RELATIVE SUCCESS OF SELF AND OUTCROSS POLLEN COMPARING MIXED- AND SINGLE-DONOR POLLINATIONS IN AQUILEGIA CAERULEA

Pollination effectiveness in a generalist plant: adding the genetic component

The pollination effectiveness of a flower visitor has traditionally been measured as the product of a quantity component that depends on the frequency of interaction and a quality component that measures the per-visit effects on plant reproduction. We propose that this could be complemented with a genetic component informing about each pollinator's contribution to the genetic diversity and composition of the plant progeny. We measured the quantity and quality components of effectiveness of most pollinator functional groups of the generalist herb Erysimum mediohispanicum. We used 10 microsatellite markers to calculate the genetic component as the diversity of sires among siblings and included it into the calculation of the pollination effectiveness. Functional groups varied in the quantity and quality components, which were shown to be decoupled. Functional groups also differed in the genetic component. This component changed the estimates of pollination effectiveness, increasing the differences between some functional groups and modifying the pollination effectiveness landscape. We demonstrate that including the genetic component in the calculation of the pollination effectiveness may allow a more complete quantification of the contribution of each pollinator to the reproductive success of a plant, providing information on its mating patterns and long-term fitness.

Evaluation of pollinator effectiveness based on pollen deposition and seed production in a gynodieocious alpine plant, Cyananthus delavayi

Examining variations in pollinator effectiveness can enhance our understanding of how pollinators and plants interact. Pollen deposition and seed production after a single visit by a pollinator are often used to estimate pollinator effectiveness. However, seed production is not always directly related to pollen deposition because not all pollen grains that are deposited on a stigma are compatible or conspecific. In the field, we tested pollinator effectiveness based on pollen deposition and the resulting seed production after single visits by different pollinator groups in a gynodieocious alpine plant Cyananthus delavayi (Campanulaceae). Our results showed that mean pollen deposition was generally inconsistent with mean seed production when comparisons were performed among different pollinator groups and sexes. In general, the correlations were not significant between pollen deposition and seed production in both perfect and female flowers after single visits by halictid bees, bumble bees, and hoverflies. We suggest seed set of virgin flowers after single visits is a more reliable indicator of pollinator effectiveness than pollen deposition and would be a better indicator of pollinator effectiveness for future studies.

Does the morphological fit between flowers and pollinators affect pollen deposition? An experimental test in a buzz-pollinated species with anther dimorphism

Some pollination systems, such as buzz‐pollination, are associated with floral morphologies that require a close physical interaction between floral sexual organs and insect visitors. In these systems, a pollinator's size relative to the flower may be an important feature determining whether the visitor touches both male and female sexual organs and thus transfers pollen between plants efficiently. To date, few studies have addressed whether in fact the “fit” between flower and pollinator influences pollen transfer, particularly among buzz‐pollinated species. Here we use Solanum rostratum, a buzz‐pollinated plant with dimorphic anthers and mirror‐image flowers, to investigate whether the morphological fit between the pollinator's body and floral morphology influences pollen deposition. We hypothesized that when the size of the pollinator matches the separation between the sexual organs in a flower, more pollen should be transferred to the stigma than when the visitor is either too small or too big relative to the flower. To test this hypothesis, we exposed flowers of S. rostratum with varying levels of separation between sexual organs, to bumblebees (Bombus terrestris) of different sizes. We recorded the number of visits received, pollen deposition, and fruit and seed production. We found higher pollen deposition when bees were the same size or bigger than the separation between anther and stigma within a flower. We found a similar, but not statistically significant pattern for fruit set. In contrast, seed set was more likely to occur when the size of the flower exceeded the size of the bee, suggesting that other postpollination processes may be important in translating pollen receipt to seed set. Our results suggest that the fit between flower and pollinator significantly influences pollen deposition in this buzz‐pollinated species. We speculate that in buzz‐pollinated species where floral morphology and pollinators interact closely, variation in the visitor's size may determine whether it acts mainly as a pollinator or as a pollen thief (i.e., removing pollen rewards but contributing little to pollen deposition and fertilization).

Attack of the clones: reproductive interference between sexuals and asexuals in the Crepis agamic complex

Negative reproductive interactions are likely to be strongest between close relatives and may be important in limiting local coexistence. In plants, interspecific pollen flow is common between co-occurring close relatives and may serve as the key mechanism of reproductive interference. Agamic complexes, systems in which some populations reproduce through asexual seeds (apomixis), while others reproduce sexually, provide an opportunity to examine effects of reproductive interference in limiting coexistence. Apomictic populations experience little or no reproductive interference, because apomictic ovules cannot receive pollen from nearby sexuals. Oppositely, apomicts produce some viable pollen and can exert reproductive interference on sexuals by siring hybrids. In the Crepis agamic complex, sexuals co-occur less often with other members of the complex, but apomicts appear to freely co-occur with one another. We identified a mixed population and conducted a crossing experiment between sexual diploid C. atribarba and apomictic polyploid C. barbigera using pollen from sexual diploids and apomictic polyploids. Seed set was high for all treatments, and as predicted, diploid-diploid crosses produced all diploid offspring. Diploid-polyploid crosses, however, produced mainly polyploidy offspring, suggesting that non-diploid hybrids can be formed when the two taxa meet. Furthermore, a small proportion of seeds produced in open-pollinated flowers was also polyploid, indicating that polyploid hybrids are produced under natural conditions. Our results provide evidence for asymmetric reproductive interference, with pollen from polyploid apomicts contributing to reduce the recruitment of sexual diploids in subsequent generations. Existing models suggest that these mixed sexual-asexual populations are likely to be transient, eventually leading to eradication of sexual individuals from the population.

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.

Abortion of reproductive organs as an adaptation to fluctuating daily carbohydrate production

Excess flower production is a common phenomenon in hermaphrodite plants. The tropical pioneer shrub Melastoma malabathricum (Melastomataceae) frequently aborts not only young ovaries just after flowering, but also flower buds and developed ovaries. We tested a hypothesis that the excess production of reproductive organs and their abortion in this species is an adaptation to environmental fluctuations over shorter time scales than had previously been reported in other plants. To calculate the daily demand for carbohydrate and water by reproductive organs at the level of individual plants, we measured the respiration and transpiration of the reproductive organs at various stages and monitored their growth and abortion. To determine the daily supply of carbohydrate and water, we measured the photosynthetic productivity of leaf area, solar radiation and rainfall. The daily carbohydrate demands of the reproductive organs were significantly correlated with total photosynthetic productivity per leaf area during the previous 1, 3 and 5 days, but no correlations were found between the demands for water and accumulated rainfall or radiation. The daily abortion rates of the population were also correlated with demand for carbohydrates on the previous day per total photosynthetic productivity per leaf area. In brief, it was considered that this species produced and grew more reproductive organs when more resources were supplied and that the abortion occurred when demands for carbohydrate were large. Therefore our hypothesis was supported. We concluded that this reproductive strategy was an adaptation for pioneers characterized by continuous reproduction in aseasonal tropics. In our study, the adaptive consequence of excess production was determined by measuring natural environmental fluctuation.

Flexible stigma presentation assists context-dependent pollination in a wild columbine

Stigma presentation has received much less attention than pollen presentation. Investigation of the various forms of stigma presentation may reveal previously undescribed reproductive mechanisms in plants. Here we investigated stigma presentation in the spring-flowering herb Aquilegia yabeana, whose linear stigma gradually develops over the lifetime of the flower. The degree of stigma development and recurvature are influenced by the timing of pollen deposition, apparently to favor cross-pollination. In this species, autogamous self-pollination was found to occur in the middle of the receptive period of the stigma if prior cross-pollination did not occur. However, because later-arriving cross-pollen was deposited closer to the ovary, it reached the base of the style before self-pollen which had been deposited earlier. Flexible stigma presentation helps the flower to maximize pollen resources under unreliable pollination conditions, taking advantage of both selfing and outcrossing. This is the first description of stigma development regulated by pollen deposition in a wild plant.