The ecological consequences of herbivore-induced plant responses on plant-pollinator interactions

Plant induced responses to herbivory have long been found to function as plant direct and indirect defenses and to be major drivers of herbivore community and population dynamics. While induced defenses are generally understood as cost-saving strategies that allow plants to allocate valuable resources into defense expression, it recently became clear that, in particular, induced metabolic changes can come with significant ecological costs. In particular, interactions with mutualist pollinators can be significantly compromised by herbivore-induced changes in floral morphology and metabolism. We review recent findings on the evidence for ecological conflict between defending against herbivores and attracting pollinators while using similar modes of information transfer (e.g. visual, olfactory, tactile). Specifically, we discuss plant traits and mechanisms through which plants mediate interactions between antagonists and mutualist and present functional hypotheses for how plants can overcome the resulting conflicts.

Comparison of the Micromorphology and Ultrastructure of Pollen Grains of Selected Rubus idaeus L. Cultivars Grown in Commercial Plantation

The genus Rubus is one of the largest taxonomically diverse and complex genera in the family Rosaceae. Morphology of pollen grains (equatorial and polar axes length, shape and size, aperture position, exine sculpture, perforations) is regarded as one of its main diagnostic features for identification of species and varieties. An attempt was made to fill the gap concerning the pollen micromorphology and ultrastructure of R. idaeus L. using light, scanning, and electron transmission microscopy. This study is a comparative analysis of micromorphological and ultrastructural traits of pollen from six raspberry cultivars. The pollen grains were classified as small or medium of shape prolato-spheroids. The parallel striae in the equatorial view in the exine sculpture were sometimes branched dichotomously in ‘Glen Ample’, ‘Polka’, and ‘Polana’, arcuate in ‘Laszka’ and ‘Pokusa’, or irregularly overlapping in ‘Radziejowa’. The width of exine striae of biennial fruiting cultivars was much larger than in repeated fruiting cultivars. In terms of the increasing number of perforations per unit area of the exine surface, the cultivars were ranked as follows: ‘Pokusa’ < ‘Glen Ample’ < ‘Laszka’ < ‘Polka’ < ‘Polana’ < ‘Radziejowa’. The thickest tectum, the highest and thickest columellae with the largest distances between them, and the thicker foot layer were demonstrated in ‘Glen Ample’. The ectoexine constituted on average ca. 78–90% of the exine thickness. The findings may constitute auxiliary traits i.a. for identification of related taxa, interpretation of phylogenetic relationships, and pollination biology.

Pollen colour morphs take different paths to fitness

Colour phenotypes are often involved in communication and are thus under selection by species interactions. However, selection may also act on colour through correlated traits or alternative functions of biochemical pigments. Such forms of selection are instrumental in maintaining petal colour diversity in plants. Pollen colour also varies markedly, but the maintenance of this variation is little understood. In Campanula americana, pollen ranges from white to dark purple, with darker morphs garnering more pollinator visits and exhibiting elevated pollen performance under heat stress. Here, we generate an F2 population segregating for pollen colour and measure correlations with floral traits, pollen attributes and plant-level traits related to fitness. We determine the pigment biochemistry of colour variants and evaluate maternal and paternal fitness of light and dark morphs by crossing within and between morphs. Pollen colour was largely uncorrelated with floral traits (petal colour, size, nectar traits) suggesting it can evolve independently. Darker pollen grains were larger and had higher anthocyanin content (cyanidin and peonidin) which may explain why they outperform light pollen under heat stress. Overall, pollen-related fitness metrics were greater for dark pollen, and dark pollen sires generated seeds with higher germination potential. Conversely, light pollen plants produce 61% more flowers than dark, and 18% more seeds per fruit, suggesting a seed production advantage. Results indicate that light and dark morphs may achieve fitness through different means-dark morphs appear to have a pollen advantage whereas light morphs have an ovule advantage-helping to explain the maintenance of pollen colour variation.

Effect of flower orientation on the male and female traits of Myrtillocactus geometrizans (Cactaceae)

Intra-individual variation in the production and size of reproductive traits has been documented in columnar cacti, being higher in equator-facing flowers. Such variation is attributed to the high amount of PAR intercepted by stems oriented towards the equator. Most studies focused on this phenomenon have documented the existence of intra-individual variation on traits associated with the female function; however, its impact on traits associated with the male function has been neglected. We tested the hypothesis that equator-facing flowers of Myrtillocactus geometrizans exhibit higher values on traits associated with both male and female functions than flowers facing against it. Number and size of anthers and ovaries, pollen:ovule ratio and number and quality of pollen grains (diameter, germinability, viability and pollen tube length) were estimated from reproductive structures facing north and south, and compared with t-tests between orientations. Number of anthers per flower, number of pollen grains per anther and per floral bud; pollen size, viability and germinability; pollen tube length; ovary length and pollen:ovule ratio were significantly higher in reproductive structures oriented towards the south (i.e. equator). These findings suggest that intra-individual variation in floral traits of M. geometrizans might be associated with different availability of resources in branches with contrasting orientation. Our results provide new evidence of the existence of a response to an orientation-dependent extrinsic gradient. To our knowledge, this is the first study documenting the existence of intra-individual variation on pollen quality and P:O ratio in Cactaceae species.

Flower thermoregulation facilitates fertilization in Asian sacred lotus

BACKGROUND AND AIMS

The thermoregulatory flower of the Asian sacred lotus (Nelumbo nucifera) can maintain a relatively stable temperature despite great variations in ambient temperature during anthesis. The thermoregulation has been hypothesized to offer a direct energy reward for pollinators in lotus flowers. This study aims to examine whether the stable temperature maintained in the floral chamber influences the fertilization process and seed development.

METHODS

An artificial refrigeration instrument was employed to cool flowers during the fertilization process and post-fertilization period in an experimental population. The effect of temperature on post-pollination events was also examined by removing petals in two field populations.

KEY RESULTS

Treatments with low floral temperature did not reduce stigma receptivity or pollen viability in undehisced anthers. Low temperature during the fertilization period significantly decreased seed set per flower but low temperature during the phase of seed development had no effect, suggesting that temperature regulation by lotus flowers facilitated fertilization success. Hand-pollination treatments in two field populations indicated that seed set of flowers with petals removed was lower than that of intact flowers in north China, where ambient temperatures are low, but not in south China, confirming that reducing the temperature of carpels did influence post-pollination events.

CONCLUSIONS

The experiments suggest that floral thermoregulation in lotus could enhance female reproductive success by facilitating fertilization.

Displacement of a native by an alien bumblebee: lower pollinator efficiency overcome by overwhelmingly higher visitation frequency

Biological invasions might constitute a major threat to mutualisms. Introduced pollinators might competitively displace their native counterparts, which in turn affects the pollination of native plants, if native and alien visitors differ in pollinator effectiveness. Since its invasion in 1994 into south-west Argentina, the introduced European bumblebee Bombus ruderatus has continuously increased in abundance, along with a simultaneous decrease in the abundance of the native Bombus dahlbomii. The latter is the only native bumblebee species of the temperate forests of southern South America, and the main pollinator of the endemic herb Alstroemeria aurea. In order to evaluate the impact of the ongoing displacement of the native by the alien bumblebee, we compared the pollinator effectiveness (i.e., the combination of pollinator efficiency per visit and visitation frequency) between both bumblebee species, as well as related pollinator traits that might account for potential differences in pollinator efficiency. Native Bombus dahlbomii, which has a larger body and spent more time per flower, was the more efficient pollinator compared to Bombus ruderatus, both in terms of quantity and quality of pollen deposited per visit. However, Bombus ruderatus was a much more frequent flower visitor than Bombus dahlbomii. As a consequence, Bombus ruderatus is nowadays a more effective pollinator of A. aurea than its native congener. Despite the lack of evidence of an increase in seed set at the population level, comparisons with historical records of Bombus dahlbomii abundances prior to Bombus ruderatus' invasion suggest that the overall pollination intensity of A. aurea might in fact have risen as a consequence of this invasion. Field experiments like these, that incorporate the natural variation in abundance of native and alien species, are powerful means to demonstrate that the consequences of invasions are more complex than previous manipulated and controlled experiments have suggested.

Environmental effects on pollen-pistil compatibility between Phlox cuspidata and P. drummondii (Polemoniaceae): implications for hybridization dynamics

Postpollination mechanisms of reproductive isolation can critically influence the amount of gene flow between hybridizing species. While much evidence exists for genetically based pollen-pistil incompatibility, we show that environmental variation also influences the postpollination performance of heterospecific pollen in the annual Phlox hybrid system. Thus, the environmental segregation of species can influence hybridization dynamics. We found that P. cuspidata was restricted to soils of low Ca concentrations in the field and performed better under experimentally low Ca; P. drummondii was able to inhabit high-Ca soils and sometimes performed better in this environment. To determine whether soil Ca influenced pollen-pistil compatibility in a manner that alters pollen siring success, single-donor pollinations were performed in a completely factorial crossing design between species, maternal Ca environments, and paternal Ca environments. Maternal and paternal environments interacted in their effects on pollen-pistil compatibility for both inter- and intraspecific crosses, such that pollen performance was highest when mothers and fathers were grown in different soil Ca environments. These results suggest that when Phlox species predictably inhabit different environments, environmental heterogeneity can impede the processes of speciation and local adaptation by enhancing the performance of pollen dispersed across species and environments.

Ecological limits to plant phenotypic plasticity

Phenotypic plasticity is considered the major means by which plants cope with environmental heterogeneity. Although ubiquitous in nature, actual phenotypic plasticity is far from being maximal. This has been explained by the existence of internal limits to its expression. However, phenotypic plasticity takes place within an ecological context and plants are generally exposed to multifactor environments and to simultaneous interactions with many species. These external, ecological factors may limit phenotypic plasticity or curtail its adaptive value, but seldom have they been considered because limits to plasticity have typically addressed factors internal to the plant. We show that plastic responses to abiotic factors are reduced under situations of conservative resource use in stressful and unpredictable habitats, and that extreme levels in a given abiotic factor can negatively influence plastic responses to another factor. We illustrate how herbivory may limit plant phenotypic plasticity because damaged plants can only rarely attain the optimal phenotype in the challenging environment. Finally, it is examined how phenotypic changes involved in trait-mediated interactions can entail costs for the plant in further interactions with other species in the community. Ecological limits to plasticity must be included in any realistic approach to understand the evolution of plasticity in complex environments and to predict plant responses to global change.

Pollen performance and male fitness in an anemophilous, monoecious tree, Betula pendula

Betula pendula Roth clones were examined to study the relationship between pollen grain size and pollen tube growth rate and pollen and seed performance. Two hypotheses were tested: (i) pollen donors with larger pollen grains have faster growing pollen tubes and (ii) maternal plants produce more seeds when inflorescences have been pollinated by pollen from donors with fast pollen tube growth. Pollen from 15 plants was collected, and pollen grain sizes and pollen tube growth rates were studied in vitro. Eight maternal clones were pollinated by pollen from seven paternal clones, pollen tube growth rates of the pollen donors were measured on each maternal plant, and the number and mass of the resulting seeds were analysed. There was no correlation between pollen grain size and pollen tube growth rate in vitro. Also, there was no relationship between pollen tube growth rate and the number of produced or aborted seeds. Thus, the hypothesis that larger pollen grains would have longer pollen tubes was not supported by the present study. The results of the present study do not support the idea that pollen donors with fast pollen tube growth should sire more and better seeds.Key words: Betula pendula, female choice, nonrandom abortion, pollen competition, sexual selection.

Inbreeding influences herbivory in Cucurbita pepo ssp. texana (Cucurbitaceae)

In a series of field experiments Diabrotica beetle herbivory was found to influence the magnitude of inbreeding depression in Cucurbita pepo ssp. texana, an annual monoecious vine. Beetles damage flowers and fruits and chew dime-sized holes in leaf tissue between major veins. Inbred plants were found to be more likely to be damaged by beetles and to have more leaves damaged per plant than outcrossed plants. A positive linear association was found between the coefficient of inbreeding and the magnitude of leaf damage, whereas a negative association was found between coefficient of inbreeding and several male and female fitness traits. When pesticides were used to control beetle herbivory, the interaction between coefficient of inbreeding and pesticide treatment was significant for fruit production and marginally significant for pollen quantity per anther. Therefore, the magnitude of inbreeding depression in C. pepo ssp. texana varies depending on the severity of beetle herbivory.

Sunny-side up: flower heliotropism as a source of parental environmental effects on pollen quality and performance in the snow buttercup, Ranunculus adoneus (Ranunculaceae)

Floral traits affect mating success via their influence on the microenvironment in which sexual reproduction occurs as well as their impact on pollinator attraction. Here we investigate the importance of flower heliotropism as a source of parental environmental effects on pollen quality and performance. Flowers of the snow buttercup, Ranunculus adoneus, closely track the sun's rays. We experimentally restrained flowers to test for effects of heliotropism on pollen quality and performance after pollination. When equivalent amounts of pollen were transferred to recipient pistils, pollen from solar-tracking donor flowers exhibited a 32% advantage in germination compared to pollen from stationary (tethered) donor flowers. By the end of anthesis, pistils of tracking flowers contained 40% more germinating pollen grains and 44% more pollen tubes midway down the style than pistils of stationary ones. Solar tracking had no direct effect on pollen tube growth. The greater amount of germinating pollen in tracking flowers accounted for the treatment effect on pollen tube density. A survey of pollen receipt and pollen germination in naturally tracking flowers indicated that solar tracking primarily affects pollen tube density by promoting pollen germination rather than pollen deposition. We conclude that flower heliotropism, by enhancing the paternal environment for pollen development and the maternal environment for pollen germination, represents a source of positive parental environmental effects on pollen performance in snow buttercups.

Interaction of pollinators and herbivores on plant fitness suggests a pathway for correlated evolution of mutualism- and antagonism-related traits

Different kinds of plant-animal interactions are ordinarily studied in isolation, yet considering the combined fitness effects of mutualistic and antagonistic interactions is essential to understanding plant character evolution. Functional, structural, or phylogenetic associations between attractive and defensive traits may be nonadaptive or result from correlational selection on sets of herbivory- and pollination-linked traits. Nonadditivity of fitness effects of mutualists and antagonists, a requisite for correlational selection, was experimentally tested in the field. We created experimental populations of the insect-pollinated perennial herb, Helleborus foetidus, at 16 different locations distributed among three regions in the Iberian Peninsula. Plants experienced one of four possible selective regimes generated by independently weakening the effects of pollinators and herbivores (flower and fruit predators) according to a two-way fully factorial design. Effects were assessed in terms of number of next-generation offspring recruited per mother plant under natural field conditions. Differences among H. foetidus plants in the strength of their interactions with pollinators and herbivores translated into differential fitness, as measured in terms of recruited offspring, and subsequent changes in plant population densities. A strong, geographically consistent nonadditivity in the fitness consequences of pollinators and herbivores was found also. Plants possessing the particular combination of "traits" simultaneously enhancing pollination and escape from herbivores enjoyed a disproportionate fitness advantage over plants possessing any of the other three possible "trait" combinations. Results suggest a simple, possibly widespread ecological pathway favoring the adaptive correlated evolution of mutualism- and antagonism-related plant traits in pollinator-dependent plants suffering intense flower and fruit herbivory.

The influence of light on paternal plants in Campanula americana (Campanulaceae): pollen characteristics and offspring traits

Offspring trait expression is determined by the combination of parental genes and parental environments. Although maternal environmental effects have been widely characterized, few studies have focused on paternal environmental effects. To determine whether light availability influences pollen and offspring traits in the woodland herb Campanula americana, we reared clones of 12 genotypes in two light levels. In the parental generation we measured pollen number and size. Plants grown under high light produced more pollen grains per flower than those grown under low light. However, the response was genotype specific; some individuals responded little to changes in light availability while others substantially reduced pollen production. As a consequence, paternity ratios may vary between light environments if more pollen is associated with greater siring success. We crossed a subset of these plants to produce the offspring generation. The paternal and maternal light environments influenced offspring seed mass, percentage germination, and days to germination, while only maternal light levels influenced later life traits, such as leaf number and size. Maternal and paternal environmental effects had opposite influences on seed mass, percentage germination and days to germination. Finally, there was no direct relationship between light effects on pollen production and offspring trait expression.

Reproductive processes in two oak (Quercus) contact zones with different levels of hybridization

Patterns of reproductive isolation between two sympatric species of oaks, Quercus gambelii and Q. grisea, that exhibit strong ecological differentiation were examined. A full diallel cross using four trees of each species (i.e. all possible pollinations among eight trees) was performed. This design was repeated at two sites that represent different outcomes of sympatry: (1) a xeric mountain ridge where many hybrids are established (HZ); and (2) a mesic valley bottom where virtually no hybrids are established (MOCYN). By measuring fruit survival at several developmental stages, both the timing and strength of reproductive barriers within and between sites, species, cross types, and pollen dosage levels were examined. In three of four cases, heterospecific fruit set was significantly reduced compared to conspecific fruit set. This reduction occurred after the time of fertilization, but before the onset of embryo growth. Increasing the dose of pollen from an average of 9-194 grains/stigma did not affect this result. Thus, early postfertilization processes play a strong role in species fidelity in these oaks. Quercus gambelii experienced a five-fold decrease in conspecific fruit set at HZ relative to MOCYN. In contrast, heterospecific fruit set of Q. gambelii was the same at both sites. Poor Q. gambelii pollen performance is implicated as playing the major role in this result. One Q. gambelii individual at HZ was highly fecund, and had higher heterospecific than conspecific fruit set; slight introgression in this tree was detected uisng RAPD markers. The Environmental Emasculation Hypothesis that posits that environmental stress can increase the probability of hybrid formation by reducing the competitive ability of male gametes of one species is proposed.

The effect of maternal and paternal environments on seed characters in the herbaceous plant Campanula Americana (Campanulaceae)

Maternal environments typically influence the phenotype of their offspring. However, the effect of the paternal environment or the potential for joint effects of both parental environments on offspring characters is poorly understood. Two populations of Campanula americana, a woodland herb with a variable life history, were used to determine the influence of maternal and paternal light and nutrient environments on offspring seed characters. Families were grown in the greenhouse in three levels of light or three levels of nutrients. Crosses were conducted within each environmental gradient to produce seeds with all combinations of maternal and paternal environments. On average, increasing maternal nutrient and light levels increased seed mass and decreased percentage germination. The paternal environment affected seed mass, germination time, and percentage germination. However, the influence of the paternal environment varied across maternal environments, suggesting that paternal environmental effects should be evaluated in the context of maternal environments. Significant interactions between family and the parental environments for offspring characters suggest that parental environmental effects are genetically variable. In C. americana, the timing of germination determines life history. Therefore parental environmental effects on germination timing, and genetic variation in those parental effects, suggest that parental environments may influence life history evolution in this system.

High and Dry: Drought Stress, Sex-Allocation Trade-offs, and Selection on Flower Size in the Alpine Wildflower Polemonium viscosum (Polemoniaceae)

Sex-allocation trade-offs may maintain variation in secondary sexual characteristics if such traits vary in their benefits or costs in association with different genders. In Polemonium viscosum, large flowers benefit both male and female aspects of reproduction. In this study, I explore how resource investment in flower size influences the cost of allocation to male and female function. Large flowers exact a water cost in P. viscosum under dry conditions. In an extreme drought in 1997, experimentally watered plants had higher survival and fecundity than controls. By comparing allocation patterns between plants dying from drought and survivors, I tested whether the demographic cost of large flowers increases with allocation to fecundity. Controls that died showed a positive relationship between flower size and fruit production, while survivors showed a negative relationship or trade-off. Watered plants showed no such trade-off. To test whether drought affects the relationship of corolla size to male function, I used leaf-water potential in 1998 to classify plants as stressed or unstressed. Corolla size showed positive correlations to pollen per flower regardless of drought stress. I conclude that under drought the demographic cost of producing large flowers is gender dependent, such that viability selection favors either small-flowered plants with female-biased reproduction or larger-flowered plants with male-biased reproduction.