Pollination of Ipomopsis aggregata (Polemoniaceae): effects of intra- vs. interspecific competition

Although plants may simultaneously experience intra- and interspecific competition for pollination, their relative strength has rarely been experimentally evaluated. Yet because intra- and interspecific competition can be caused by different mechanisms, their effect on the ecology and evolution of plants may differ. To determine the relative strength of intra- and interspecific competition for pollination, I manipulated the presence of heterospecifics and density of conspecifics using Ipomopsis aggregata as the focal species. All plots contained I. aggregata and Castilleja linariaefolia, but C. linariaefolia inflorescences were removed from half of the plots to create the heterospecifics-absent treatment. Within each plot, all I. aggregata inflorescences were removed from a 5-m radius around a focal plant to create a low conspecific density experimental unit, and a group of 12 I. aggregata plants/1 m² was designated as a high conspecific density unit. Conspecific pollen deposition was reduced when C. linariaefolia was present but was not influenced by I. aggregata density. Although seed set per fruit was reduced by 17% when C. linariaefolia was present, it was not significantly influenced by either treatment. Interspecific competition for pollination is stronger than intraspecific competition in the I. aggregata–C. linariaefolia system, but neither process appears to influence plant fitness.     

Regrowth following ungulate herbivory in Ipomopsis aggregata: geographic evidence for overcompensation

Studies were conducted on eight populations of scarlet gilia, Ipomopsis aggregata, across Colorado and in northern Arizona, to assess the fitness consequences of natural and simulated herbivory. To date, geographic studies have failed to incorporate treatment groups that included naturally browsed plants along with clipping treatments. The results presented here clearly demonstrate the importance of assessing, a priori, whether or not clipping experiments accurately reflect natural patterns of herbivory. Although because of the timing of the clip no evidence of overcompensation was found in any of the Colorado populations when ungulate herbivory was simulated in experimental clipping treatments, evidence for overcompensation was found in more than half the populations when plants that were naturally browsed and matched for size using root diameters were included early in the season. Matching plants for size based on root diameter late in the season would be problematic because root diameters significantly increased in size in all eight populations following high levels of ungulate herbivory and/or experimental clipping. Results from this study, and other recent studies on another biennial herb, the field gentian, clearly demonstrate that overcompensation is not only a real phenomenon but also is more widespread, both taxonomically and geographically, than previously thought. Received: 15 July 1998 / Accepted: 9 November 1998     

Multiple paternity in fruits of Ipomopsis aggregata (Polemoniaceae)

Two different mechanisms can result in multiple paternity within fruits: deposition of a mixed pollen load due to carryover of pollen from flower to flower and multiple pollinator visits in close succession. I investigated the extent of multiple paternity within fruits of Ipompsis aggregata containing from 2 to 14 seeds. A paternity analysis based on ten polymorphic isozyme markers revealed multiple paternity in a minimum of 68% (based on simple paternity exclusion) and up to 100% (based on identification of the most likely father) of the multiseeded fruits. The estimated number of fathers increased with the number of seeds in a fruit, with an average of four sires, and up to nine sires, represented in a single fruit. To explore whether this level of multiple paternity could be explained solely by simultaneous deposition of a mixed pollen load, I constructed a computer simulation model based on previous measurements of movement patterns and pollen carryover by the hummingbird pollinators. Model predictions provided a good match to observed values for number of sires per fruit. Thus, the extensive pollen carryover in this species and consequent mixed pollen loads can explain the high levels of multiple paternity in natural populations.     

Differential selection on floral traits of Ipomopsis aggregata growing in contrasting environments

Interactions for pollination between co-flowering plant species have been hypothesized to shape the evolution of their floral traits, but this hypothesis has rarely been tested. I tested the prediction that the presence of a co-flowering plant species influences the strength and/or direction of pollinator-mediated selection on floral traits. I measured phenotypic selection via female fitness on four floral traits of Ipomopsis aggregata in five populations. Three contained only conspecifics ( I only ) and two also contained the co-flowering species Penstemon barbatus ( P + I ). Directional selection via fruits/plant on corolla length and width differed in both strength and direction between P + I and I only populations. On average, selection on corolla length and width (1) was stronger in P + I than I only populations and (2) was consistently negative in P + I populations, but consistently positive in I only populations. However, these differences in selection on I . aggregata between P + I and I only populations were not caused by interactions for pollination with P . barbatus . Although plants in P + I populations received approximately 31% less conspecific pollen/flower than plants in I only populations, this difference in pollination did not translate into differences in reproductive success, which indicates that P . barbatus and I . aggregata do not strongly compete for pollination. In addition, selection via fruits/plant and conspecific pollen deposited/flower was not congruent. For example, selection on corolla length via pollen/flower was uniformly positive and did not differ between P + I and I only populations. These data suggest that the presence of P . barbatus does influence selection on floral traits of I . aggregata , but not by influencing pollination. Instead, differences in selection between P + I and I only populations appear to be the result of post-pollination modification of selection by a factor correlated with the presence of P . barbatus .     

Maternal flower color,ultraviolet protection,and germination in Ipomopsis aggregata (Polemoniaceae)

Transgenerational interactions between flower color, seed quality, and seedling performance have rarely been investigated. The ecological model, Ipomopsis aggregata, is a great candidate for examining the maternal effects of flower color because it is a mostly scarlet-flowering plant which shows color polymorphism within natural populations. Anthocyanin, the red flavonoid pigment which gives these flowers color, has been shown to act as an ultraviolet (UV) protectant by shielding chloroplasts and acting as an antioxidant. This study was conducted on scarlet- and fuchsia-flowering maternal plants and their seeds from natural populations in Colorado. Dark-flowering (scarlet) maternal plants from these populations had consistently higher foliar anthocyanin content, photosystem efficiency, and chlorophyll content than light-flowering (fuchsia) plants over a 3-year period in the field. Seeds from a subset of these maternal plants were counted, weighed, and germinated in a growth chamber. Photosystem efficiency, vegetative anthocyanin content, chlorophyll content, and biomass were measured on germinated seedlings after the germination census was completed. Dark-flowering maternal plants yielded seeds and seedlings with higher biomass than light-flowering ones. Seeds from dark-flowering maternal plants also germinated faster than those from light-flowering maternal plants and seedlings had higher vegetative anthocyanin content. The hereditary nature of anthocyanin content thus suggests that higher anthocyanin levels (both floral and vegetative) are potentially linked to measures of fitness such as increased seed weight, germination rate, and seedling biomass. These data suggest that UV protection provided by anthocyanins potentially increases the realized fitness of maternal plants, thereby influencing life history.     

Characterization of microsatellite loci in Ipomopsis (Polemoniaceae) wildflowers

Members of the Ipomopsis aggregata species complex in the phlox family (Polemoniaceae) often hybridize when they occur in sympatry, and thus have been extensively studied to examine processes involved in plant speciation. I developed 12 microsatellite loci in I. aggregata that are also polymorphic in closely related Ipomopsis tenuituba, producing an average of eight alleles per locus in test populations of 12 individuals per species. Several of these markers also successfully amplified in other Ipomopsis species and more distant members of the Polemoniaceae, suggesting they should prove useful for a broad range of evolutionary studies in this widely distributed system.     

The evolution of compensation to herbivory in scarlet gilia, Ipomopsis aggregata: herbivore-imposed natural selection and the quantitative genetics of tolerance

Tolerance is the ability of plants to maintain fitness after experiencing herbivore damage. We investigated scarlet gilia tolerance to browsing in the framework of phenotypic plasticity using both an operational and candidate trait approach. Individuals from full-sib families were split into an artificial clipping treatment, a natural-damage treatment, or left as controls. We tested for genetic variation in tolerance by evaluating family x herbivory treatment interactions on fitness in a mixed model analysis of variance. In addition, we used selection analyses to assess the function of flowering phenology and compensatory regrowth (via branch production) as candidate tolerance traits. We found a strong detrimental fitness effect of browsing and considerable variation among sire half-sib families in levels of tolerance (25% to 63% of the fitness of controls). There was no evidence of overcompensation at either the population or family level and no additive genetic variation in operationally defined tolerance. Phenotypic selection analyses provide evidence that early flowering and compensatory regrowth function as tolerance characters. We found strong linear and correlational selection for early flowering and increased branch production for damaged plants and linear selection for apical dominance (reduced branchiness) and early flowering in control plants. Moreover, reduced phenological delay and increased plasticity in branch production were correlated with tolerance. We detected significant additive genetic variation in flowering phenology in both treatments and a positive genetic correlation between the phenology of control and damaged plants. We found significant additive genetic variation in branch production in undamaged and naturally damaged plants, but not in clipped plants. Damaged plants exhibited marginally significant additive genetic variance in fitness, although its heritability was very low (approximately 3.6%). We failed to find additive genetic variation in the fitness of control plants. Our results suggest that tolerance traits are under herbivore-imposed natural selection in this population, but that responses to selection are limited by available genetic variation and selective constraints.     

PERMANENT GENETIC RESOURCES: Identification of nuclear microsatellite loci for Ipomopsis aggregata and the distribution of pairwise relatedness in a natural population

Nine microsatellite loci were developed from enriched libraries of scarlet gilia (Ipomopsis aggregata). A screen of 160 individuals from a population identified reduced levels of heterozygosity, low levels of relatedness, and weak spatial genetic patterns. The population inbreeding coefficient was calculated to be 0.19 (SE = 0.04). These patterns are consistent with those expected from low levels of biparental inbreeding in an obligate outcrosser and extensive seed and pollen dispersal. These preliminary data confirm the usefulness of microsatellite markers for breeding system studies of I. aggregata.     

A meta‐analysis of the agents of selection on floral traits

Floral traits are hypothesized to evolve primarily in response to selection by pollinators. However, selection can also be mediated by other environmental factors. To understand the relative importance of pollinator‐mediated selection and its variation among trait and pollinator types, we analyzed directional selection gradients on floral traits from experiments that manipulated the environment to identify agents of selection. Pollinator‐mediated selection was stronger than selection by other biotic factors (e.g., herbivores), but similar in strength to selection by abiotic factors (e.g., soil water), providing partial support for the hypothesis that floral traits evolve primarily in response to pollinators. Pollinator‐mediated selection was stronger on pollination efficiency traits than on other trait types, as expected if efficiency traits affect fitness via interactions with pollinators, but other trait types also affect fitness via other environmental factors. In addition to varying among trait types, pollinator‐mediated selection varied among pollinator taxa: selection was stronger when bees, long‐tongued flies, or birds were the primary visitors than when the primary visitors were Lepidoptera or multiple animal taxa. Finally, reducing pollinator access to flowers had a relatively small effect on selection on floral traits, suggesting that anthropogenic declines in pollinator populations would initially have modest effects on floral evolution.     

Factors limiting rosette recruitment in scarlet gilia, Ipomopsis aggregata: seed and disturbance limitation

We performed a factorial combination of seed additions, surface soil disturbance, and protection from potential seed predation in experimental plots to elucidate the relative importance of each factor in the recruitment of scarlet gilia seedlings. We detected a significant interaction between seed addition and surface soil disturbance on seedling recruitment over 2 years of establishment. Plots that received both supplemental seed addition and disturbance established 10.5 times as many rosettes as control plots. We did not detect main or interactive effects of caging on seedling establishment. We explored the importance of density-dependent recruitment by investigating the number of rosettes per experimental plot through time. In addition, we used planned contrasts to compare the absolute and percent changes in control and treatment plot rosette densities between the study years. We found significant downward concave curvature to the seedling recruitment curve, suggesting negative density-dependent recruitment across the experimental range of rosette densities. We found a significant difference between the control plots and the seed addition-disturbance plots in terms of absolute changes in plot density from 1997 to 1998. Greater net mortality occurred in plots that received both seed addition and disturbance. However, we found no differences among treatments in the percent change in rosette density across the study years. Our study demonstrates that seedling recruitment in natural plant populations may be limited by the interaction of seed and microsite availability, and that seedling recruitment in scarlet gilia may be negatively density dependent. Our results suggest that the fecundity effects of particular plant-animal interactions (e.g., pollination, herbivory) may affect scarlet gilia population dynamics, particularly under conditions of high soil disturbance. Received: 11 June 1999 / Accepted: 24 November 1999     

Adaptive differentiation in floral traits in the presence of high gene flow in scarlet gilia (Ipomopsis aggregata)

Plant–pollinator interactions are thought to be major drivers of floral trait diversity. However, the relative importance of divergent pollinator‐mediated selection vs. neutral processes in floral character evolution has rarely been explored. We tested for adaptive floral trait evolution by comparing differentiation at neutral genetic loci to differentiation at quantitative floral traits in a putative Ipomopsis aggregata hybrid zone. Typical I. aggregata subsp. candida displays slender white tubular flowers that are typical of flowers pollinated by hawkmoths, and subsp. collina displays robust red tubular flowers typical of flowers pollinated by hummingbirds; yet, hybrid flower morphs are abundant across the East Slope of the Colorado Rockies. We estimated genetic differentiation (F_ST) for nuclear and chloroplast microsatellite loci and used a half‐sib design to calculate quantitative trait divergence (Q_ST) from collection sites across the morphological hybrid zone. We found little evidence for population structure and estimated mean F_ST to be 0.032. Q_ST values for several floral traits including corolla tube length and width, colour, and nectar volume were large and significantly greater than mean F_ST. We performed multivariate comparisons of neutral loci to genetic correlations within and between populations and found a strong signal for divergent selection, suggesting that specific combinations of floral display and reward traits may be the targets of selection. Our results show little support for historical subspecies categories, yet floral traits are more diverged than expected due to drift alone. Non‐neutral divergence for multivariate quantitative traits suggests that selection by pollinators is maintaining a correlation between display and reward traits.     

Species-independent, geographical structuring of chloroplast DNA haplotypes in a montane herb Ipomopsis (Polemoniaceae)

Two hypotheses have been proposed to explain the occurrence of hybrid zones between red-flowered Ipomopsis aggregata and white-flowered I. tenuituba . Either local adaptation to hummingbird and hawkmoth pollinators has given rise to sympatric (or parapatric) divergence of flower colour and morphology (primary intergradation at hybrid zones), or alternatively two previously allopatric species are coming into contact at several geographical areas of secondary intergradation. We examined restriction site patterns in nuclear DNA (nrDNA), chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) from populations of I. aggregata and I. tenuituba representing seven zones of sympatry. No variation was detected in a 350-bp fragment of mtDNA and uninformative levels of variation were observed for nrDNA. We detected 22 potentially informative restriction site polymorphisms in cpDNA, all of which united geographical areas containing populations of both species. We detected no informative species-specific markers. Studies of other species (e.g. oaks) have detected similar species-independent geographical structure of cpDNA. However, in these cases secondary interegradation could be inferred from species-specific nuclear alleles. The pattern in Ipomopsis is consistent with both primary intergradation (independent speciation in each area of sympatry) or secondary intergradation involving complete cytoplasmic replacement. Thus, additional data are needed to explain the origin of hybrid zones in Ipomopsis .     

The consequences of direct versus indirect species interactions to selection on traits: pollination and nectar robbing in Ipomopsis aggregata

Organisms experience a complex suite of species interactions. Although the ecological consequences of direct versus indirect species interactions have received attention, their evolutionary implications are not well understood. I examined selection on floral traits through direct versus indirect pathways of species interactions using the plant Ipomopsis aggregata and its pollinators and nectar robber. Using path analysis and structural equation modeling, I tested competing hypotheses comparing the relative importance of direct (pollinator-mediated) versus indirect (robber-mediated) interactions to trait selection through female plant function in 2 years. The hypothesis that provided the best fit to the observed data included robbing and pollination, suggesting that both interactors are important in driving selection on some traits; however, the direction and intensity of selection through robbing versus pollination varied between years. I then increased my scope of inference by assessing traits and species interactions across more years. I found that the potential for temporal variation in the direction and intensity of selection was pronounced. Taken together, results suggest that assessing the broader context in which organisms evolve, including both direct and indirect interactions and across multiple years, can provide increased mechanistic understanding of the diversity of ways that animals shape floral and plant evolution.     

Environment and pollinator-mediated selection on parapatric floral races of Mimulus aurantiacus

We tested whether selection by pollinators could explain the parapatric distribution of coastal red- and inland yellow-flowered races of Mimulus aurantiacus (Phrymaceae) by examining visitation to natural and experimental populations. As a first step in evaluating whether indirect selection might explain floral divergence, we also tested for local adaptation in early life stages using a reciprocal transplant experiment. Hummingbirds visited flowers of each race at similar rates in natural populations but showed strong (>95%) preference for red flowers in all habitats in experimental arrays. Hawkmoths demonstrated nearly exclusive (>99% of visits) preference for yellow flowers and only visited in inland regions. Strong preferences for alternative floral forms support a direct role for pollinators in floral divergence. Despite these preferences, measures of plant performance across environments showed that red-flowered plants consistently survived better, grew larger and received more overall pollinator visits than yellow-flowered plants. Unmeasured components of fitness may favour the yellow race in inland habitats. Alternatively, we document a marked recent increase in inland hummingbird density that may have caused a change in the selective environment, favouring the eastward advance of red-flowered plants.     

Morph‐specific selection on floral traits in a polymorphic plant

Correlations between phenotypic traits are common in many organisms, but the relative importance of nonadaptive mechanisms and selection for the evolution and maintenance of such correlations are poorly understood. In polymorphic species, morphs may evolve quantitative differences in additional characters as a result of morph‐specific selection. The perennial rosette herb Primula farinosa is polymorphic for scape length. The short‐scaped morph is less damaged by grazers and seed predators but is more strongly pollen limited than the long‐scaped morph. We examined whether morph‐specific differences in biotic interactions are associated with differences in selection on two other traits affecting floral display (number of flowers and petal size) and on one trait likely to affect pollination efficiency (corolla tube width) in three P. farinosa populations. Differences in selection between morphs were detected in one population. In this population, selection for more flowers and larger petals was stronger in the short‐scaped than in the long‐scaped morph, and although there was selection for narrower corolla tubes in the short‐scaped morph, no statistically significant selection on corolla tube width could be detected in the long‐scaped morph. In the study populations, the short‐scaped morph produced more and larger flowers and wider corolla tubes. Current morph‐specific selection was thus only partly consistent with trait differences between morphs. The results provide evidence of morph‐specific selection on traits associated with floral display and pollination efficiency, respectively.     

Quantifying hummingbird preference for floral trait combinations: The role of selection on trait interactions in the evolution of pollination syndromes

Darwin recognized the flower's importance for the study of adaptation and emphasized that the flower's functionality reflects the coordinated action of multiple traits. Here we use a multitrait manipulative approach to quantify the potential role of selection acting on floral trait combinations underlying the divergence and maintenance of three related North American species of Silene (Caryophyllaceae). We artificially generated 48 plant phenotypes corresponding to all combinations of key attractive traits differing among the three Silene species (color, height, inflorescence architecture, flower orientation, and corolla‐tube width). We quantified main and interaction effects of trait manipulation on hummingbird visitation preference using experimental arrays. The main effects of floral display height and floral orientation strongly influenced hummingbird visitation, with hummingbirds preferring flowers held high above the ground and vertically to the sky. Hummingbirds also prefer traits in a nonadditive manner as multiple two‐way and higher order interaction effects were important predictors of hummingbird visitation. Contemporary trait combinations found in hummingbird pollinated S. virginica are mostly preferred. Our study demonstrates the likelihood of pollination syndromes evolving due to selection on trait combinations and highlights the importance of trait interactions in understanding the evolution of complex adaptations.     

Pollen precedence and stigma closure: a mechanism of competition for pollination between Delphinium nelsonii and Ipomopsis aggregata

Summary Previous experiments showed that the sympatric herbs Delphinium nelsonii and Ipomopsis aggregata compete for hummingbird pollination and that deleterious effects of the former species on seed set of the latter involve interspecific pollen transfer. However, seed set was not reduced when pollen of both species was applied simultaneously to I. aggregata stigmas. Hence a competitive effect may require arrival of foreign pollen before conspecific pollen. To explore this possibility we subjected I. aggregata flowers to a competition treatment in which they received D. nelsonii pollen 6 h before I. aggregata pollen, or to a control in which they received only the conspecific pollen. Foreign pollen precedence decreased mean seed set by almost 50%, which is consistent with effects observed in previous experiments. Reduced seed set can be explained by the fact that foreign pollen often caused stigma lobes to close together within 1.5–6 h, reducing subsequent receptivity. Stigma closure was also elicited by conspecific pollen, but not by mechanical stimulation, and was influenced by size of the pollen load and identity of the plant being pollinated.     

Functional differentiation in pollination processes among floral traits in Serapias species (Orchidaceae)

Floral displays, influencing attractiveness to insects, increase the number of pollinator visits and the efficiency of each visit in terms of pollen exchange and thus affect the plant reproductive success. Here, we conducted an in situ manipulation experiment to investigate whether the floral modifications affect reproductive success in natural orchid populations of Serapias lingua and Serapias vomeracea. We estimated male and female reproductive success of three treatment groups, disassembly of floral tube, cutting of lip, and painting of the callus surface, in terms of pollinaria removed/deposited and fruit production. Results revealed that phenotypic modification had opposite effects on reproductive success of two examine species. Indeed, reproductive success was significantly increased by the detached of the petals and sepals, and decreased, due to callus painting and lip removal, in S. lingua. On the contrary, unmanipulated plants of S. vomeracea showed significantly higher value of pollinaria removed and deposited and fruit set than manipulated ones. The differences between S. lingua and S. vomeracea agree to the different pollination strategy of examined species. S. vomeracea shows shelter imitation strategy, and thus, the disassembly of tunnel‐like corolla does not allow the insects to use the flower as a refuge, while S. lingua is a sexually deceptive orchid and therefore the opening of the flower made more visible callus (visible at a greater distance) increasing the pollinators attraction. This study provides evidence that pollinators were largely sensitive to the experimental modification of the flower phenotype, which is consistent with the presence of significant selection on individual floral characters. Our experimental investigations of the effects of variation in display on pollinator visitation provide insights into the evolution of floral morphology in orchid with shelter imitation strategy.     

The effects of grazers on the performance of individuals and populations of scarlet gilia,Ipomopsis aggregata

Summary I. aggregata exhibits considerable powers of regrowth following removal of its primary shoot by herbivores, but we found no evidence of overcompensation (i.e. of significantly higher plant performance where plants were exposed to ungulate herbivory) in a comparison between individuals on grazed and ungrazed sides of exclosure fences, in a comparison between artificially clipped and control plants in one population in the Okanagan National Forest, or in comparisons between grazed and ungrazed plants in 14 natural populations. We tested whether ungulate grazing affects the population size of Ipomopsis aggregata by comparing populations inside and outside deer exclosures at 7 sites in the Western United States. We found consistent, highly significant differences in plant population density on the grazed and ungrazed sides of these exlosure fences. Plant density was a modal 25-fold higher on the protected side of the fence, suggesting that exposure to ungulate grazing increases plant death rates at some stage in the life cycle. Our results show that the presence of ungulate grazers leads to a substantial decrease in plant density despite the fact that grazing on young bolting shoots has very little influence on fruit production. Since this decrease in population density is not correlated with a decrease in the fecundity of individuals, it must instead be due to other direct and indirect effects of ungulate grazers.