POLLINATION DYNAMICS IN EPILOBIUM CANUM (ONAGRACEAE): CONSEQUENCES FOR GAMETOPHYTIC SELECTION

Intraspecific variation in realized dispersal probability and host quality shape nectar microbiomes

Epiphytic microbes frequently affect plant phenotype and fitness, but their effects depend on microbe abundance and community composition. Filtering by plant traits and deterministic dispersal-mediated processes can affect microbiome assembly, yet their relative contribution to predictable variation in microbiome is poorly understood. We compared the effects of host-plant filtering and dispersal on nectar microbiome presence, abundance, and composition. We inoculated representative bacteria and yeast into 30 plants across four phenotypically distinct cultivars of Epilobium canum. We compared the growth of inoculated communities to openly visited flowers from a subset of the same plants. There was clear evidence of host selection when we inoculated flowers with synthetic communities. However, plants with the highest microbial densities when inoculated did not have the highest microbial densities when openly visited. Instead, plants predictably varied in the presence of bacteria, which was correlated with pollen receipt and floral traits, suggesting a role for deterministic dispersal. These findings suggest that host filtering could drive plant microbiome assembly in tissues where species pools are large and dispersal is high. However, deterministic differences in microbial dispersal to hosts may be equally or more important when microbes rely on an animal vector, dispersal is low, or arrival order is important.

Breeding systems of naturalized versus indigenous species provide support for Baker's law on Pohnpei island

The factors that facilitate successful colonization of islands should be especially evident where the establishment filter is strongest. Colonizers of small, remote oceanic islands should be initially rare, extremely mate-limited and often without pollinators. Hence, plant communities on such islands should reflect an establishment history in which young 'naturalized' species are most likely to display self-compatibility and autonomous selfing, whereas 'indigenous' species may exhibit more diverse reproductive strategies. To test this prediction, we characterized breeding systems of 28 species on Pohnpei, in the Federated States of Micronesia, a group of remote Pacific islands that are considered a global biodiversity hotspot. Three families with both naturalized and indigenous species were selected-Fabaceae, Malvaceae and Melastomataceae. Measurements included field observations of dichogamy/herkogamy and floral attraction traits, pollen:ovule (P:O) ratios and experimental hand-pollinations for self-compatibility and pollen limitation. Phylogenetic generalized least squares analyses tested for trait correlations between naturalized and indigenous species. Flowers of all 28 species were bisexual, and pollinator attraction features were common. Pollen:ovule ratios ranged from 9 to 557 (median = 87), and all 11 hand-pollinated species were self-compatible. All species had >5 ovules and <3500 pollen grains per flower. Indigenous species did not differ significantly from naturalized species for any trait. There is a dearth of data from remote islands bearing on the question of establishment history. In this study, we inferred all species to have some degree of autogamy and indigenous species were no more likely than naturalized species to display outcrossing mechanisms. On Pohnpei, high ovule numbers, and the inaccessibility of wind pollination and obligate outcrossing strategies, reflect the importance of retaining reproductive assurance mechanisms in the face of pollinator uncertainty.

Microbial abundance, composition, and function in nectar are shaped by flower visitor identity

Microbial dispersal is essential for establishment in new habitats, but the role of vector identity is poorly understood in community assembly and function. Here, we compared microbial assembly and function in floral nectar visited by legitimate pollinators (hummingbirds) and nectar robbers (carpenter bees). We assessed effects of visitation on the abundance and composition of culturable bacteria and fungi and their taxonomy and function using shotgun metagenomics and nectar chemistry. We also compared metagenome-assembled genomes (MAGs) of Acinetobacter, a common and highly abundant nectar bacterium, among visitor treatments. Visitation increased microbial abundance, but robbing resulted in 10× higher microbial abundance than pollination. Microbial communities differed among visitor treatments: robbed flowers were characterized by predominant nectar specialists within Acetobacteraceae and Metschnikowiaceae, with a concurrent loss of rare taxa, and these resulting communities harbored genes relating to osmotic stress, saccharide metabolism and specialized transporters. Gene differences were mirrored in function: robbed nectar contained a higher percentage of monosaccharides. Draft genomes of Acinetobacter revealed distinct amino acid and saccharide utilization pathways in strains isolated from robbed versus pollinated flowers. Our results suggest an unrecognized cost of nectar robbing for pollination and distinct effects of visitor type on interactions between plants and pollinators. Overall, these results suggest vector identity is an underappreciated factor structuring microbial community assembly and function.

Evolution of development of pollen performance

With the origin of pollination in ancient seed plants, the male gametophyte ("pollen") began to evolve a new and unique life history stage, the progamic phase, a post-pollination period in which pollen sexual maturation occurs in interaction with sporophyte-derived tissues. Pollen performance traits mediate the timing of the fertilization process, often in competition with other pollen, via the speed of pollen germination, sperm development, and pollen tube growth. Studies of pollen development rarely address the issue of performance or its evolution, which involves linking variation in developmental rates to relative fitness within populations or to adaptations on a macroevolutionary scale. Modifications to the pollen tube pathway and changes in the intensity of pollen competition affect the direction and strength of selection on pollen performance. Hence, pollen developmental evolution is always contextual-it involves both the population biology of pollen reaching stigmas and the co-evolution of sporophytic traits, such as the pollen tube pathway and mating system. For most species, performance evolution generally reflects a wandering history of periods of directional selection and relaxed selection, channeled by developmental limitations, a pattern that favors the accumulation of diversity and redundancy in developmental mechanisms and the genetic machinery. Developmental biologists are focused on finding universal mechanisms that underlie pollen function, and these are largely mechanisms that have evolved through their effects on performance. Here, we suggest ways in which studies of pollen performance or function could progress by cross-fertilization between the "evo" and "devo" fields.

PARENTAL EFFECTS ON SEED DEVELOPMENT AND SEED YIELD IN RAPHANUS RAPHANISTRUM: IMPLICATIONS FOR NATURAL AND SEXUAL SELECTION

The possibility that sexual selection operates in angiosperms to effect evolutionary change in polygenic traits affecting male reproductive success requires that there is additive genetic variance for these traits. I applied a half-sib breeding design to individuals of the annual, hermaphroditic angiosperm, wild radish (Raphanus raphanistrum: Brassicaceae), to estimate paternal genetic effects on, or, when possible, the narrow-sense heritability of several quantitative traits influencing male reproductive success. In spite of significant differences among pollen donors with respect to in vitro pollen tube growth rates, I detected no significant additive genetic variance in male performance with respect to the proportion of ovules fertilized, early ovule growth, the number of seeds per fruit, or mean individual seed weight per fruit. In all cases, differences among maternal plants in these traits far exceeded differences among pollen donors. Abortion rates of pollinated flowers and fertilized ovules also differed more among individuals as maternal plants than as pollen donors, suggesting strong maternal control over these processes. Significant maternal phenotypic effects in the absence of paternal genetic or phenotypic effects on reproductive traits may be due to maternal environmental effects, to non-nuclear or non-additive maternal genetic effects, or to additive genetic variance in maternal control over offspring development, independent of offspring genotype. While I could not distinguish among these alternatives, it is clear that, in wild radish, the opportunity for natural or sexual selection to effect change in seed weight or seed number per fruit appears to be greater through differences in female performance than through differences in male performance.

Pollen performance traits reveal prezygotic nonrandom mating and interference competition in Arabidopsis thaliana

PREMISE

The lack of ability to measure pollen performance traits in mixed pollinations has been a major hurdle in understanding the mechanisms of differential success of compatible pollen donors. In previous work, we demonstrated that nonrandom mating between two accessions of Arabidopsis thaliana, Columbia (Col) and Landsberg (Ler), is mediated by the male genotype. Despite these genetic insights, it was unclear at what stage of reproduction these genes were acting. Here, we used an experimental strategy that allowed us to differentiate different pollen populations in mixed pollinations to ask: (1) What pollen performance traits differed between Col and Ler accessions that direct nonrandom mating? (2) Is there evidence of interference competition?

METHODS

We used genetically marked pollen that can be visualized colorimetrically to quantify pollen performance of single populations of pollen in mixed pollinations. We used this and other assays to measure pollen viability, germination, tube growth, patterns of fertilization, and seed abortion. Finally, we assessed interference competition.

RESULTS

In mixed pollinations on Col pistils, Col pollen sired significantly more seeds than Ler pollen. Col pollen displayed higher pollen viability, faster and greater pollen germination, and faster pollen tube growth. We saw no evidence of nonrandom seed abortion. Finally, we found interference competition occurs in mixed pollinations.

CONCLUSION

The lack of differences in postzygotic processes coupled with direct observation of pollen performance traits indicates that nonrandom mating in Arabidopsis thaliana is prezygotic, due mostly to differential pollen germination and pollen tube growth rates. Finally, this study unambiguously demonstrates the existence of interference competition.

Can selection on a male mating character result in evolutionary change? A selection experiment on California wild radish, Raphanus sativus

PREMISE OF THE STUDY

Whenever more pollen grains arrive on stigmas than necessary to fertilize ovules, sexual selection is possible. However, the role of sexual selection remains controversial, in part because of lack of evidence on genetic bases of traits and the response of relevant characters to selection.

METHODS

In an experiment with Raphanus sativus, we selected on tendency to sire seeds in the stylar or basal regions of fruits. This character is likely related to pollen tube growth rate, and seed position affects rates of abortion and seed predation. We measured differences among families in seed siring and related characters and evaluated responses to selection.

KEY RESULTS

All replicates showed strong effects of pollen donor family on proportion of seeds sired per fruit in mixed pollinations. Most also showed effects of pollen donor family on number of pollen grains per flower and pollen diameter. Two of four replicates showed a response to selection on position of seeds sired. In responding replicates, we found trade-offs in pollen grain size and number; plants with larger pollen grains sired more seeds in the basal region.

CONCLUSIONS

Our data suggest a genetic basis for pollen donor ability to sire seeds in competition. The significant response to selection in two replicates shows that position of seeds sired can respond to selection. Thus, all components for sexual selection to occur and affect traits are present. Variation in results among replicates might be due to changes in greenhouse conditions. Environmental effects may contribute to the maintenance of variation in these fitness-related characters.

The influence of herbivory on pre- and postzygotic stages of reproduction following open, self, and outcross pollination

PREMISE OF THE STUDY

Herbivory affects pollination success and reproductive output in plants. However, the different stages in the process from pollination to seed maturation have hardly been investigated within the context of herbivory. Herbivory might affect these stages via its effect on geitonogamous pollination and thereby the proportion of self pollen delivered to the stigma and/or via its effect on the nutritional capacity of the maternal plant.

METHODS

Plants of monocarpic Cynoglossum officinale were experimentally subjected to root herbivory and exposed to natural open pollination in combination with self and outcross hand pollination. We quantified pollen germination, pollen tube competition intensity, pollen tube attrition, fruit set, and seed initiation, abortion, and maturation.

KEY RESULTS

Although root herbivory did not affect pollen germination or pollen tube attrition, fruit set and seed initiation and maturation were negatively affected by herbivory, but for seed initiation only in the case of outcross- and open-pollinated flowers. The intensity of pollen tube competition positively affected seed initiation, but only in plants infested with the herbivore.

CONCLUSIONS

Our study demonstrates that herbivory did not affect the early stages following pollination, but significantly impacted later postpollination stages such as fruit set and seed maturation and selection based on pollen tube competition intensity on zygote development. Our findings suggest that decreased nutritional capacity of the mother plant in response to root herbivory rather than herbivory effects on pollen quality was responsible for the lower fruit and seed production in infested plants.

Pollen Performance in Clarkia Taxa with Contrasting Mating Systems: Implications for Male Gametophytic Evolution in Selfers and Outcrossers

We tested three predictions regarding the joint evolution of pollen performance and mating system. First, due to the potential for intense intrasexual competition in outcrossing populations, we predicted that outcrossers would produce faster-growing pollen than their selfing relatives. Second, if elevated competition promotes stronger selection on traits that improve pollen performance, then, among-plant variation in pollen performance would be lower in outcrossers than in selfers. Third, given successive generations of adaptation to the same maternal genotype in selfers, we predicted that, in selfing populations (but not in outcrossing ones), pollen would perform better following self- than cross-pollinations. We tested these predictions in field populations of two pairs of Clarkia (Onagraceae) sister taxa. Consistent with our predictions, one outcrosser (C. unguiculata) exhibited faster pollen germination and less variation in pollen tube growth rate (PTGR) among pollen donors than its selfing sister species, C. exilis. Contrary to our predictions, the selfing C. xantiana ssp. parviflora exhibited faster PTGR than the outcrossing ssp. xantiana, and these taxa showed similar levels of variation in this trait. Pollen performance following self- vs. cross-pollinations did not differ within either selfing or outcrossing taxa. While these findings suggest that mating system and pollen performance may jointly evolve in Clarkia, other factors clearly contribute to pollen performance in natural populations.

Persistently low fruiting success in the Mediterranean pipevine Aristolochia baetica (Aristolochiaceae): a multi-year study

Low fruit set is common in many plant species and may be caused by a variety of factors, such as predation, resource limitation or deficient pollination, or it may be an evolutionary strategy. In this paper, we investigate factors that affect fruit set in Aristolochia baetica (Aristolochiaceae), a Mediterranean pipevine found in southwest Spain. Fruit production was monitored in two populations over 4 years (2002-2005), and the causes of flower or fruit loss were determined. Experimental hand-pollinations were performed, and germinated pollen grains on the stigmas of open-pollinated flowers were quantified. Fruit set was always very low (4-14%). Floral abscission initially reduced reproductive output by more than 50%; then herbivory (6-12%) and fruit abortion (8-26%) caused further reductions. Given that the number of efficiently pollinated flowers was always higher than that of ripe fruits, and that xenogamous hand-pollination did not increase fruit set in relation to open-pollination, the final fruit production of A. baetica seems not to be pollen-limited. Fruit abortion of effectively pollinated flowers supports the idea that resource availability limits fruit set. In A. baetica, fruit abortion could lead to mate selection of the best quality fruits. Moreover, the initiated fruits that finally abort could also satiate predators, contributing to increase progeny fitness.

Multiple pollinator visits to Mimulus ringens (Phrymaceae) flowers increase mate number and seed set within fruits

The timing and effectiveness of pollinator visitation to flowers is an important factor influencing mating patterns and reproductive success. Multiple pollinator probes to a flower may increase both the quantity and genetic diversity of progeny, especially if single probes deposit insufficient pollen for maximal seed set or if the interval between probes is brief. When pollen carryover is limited, sequential pollen loads may also differ markedly in sire representation. We hypothesized that these conditions help explain high levels of multiple paternity in Mimulus ringens fruits. We documented all bee visits to individual flowers, quantified resulting seed set, and determined paternity for 20 seeds per fruit. Most (76%) flowers received multiple probes, and the interval between probes was usually <30 min. Flowers probed multiple times produced 44% more seeds than flowers probed once. All fruits were multiply sired. Flowers receiving a single probe averaged 3.12 outcross sires per fruit, indicating that single probes deposit pollen from several donors. Multiple paternity was even greater after three or more probes (4.92 outcross sires), demonstrating that sequential visits bring pollen from donors not represented in the initial probe.

Patterns of multiple paternity in fruits of Mimulus ringens (Phrymaceae)

Multiply sired fruits provide unambiguous evidence that pollen from two or more donors was deposited on a stigma and successfully fertilized ovules. Such multiple paternity within fruits can have important consequences for both parental and offspring fitness, but little is known about the frequency of multiple paternity or the mechanisms causing it. In this study we quantify the extent of multiple paternity in replicate experimental arrays of Mimulus ringens (square-stem monkeyflower) and use observations of pollinator behavior to infer mechanisms generating multiply sired fruits. In each array, floral displays were trimmed to two, four, eight, or 16 flowers per plant to span the range of display sizes observed in nature. In our sample of 204 fruits, more than 95% had two or more outcross pollen donors. The number of sires per fruit averaged 4.63 ± 0.10 (mean ± 1 SE), including selfs, and did not vary significantly with floral display treatment. Patterns of bumble bee foraging, combined with limited pollen carryover, suggest that observed levels of multiple paternity cannot be fully explained by single probes that deposited mixed pollen loads. Multiple probes to flowers, each delivering pollen from 1-3 different sires, are more likely to have caused the observed patterns. These sequential visits may reduce the potential for pollen competition and female choice based on pollen tube growth rate.

Does pollen competition reduce the cost of inbreeding?

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

Distribution ecology of pollen tubes: fine-grained, labile spatial mosaics in southern Spanish Lamiaceae

•  Patterns of intraspecific variation in the number of pollen tubes per style in naturally pollinated plants are poorly known, yet that information is essential for assessing the frequency of occurrence and evolutionary implications of microgametophyte competition in the wild. •  This paper analyses intraspecific variation in the number of pollen tubes per style for six species of southern Spanish insect-pollinated Lamiaceae (Ballota hirsuta, Lavandula latifolia, Marrubium supinum, Phlomis lychnitis, Rosmarinus officinalis and Teucrium rotundifolium) differing in growth form, phenology, flower size and pollinators. •  Number of pollen tubes exceeded number of fertilizable ovules in 26-87% of styles, and mean number per pollen tube of other pollen tubes in the same style varied between five and 12. Within-plant variation in pollen tube number was extensive in all species, accounting for 68-92% of total population-level variance. In L. latifolia mean pollen tube number per style differed between populations and between plants within populations, but such differences were not consistent among years. •  It is concluded that opportunities for microgametophyte competition and selection on competitive ability are considerable in all species studied, although extreme spatial fine-graininess and marked stochasticity in the variation of pollen tube numbers, including temporal inconstancy of individual differences, will greatly reduce the opportunity for selection on sporophytic characters influencing degree of microgametophyte competition.

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.

Effects of pollen load size and composition on pollen donor performance in wild radish, Raphanus sativus (Brassicaceae)

A critical concern in the debate over the importance of sexual selection in plants is whether the nonrandom mating demonstrable in greenhouse crosses can occur in the field. Field populations likely experience smaller and more variable pollen load sizes than those that have been used in many greenhouse experiments. Therefore, we performed a greenhouse experiment in which we varied both pollen load size and composition in wild radish, Raphanus sativus, and examined the paternity of seeds. We used five maternal plants and four pairs of pollen donors. We were able to produce pollen loads of 40, 118, and 258 grains per stigma. The smallest of the pollen loads was scant enough to result in a slight, but significant reduction in seed number per fruit. While variation in pollen load composition significantly affected the proportions of seeds fathered by different donors, variation in pollen load size did not. The relative performance of different donors was constant across pollen load sizes, suggesting that, for this species, differential performance of pollen donors can occur at pollen load sizes that are likely to occur in field populations.

Variability in Pollen Load: Implications for Reproduction and Seedling Vigor in a Rare Plant, Silene douglasii var. Oraria

Habitat fragmentation can markedly influence the levels of pollen deposition and seed production in natural populations, and rare plants may be especially susceptible to any associated reductions in pollen quantity and quality. In order to ascertain the potential for pollen limitation of maternal fitness in a rare plant, Silene douglasii var. oraria, which is endemic to western coastal prairies, we counted ovules and measured conspecific and heterospecific pollen deposition on stigmas collected from open-pollinated plants. We further investigated the effect of increasing pollen intensity on fruit production, seed number and weight, as well as several measures of progeny vigor. Three levels of outcross pollen were added to plant stigmas for comparison with autogamous and open pollination in the largest naturally occurring population. Both seed and fruit production were significantly greater (P<0.05) for supplemented versus nonsupplemented stigmas, but flowers receiving different levels of pollen addition were statistically indistinguishable. Seed germination and seedling survival were also lowest for the offspring of nonsupplemented flowers; however, in natural populations, opportunities for pollen competition are very limited since open-pollinated flowers averaged fewer viable pollen grains than ovules. Seed production was equivalent for open- and autogamously pollinated flowers in 1996, indicating that natural pollen transfer may have involved mostly self pollen. Overall, the low reproductive success of var. oraria likely reflects both low pollen quantity and quality. Multiyear empirical studies of pollen intensity in field populations are needed so that we can better understand the fitness consequences of pollen limitation in rare perennials.

Seed abortion in Pongamia pinnata (Fabaceae)

In Pongamia pinnata only one of the two ovules develops into a seed in most of the pods. Since pollen was not found to be limiting and reduced fertilization could not completely explain the observed frequency of seed abortion, it implied an effect of postfertilization factors. Aqueous extracts of developing seeds and maternal tissue (placenta) did not influence abortion in vitro, suggesting that abortion may not be mediated by a chemical. Experimental uptake of (14)C sucrose in vitro indicated that both the stigmatic and the peduncular seed have similar inherent capacities of drawing resources, but the peduncular seed is deprived of resources in the presence of the stigmatic seed. This deprivation of the peduncular seed could be offset by supplying an excess of hormones leading to the subsequent formation of two seeds in a pod. The prevalence of single-seeded pods in P. pinnata seems therefore to be a result of competition between the two seeds for maternal resources. The evolutionary significance of single-seeded pods in P. pinnata is discussed with respect to possible dispersal advantage enjoyed by such pods.

How important is biotic pollination and dispersal to the success of the angiosperms?

The rise to dominance of the angiosperms has frequently been hypothesized to be due to reproductive innovations, especially those involving coevolution with biotic gene dispersers. Wind pollination is considered inefficient in comparison with insect pollination. The scope for speciation is considered to be greater when biotic gene dispersers are involved. The biogeographical restriction of extant conifers to stressful environments is also considered to be due to the inefficiency of wind pollination in species-rich environments. We consider both fossil and contem porary evidence and conclude that biotic gene dispersal has not been as im portant as have innovations affecting growth rates in explaining the rise of angiosperms. We conclude that differences in growth rates, especially in the regeneration phase, can explain the rise of the angiosperms and the extant biogeography of gymnosperms.

Limitations on reproduction in alpine Ranunculus acris

This study examines experimentally the influence of limiting factors on seed to ovule ratio and seed weight in three alpine populations of the perennial herb Ranunculus acris L. at Finse, southwest Norway. To test for pollen and resource limitation, seed production of supplementally pollinated or completely defoliated plants was compared to that of untreated plants. In one population the experiment was repeated twice during the season. In mid-season, supplemental hand-pollination had no significant effect on seed to ovule ratio, but significantly increased the seed weight, whereas complete defoliation resulted in a significant decrease in seed to ovule ratio and a marginally significant decrease in seed weight. These results indicate that pollen limitation occurred on seed weight and resource limitation mainly on seed to ovule ratio. Seed production in late-flowering control and defoliated plants in one population was nearly completely absent, whereas supplemental pollination increased seed production substantially. Most seeds were produced at the lowest elevation, and frost damage on developing seeds was most pronounced at the highest elevation population, suggesting that severe weather conditions at the highest altitudes had a negative influence on reproductive processes. Significant positive relationships between seed number and seed weight were found, possibly reflecting a large among-plant variation in total resource availability. Key words: altitude gradient, climate, defoliation, pollen and resource availability, seasonal variation, seed size.

Demographics of pollen tube growth inCucurbita pepo

Pollen tube growth rates and pollen tube attrition were examined in Cucurbita pepo plants grown in an experimental garden. Two separate studies were performed utilizing controlled hand-pollinations. First, female flowers were hand-pollinated and collected at intervals over a 22-h period. Examination of stained pollen tubes in these pistils under reflected UV light revealed variation in growth rate: after approximately 7 h, the fastest growing pollen tubes grew at nearly twice the rate of the mass of the pollen tubes. The faster growing tubes reached the top of the ovary well ahead of most of the pollen tubes. Second, density of pollen application was varied and counts of germinated pollen and pollen tubes were made at four points from the stigmatic surface to the receptacle. Attrition was high for all pollen densities, but significantly higher rates occurred at high and medium densities just below the stigmatic surface. Pollen tube attrition at other points in the pistil was associated with constrictions in the stylar tissue and possibly with levels of nutrients in the pistil. Key words: Cucurbita pepo, pollen competition, pollen tube attrition.

Sporophytic response to pollen selection for Alachlor tolerance in maize

In order to assess the efficiency of male gametophytic selection (MGS) for crop improvement, pollen selection for tolerance to herbicide was applied in maize. The experiment was designed to test the parallel reactivity to Alachlor of pollen and plants grown in controlled conditions or in the field, the response to pollen selection in the sporophytic progeny, the response to a second cycle of MGS, and the transmission of the selected trait to the following generations. The results demonstrated that pollen assay can be used to predict Alachlor tolerance under field conditions and to monitor the response to selection. A positive response to selection applied to pollen in the sporophytic progeny was obtained in diverse genetic backgrounds, indicating that the technique can be generally included in standard breeding programs; the analysis of the data produced in a second selection cycle indicated that the selected trait is maintained in the next generation.

Pollen selection

In recent years a large number of reports have been published hinting about the possible role of selection at the male gametophyte level in natural conditions and its possible use as a tool in plant breeding programs. In this work, the evidence that has so far emerged on gametophytic selection is reviewed, and its relationship to pollen competition under natural conditions is analyzed. Information supporting an overlap in the genetic expression between the gametophytic and sporophytic phases is also reported, mainly in terms of isozymes and RNA hybridization. The correspondence between both phases is reviewed through their common response to different external agents such as temperature, salinity, metals, herbicides, fungal toxins, and other factors. Finally, the implications that pollen selection may have in plant breeding are evaluated.

Evolution of Pollen Morphology

Over evolutionary time, the morphology of angiosperm pollen has evolved toward an increasing number of apertures, among other things. From a neo-Darwinian point of view, this means that (i) some polymorphism for aperture number must exist and (ii) there must be some fitness increase associated with increasing the aperture number. Pollen types with different aperture numbers often occur in the same species. Such is the case in Viola diversifolia. Comparison of pollen with three and four apertures in this species showed that four-apertured grains germinated faster than three-apertured ones but that the four-apertured ones experienced other disadvantages. These results obtained on the gametophyte can be interpreted in terms of strategies of the sporophyte.

Effects of parentage, prior fruit set and pollen load on fruit and seed production in Campanula americana L

We conducted a controlled crossing experiment to examine the effects of maternal and paternal parentage, the size of the pollen load, and prior fruit production on the proportion of flowers that set fruit, seed number per fruit and seed weight in a natural population of Campanula americana. Effects due to the maternal parent were large for all measures of fruit and seed production, while the paternal parent had a significant effect only upon mean seed weight. As the number of prior fruits on the maternal plant increased the probability that a flower would produce a mature fruit, the number of seeds per fruit, and total seed weight per fruit all decreased. We found no effect of the size of the pollen loads used in this study on fruit or seed production. These results are consistent with those of other studies that suggest in natural plant populations maternal effects, especially environmental maternal effects, can have an overwhelming effect on fruit and seed production and on seed characteristics.

EFFECTS OF POLLINATION INTENSITY IN CAMPSIS RADICANS

I examined effects of pollination intensity on fruit, seed and seedling characteristics in Campsis radicans, and joint effects of pollen donor and pollination intensity on fruit production. Large pollen loads were more likely to initiate fruit production than small pollen loads, and the former fruits contained more seeds and a greater total seed mass. No further increases in seed number or mass occurred for pollen loads above 4,000 grains. The weight of individual seeds was unaffected by pollen load. Effects of pollen donor were generally larger than effects of pollen load, and fruit production from small loads of pollen from one donor were sometimes equal to fruit production from larger pollen loads from another donor. The ratio of pollen grains deposited to resultant seeds increased with pollen load, and several explanations are proposed. Seeds from heavy pollinations emerged better than seeds from light pollinations, but did not differ in speed of germination or in the performance of seedlings up to 126 days. The emergence differences are probably due to differing intensities of pollen tube competition.