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.     

Differential siring success of Pgi genotypes in Clarkia Unguiculata (Onagraceae)

Competition among pollen grains for the fertilization of ovules can play an important role in determining the male and female reproductive success of flowering plants. To examine the influence of pollen-donor genotype on male reproductive success, hand-pollinations were conducted on Clarkia unguiculata and the siring success of pollen-donor plants was compared between donors homozygous for different allelomorphs of the allozyme PGI (phosphoglucoisomerase). Donors homozygous for the B allele sired more seeds than C-allele donors. Single-donor crosses indicated that C-donor-sired seeds are aborted more often than are B-donor-sired seeds, suggesting that the B-allele donor's advantage in mixed pollinations was a result of differential abortion. A negative relationship between pollen load and the siring success of B-allele donors implies that pollen from B-allele donors has reduced performance relative to C-allele donors when pollen loads are high. These data demonstrate consistent differences in siring success between individuals homozygous for different alleles at a single locus and suggest that variation at the Pgi locus may be maintained by a post-pollination trade-off.     

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.     

DOES INTERFERENCE COMPETITION AMONG POLLEN GRAINS OCCUR IN WILD RADISH?

Interest in the possibility of sexual selection in plants has focused primarily on competition among pollen donors based on the speed of pollen-tube growth. However, when pollen arrives on stigmas, there is the opportunity for both races for access to ovules (exploitation competition) and interference with the germination and growth of pollen from other donors (interference competition). We considered whether this second form of competition might occur among pollen grains of wild radish in two experiments. In the first, interference likely occurred because the amount of pollen germination was less in mixed-donor than in single-donor pollinations. This result was duplicated in a second experiment, which also showed that interference occurred only when pollen grains from different donors were in direct contact with each other. In addition, in the second experiment, the opportunity for interference affected the frequency of seeds sired by different pollen donors. Because pollen loads are often mixed in nature, interference competition among pollen grains may be important in the ecology and evolution of plant reproduction.     

Trade-offs between male and female reproduction associated with allozyme variation in phosphoglucoisomerase in an annual plant (Clarkia unguiculata: Onagraceae)

The genotype of an individual for allozymes such as phosphoglucoisomerase (Pgi) is often not neutral with regard to fitness. Studies of several taxa have found consistent fitness differences among Pgi genotypes expressing different allozymes. We conducted a greenhouse experiment with Clarkia unguiculata to determine whether allelic variation at the Pgi-C1 locus may affect components of male and female function. We found significant differences in siring success between pollen donors homozygous for different Pgi alleles. When a mixture of pollen was applied to stigmas under conditions of gametophytic competition (more pollen deposited on stigmas than there are ovules available to fertilize), donors homozygous for the C allele of Pgi sired more seeds per fruit than B-allele donors. Differences between genotypes with respect to female fertility per fruit contrasted with the male advantage associated with the C allele. Recipients homozygous for the C allele produced fruits with more aborted seeds and fewer viable seeds than recipients homozygous for the B allele. These results suggest that allelic variation at a single locus may have opposing effects on male and female reproductive success in C. unguiculata, and that trade-offs between the two types of reproductive success could contribute to the maintenance of variation at the Pgi-C1 locus.     

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.     

Effects of mycorrhizal infection and soil phosphorus availability on in vitro and in vivo pollen performance in Lycopersicon esculentum (Solanaceae)

The effects of mycorrhizal infection and soil P availability on in vitro and in vivo pollen performance were studied in two cultivars of tomato (Lycopersicon esculentum). In the first study, plants were grown in a greenhouse under three treatment combinations: nonmycorrhizal, low P (NMPO); nonmycorrhizal, high P (NMP3); and mycorrhizal, low P (MPO). Mycorrhizal infection and high soil P conditions significantly increased in vitro pollen tube growth rates but not percentage of germination. In addition, pollen from NMP3 and MPO plants sired significantly more seeds than pollen from NMPO plants in pollen mixture studies. In the second study, plants were grown initially in a greenhouse under two treatment combinations: NMPO and MPO. After all plants began to flower, they were placed in experimental arrays in the field. Under open pollination, pollen from MPO plants sired significantly more seeds than pollen from NMPO plants. This result was primarily attributed to increased flower production (and thus pollen production) in MPO plants. Thus, mycorrhizal infection and high soil P conditions can increase pollen quality (in vitro and in vivo pollen performance) as well as pollen quantity, thereby enhancing fitness through the male function. Anthocyanin production (used to determine paternity) also affected pollen performance.     

Is relative pollen production or removal a good predictor of relative male fitness? an experimental exploration with a wild strawberry (Fragaria virginiana,Rosaceae)

Understanding plant reproduction requires knowledge of genetic contributions through pollen and seeds. Since direct genetic assessments of fitness through pollen are often intractable, reproductive ecologists use components of male fitness such as pollen production and pollen removal as surrogates tot paternity. However, we know little of the strength of the relationship between these components and actual paternity. Here, I report on a study undertaken to examine the relationship of pollen production and removal with paternity in Fragaria virginiana, a wild strawberry. A morphological marker was used to track paternity in experimental arrays exposed to native pollinators. Relative pellen production proved to be a poor predictor of relative paternity in most arrays, and over all arrays there was no significant correlation between relative paternity and relative pellen production. In contrast, relative pollen removed correlated significantly and positively with proportion of seeds sired, suggesting that a plant's contribution to the pool of removed pellen is a good predictor of its male reproductive success. Deviations from expected paternity based on relative pollen removal suggest a systematic overestimation of the siring success of plants with low pollen removal. And, in at least one specific case, low pellen removal may be explained by delayed anther dehiscence, which could lower the effectiveness of the removed pellen.     

Selection on pollen donors by Echium vulgare (Boraginaceae)

Fewer than 25% of the ovules of Echium vulgare in the field develop into viable seeds, even with no pollen limitation. The loss of ovules could enhance the fitness of the maternal parent, if less fit embryos are selectively aborted. Two pollination experiments were performed to examine the selectivity of maternal parents on self-pollen and different cross-pollen sources. Pollinated with one pollen genotype per flower, self-pollen was, on average, as successful in siring seeds as cross-pollen. However, the relative success of self-pollen compared to outcross-pollen differed significantly among the maternal parents. These results suggest that, under certain conditions, selfing can produce a greater number of seeds than cross-pollination. Pollen donors differed significantly in outcrossing success. The plants that were more successful in selfing were also more successful pollen donors in outcross-pollinations. No significant interaction between maternal parent and paternal genotype was found. Pollinations with a pollen mixture produced selfed and outcrossed seeds in the same ratios as the single-donor experiment. Overall, only slight differences were found between the single- and mixed-donor experiments. Pollen tube growth did not show a significant correlation with the success of the parental genotypes in the mixed-donor experiment, indicating that pollen tube growth is not the determining factor controlling the paternity of the seeds. These results are discussed with reference to possible mediating mechanisms.     

GAMETOPHYTIC SELECTION IN RAPHANUS RAPHANISTRUM: A TEST FOR HERITABLE VARIATION IN POLLEN COMPETITIVE ABILITY

Competition among many microgametophytes for a limited number of ovules can lead to both nonrandom fertilization by pollen genotypes and selection for greater sporophytic vigor. The evolutionary implications of this process depend on the extent of heritable genetic variation for pollen competitive ability. Using flower color in wild radish as a genetic marker, we demonstrate differences among pollen donors in competitive ability. Significant differences were found in four out of five pairs of donors. For three pairs of donors, competitive differences were observed in certain maternal plants but not others. To test for heritability of pollen performance, we conducted a selection experiment. We manipulated the intensity of pollen competition for two generations and then tested for differences in the performance of pollen from two selected lines. Competitive ability of pollen derived from each line was assessed relative to a standard unrelated pollen donor, using pollen mixtures on six wild maternal plants. The intensity of previous pollen competition had no overall effect on the proportion of seeds sired by each selected line. In two maternal plants, pollen from intense previous competition was actually inferior, contrary to expectation. Thus, we found no evidence for heritable variation in this trait. Other factors, such as male-female interactions, may influence the outcome of pollen competition. Prevailing theory on the genetic basis of effects of pollen competition on subsequent generations is not supported by our results. Improved protocols for future experiments are discussed.     

Effects of foliar herbivory by insects on the fitness of Raphanus raphanistrum: damage can increase male fitness

Generally, effects of herbivory on plant fitness have been measured in terms of female reproductive success (seed production). However, male plant fitness, defined as the number of seeds sired by pollen, contributes half of the genes to the next generation and is therefore crucial to the evolution of natural plant populations. This is the first study to examine effects of insect herbivory on both male and female plant reproductive success. Through controlled field and greenhouse experiments and genetic paternity analysis, we found that foliar damage by insects caused a range of responses by plants. In one environment, damaged plants had greater success as male parents than undamaged plants. Neither effects on pollen competitive ability nor pollinator visitation patterns could explain the greater siring success of these damaged plants. Success of damaged plants as male parents appeared to be due primarily to changes in allocation to flowers versus seeds after damage. Damaged plants produced more flowers early in the season, but not more seeds, than undamaged plants. Based on total seed production, male fitness measures from the first third of the season, and flower production, we estimated that damaged and undamaged plants had equal total reproductive success at the end of the season in this environment. In a second, richer environment, damaged and undamaged plants had equal male and female plant fitness, and no traits differed significantly between the treatments. Equal total reproductive success may not be ecologically or evolutionarily equivalent if it is achieved differentially through male versus female fitness. Genes from damaged plants dispersed through pollen may escape attack from herbivores, if such attack is correlated spatially from year to year.     

Do differences in plant and flower age change mating patterns and alter offspring fitness in Raphanus sativus (Brassicaceae)?

When more pollen is present on stigmas than needed to fertilize all ovules, selection among pollen grains may occur due to effects of both pollen donors and maternal plants. We asked whether increasing plant age and flower age, two changes in maternal condition, altered the pattern of seed paternity after mixed pollination. We also asked whether changes in seed paternity affected offspring success in an experimental garden. While flower age did not affect seed paternity, there was a dramatic shift in pollen donor performance as plants aged. These differences were seen in the offspring as well, where the offspring of one pollen donor, which sired more seeds on young plants, flowered earlier in the season, and the offspring of another pollen donor, which sired more seeds on old plants, flowered later in the season. Thus, change in maternal condition resulted in altered seed paternity, perhaps because the environment for pollen tube growth was different. The pattern of seed paternity and offspring performance suggests that pollen donors may show temporal specialization.     

Why do stigmas move in a flexistylous plant?

Flexistyly is a recently documented stylar polymorphism involving both spatial and temporal segregation of sex roles within hermaphroditic flowers. Using the experimental manipulation of stigma movement in self-compatible Alpinia mutica, we tested the hypothesis that selection for reducing interference between male and female function drives the evolution and/or maintenance of stigma movement. In experimental arrays, anaflexistylous (protogynous) flowers served as pollen donors competing for mating opportunities on cataflexistylous (protandrous) flowers. The pollen donors were either manipulated so their stigmas could not move or were left intact, and their success was determined using allozymes to assess the paternity of recipient seeds. We found that manipulated flowers sired a significantly smaller proportion of seeds, showing that stigma movement in unmanipulated plants increased male fitness. This result was strongest under conditions in which pollen competition was expected to be highest, specifically when pollinators visited multiple donor plants before visiting recipient flowers.     

Effects of pollination timing on seed paternity and seed mass in Silene latifolia (Caryophyllaceae)

Background and Aims

Competition among genetically different pollen donors within one recipient flower may play an important role in plant populations, increasing offspring genetic diversity and vigour. However, under field conditions stochastic pollen arrival times may result in disproportionate fertilization success of the first-arriving pollen, even to the detriment of the recipient plant''s and offspring fitness. It is therefore critical to evaluate the relative importance of arrival times of pollen from different donors in determining siring success.

Methods

Hand pollinations and genetic markers were used to investigate experimentally the effect of pollination timing on seed paternity, seed mass and stigmatic wilting in the the dioecious plant Silene latifolia. In this species, high prevalence of multiply-sired fruits in natural populations suggests that competition among different donors may often take place (at fertilization or during seed development); however, the role of variation due to pollen arrival times is not known.

Key Results

First-arriving pollen sired significantly more seeds than later-arriving pollen. This advantage was expressed already before the first pollen tubes could reach the ovary. Simultaneously with pollen tube growth, the stigmatic papillae wilted visibly. Individual seeds were heavier in fruits where one donor sired most seeds than in fruits where both donors had more even paternity shares.

Conclusions

In field populations of S. latifolia, fruits are often multiply-sired. Because later-arriving pollen had decreased chances of fertilizing the ovules, this implies that open-pollinated flowers often benefit from pollen carry-over or pollinator visits within short time intervals, which may contribute to increase offspring genetic diversity and fitness.Key words: Reproduction, reproductive success, pollen, siring success, microsatellite DNA, paternity, pollen tube growth, seed mass, Silene alba, stigma wilting     

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.     

Floral sex allocation and reproductive success within inflorescences of Hosta ventricosa,a pseudogamous apomict

Aims Within inflorescences of sexually reproducing hermaphrodites, the production of ovules, fruits and seeds commonly declines from basal (early-opening) to distal (late-opening) flowers, while pollen production remains constant or only changes slightly, with the result that distal (late-opening) flowers become functionally male. However, few empirical studies have specifically examined whether or not changes in allocation to pollen production actually lead to changes in the number of seeds sired, a more direct measure of male fitness. In pseudogamous apomicts, fitness depends on the number of seeds produced; thus, a contrasting pattern of variation in the pollen-to-ovule (P/O) ratio within inflorescences might be expected.Methods We investigated floral sex allocation and reproductive success within racemes of Hosta ventricosa, a pseudogamous apomictic hermaphrodite possessing flowers that open acropetally. We quantified variations in pollen number, ovule number, the P/O ratio and fruit and seed production, from 2007 to 2011, among flowers within racemes of four populations of H. ventricosa in southwest China. Ecological causes for fruit and seed production were evaluated by observing patterns of pollen deposition, flower removal and supplemental pollination.Important findings Pollen number, ovule number and the P/O ratio declined from basal-to-distal positions in all sampled populations (years). Fruit and seed production decreased distally in most populations (years). Low fruit and seed set of distal flowers was not due to pollen limitation because pollen deposition never declined distally and the low fruit and seed set of distal flowers remained even after supplementary pollination was provided. The flower-removal experiment indicated that inter-fruit competition for resources among flowers was common. The low P/O ratio of distal flowers in H. ventricosa might be favored because they were unable to obtain fitness by donating pollen and siring seeds on other plants. Our study may help to understand the adaptive significance of sex allocation among flowers within inflorescences of sexually reproducing hermaphrodites.     

THE EFFECT OF FLOWER PRODUCTION ON MALE REPRODUCTIVE SUCCESS IN WILD RADISH POPULATIONS

Flower production is the major determinant of pollen yield and an important component in pollinator attraction. Consequently differences among plants in flower production are expected to have a substantial impact on their relative success at fathering seed. We examined this prediction using one natural and three structured populations of wild radish. We found that a plant's relative success at fathering seed on another plant in the population (male fertility) increased with flower production. Nonetheless, the increase in fertility exhibited a diminishing marginal gain, with the relationship varying among populations. The relationship between the estimates of total number of seeds sired and flower production varied substantially among the populations examined, ranging from a weakly linear to strongly negative quadratic. Not surprisingly, the spatial structure of the population with respect to seed yield had a powerful effect on the total number of seeds sired because male fertility decreased exponentially with intermate distance. This exponential relationship occurred in all populations examined. Other covariates important to male fertility were flower color, time, the specific identity of the male parent, and male by female interaction. The identity of the male parent consistently accounted for a large portion of the variation in male fertility, indicating that other unmeasured features of the plant influenced its success.     

Mechanisms of differential pollen donor performance in wild radish, Raphanus sativus (Brassicaceae)

In order to understand the characters on which sexual selection might operate in plants, it is critical to assess the mechanisms by which pollen competition and mate choice occur. To address this issue we measured a number of postpollination characters, ranging from pollen germination and pollen tube growth to final seed paternity, in wild radish. Crosses were performed using four pollen donors on a total of 16 maternal plants (four each from four families). Maternal plants were grown under two watering treatments to evaluate the effects of maternal tissue on the process of mating. The four pollen donors differed significantly in number of seeds sired and differed overall in the mating characters measured. However, it was difficult to associate particular mechanistic characters with ability to sire seeds, perhaps because of interactions among pollen donors within styles or among pollen donors and maternal plants. The process of pollen tube growth and fertilization differed substantially among maternal watering treatments, with many early events occurring more quickly in stressed plants. Seed paternity, however, was somewhat more even among pollen donors used on stressed maternal plants, suggesting that when maternal tissue is more competent, mating is slowed and is more selective.     

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.     

ADAPTIVE SIGNIFICANCE OF FLOWER COLOR AND INTER-TRAIT CORRELATIONS IN AN IPOMOPSIS HYBRID ZONE

Flower color is often viewed as a trait that signals rewards to pollinators, such that the relationship between flower color and plant fitness might result from its association with another trait. We used experimental manipulations of flower color and nectar reward to dissociate the natural character correlations present in a hybrid zone between Ipomopsis aggregata and Ipomopsis tenuituba. Isozyme markers were used to follow the male and female reproductive success of these engineered phenotypes. One field experiment compared fitnesses of I. aggregata plants that varied only in flower color. Plants with flowers painted red received more hummingbird visits and sired more seeds than did plants with flowers painted pink or white to match those of hybrids and I. tenuituba. Our second field experiment compared fitnesses of I. aggregata, I. tenuituba, and hybrid plants in an unmanipulated array and in a second array where all flowers were painted red. In the unmanipulated array, I. aggregata received more hummingbird visits, set more seeds per flower, and sired more seeds per flower. These fitness differences largely disappeared when the color differences were eliminated. The higher male fitness of I. aggregata was due to its very high success at siring seeds on conspecific recipients. On both I. tenuituba and hybrid recipients, hybrid plants sired the most seeds, despite showing lower pollen fertility than I. aggregata in mixed donor pollinations in the greenhouse. Ipomopsis tenuituba had a fitness of only 13% relative to I. aggregata when traits varied naturally, compared to a fitness of 36% for white relative to red flowers when other traits were held constant.