Pollinators differ in their contribution to the male fitness of a self-incompatible composite

Premise

Reproductive fitness in plants is often determined by the quantity and quality of pollen transferred by pollinators. However, many fitness studies measure only female fitness or rely on proxies for male fitness. Here we assessed how five bee taxon groups affect male fitness in a prairie plant by quantifying pollen removal, visitation, and siring success using paternity assignments and a unique pollinator visitation experiment.

Methods

In Echinacea angustifolia, we measured per-visit pollen removal for each pollinator taxon and estimated the number of pollen grains needed for successful ovule fertilization. Additionally, we directly measured pollinator influence on siring by allowing only one bee taxon to visit each pollen-donor plant, while open-pollinated plants acted as unrestricted pollen recipients. We genotyped the resulting offspring, assigned paternity, and used aster statistical models to quantify siring success.

Results

Siring success of pollen-donor plants differed among the five pollinator groups. Nongrooming male bees were associated with increased siring success. Bees from all taxa removed most of the flowering head's pollen in one visit. However, coneflower-specialist bee Andrena helianthiformis removed the most pollen per visit. Female fitness and proxy measures of male fitness, such as pollinator visitation and pollen removal, did not align with our direct quantifications of male fitness.

Conclusions

Our results illustrate the need for more studies to directly quantify male fitness, and we caution against using male fitness proxy measures. In addition, conservation efforts that preserve a diverse pollinator community can benefit plants in fragmented landscapes.

     

Adaptive function of duodichogamy: Why do chestnut trees have two pollen emission phases?

Premise

Intersexual mating facilitation in flowering plants has been largely underexplored. Duodichogamy is a rare flowering system in which individual plants flower in the sequence male-female-male. We studied the adaptive advantages of this flowering system using chestnuts (Castanea spp., Fagaceae) as models. These insect-pollinated trees produce many unisexual male catkins responsible for a first staminate phase and a few bisexual catkins responsible for a second staminate phase. We hypothesized that duodichogamy increases female mating success by facilitating pollen deposition on stigmas of the rewardless female flowers through their proximity with attractive male flowers responsible for the minor staminate phase.

Methods

We monitored insect visits to 11 chestnut trees during the entire flowering period and explored reproductive traits of all known duodichogamous species using published evidence.

Results

In chestnuts, insects visited trees more frequently during the first staminate phase but visited female flowers more frequently during the second staminate phase. All 21 animal-pollinated duodichogamous species identified are mass-flowering woody plants at high risk of self-pollination. In 20 of 21 cases, gynoecia (female flower parts) are located close to androecia (male flower parts), typically those responsible for the second minor staminate phase, whereas androecia are often distant from gynoecia.

Conclusions

Our results suggest that duodichogamy increases female mating success by facilitating pollen deposition on stigmas by means of the attractiveness of the associated male flowers while effectively limiting self-pollination.

     

Beyond pollen:ovule ratios: Evolutionary consequences of pollinator dependence and pollination efficiency for pollen and ovule production in angiosperms

Premise

The relative per-flower production of ovules and pollen varies broadly with angiosperm mating systems, with outcrossing types commonly producing more pollen grains per ovule than selfing types. The evolutionary causes of this variation are contentious, especially the relevance of pollination risk. Resolution of this debate may have been hampered by its focus on pollen:ovule (P:O) ratios rather than on the evolution of pollen and ovule numbers per se.

Methods

Using published mean ovule and pollen counts, we analyzed associations with the proportion of removed pollen that reaches stigmas (pollen-transfer efficiency) and differences between pollinator-dependent and autogamous forms within and among species. Analyses involved Bayesian methods that simultaneously considered variation in pollen and ovule numbers and accounted for phylogenetic relatedness. We also assessed the utility of P:O ratios as mating-system proxies and their association with female outcrossing rates.

Results

Median pollen number declined consistently with pollen-transfer efficiency among species, whereas median ovule number did not. Similarly, in both intraspecific and interspecific analyses, pollinator-dependent plants produced more pollen than autogamous plants, whereas ovule production did not differ statistically. Distributions of P:O ratios overlapped extensively for self-incompatible and self-compatible species and for different mating-system classes, and P:O ratios correlated weakly with outcrossing rate.

Conclusions

Our findings demonstrate that pollinator dependence and pollination efficiency commonly influence the evolution of pollen number per flower but have more limited effects on ovule number. P:O ratios provide ambiguous, possibly misleading, information about mating systems, especially when compared among clades.

     

Changes in endogenous abscisic acid and stomata of the resurrection fern,Pleopeltis polypodioides,in response to de- and rehydration

Premise

While angiosperms respond uniformly to abscisic acid (ABA) by stomatal closure, the response of ferns to ABA is ambiguous. We evaluated the effect of endogenous ABA, hydrogen peroxide (H₂O₂), nitric oxide (NO), and Ca²⁺, low and high light intensities, and blue light (BL) on stomatal opening of Pleopeltis polypodioides.

Methods

Endogenous ABA was quantified using gas chromatography-mass spectrometry; microscopy results and stomatal responses to light and chemical treatments were analyzed with Image J.

Results

The ABA content increases during initial dehydration, peaks at 15 h and then decreases to one fourth of the ABA content of hydrated fronds. Following rehydration, ABA content increases within 24 h to the level of hydrated tissue. The stomatal aperture opens under BL and remains open even in the presence of ABA. Closure was strongly affected by BL, NO, and Ca²⁺, regardless of ABA, H₂O₂ effect was weak.

Conclusions

The decrease in the ABA content during extended dehydration and insensitivity of the stomata to ABA suggests that the drought tolerance mechanism of Pleopeltis polypodioides is independent of ABA.

     

Functional traits and trait coordination change over the life of a leaf in a tropical fern species

Premise

Plant ecological strategies are often defined by the integration of underlying traits related to resource acquisition, allocation, and growth. Correlations between key traits across diverse plants suggest that variation in plant ecological strategies is largely driven by a fast–slow continuum of plant economics. However, trait correlations may not be constant through the life of a leaf, and it is still poorly understood how trait function varies over time in long-lived leaves.

Methods

Here, we compared trait correlations related to resource acquisition and allocation across three different mature frond age cohorts in a tropical fern species, Saccoloma inaequale.

Results

Fronds exhibited high initial investments of nitrogen and carbon, but with declining return in photosynthetic capacity after the first year. In the youngest fronds, we found water-use efficiency to be significantly lower than in the oldest mature fronds due to increased transpiration rates. Our data suggest that middle-aged fronds are more efficient relative to younger, less water-use efficient fronds and that older fronds exhibit greater nitrogen investments without higher photosynthetic return. In addition, several trait correlations expected under the leaf economics spectrum (LES) do not hold within this species, and some trait correlations only appear in fronds of a specific developmental age.

Conclusions

These findings contextualize the relationship between traits and leaf developmental age with those predicted to underlie plant ecological strategy and the LES and are among the first pieces of evidence for when relative physiological trait efficiency is maximized in a tropical fern species.

     

Soil phosphorus availability determines the contribution of small,individual grassland remnants to the conservation of landscape-scale biodiversity

Questions

Small, remnant habitats embedded in degraded, human-dominated landscapes are generally not a priority in conservation, despite their potential role in supporting landscape-scale biodiversity. To warrant their inclusion in conservation management and policy, we question under which conditions they may exhibit the largest conservation value.

Location

Nine landscapes spread across the counties of Stockholm and Södermanland, Sweden.

Methods

Per landscape, plant communities were surveyed in 6 and 12 1 × 1 m² plots across large, intact semi-natural grasslands and small remnant grasslands, respectively. These two contrasting grassland types served as a model system. A topsoil sample was taken in each plot to determine habitat quality in terms of soil pH, plant-available P, and C:N ratio. We used a joint species distribution model to analyse the extent to which grassland type and habitat quality define and predict resident community diversity and composition, including whether they support grassland specialists.

Results

At the landscape scale, the combined remnant grasslands sustained diverse plant communities which did include a significant subset of habitat specialists. Yet, the contribution of individual remnants clearly varied with local-scale habitat quality; soil phosphorus availability lowered plot-level species richness, mostly by constraining the occurrence of grassland specialists. Semi-natural grassland communities were comparatively insensitive to variation in soil phosphorus availability.

Conclusions

The combined habitat amount and the significant number of habitat specialists sustained by remnant grasslands with high habitat quality, shows they can represent a valuable resource to support landscape-scale biodiversity conservation. This offers no wildcard to neglect the continued biotic and abiotic threats on semi-natural grassland plant diversity such as chronic and accumulating P eutrophication, discontinuation of management or poor matrix permeability, as semi-natural grasslands harbour the majority of habitat specialists, while sourcing surrounding remnant grassland communities.

     

Individual bee foragers are less-efficient transporters of pollen for plants from which they collect the most pollen in their scopae

Premise

Bees provision most of the pollen removed from anthers to their larvae and transport only a small proportion to stigmas, which can negatively affect plant fitness. Though most bee species collect pollen from multiple plant species, we know little about how the efficiency of bees' pollen transport varies among host plant species or how it relates to other aspects of generalist bee foraging behavior that benefit plant fitness, such as specialization on individual foraging bouts.

Methods

We compared the pollen collected and transported by three bee species for 46 co-occurring plant species. Specifically, we compared the relative abundance of pollen taxa in the individual bees' scopae, structures where bees store pollen to provision larvae, with the relative abundance of pollen taxa on the rest of bees' bodies, which is more likely to be transferred to stigmas.

Results

Bees carried five times more pollen grains in their scopae than elsewhere on their bodies. Within foraging bouts, bees were relatively specialized in their pollen collection, but transported proportionally less pollen for the host plants on which they specialized. Across foraging bouts, two bee species transported proportionally less pollen for some of their host plants than for others, though differences didn't consistently follow the same trend as at the foraging bout scale.

Conclusions

Our results suggest that foraging-bout specialization, which is known to reduce heterospecific pollen transfer, also results in less-efficient pollen transport. Thus, bee foragers that visit predominantly one plant species may have contrasting effects on that plant's fitness.

     

Hawkmoth and bee pollinators impact pollen dispersal at the landscape but not local scales in two species of Oenothera

Premise

Animal pollinators play an important role in pollen dispersal. Here, we assessed differences in pollen and seed dispersal and the role of pollinator functional groups with different foraging behaviors in generating patterns of genetic diversity over similar geographic ranges for two closely related taxa. We focused on two members of Oenothera section Calylophus (Onagraceae) that co-occur on gypsum outcrops throughout the northern Chihuahuan Desert but differ in floral phenotype and primary pollinator: Oenothera gayleana (bee) and O. hartwegii subsp. filifolia (hawkmoth).

Methods

We measured breeding system and floral traits and studied gene flow and population differentiation at the local (<13 km; four populations) and landscape (60–440 km; five populations) scales using 10–11 nuclear (pollen dispersal) and three plastid (seed dispersal) microsatellite markers.

Results

Both taxa were self-incompatible and floral traits were consistent with expectations for different pollinators. Seed and pollen dispersal patterns were distinctly different for both species. We found no evidence of genetic structure at the local scale but did at the landscape scale; O. gayleana showed greater differentiation and significant isolation by distance than in O. hartwegii subsp. filifolia. The plastid data were consistent with gravity dispersal of seeds and suggest that pollen dispersal is the principal driver of genetic structure in both species.

Conclusions

We demonstrated that pollinator functional groups can impact genetic differentiation in different and predictable ways. Hawkmoths, with larger foraging distances, can maintain gene flow across greater spatial scales than bees.

     

Perception,regulation, and fitness effects of pollen phytosterols in the bumble bee,Bombus terrestris

Premise

Many flowering plants depend on insects for pollination and thus attract pollinators by offering rewards, mostly nectar and pollen. Bee pollinators rely on pollen as their main nutrient source. Pollen provides all essential micro- and macronutrients including substances that cannot be synthesized by bees themselves, such as sterols, which bees need for processes such as hormone production. Variations in sterol concentrations may consequently affect bee health and reproductive fitness. We therefore hypothesized that (1) these variations in pollen sterols affect longevity and reproduction in bumble bees and (2) can thus be perceived via the bees' antennae before consumption.

Methods

We studied the effect of sterols on longevity and reproduction of Bombus terrestris workers in feeding experiments and investigated sterol perception using chemotactile proboscis extension response (PER) conditioning.

Results

Workers could perceive several sterols (cholesterol, cholestenone, desmosterol, stigmasterol, β-sitosterol) via their antennae but not differentiate between them. However, when sterols were presented in pollen, and not as a single compound, the bees were unable to differentiate between pollen differing in sterol content. Additionally, different sterol concentrations in pollen neither affected pollen consumption nor brood development or worker longevity.

Conclusions

Since we used both natural concentrations and concentrations higher than those found in pollen, our results indicate that bumble bees may not need to pay specific attention to pollen sterol content beyond a specific threshold. Naturally encountered concentrations might fully support their sterol requirements and higher concentrations do not seem to have negative effects.

     

Phenotype–fitness relationships and pollen-transfer efficiency of five orchid species with different pollination strategies

Premise

Deceptive pollination, a fascinating mechanism that independently originated in several plant families for benefiting from pollinators without providing any reward, is particularly widespread among orchids. Pollination efficiency is crucial in orchids due to the aggregated pollen in a pollinarium, which facilitates pollen transfer and promotes cross-pollination as pollinators leave after being deceived.

Methods

In this study, we compiled data on reproductive ecology from five orchid species with different pollination strategies: three deceptive-strategy species (shelter imitation, food deception, sexual deception), one nectar-rewarding species, and one shelter-imitation but spontaneously selfing species. We aimed to compare the reproductive success (female fitness: fruit set; male fitness: pollinarium removal) and pollination efficiency of species representing these strategies. We also investigated pollen limitation and inbreeding depression among the pollination strategies.

Results

Male and female fitness were strongly correlated in all species but the spontaneously selfing species, which had high fruit set and low pollinarium removal. As expected, pollination efficiency was highest for the rewarding species and the sexually deceptive species. Rewarding species had no pollen limitation but did have high cumulative inbreeding depression; deceptive species had high pollen limitation and moderate inbreeding depression; and spontaneously selfing species did not have pollen limitation or inbreeding depression.

Conclusions

Pollinator response to deception is critical to maintain reproductive success and avoid inbreeding in orchid species with non-rewarding pollination strategies. Our findings contribute to a better understanding of the trade-offs associated with different pollination strategies in orchids and highlight the importance of pollination efficiency in orchids due to the pollinarium.

     

Genomic analysis unveils reduced genetic variability but increased proportion of heterozygotic genotypes of the intensively managed mezcal agave,Agave angustifolia

Premise

The central Oaxaca Basin has a century-long history of agave cultivation and is hypothesized to be the region of origin of other cultivated crops. Widely cultivated for mezcal production, the perennial crop known as “espadín” is putatively derived from wild Agave angustifolia. Nevertheless, little is known about its genetic relationship to the wild A. angustifolia or how the decades-long clonal propagation has affected its genetics.

Methods

Using restriction-site-associated DNA sequencing and over 8000 single-nucleotide polymorphisms, we studied aspects of the population genomics of wild and cultivated A. angustifolia in Puebla and Oaxaca, Mexico. We assessed patterns of genetic diversity, inbreeding, distribution of genetic variation, and differentiation among and within wild populations and plantations.

Results

Genetic differentiation between wild and cultivated plants was strong, and both gene pools harbored multiple unique alleles. Nevertheless, we found several cultivated individuals with high genetic affinity with wild samples. Higher heterozygosity was observed in the cultivated individuals, while in total, they harbored considerably fewer alleles and presented higher linkage disequilibrium compared to the wild plants. Independently of geographic distance among sampled plantations, the genetic relatedness of the cultivated plants was high, suggesting a common origin and prevalent role of clonal propagation.

Conclusions

The considerable heterozygosity found in espadín is contained within a network of highly related individuals, displaying high linkage disequilibrium generated by decades of clonal propagation and possibly by the accumulation of somatic mutations. Wild A. angustifolia, on the other hand, represents a significant genetic diversity reservoir that should be carefully studied and conserved.

     

Molecular phylogeny and evolution of inflorescence types in Eperua

Premise

The Amazonian hyperdominant genus Eperua (Fabaceae) currently holds 20 described species and has two strongly different inflorescence and flower types, with corresponding different pollination syndrome. The evolution of these vastly different inflorescence types within this genus was unknown and the main topic in this study.

Methods

We constructed a molecular phylogeny, based on the full nuclear ribosomal DNA and partial plastome, using Bayesian inference and maximum likelihood methods, to test whether the genus is monophyletic, whether all species are monophyletic and if the shift from bat to bee pollination (or vice versa) occurred once in this genus.

Results

All but two species are well supported by the nuclear ribosomal phylogeny. The plastome phylogeny, however, shows a strong geographic signal suggesting strong local hybridization or chloroplast capture, rendering chloroplast barcodes meaningless in this genus.

Conclusions

With our data, we cannot fully resolve the backbone of the tree to clarify sister genera relationships and confirm monophyly of the genus Eperua. Within the genus, the shift from bat to bee and bee to bat pollination has occurred several times but, with the bee to bat not always leading to a pendant inflorescence.

     

Testing the effect of individual scent compounds on pollinator attraction in nature using quasi-isogenic Capsella lines

Premise

Floral scent, usually consisting of multiple compounds, is a complex trait, and its role in pollinator attraction has received increasing attention. However, disentangling the effect of individual floral scent compounds is difficult due to the complexity of isolating the effect of single compounds by traditional methods.

Methods

Using available quasi-isogenic lines (qILs) that were generated as part of the original mapping of the floral scent volatile-related loci CNL1 (benzaldehyde) and TPS2 (β-ocimene) in Capsella, we generated four genotypes that should only differ in these two compounds. Plants of the four genotypes were introduced into a common garden outside the natural range of C. rubella or C. grandiflora, with individuals of a self-compatible C. grandiflora line as pollen donors, whose different genetic background facilitates the detection of outcrossing events. Visitors to flowers of all five genotypes were compared, and the seeds set during the common-garden period were collected for high-throughput amplicon-based sequencing to estimate their outcrossing rates.

Results

Benzaldehyde and β-ocimene emissions were detected in the floral scent of corresponding genotypes. While some pollinator groups showed specific visitation preferences depending on scent compounds, the outcrossing rates in seeds did not vary among the four scent-manipulated genotypes.

Conclusions

The scent-manipulated Capsella materials constructed using qILs provide a powerful system to study the ecological effects of individual floral scent compounds under largely natural environments. In Capsella, individual benzaldehyde and β-ocimene emission may act as attractants for different types of pollinators.

     

Fewer chromosomes,more co-occurring species within plant lineages: A likely effect of local survival and colonization

Premise

Plant lineages differ markedly in species richness globally, regionally, and locally. Differences in whole-genome characteristics (WGCs) such as monoploid chromosome number, genome size, and ploidy level may explain differences in global species richness through speciation or global extinction. However, it is unknown whether WGCs drive species richness within lineages also in a recent, postglacial regional flora or in local plant communities through local extinction or colonization and regional species turnover.

Methods

We tested for relationships between WGCs and richness of angiosperm families across the Netherlands/Germany/Czechia as a region, and within 193,449 local vegetation plots.

Results

Families that are species-rich across the region have lower ploidy levels and small monoploid chromosomes numbers or both (interaction terms), but the relationships disappear after accounting for continental and local richness of families. Families that are species-rich within occupied localities have small numbers of polyploidy and monoploid chromosome numbers or both, independent of their own regional richness and the local richness of all other locally co-occurring species in the plots. Relationships between WGCs and family species-richness persisted after accounting for niche characteristics and life histories.

Conclusions

Families that have few chromosomes, either monoploid or holoploid, succeed in maintaining many species in local communities and across a continent and, as indirect consequence of both, across a region. We suggest evolutionary mechanisms to explain how small chromosome numbers and ploidy levels might decrease rates of local extinction and increase rates of colonization. The genome of a macroevolutionary lineage may ultimately control whether its species can ecologically coexist.

     

Using geometric morphometrics to determine the “fittest” floral shape: A case study in large-flowered,buzz-pollinated Meriania hernandoi

Premise

Floral shape (relative arrangement and position of floral organs) is critical in mediating fit with pollinators and maximizing conspecific pollen transfer particularly in functionally specialized systems. To date, however, few studies have attempted to quantify flowers as the inherently three-dimensional (3D) structures they are and determine the effect of intraspecific shape variation on pollen transfer. We here addressed this research gap using a functionally specialized system, buzz pollination, in which bees extract pollen through vibrations, as a model. Our study species, Meriania hernandoi (Melastomataceae), undergoes a floral shape change from pseudocampanulate corollas with more actinomorphically arranged stamens (first day) to open corollas with a more zygomorphic androecium (second day) over anthesis, providing a natural experiment to test how variation in floral shape affects pollination performance.

Methods

In one population of M. hernandoi, we bagged 51 pre-anthetic flowers and exposed half of them to bee pollinators when they were in either stage of their shape transition. We then collected flowers, obtained 3D flower models through x-ray computed tomography for 3D geometric morphometric analyses, and counted the pollen grains remaining per stamen (male pollination performance) and stigmatic pollen loads (female pollination performance).

Results

Male pollination performance was significantly higher in open flowers with zygomorphic androecia than in pseudo-campanulate flowers. Female pollination performance did not differ among floral shapes.

Conclusions

These results suggest that there is an “optimal” shape for male pollination performance, while the movement of bees around the flower when buzzing the spread-out stamens results in sufficient pollen deposition regardless of floral shape.

     

Pollen layering and male–male competition: Quantum dots demonstrate that pollen grains compete for space on pollinators

Premise

Almost nothing is known about what happens to pollen grains once they attach to pollinators, although some have postulated that pollen from different donors may form complex, two- or three-dimensional landscapes (e.g., layers or mosaics) that can facilitate male–male competition. For example, pollen that is already on pollinators may preclude the deposition of subsequent pollen grains.

Methods

Using quantum dots to mark the pollen of individual flowers, we explored the possibilities of layering and preclusion in a fly-pollinated iris, Moraea lurida.

Results and Conclusions

The proportion of labeled pollen from the last flower visited diminished in sequential pollen samples taken from the top to the bottom of the pollen load, representing the first empirical evidence for pollen layering. However, the consequences in terms of pollen preclusion were equivocal: Although the pre-existing pollen load size was not a good predictor of new pollen receipt, labeled pollen loads from the last flower visited were significantly smaller than pollen loads from the previous flower visited. Thus, pollen from the previous flower may preclude pollen placement from a subsequently visited flower, and pollen from different flowers may compete for space on pollinators.

     

Hidden cargo: The impact of historical shipping trade on the recent-past and contemporary non-native flora of northeastern United States

Premise

Understanding establishment and spread of non-native plants is important in the face of a homogenizing global flora. While many studies focus on successful, invasive species, fewer have studied failed plant introductions. Until the early 1900s, large quantities of ship ballast, often containing foreign plant propagules, were deposited in New Jersey (USA). The resulting ballast flora is documented in extensive herbarium records, providing us a unique opportunity to analyze successes and failures of novel plant species introductions.

Methods

We used digitized specimens from 75 herbaria to study 264 non-native species introduced into New Jersey through 19th century ballast deposition. We used spatial (density-based clustering; HDBSCAN) and temporal analyses of species retention and geographic spread to quantify disappearance rate, survival, and dispersion through time and define trajectory groups.

Results

Four distinct trajectory groups were identified: waif (only present during import; 32% of species), short-term (disappeared quickly; 20%), established–limited spread (survives locally, 30%), and established–widespread (widespread, 18%). Species disappearance rate was highest during ballast deposition and decreased soon after deposition stopped around 1900. Spatial patterns showed a strong association with 19th century railroads for inland dispersal from ports. The disappearance rate and spatial analyses are robust to herbarium collection bias.

Conclusions

This study using New Jersey as a model is one of the few documenting multispecies successes and failures in inadvertent plant introductions. Results reveal distinct trends in species establishment and geographic spread and highlight the utility of herbarium specimens in answering questions that span large time scales.

     

Dispersal and establishment traits provide a colonization advantage for a polyploid apomictic plant

Premise

Apomictic plants (reproducing asexually through seed) often have larger ranges and occur at higher latitudes than closely related sexuals, a pattern known as geographical parthenogenesis (GP). Explanations for GP include differences in colonizing ability due to reproductive assurance and direct/indirect effects of polyploidy (most apomicts are polyploid) on ecological tolerances. While life history traits associated with dispersal and establishment also contribute to the potential for range expansion, few studies compare these traits in related apomicts and sexuals.

Methods

We investigated differences in early life history traits between diploid-sexual and polyploid-apomictic Townsendia hookeri (Asteraceae), which displays a classic pattern of GP. Using lab and greenhouse experiments, we measured seed dispersal traits, germination success, and seedling size and survival in sexual and apomictic populations from across the range of the species.

Results

While theory predicts that trade-offs between dispersal and establishment traits should be common, this was largely not the case in T. hookeri. Apomictic seeds had both lower terminal velocity (staying aloft longer when dropped) and higher germination success than sexual seeds. While there were no differences in seedling size between reproductive types, apomicts did, however, have slightly lower seedling survival than sexuals.

Conclusions

These differences in early life history traits, combined with reproductive assurance conferred by apomixis, suggest that apomicts achieve a greater range through advantages in their ability to both spread and establish.

     

Conflicting constraints on male mating success shape reward size in pollen-rewarding plants

Premise

Pollen-rewarding plants face two conflicting constraints: They must prevent consumptive emasculation while remaining attractive to pollen-collecting visitors. Small pollen packages (the quantity of pollen available in a single visit) may discourage visitors from grooming (reducing consumptive loss) but may also decrease a plant's attractiveness to pollen-collecting visitors. What package size best balances these two constraints?

Methods

We modeled the joint effects of pollinators' grooming behaviors and package size preferences on the optimal package size (i.e., the size that maximizes pollen donation). We then used this model to examine Darwin's conjecture that selection should favor increased pollen production in pollen-rewarding plants.

Results

When package size preferences are weak, minimizing package size reduces grooming losses and should be favored (as in previous theoretical studies). Stronger preferences select for larger packages despite the associated increase to grooming loss because loss associated with nonremoval of smaller packages is even greater. Total pollen donation increases with production (as Darwin suggested). However, if floral visitation declines or packages size preference increases with overall pollen availability, the fraction of pollen donated may decline as per-plant pollen production increases. Hence, increasing production may result in diminishing returns.

Conclusions

Pollen-rewarding plants can balance conflicting constraints on pollen donation by producing intermediate-sized pollen packages. Strictly pollen-rewarding plants may have responded to past selection to produce more pollen in total, but diminishing returns may limit the strength of that selection.