that extensive long-distance pollen dispersal plays a primary role in maintaining low genetic differentiation among natural populations of P. glauca and helps maintain genetic diversity and minimize inbreeding in small stands in a fragmented landscape.     

Complex sex determination in the stinging nettle Urtica dioica

The dioecious species Urtica dioica harbours wide variation in sex ratio of seeds. We conducted a series of crosses to analyse the genetic basis of sex determination in this species. Dutch populations of U. dioica contain low proportions of monoecious individuals beside male and female plants. Self-pollination of monoecious plants always yielded female, male and monoecious plants, generally in a ratio of one female to three male/monoecious individuals. This motivated us to write down a simple model in which gender is determined by one major sex-determination locus with four alleles. In the model males and monoecious plants have distinct genotypes but are both heterozygous at the sex-determination locus. We first made crosses among progeny obtained after self-pollination of monoecious plants. These crosses showed that the monoecious trait generally showed Mendelian inheritance and was passed on to the next generation via both pollen and seeds. Further crosses between monoecious plants and plants from dioecious system indicated that alleles from the dioecious system are often dominant. However, many exceptions to our genetic model are observed which suggest that dominance is incomplete and/or that more genes are involved in sex determination. We discuss to what extent sex determination genes explain the strongly biased seed sex ratios and argue that additional genes, for instance genes for female choice, must also be involved.     

Effects of pollen load size on seed paternity in wild radish: the roles of pollen competition and mate choice

For sexual selection to be important in plants, it must occur at pollen load sizes typical of field populations. However, studies of the impact of pollen load size on pollen competition have given mixed results, perhaps because so few of these studies directly examined the outcome of mating when pollen load size was varied. We asked whether seed paternity after mixed pollination of wild radish was affected by pollen load sizes ranging from 22 to 220 pollen grains per stigma. We examined the seed siring abilities of 12 pollen donors across 11 maternal plants. Seed paternity was statistically indistinguishable across the pollen load sizes even though, overall, the pollen donors sired different numbers of seeds. This lack of effect of pollen load size on seed paternity may have occurred because fruit abortion and early abortion or failure of fertilization of seeds increased as load size decreased. Thus, failures of fruits and seeds sired by poorer pollen donors may keep seed paternity constant across pollen load sizes.     

NONRANDOM MATING IN WILD RADISH: VARIATION IN POLLEN DONOR SUCCESS AND EFFECTS OF MULTIPLE PATERNITY AMONG ONE- TO SIX-DONOR POLLINATIONS

Nonrandom mating in plants may involve differential success of self- vs. outcross pollen, of pollen from relatives vs. nonrelatives, and of pollen from several compatible donors. Of these, the last is least understood. Wild radish plants were crossed with six pollen donors in all possible combinations to investigate differences in pollen donor performance and to compare multiply sired fruits with different numbers and identities of fathers. Pollen donors differed in number of seeds sired, position of seeds sired within fruits, and mass of seeds sired. Performance in single pollination was not a good predictor of performance in mixed pollination. Performance in two-donor pollinations was a better predictor of the outcome of multidonor pollinations. Multiply sired fruits were selectively filled, but not all multiply sired fruits were equal. Both the number and identity of donors affected fruit filling. These results and others with wild radish suggest that variation in pollen donor ability to sire seeds is relatively common in wild radish. Since wild radish plants in the field typically receive pollen from several donors, there may be considerable opportunity for selection at this stage of the life cycle in wild radish.     

Pollination by sexual deception promotes outcrossing and mate diversity in self‐compatible clonal orchids

The majority of flowering plants rely on animals as pollen vectors. Thus, plant mating systems and pollen dispersal are strongly influenced by pollinator behaviour. In Australian sexually deceptive orchids pollinated by male thynnine wasps, outcrossing and extensive pollen flow is predicted due to floral deception, which minimizes multiple flower visitations within patches, and the movement of pollinators under mate‐search rather than foraging behaviours. This hypothesis was tested using microsatellite markers to reconstruct and infer paternity in two clonal, self‐compatible orchids. Offspring from naturally pollinated Chiloglottis valida and C. aff. jeanesii were acquired through symbiotic culture of seeds collected over three seasons. In both species, outcrossing was extensive (t_m = 0.924–1.00) despite clone sizes up to 11 m wide. The median pollen flow distance based on paternity for both taxa combined was 14.5 m (n = 18, range 0–69 m), being larger than typically found by paternity analyses in other herbaceous plants. Unexpectedly for orchids, some capsules were sired by more than one father, with an average of 1.35 pollen donors per fruit. This is the first genetic confirmation of polyandry in orchid capsules. Further, we report a possible link between multiple paternity and increased seed fitness. Together, these results demonstrate that deceptive pollination by mate‐searching wasps enhances offspring fitness by promoting both outcrossing and within‐fruit paternal diversity.     

The influence of distinct pollinators on female and male reproductive success in the Rocky Mountain columbine

Although there are many reasons to expect distinct pollinator types to differentially affect a plant's reproductive success, few studies have directly examined this question. Here, we contrast the impact of two kinds of pollinators on reproductive success via male and female functions in the Rocky Mountain columbine, Aquilegia coerulea . We set up pollinator exclusion treatments in each of three patches where Aquilegia plants were visited by either day pollinators (majority bumble bees), by evening pollinators (hawkmoths), or by both (control). Day pollinators collected pollen and groomed, whereas evening pollinators collected nectar but did not groom. Maternal parents, potential fathers and progeny arrays were genotyped at five microsatellite loci. We estimated female outcrossing rate and counted seeds to measure female reproductive success and used paternity analysis to determine male reproductive success. Our results document that bumble bees frequently moved pollen among patches of plants and that, unlike hawkmoths, pollen moved by bumble bees sired more outcrossed seeds when it remained within a patch as opposed to moving between patches. Pollinator type differentially affected the outcrossing rate but not seed set, the number of outcrossed seeds or overall male reproductive success. Multiple visits to a plant and more frequent visits by bumble bees could help to explain the lack of impact of pollinator type on overall reproductive success. The increase in selfing rate with hawkmoths likely resulted from the abundant pollen available in experimental flowers. Our findings highlighted a new type of pollinator interactions that can benefit a plant species.     

Gene flow inferred from seed dispersal and pollinator behaviour compared to DNA analysis of restriction site variation in a patchy population of Lotus corniculatus L.

Summary Gene flow was investigated in a natural population of Lotus corniculatus L. (Fabaceae) using a combination of pollen and seed dispersal studies and a recombinant DNA technique. The population is spatially heterogeneous and grows with Empetrum nigrum. L. corniculatus is pollinated by the pollen-collecting bumblebee Bombus lapidarius L. Most pollinator flights occurred within patches, as bees usually visit nearest-neighbour plants, show no marked directionality, and forage mostly within patches. Gene flow by seeds is also limited, reinforcing the pattern of gene flow within patches. However, 2.6% of pollinator flights are between patches and considerable pollen carryover also occurs. Thus, gene flow between patches is potentially sufficient to retard or prevent genetic differentiation in spite of the patchy sub-structuring of the population. A sub-set of the population was analysed for restriction fragment length polymorphisms (RFLPs) to document the actual gene flow pattern of the population. The DNA analysis revealed significant levels of genetic differentiation between the patches. The level of gene flow that can be inferred from the distribution of genetic variation is surprisingly restricted, as compared to gene flow inferred from pollinator behaviour, and emphasizes that stochastic processes like genetic drift and founder effects may have a strong impact on the prevailing genetic structure.     

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.     

EFFECTS OF POLLEN DONOR IDENTITY ON OFFSPRING QUALITY IN WILD RADISH,RAPHANUS SATIVUS

If pollen donor performance during mating correlates with differences in offspring growth and fitness, processes that sort among potential mates may directly improve offspring fitness. Here seeds sired by three pollen donors on ten maternal plants were grown for eight weeks in the greenhouse. The performance of the pollen donors during pollination and fertilization was known from a previous experiment. There were significant effects of paternity on two measures of early growth: leaf number and plant height. Paternal effects on three measures more closely related to fitness; final plant weight, day of first flower production, and total flower number were also significant. Under the conditions of this experiment, final plant weight was probably the best predictor of fitness. The pollen donor that sired the largest seeds in the previous experiment sired offspring that were largest after 8 weeks of growth. Half of the plants were grown under low-water conditions. Paternal effects on growth were not masked by the environmental effects. In fact, some paternal effects became stronger under stress. This suggests that paternal effects could also be important in the field. Plants sired by donor A bolted very early when water was limited and would probably have an advantage in a season that was very short due to an early and severe drought. During fertilization and seed filling, seeds sired by this donor were more frequent on water-stressed maternal plants than on control maternal plants (Marshall, 1988). The data from this experiment indicate a connection between pollen donor performance during mating and offspring growth. These results suggest that the processes that sort among potential fathers during pollination, fertilization, and seed filling, may improve offspring quality.     

Influence of pollinator grooming on pollen‐mediated gene dispersal in Mimulus ringens (Phrymaceae)

Pollinator foraging patterns and the dynamics of pollen transport influence the quality and diversity of flowering plant mating opportunities. For species pollinated by grooming pollinators, such as bees, the amount of pollen carried between a donor flower and potential recipient flowers depends on how grooming influences pollen transfer. To investigate the relationship between grooming and pollen‐mediated gene dispersal, we studied bumblebee (Bombus fervidus) foraging behavior and resulting gene dispersal in linear arrays of Mimulus ringens. Each of the 14 plants in an array had a unique multilocus genotype, facilitating unambiguous assignment of paternity to 1050 progeny. Each plant was trimmed to a single flower so that pollinator movements could be linked directly to resulting gene dispersal patterns. Pollen‐mediated gene dispersal was very limited. More than 95% of the seeds sired by a donor flower were distributed over the first three recipient flowers in the visitation sequence. However, seeds were occasionally sired on flowers visited later in the pollinator's floral visitation sequence. Intensive grooming immediately following pollen removal from a donor flower significantly increased the decay rate of the donor flower's gene dispersal curve. These results suggest that the frequency and relative intensity of grooming can have significant effects on patterns of pollen‐mediated gene dispersal from individual pollen donors.     

PATERNITY ANALYSIS IN A NATURAL POPULATION OF ASCLEPIAS EXALTATA: MULTIPLE PATERNITY,FUNCTIONAL GENDER,AND THE “POLLEN-DONATION HYPOTHESIS”

Recently, some evolutionary biologists have argued that selection on the male component of fitness shapes the evolution of reproductive characters in angiosperms. Floral features, such as inflorescence size, that lead to increased insect visitation without a concomitant increase in seed production are viewed as adaptations to enhance the probability of fathering seeds on other plants. In tests of this “pollen donation hypothesis,” male reproductive success has usually been measured indirectly by flower production, pollinator visitation, or pollen removal. We tested the pollen donation hypothesis directly by quantifying the number of seeds sired by individual genotypes in a natural population of poke milkweed, Asclepias exaltata, in southwestern Virginia. Multiple paternity was low within fruits, a fact which allowed us to use genotypes of progeny arrays to identify a unique pollen parent for 85% of the fruits produced in the population. Seeds sired (male success) and seeds produced (female success) were significantly correlated with flower number per plant (for male success, r = 0.32, P > 0.05; for female success, r = 0.66, P > 0.001). While the number of pollinaria removed, the usual estimator of male success in milkweeds, was highly correlated with numbers of seeds sired (r = 0.47; P > 0.001), it was even more highly correlated with numbers of seeds produced (r = 0.71, P > 0.001). Analysis of functional gender indicated that plants with many flowers did not behave primarily as males. In fact, individuals with the highest total reproductive success contributed equally as males and females. Furthermore, estimates of gender based on numbers of flowers produced or pollinaria removed overestimated the number of functional males in the population. In pollen-limited species, such as many milkweeds, proportional increases in both male and female reproductive success indicate the potential for selection to shape the evolution of large floral displays through both male and female functions.     

POLLEN DISPERSAL DYNAMICS IN AN ALPINE WILDFLOWER,POLEMONIUM VISCOSUM

The alpine wildflower, Polemonium viscosum, depends on insect visitors for effective pollination. Here, I examine experimentally the effects of pollinator visitation on pollen removal, pollen dispersal success, paternity, and gene flow. Bumble bee pollinators visited donor individuals homozygous for marker alleles at an isozyme (GOT-2) encoding locus and then were presented with arrays of recipient plants lacking the marker alleles. Four aspects of male fitness were estimated for each donor: the number of pollen grains dispersed to flowers of the first recipient visited, the number of offspring sired on that recipient, the proportion of offspring sired in the full array, and the proportion of mates in the array bearing seeds of the donor. Pollen removal was strongly influenced by the number of bee visits to donor flowers. The amount of pollen removed in turn significantly affected the number of pollen grains reaching flowers of the first recipient. However, because seed production decelerates with stigma pollen load, the relationship between pollen export and paternal success at this proximate scale showed diminishing returns. The probability of reaching mates within the array also increased with pollen export. These findings show that floral characters enhancing pollinator visitation rate in P. viscosum have positive effects on paternity and gene flow.     

POLLEN AND OVULE SOURCES AFFECT SEED PRODUCTION OF LOBELIA CARDINALIS (LOBELIACE AE)

To examine the breeding system and components of male and female reproductive success in the hermaphroditic plant Lobelia cardinalis, we performed three crossing experiments with plants taken from natural populations. The experiments were designed to determine if the crossing success of plants as pollen and ovule parents was affected by the distance among mates, including self-pollinations and pollinations between populations; to determine if plants differed in their abilities to sire or mature seed; and to determine if there was a correlation between a plant's success at siring and maturing seed. Selfpollinations resulted in significantly fewer seeds per fruit and significantly smaller seeds. There were no significant differences in germinability between selfed and outcrossed seeds. Distance among parents within a population did not affect any of the traits. Outcrosses within and between population produced similar numbers of seeds per fruit, similar seed weights, and similar germination success. There were highly significant differences among maternal plants in all three experiments in the number of seeds they matured, mean seed weight, and seed germinability. The maternal parent was the most important factor determining seed production, but there were also significant differences among paternal plants in the number of seeds they sired (all three experiments), in the germinability of the seeds they sired (two experiments), and in the sizes of seeds they sired (one experiment). Our results indicate that differences in success of Lobelia plants as male parents cannot be due solely to their relatedness to the female parent.     

Effects of soil phosphorus on pollen production,pollen size,pollen phosphorus content,and the ability to sire seeds in Cucurbita pepo (Cucurbitaceae)

To determine the effects of soil phosphorus on pollen production, pollen grain size, phosphate concentration per pollen grain, and the siring ability of pollen, two cultivars of the common zucchini (Cucurbita pepo) were grown under two soil phosphorus conditions in an experimental garden. Overall, soil phosphorus availability had a significant effect on reproductive output through the female function and on traits affecting the male function of plants (staminate flower production, pollen production per flower, and pollen grain size). In addition, pollen produced by plants in the high phosphorus soils had a higher phosphate concentration than pollen produced by plants in the low phosphorus soils. A pollen mixture experiment revealed that pollen produced by plants in the high phosphorus treatment sired significantly more seeds than pollen produced by plants in the low phosphorus treatment. This study showed that growing conditions such as soil phosphorus can influence the size of a pollen grain and its chemical composition, which, in turn, can affect its ability to sire mature seeds.     

THE EFFECT OF INFLORESCENCE SIZE ON MALE FITNESS: EXPERIMENTAL TESTS IN THE ANDROMONOECIOUS LILY,ZIGADENUS PANICULATUS

We studied the relationship between inflorescence size and male fitness in the andromonoecious lily Zigadenus paniculatus, using experimentally manipulated inflorescences to eliminate possible correlations between flower number, resource availability, and other floral traits. Allozyme markers were used to determine the siring success of large versus small plants in 14 arrays of plants, each array containing five large and five small plants. The inflorescence size of small plants was held constant both within and among arrays; the size of large plants was held constant within an array but was varied among arrays. Large plants sired more than half the seeds in 12 of the 14 arrays, and significantly more than half in six of these 12. However, in eight of the arrays, large plants sired significantly fewer seeds than expected on the basis of their size advantage. Furthermore, there was no significant relationship between relative size and relative siring success in comparisons among arrays. A maximum-likelihood model estimated that 28% of seeds were sired by imported pollen, with 95% confidence limits of 13% and 50%. Within these limits, high import rates tended to mask the relative success of large plants in several arrays. These results suggest that the evolution of inflorescence size in Z. paniculatus is at least partly driven by selection for increased male success, assuming genetic variation for flower number. However, the data also support a growing body of evidence that estimates of male fitness in plants can be highly variable. We discuss the sources of this variability and the possible effects of inflorescence design on the relationship between inflorescence size and fitness.     

Inbreeding depression affects pollen performance in Cucurbita texana

Most studies of the effects of inbreeding focus on vegetative vigor and reproductive output through the female (fruit and seed) function. This study not only examines the effects of inbreeding on the female function but it also examines the effects of inbreeding on pollen performance both in vitro and in vivo. This study used Cucurbita texana, a wild gourd, and was performed under field conditions. In vivo performance was assessed by placing equal amounts of pollen from either the inbred or outcrossed plants onto a stigma together with pollen from a tester line. As with other studies, we found that outcrossed plants had greater reproductive output (male flowers and fruits) than plants produced from self pollinations. Unlike most studies of inbreeding depression, which mostly ignore the male function of plants, we also found that the pollen produced by outcrossed plants had faster pollen tube growth in vitro than the pollen produced on selfed plants. Moreover, pollen from selfed plants sired significantly fewer seeds than pollen from outcrossed plants under conditions of pollen competition (i.e. the number of pollen grains deposited onto the stigmas was larger than the number of available ovules). These findings indicate that inbreeding affects the performance of the resulting sporophytic generation and the microgametophytes they produce.     

THE INFLUENCE OF POPULATION SIZE AND ISOLATION ON GENE FLOW BY POLLEN IN SILENE ALBA

In a series of experiments conducted over two seasons, we used arrays of experimental populations to examine the effects of flower number and distance between patches on gene flow by pollen. For this study we used the dioecious, short-lived perennial plant Silene alba (Caryophyllaceae). This species lives in disturbed roadside and agricultural habitats and displays a weedy population dynamic with high colonization and extinction rates. The motivation for the study was to understand what factors may be influencing genetic connectedness among newly colonized populations within a regional metapopulation. By using experimental populations composed of genotypes homozygous at a diagnostic locus, it was possible to identify explicitly pollen movement into a focal patch as a function of flower number and distance to the nearest neighboring patch. Overall, the mean immigration rate (measured as the fraction of seeds sired by males outside the focal patch) at 20 m was just over 47%, whereas at 80 m immigration rates were less than 6%. In addition, by knowing the context in which each of these gene-flow events occurred, it was possible to understand some of the factors that influenced the exchange of genes. Both the number of flowers in the focal population (target) and in the neighboring populations (source) had a significant effect on the frequency of gene flow. Our experimental data also demonstrate that factors that influence gene flow at one spatial scale may not act in the same way at another. Specifically, the influence of target size and the relative size of the target and source patches on rates of gene flow depended on whether the patches were separated by 20 m or 80 m. These data suggest that the patterns of gene flow within a metapopulation system can be complex and may vary within a growing season.     

EVIDENCE FOR LONG-DISTANCE POLLEN DISPERSAL IN MILKWEEDS (ASCLEPIAS EXALTATA)

We investigated pollen-mediated gene flow and interspecific matings in natural populations of poke milkweed, Asclepias exaltata. Genetic data from 16 polymorphic isozymes allowed unambiguous identification of the diploid paternal genotype for 225 singly sired fruits collected from six populations in Shenandoah National Park, Virginia. Paternity analysis of progeny arrays revealed that 29%–50% (39.2%, mean) of the seeds produced in these populations were sired by plants located outside each target population. Variation among populations appears to be related to isolation distance (Kendall's τ = ?0.69, N = 6, P > 0.1), although the negative correlation was statistically significant only when the results from a previous study of A. exaltata were also included in the analysis (Kendall's τ = ?0.78, N = 7, P < 0.05). The coupling of standard paternity exclusion analysis on individual seeds (12 seeds/fruit) with estimation of cryptic gene flow from Monte Carlo simulations accounted for only 65% of the gene flow detected using the progeny-array analysis. These results suggest that standard paternity exclusion, incorporating cryptic gene flow, will inherently underestimate actual gene flow rates when the number of paternal parents is smaller than the number of seeds per fruit (i.e., matings are correlated). Indirect estimates of the number of migrants per generation (Nm ≈ 2.3) obtained from Nei's genetic diversity statistic (G_ST) are much smaller than realized gene flow measured in 1990 (Nm = 9.4, progeny-array method). In addition, interspecific hybridization with common milkweed, Asclepias syriaca, via long-distance pollen dispersal was detected in two fruits (0.8%). Thus, pollen-mediated gene flow and infrequent interspecific matings provide mechanisms to lower genetic differentiation among populations to maintain novel, low-frequency alleles within populations of A. exaltata.