Specialization of flowers: is floral orientation an overlooked first step?

Feeding efficiency of two coexisting nectarivorous bat species (Phyllostomidae: Glossophaginae) at flowers of two key-resource plants

Animals should maximize their energy uptake while reducing the costs for foraging. For flower-visitors these costs and benefits are rather straight forward as the energy uptake equals the caloric content of the consumed nectar while the costs equal the handling time at the flower. Due to their energetically demanding lifestyle, flower-visiting bats face particularly harsh energetic conditions and thus need to optimize their foraging behavior at the flowers of the different plant species they encounter within their habitat. In flight cage experiments we examined the nectar-drinking behavior (i.e. hovering duration, nectar uptake, and the resulting feeding efficiency) of the specialized nectar-feeding bat Hylonycteris underwoodi and the more generalistic Glossophaga commissarisi at flowers of two plant species that constitute important nectar resources in the Caribbean lowland rainforests of Costa Rica and compared nectar-drinking behavior between both bat species and at both plant species. We hypothesized that the 1) specialized bat should outperform the more generalistic species and that 2) bats should generally perform better at flowers of the nectar-rich flowers of the bromeliad Werauhia gladioliflora than at the relatively nectar-poor flowers of the Solanaceae Merinthopodium neuranthum that has an extremely long flowering phase and therefore is an extremely reliable nectar resource, particularly for the specialized Hylonycteris. While we did not find substantial differences in the feeding efficiency of the generalist G. commissarisi, we observed an increased feeding efficiency of the specialized H. underwoodi at flowers of the nectar-poor M. neuranthum. This suggests that familiarity and ecological importance are more important determinants of the interaction than just morphological traits. Our results demonstrate that in addition to morphology, behavioral adaptations are also important drivers that determine the fitness of nectar-feeding bats. Both familiarity with and the ecological importance of a resource seem to contribute to shaping the interactions between pollinating bats and their plants.

Incomplete resupination during floral development leads to pollination failure in a slipper orchid

In many families, plants undergo floral resupination by twisting through approximately 180° during floral development so that the flower is effectively positioned upside down. In most orchids, resupination results in the median petal (i.e., the labellum) becoming lowermost, which plays a crucial role in pollination by serving as a landing platform or as a trapping device, or both. Incomplete resupination is predicted to lead to reduced pollination, although tests of this assumption are still lacking. We investigated the effect of resupination using Phragmipedium vittatum, a rare lady's slipper orchid whose specialized labellum forms a trapping device. First, we surveyed the natural occurrence of incomplete resupination. Then we manipulated flowers into non- (≈0°), half- (≈90°), and fully resupinate (≈180°) positions to test the effect of orientation on pollen smear removal and deposition by pollinators (female hoverflies). We found that ca. 10% of flowers in the natural population were not fully resupinate, being either non- (upward, 0-60°) or half-resupinate (sideward, 60-120°). The change in orientation prevented the effectiveness of pollination by hoverflies since no pollen smear removal or deposition were found in flowers from non- and half-resupinate treatments. Although these flowers still attracted hoverflies, they were not trapped effectively. As this orchid is incapable of autonomous self-pollination, flowers that do not resupinate fail to set fruits. These results highlight the importance of correct floral orientation provided by resupination to ensure pollination in orchids and other resupinate flowers.

Flower orientation and corolla length as reproductive barriers in the pollinator-driven divergence of Erica shannonea and Erica ampullacea

A variety of reproductive barriers can enable reproductive isolation and stable coexistence of plant species. Differing floral traits might play an important role in reproductive isolation imposed by pollinators. Such shifts in pollinator use have been hypothesized to contribute to the radiation of Erica (Ericaceae) in the Cape Floristic Region, South Africa. The sister species Erica shannonea and Erica ampullacea co-occur and overlap in flowering phenology. Both have unscented long-tubed flowers consistent with adaptations for pollination by long-proboscid flies (LPFs), but differences in flower orientation and corolla tube length are indicative of a shift in pollinator species. We conducted controlled pollination experiments and pollinator observations to determine the breeding system and pollinators of the two species. Both species are self-incompatible and require pollinator visits for seed production, suggesting that pollinators could strongly influence flower evolution. The horizontally orientated flowers of E. shannonea were found to be pollinated by Philoliche rostrata (Tabanidae), which has a long, fixed forward-pointing proboscis, while the vertically upright orientated flowers of E. ampullacea were found to be pollinated by Prosoeca westermanni (Nemestrinidae), which has a shorter proboscis that can swivel downwards. The nemestrinid fly's proboscis is too short to access the nectar in the relative long-tubed flowers of E. shannonea and the tabanid fly's proboscis cannot swivel down to access the upright flowers of E. ampullacea. Consequently, these traits are likely to act as reproductive barriers between the two Erica species and thereby might have contributed to speciation and enable stable coexistence.

Global distribution and evolutionary transitions of floral symmetry in angiosperms

Floral symmetry plays an important role in plant-pollinator interactions and may have remarkable impacts on angiosperm diversification. However, spatiotemporal patterns in floral symmetry and drivers of these patterns remain unknown. Here, using newly compiled floral symmetry (actinomorphy versus zygomorphy) data of 279,877 angiosperm species and their distributions and phylogenies, we estimated global geographic patterns and macroevolutionary dynamics of floral symmetry. We found that frequency of actinomorphic species increased with latitude, while that of zygomorphic species decreased. Solar radiation, present-day temperature, and Quaternary temperature change correlated with geographic variation in floral symmetry frequency. Evolutionary transitions from actinomorphy to zygomorphy dominated floral symmetry evolution, although the transition rate decreased with decreasing paleotemperature throughout the Cenozoic. Notably, we found that zygomorphy may not favor diversification of angiosperms as previously observed in some clades. Our study demonstrates the influence of (paleo)climate on spatiotemporal patterns in floral symmetry and challenges previous views about role of flower symmetry in angiosperm diversification.

CYCLOIDEA-like genes control floral symmetry, floral orientation, and nectar guide patterning

Actinomorphic flowers usually orient vertically (relative to the horizon) and possess symmetric nectar guides, while zygomorphic flowers often face horizontally and have asymmetric nectar guides, indicating that floral symmetry, floral orientation, and nectar guide patterning are correlated. The origin of floral zygomorphy is dependent on the dorsoventrally asymmetric expression of CYCLOIDEA (CYC)-like genes. However, how horizontal orientation and asymmetric nectar guides are achieved remains poorly understood. Here, we selected Chirita pumila (Gesneriaceae) as a model plant to explore the molecular bases for these traits. By analyzing gene expression patterns, protein-DNA and protein-protein interactions, and encoded protein functions, we identified multiple roles and functional divergence of 2 CYC-like genes, i.e. CpCYC1 and CpCYC2, in controlling floral symmetry, floral orientation, and nectar guide patterning. CpCYC1 positively regulates its own expression, whereas CpCYC2 does not regulate itself. In addition, CpCYC2 upregulates CpCYC1, while CpCYC1 downregulates CpCYC2. This asymmetric auto-regulation and cross-regulation mechanism might explain the high expression levels of only 1 of these genes. We show that CpCYC1 and CpCYC2 determine asymmetric nectar guide formation, likely by directly repressing the flavonoid synthesis-related gene CpF3'5'H. We further suggest that CYC-like genes play multiple conserved roles in Gesneriaceae. These findings shed light on the repeated origins of zygomorphic flowers in angiosperms.

Honesty, reliability, and information content of floral signals

Plants advertise their presence by displaying attractive flowers, which pollinators use to locate a floral reward. Understanding how floral traits scale with reward status lies at the heart of pollination biology, because it connects the different interests of plants and pollinators. Studies on plant phenotype-reward associations often use different terms and concepts, which limits developing a broader synthesis. Here, we present a framework with definitions of the key aspects of plant phenotype-reward associations and provide measures to quantify them across different species and studies. We first distinguish between cues and signals, which are often used interchangeably, but have different meanings and are subject to different selective pressures. We then define honesty, reliability, and information content of floral cues/signals and provide ways to quantify them. Finally, we discuss the ecological and evolutionary factors that determine flower phenotype-reward associations, how context-dependent and temporally variable they are, and highlight promising research directions.

Pollen production per flower increases with floral display size across animal-pollinated flowering plants

PREMISE

The number of open flowers on a plant (i.e., floral display size) can influence plant fitness by increasing pollinator attraction. However, diminishing marginal fitness returns with increasing floral display are expected as pollinators tend to visit more flowers per plant consecutively. An extended flower visitation sequence increases the fraction of ovules disabled by self-pollination (ovule discounting) and reduces the fraction of a plant's own pollen that is exported to sire seeds in other plants (pollen discounting). Hermaphroditic species with a genetic system that prevents self-fertilization (self-incompatibility) would avoid ovule discounting and its fitness cost, whereas species without such a genetically based barrier would not. Contrarily, pollen discounting would be an unavoidable consequence of a large floral display irrespective of selfing barriers. Nevertheless, the increasing fitness costs of ovule and pollen discounting could be offset by respectively increasing ovule and pollen production per flower.

METHODS

We compiled data on floral display size and pollen and ovule production per flower for 1241 animal-pollinated, hermaphroditic angiosperm species, including data on the compatibility system for 779 species. We used phylogenetic general linear mixed models to assess the relations of pollen and ovule production to floral display size.

RESULTS

Our findings provide evidence of increasing pollen production, but not of ovule production, with increasing display size irrespective of compatibility system and even after accounting for potentially confounding effects like flower size and growth form.

CONCLUSIONS

Our comparative study supports the pollen-discount expectation of an adaptive link between per-flower pollen production and floral display across animal-pollinated angiosperms.

Effects of floral symmetry and orientation on the consistency of pollinator entry angle

Since the publication of Sprengel's (1793) observations, it has been considered that flowers with zygomorphic (or bilaterally symmetrical) corollas evolved to restrict the movement of pollinators into the flower by limiting the pollinator's direction of approach. However, little empirical support has been accumulated so far. Our aim was to build on previous research that showed zygomorphy reduces variance in pollinator entry angle, aiming to observe whether floral symmetry or orientation had an impact on pollinator entry angle in a laboratory experiment using bumble bees, Bombus ignitus. Using nine different combinations of artificial flowers created from three symmetry types (radial, bilateral and disymmetrical) and three orientation types (upward, horizontal, and downward), we tested the effects of these two floral aspects on the consistency of bee entry angle. Our results show that horizontal orientation significantly reduced the variance in entry angle, while symmetry had little effect. We also found either little or no significant interactions between angle and symmetry in their effect on entry angle. Thus, our results suggest that horizontal orientation forces the bees to orient themselves relative to gravity rather than the corolla and stabilizes their flower entry. This stabilizing effect may have been mistaken for the effect of zygomorphic corolla as it is presented horizontally in most species. Consequently, we suggest that the evolution of horizontal orientation preceded that of zygomorphy as indicated by some authors, and that the reason behind the evolution of zygomorphy should be revisited.

Eighteen new species of Neotropical Costaceae (Zingiberales)

In preparation for a full taxonomic revision of the Neotropical genera of Costaceae (i.e., Chamaecostus, Costus, Dimerocostus, and Monocostus), we present the description of 17 new species of Neotropical Costus and one new species of the Neotropic endemic genus Chamaecostus with notes on their distribution and ecology, vernacular names (when known), and diagnostic characters for identification. Distribution maps are included for all species, and each description is accompanied by photographic plates illustrating diagnostic characters.

Floral orientation affects outcross-pollen deposition in buzz-pollinated flowers with bilateral symmetry

PREMISE

Floral orientation is central to plant-pollinator interactions and is commonly associated with floral symmetry. Bilaterally symmetrical flowers are often oriented horizontally for optimal pollinator positioning and pollen transfer efficiency, while the orientation of radially symmetrical flowers is variable. Buzz-pollinated species (pollinated by vibration-producing bees) include bilateral, horizontally oriented flowers, and radial, pendant flowers. The effect of floral orientation on pollen transfer has never been tested in buzz-pollinated species.

METHODS

Here, we examined the effect of floral orientation on bumblebee-mediated pollen deposition in three buzz-pollinated Solanum species with different floral symmetry and natural orientations: S. lycopersicum and S. seaforthianum (radial, pendant), and S. rostratum (bilateral, horizontal). We tested whether orientation affects total stigmatic pollen deposition (both self and outcross pollen) when all flowers have the same orientation (either pendant or horizontal). In a second experiment, we evaluated whether different orientations of donor and recipient flowers affects the receipt of outcross pollen by S. rostratum.

RESULTS

For the three Solanum species studied, there was no effect of floral orientation on total pollen deposition (both self and outcross) when flowers shared the same orientation. In contrast, in our experiment with S. rostratum, we found that pendant flowers received fewer outcross-pollen grains when paired with pendant donors.

CONCLUSIONS

We suggest that floral orientation influences the quality of pollen transferred, with more outcross pollen transferred to horizontally oriented recipients in the bilaterally symmetrical S. rostratum. Whether other bilaterally symmetrical, buzz-pollinated flowers also benefit from increased cross-pollination when presented horizontally remains to be established.

Horizontal orientation facilitates pollen transfer and rain damage avoidance in actinomorphic flowers of Platycodon grandiflorus

In zoophilous plants, floral orientation evolved under both biotic and abiotic pressure to enhance pollination success. However, the adaptive significance of horizontal orientation in radially symmetrical (actinomorphic) flowers remains largely unknown, although that of bilaterally symmetrical flowers has been well studied. We experimentally altered floral angle in a population of insect-pollinated Platycodon grandiflorus flowers to examine the effects of floral orientation on pollinator behaviour, pollination success and pollen rain damage avoidance. To further investigate the potential pollen damage by rain, we obtained past precipitation records for the study area during the flowering season, and experimentally tested P. grandiflorus pollen damage by water. Horizontally oriented flowers received more pollinator visits and had pollen grains on the stigma in male and/or female phases than downward- and/or upward-oriented flowers and avoided pollen damage by rainfall better than upward-oriented flowers. A pollen germination experiment showed that approximately 30% of pollen grains burst in distilled water, indicating that pollen damage by rainfall may be significant in P. grandiflorus. Our field experiments revealed that upward-oriented flowers cannot avoid pollen damage by rainfall during the flowering period, and that both upward- and downward-oriented flowers experience pollinator limitation in female success. Therefore, horizontal flower orientation appears to be adaptive in this insect-pollinated actinomorphic species that blooms during the rainy season.

Pollination-precision hypothesis: support from native honey bees and nectar bats

The evolution of floral traits is often considered to reflect selection for increased pollination efficiency. Known as the pollination-precision hypothesis, increased pollination efficiency is achieved by enhancing pollen deposition on precise areas of the pollinator. Most research to date addressing this hypothesis has examined plant species that are a priori predicted to place pollen precisely, but we still lack comparisons with species predicted to have low pollination efficiency. We studied 39 plant species with diverse floral morphologies and measured the precision of pollen placement on two pollinator groups: honey bees (genus Apis) and nectar bats (family Pteropodidae). Pollen was collected from four locations of each pollinator's body (bees: dorsal thorax, ventral thorax, dorsal abdomen, ventral abdomen; bats: crown, face, chest, wing) to calculate pollen placement precision using Pielou's evenness index. We also quantified variation in floral design by scoring floral symmetry, corolla fusion, floral orientation and stamen number. We confirm the importance of four floral character states (bilateral symmetry, fused corollas, horizontal orientation and reduced stamen number) in promoting precise pollen placement on diverse pollinators. Our findings provide phylogenetically corrected, empirical support that the evolution of the four floral characters reflect selection for enhanced precision of pollen placed on pollinators.

Flower orientation influences floral temperature, pollinator visits and plant fitness

Effective insect pollination requires appropriate responses to internal and external environmental cues in both the plant and the pollinator. Helianthus annuus, a highly outcrossing species, is marked for its uniform eastward orientation of mature pseudanthia, or capitula. Here we investigate how this orientation affects floral microclimate and the consequent effects on plant and pollinator interactions and reproductive fitness. We artificially manipulated sunflower capitulum orientation and temperature in both field and controlled conditions and assessed flower physiology, pollinator visits, seed traits and siring success. East-facing capitula were found to have earlier style elongation, pollen presentation and pollinator visits compared with capitula manipulated to face west. East-facing capitula also sired more offspring than west-facing capitula and under some conditions produced heavier and better-filled seeds. Local ambient temperature change on the capitulum was found to be a key factor regulating the timing of style elongation, pollen emergence and pollinator visits. These results indicate that eastward capitulum orientation helps to control daily rhythms in floral temperature, with direct consequences on the timing of style elongation and pollen emergence, pollinator visitation, and plant fitness.

Repeated evolution of a reproductive polyphenism in plants is strongly associated with bilateral flower symmetry

Polyphenisms are a special type of phenotypic plasticity in which the products of development are not continuous but instead are separate and distinct phenotypes produced in the same genetic background. One of the most widespread polyphenisms in the flowering plants is cleistogamy, in which the same individual plant produces both open, cross-pollinated flowers as well as highly reduced and closed, self-pollinated (cleistogamous) flowers.^1-5 Cleistogamy is not a rare evolutionary phenomenon. It has evolved independently at least 41 times.¹ But what favors the evolution of cleistogamy is still largely unknown.¹ Darwin⁶ proposed a hypothesis that has never been properly tested. He observed that cleistogamy is more common in taxa with bilaterally symmetric (zygomorphic) flowers than in those with radially symmetric (actinomorphic) flowers. Moreover, Darwin suggested that cleistogamous flowers help to ensure pollination, which he postulated is less certain in zygomorphic taxa that rely on more specialized groups of pollinators. Here, we combined the largest datasets on floral symmetry and cleistogamy and used phylogenetic approaches to show that cleistogamy is indeed disproportionately associated with zygomorphic flowers and that zygomorphic species are more likely to evolve cleistogamy than actinomorphic species. We also show that zygomorphic species are less capable of autonomous open-flower self-pollination (lower autofertility), suggesting that selection of cleistogamy via reproductive assurance in zygomorphic taxa could help account for Darwin's observation. Our results provide support for the hypothesis that polyphenisms are favored when organisms encounter contrasting environments.

Horizontal orientation of zygomorphic flowers: significance for rain protection and pollen transfer

Floral traits are recognized to have evolved under selection for abiotic and biotic factors. Complex zygomorphic flowers usually face horizontally. It has been proved that a horizontal orientation facilitates pollinator recognition and pollination efficiency, but its significance in adaptation to abiotic factors remains unknown. The floral orientation of Abelia × grandiflora naturally varies around horizontal (with an angle of -30 to +33° between the floral main axis and the horizontal). We examined whether three different floral orientations affected flower thermal conditions, response to rain and pollination. Results showed that floral orientation had no effect on diurnal variations in flower temperature. The anthers of all three flower orientations were wetted by rainfall, but the inclined upward-facing flowers contained significantly more rainwater. The horizontal flowers received significantly higher visitation by hawkmoths and had a higher stigmatic pollen load. In contrast, the upward flower orientation reduced pollination precision, while downward-facing flowers had decreased pollinator attraction. This study indicates that horizontal flowers may have evolved as a trade-off between rain protection and pollination. Zygomorphic flowers that deviate from a horizontal orientation may have lower fitness because of flower flooding by rainwater and decreased pollen transfer.

Functional consequences of flower curvature, orientation and perch position for nectar feeding by sunbirds

Mutualisms between nectarivorous birds and the plants they pollinate are functionally diverse. Nectarivorous birds which hover while feeding (the majority of hummingbirds) tend to have straight bills, while those that perch while feeding (some hummingbirds and almost all passerine nectarivores) tend to have decurved bills. Sunbirds typically use their curved bills to feed in an arc from a perching position and we thus predicted that they would prefer, and feed most efficiently on, flowers that are curved towards a perching position. To test this, we examined the responses of sunbirds to model flowers differing in curvature (straight or curved), orientation (facing upwards or downwards), and availability of a top perch (present or absent). Birds did not show preferences among model flower types in terms of number of landings to feed or number of probes. In general they preferred to use perches above model flowers, particularly those that curved upwards, but they tended to perch below model flowers that curve downwards and in such cases also took the least time to insert their bills. These results are consistent with the idea that perching birds with curved beaks will feed most efficiently from flowers that are curved towards the perching position. We discuss the implications of these results for the evolution of floral architecture, including provision of perches.

Functional significance of flower orientation and green marks on tepals in the snowdrop Galanthus nivalis (Linnaeus, 1753)

Flower shapes, colors, sizes and fragrances are shaped mostly for pollinator attraction. Flower phenotypes are, however, subjected to conflicting selection directed by both pollinators and non-pollinating agents. We investigated flower attractiveness to a model pollinator in the snowdrop (Galanthus nivalis L.) under laboratory conditions. Naïve bumblebees (Bombus terrestris L.) showed strong, innate preferences for experimentally altered upward positioned flowers, suggesting that the natural, downward orientation did not evolve to attract pollinators. Experimentally treated green marks on inner tepals decreased pollinator attraction compared with flowers expressing intact marks, suggesting that green marks serve to guide/attract pollinators. Attractiveness of green marks was significantly compromised by flower orientation; green marks were attractive only for untreated downward-oriented flowers, but they did not improve the attractiveness of upward-oriented flowers. Our results suggest that downward flowers in snowdrop evolved under conflicting selection directed by biotic and abiotic factors, and that green marks on inner tepals could evolve later to enhance flower attractiveness.

Pollination syndromes in the 21st century: where do we stand and where may we go?

Pollination syndromes, recurring suites of floral traits appearing in connection with specific functional pollinator groups, have served for decades to organise floral diversity under a functional-ecological perspective. Some potential caveats, such as over-simplification of complex plant-animal interactions or lack of empirical observations, have been identified and discussed in recent years. Which of these caveats do indeed cause problems, which have been solved and where do future possibilities lie? I address these questions in a review of the pollination-syndrome literature of 2010 to 2019. I show that the majority of studies was based on detailed empirical pollinator observations and could reliably predict pollinators based on a few floral traits such as colour, shape or reward. Some traits (i.e. colour) were less reliable in predicting pollinators than others (i.e. reward, corolla width), however. I stress that future studies should consider floral traits beyond those traditionally recorded to expand our understanding of mechanisms of floral evolution. I discuss statistical methods suitable for objectively analysing the interplay of system-specific evolutionary constraints, pollinator-mediated selection and adaptive trade-offs at microecological and macroecological scales. I exemplify my arguments on an empirical dataset of floral traits of a neotropical plant radiation in the family Melastomataceae.

Zygomorphic flowers have fewer potential pollinator species

Botanists have long identified bilaterally symmetrical (zygomorphic) flowers with more specialized pollination interactions than radially symmetrical (actinomorphic) flowers. Zygomorphic flowers facilitate more precise contact with pollinators, guide pollinator behaviour and exclude less effective pollinators. However, whether zygomorphic flowers are actually visited by a smaller subset of available pollinator species has not been broadly evaluated. We compiled 53 609 floral visitation records in 159 communities and classified the plants' floral symmetry. Globally and within individual communities, plants with zygomorphic flowers are indeed visited by fewer species. At the same time, zygomorphic flowers share a somewhat larger proportion of their visitor species with other co-occurring plants and have particularly high sharing with co-occurring plants that also have zygomorphic flowers. Visitation sub-networks for zygomorphic species also show differences that may arise from reduced visitor diversity, including greater connectance, greater web asymmetry and lower coextinction robustness of both plants and visitor species-but these changes do not necessarily translate to whole plant-visitor communities. These results provide context for widely documented associations between zygomorphy and diversification and imply that species with zygomorphic flowers may face a greater risk of extinction due to pollinator loss.

Floral reorientation: the restoration of pollination accuracy after accidents

Plants sometimes suffer mechanical injury. The nonlethal collapse of a flowering stalk, for example, can greatly reduce plant fitness if it leads to 'incorrect' floral orientation and thus reduced visitation or poor pollination. When floral orientation is important for accurate pollination, as has been suggested for bilaterally symmetrical flowers, we predict that such flowers should have developmental and/or behavioural mechanisms for restoring 'correct' orientation after accidents. We made observations and conducted experiments on 23 native and cultivated flowering plant species in Australia, South America, North America and Europe. We found that flowers with bilateral symmetry usually have the capacity to reorient after accidents, and that this is manifested through rapid bending and/or rotation of pedicels or sexual organs or slower peduncle bending. Floral reorientation restores pollination accuracy and fit with pollinators. However, experimental floral misorientation in eight species with radially symmetrical flowers showed that, with one exception, they had little capacity to reorient their flowers, in line with expectations that the orientation of radially symmetrical flowers does not substantially affect pollination accuracy. Our results suggest that quick corrective reorientation of bilaterally symmetrical flowers is adaptive, highlighting a little-studied aspect of plant-pollinator interactions and plant evolution.

A Binary-Based Matrix Model for Malus Corolla Symmetry and Its Variational Significance

Floral symmetry (corolla symmetry) has important biological significance in plant genetics and evolution. However, it is often multi-dimensional and difficult to quantify. Here, we constructed a multi-dimensional data matrix [X Y Z] by extracting three qualitative variables with binary properties (X: corolla regularity of interval and coplanarity; Y: petal regularity of shape and size; Z: petal local regularity of curling and wrinkle) from different dimensions of petals (overall to individual, and then to the local): all petals (corolla), individual petals, and local areas of petals. To quantitatively express the degree of Malus corolla symmetry, these variables were then combined with weight assignments (X: 2² > Y: 2¹ > Z: 2⁰) based on their contributions to the corolla symmetry and the algorithm rule of converting binary to decimal values, which facilitated the unification of qualitative and quantitative analyses. Our results revealed significant reduction in degrees of Malus corolla symmetry along the direction of local to overall. Species showed higher degree of corolla symmetry than cultivars; however, taxa with stronger corolla symmetry might not necessarily be species. These findings provide new insights into the circumscription of Malus controversial species. The matrix model should be reference for future evaluation of angiosperm flower symmetry (lack of corolla fusion).

Radial or Bilateral? The Molecular Basis of Floral Symmetry

In the plant kingdom, the flower is one of the most relevant evolutionary novelties. Floral symmetry has evolved multiple times from the ancestral condition of radial to bilateral symmetry. During evolution, several transcription factors have been recruited by the different developmental pathways in relation to the increase of plant complexity. The MYB proteins are among the most ancient plant transcription factor families and are implicated in different metabolic and developmental processes. In the model plant Antirrhinum majus, three MYB transcription factors (DIVARICATA, DRIF, and RADIALIS) have a pivotal function in the establishment of floral dorsoventral asymmetry. Here, we present an updated report of the role of the DIV, DRIF, and RAD transcription factors in both eudicots and monocots, pointing out their functional changes during plant evolution. In addition, we discuss the molecular models of the establishment of flower symmetry in different flowering plants.

A lever action hypothesis for pendulous hummingbird flowers: experimental evidence from a columbine

BACKGROUND AND AIMS

Pendulous flowers (due to a flexible pedicel) are a common, convergent trait of hummingbird-pollinated flowers. However, the role of flexible pedicels remains uncertain despite several functional hypotheses. Here we present and test the 'lever action hypothesis': flexible pedicels allow pendulous flowers to move upwards from all sides, pushing the stigma and anthers against the underside of the feeding hummingbird regardless of which nectary is being visited.

METHODS

To test whether this lever action increased pollination success, we wired emasculated flowers of serpentine columbine, Aquilegia eximia, to prevent levering and compared pollination success of immobilized flowers with emasculated unwired and wire controls.

KEY RESULTS

Seed set was significantly lower in wire-immobilized flowers than unwired control and wire control flowers. Video analysis of visits to wire-immobilized and unwired flowers demonstrated that birds contacted the stigmas and anthers of immobilized flowers less often than those of flowers with flexible pedicels.

CONCLUSIONS

We conclude that flexible pedicels permit the levering of reproductive structures onto a hovering bird. Hummingbirds, as uniquely large, hovering pollinators, differ from flies or bees which are too small to cause levering of flowers while hovering. Thus, flexible pedicels may be an adaptation to hummingbird pollination, in particular due to hummingbird size. We further speculate that this mechanism is effective only in radially symmetric flowers; in contrast, zygomorphic hummingbird-pollinated flowers are usually more or less horizontally oriented rather than having pendulous flowers and flexible pedicels.

The thermal ecology of flowers

BACKGROUND

Obtaining an optimal flower temperature can be crucial for plant reproduction because temperature mediates flower growth and development, pollen and ovule viability, and influences pollinator visitation. The thermal ecology of flowers is an exciting, yet understudied field of plant biology.

SCOPE

This review focuses on several attributes that modify exogenous heat absorption and retention in flowers. We discuss how flower shape, orientation, heliotropic movements, pubescence, coloration, opening-closing movements and endogenous heating contribute to the thermal balance of flowers. Whenever the data are available, we provide quantitative estimates of how these floral attributes contribute to heating of the flower, and ultimately plant fitness.

OUTLOOK

Future research should establish form-function relationships between floral phenotypes and temperature, determine the fitness effects of the floral microclimate, and identify broad ecological correlates with heat capture mechanisms.

Implications of region-specific gene expression for development of the partially fused petunia corolla

Angiosperm petal fusion (sympetaly) has evolved multiple times independently and is associated with increased specificity between plants and their pollinators. To uncover developmental genetic changes that might have led to the evolution of sympetaly in the asterid core eudicot genus Petunia (Solanaceae), we carried out global and fine-scale gene expression analyses in different regions of the corolla. We found that, despite several similarities with the choripetalous model species Arabidopsis thaliana in the proximal-distal transcriptome, the Petunia axillaris fused and proximal corolla tube expresses several genes that in A. thaliana are associated with the distal petal region. This difference aligns with variation in petal shape and fusion across ontogeny of the two species. Moreover, differential gene expression between the unfused lobes and fused tube of P. axillaris petals revealed three strong candidate genes for sympetaly based on functional annotation in organ boundary specification. Partial silencing of one of these, the HANABA TARANU (HAN)-like gene PhGATA19, resulted in reduced fusion of Petunia hybrida petals, with silencing of both PhGATA19 and its close paralog causing premature plant senescence. Finally, detailed expression analyses for the previously characterized organ boundary gene candidate NO APICAL MERISTEM (NAM) supports the hypothesis that it establishes boundaries between most P. axillaris floral organs, with the exception of boundaries between petals.

Evolution of Darwin's Peloric Gloxinia (Sinningia speciosa) Is Caused by a Null Mutation in a Pleiotropic TCP Gene

Unlike most crops, which were domesticated through long periods of selection by ancient humans, horticultural plants were primarily domesticated through intentional selection over short time periods. The molecular mechanisms underlying the origin and spread of novel traits in the domestication process have remained largely unexplored in horticultural plants. Gloxinia (Sinningia speciosa), whose attractive peloric flowers influenced the thoughts of Darwin, have been cultivated since the early 19th century, but its origin and genetic basis are currently unknown. By employing multiple experimental approaches including genetic analysis, genotype-phenotype associations, gene expression analysis, and functional interrogations, we showed that a single gene encoding a TCP protein, SsCYC, controls both floral orientation and zygomorphy in gloxinia. We revealed that a causal mutation responsible for the development of peloric gloxinia lies in a 10-bp deletion in the coding sequence of SsCYC. By combining genetic inference and literature searches, we have traced the putative ancestor and reconstructed the domestication path of the peloric gloxinia, in which a 10-bp deletion in SsCYC under selection triggered its evolution from the wild progenitor. The results presented here suggest that a simple genetic change in a pleiotropic gene can promote the elaboration of floral organs under intensive selection pressure.

Patterns of Diversity of Floral Symmetry in Angiosperms: A Case Study of the Order Apiales

Floral symmetry is widely known as one of the most important structural traits of reproductive organs in angiosperms. It is tightly related to the shape and arrangement of floral parts, and at the same time, it plays a key role in general appearance (visual gestalt) of a flower, which is especially important for the interactions of zoophilous flowers with their pollinators. The traditional classification of floral symmetry divides nearly all the diversity of angiosperm flowers into actinomorphic and zygomorphic ones. Within this system, which is useful for ecological studies, many variations of symmetry appear to be disregarded. At the same time, the diversity of floral symmetry is underpinned not only by ecological factors, but also by morphogenetic mechanisms and constraints. Sometimes it is not an easy task to uncover the adaptive or developmental significance of a change of the floral symmetry in a particular lineage. Using the asterid order Apiales as a model group, we demonstrate that such changes can correlate with the merism of the entire flower or of its particular whorl, with the relative orientation of gynoecium to the rest of the flower, with the presence of sterile floral elements and other morphological characters. Besides, in some taxa, the shape and symmetry of the flower change in the course of its development, which should be taken in consideration in morphological comparisons and evaluations of synapomorphies in a particular clade. Finally, we show that different results can be obtained due to employment of different approaches: for instance, many flowers that are traditionally described as actinomorphic turn out to be disymmetric, monosymmetric, or asymmetric from a more detailed look. The traditional method of division into actinomorphy and zygomorphy deals with the general appearance of a flower, and mainly considers the shape of the corolla, while the geometrical approach handles the entire three-dimensional structure of the flower, and provides an exact number of its symmetry planes.

Flower movement balances pollinator needs and pollen protection

Flower signaling and orientation are key characteristics that determine a flower's pollinator guild. However, many flowers actively move during their daily cycle, changing both their detectability and accessibility to pollinators. The flowers of the wild tobacco Nicotiana attenuata orientate their corolla upward at sunset and downward after sunrise. Here, we investigated the effect of different flower orientations on a major pollinator of N. attenuata, the hawkmoth Manduca sexta. We found that although flower orientation influenced the flight altitude of the moth in respect to the flower, it did not alter the moth's final flower choice. These behavioral observations were consistent with the finding that orientation did not systematically change the spatial distribution of floral volatiles, which are major attractants for the moths. Moreover, hawkmoths invested the same amount of time into probing flowers at different orientations, even though they were only able to feed and gather pollen from horizontally and upward-oriented flowers, but not from downward-facing flowers. The orientation of the flower was hence crucial for a successful interaction between N. attenuata and its hawkmoth pollinator. Additionally, we also investigated potential adverse effects of exposing flowers at different orientations to natural daylight levels, finding that anther temperature of upward-oriented flowers was more than 7°C higher than for downward-oriented flowers. This increase in temperature likely caused the significantly reduced germination success that was observed for pollen grains from upward-oriented flowers in comparison to those of downward and horizontally oriented flowers. These results highlight the importance of flower reorientation to balance pollen protection and a successful interaction of the plant with its insect pollinators by maintaining the association between flower volatiles and flower accessibility to the pollinator.

Novel Traits, Flower Symmetry, and Transcriptional Autoregulation: New Hypotheses From Bioinformatic and Experimental Data

A common feature in developmental networks is the autoregulation of transcription factors which, in turn, positively or negatively regulate additional genes critical for developmental patterning. When a transcription factor regulates its own expression by binding to cis-regulatory sites in its gene, the regulation is direct transcriptional autoregulation (DTA). Indirect transcriptional autoregulation (ITA) involves regulation by proteins expressed downstream of the target transcription factor. We review evidence for a hypothesized role of DTA in the evolution and development of novel flowering plant phenotypes. We additionally provide new bioinformatic and experimental analyses that support a role for transcriptional autoregulation in the evolution of flower symmetry. We find that 5' upstream non-coding regions are significantly enriched for predicted autoregulatory sites in Lamiales CYCLOIDEA genes-an upstream regulator of flower monosymmetry. This suggests a possible correlation between autoregulation of CYCLOIDEA and the origin of monosymmetric flowers near the base of Lamiales, a pattern that may be correlated with independently derived monosymmetry across eudicot lineages. We find additional evidence for transcriptional autoregulation in the flower symmetry program, and report that Antirrhinum DRIF2 may undergo ITA. In light of existing data and new data presented here, we hypothesize how cis-acting autoregulatory sites originate, and find evidence that such sites (and DTA) can arise subsequent to the evolution of a novel phenotype.

Transcriptome-Wide Identification and Expression Analysis of DIVARICATA- and RADIALIS-Like Genes of the Mediterranean Orchid Orchis italica

Bilateral symmetry of flowers is a relevant novelty that has occurred many times throughout the evolution of flowering plants. In Antirrhinum majus, establishment of flower dorso-ventral asymmetry is mainly due to interaction of TCP (CYC and DICH) and MYB (DIV, RAD, and DRIF) transcription factors. In the present study, we characterized 8 DIV-, 4 RAD-, and 2 DRIF-like genes from the transcriptome of Orchis italica, an orchid species with bilaterally symmetric and resupinate flowers. We found a similar number of DIV- and RAD-like genes within the genomes of Phalaenopsis equestris and Dendrobium catenatum orchids. Orchid DIV- and RAD-like proteins share conserved motifs whose distribution reflects their phylogeny and analysis of the genomic organization revealed a single intron containing many traces of transposable elements. Evolutionary analysis has shown that purifying selection acts on the DIV- and RAD-like coding regions in orchids, with relaxation of selective constraints in a branch of the DIV-like genes. Analysis of the expression patterns of DIV- and RAD-like genes in O. italica revealed possible redundant functions for some of them. In the perianth of O. italica, the ortholog of DIV and DRIF of A. majus are expressed in all tissues, whereas RAD is mainly expressed in the outer tepals and lip. These data allow for proposal of an evolutionary conserved model in which the expression of the orthologs of the DIV, RAD, and DRIF genes might be related to establishment of flower bilateral symmetry in the nonmodel orchid species O. italica.

Slow stamen movement in a perennial herb decreases male-male and male-female interference

Approximately 80 % of angiosperm species produce hermaphroditic flowers, which face the problem of male-male sexual interference (one or more anthers gets in the way of disseminating pollen from other anthers) or male-female sexual interference (the pistil interferes with disseminating pollen from the anthers by preventing the anther from touching a pollinator, or the anther prevents pollinator from depositing outcross pollen on the stigma). Slow stamen movement in hermaphrodite flowers has been interpreted as an adaptation for reducing male-male sexual interference. Using slow stamen movement in Lychnis cognata (Caryophyllaceae), this study presents new evidence that this phenomenon can reduce both male-male and male-female sexual interference. Ten stamens in L. cognata flowers vertically elongated their filaments in two batches and displayed similar patterns in pollen dispensing. More importantly, 10 stamens bend out of the floral centre by curving the filament also in 2 batches and pollen grains located at the flower centre displayed the highest viability. Thus, three stages of stamen movement can be identified, comprising two male stages (M1 and M2) and one female stage (F). We found that the main pollinator for L. cognata, Bhutanitis yulongensis (Papilionodae) generally preferred M1 flowers. Manipulation experiments show that vertical stamen movement enabled the anthers to dehisce at different times to prolong the presentation of pollen grains. Horizontal movement of the stamen decreased both male-male and male-female interference. However, vertical stamen movement had a minor role in increasing amount of pollen received by the stigma. This study provides the first direct experimental evidence of concurrent male-male and male-female interference in a flower. We suggest that the selection pressure to reduce such interference might be a strong force in floral evolution. We also propose that other selective pressure, including pollen dispensing mechanisms, pollen longevity, pollinator behaviour and weather, might contribute to floral evolution.

Fitness consequences of altering floral circadian oscillations for Nicotiana attenuata

Ecological interactions between flowers and pollinators are all about timing. Flower opening/closing and scent emissions are largely synchronized with pollinator activity, and a circadian clock regulates these rhythms. However, whether the circadian clock increases a plant's reproductive success by regulating these floral rhythms remains untested. Flowers of Nicotiana attenuata, a wild tobacco, diurnally and rhythmically open, emit scent and move vertically through a 140° arc to interact with nocturnal hawkmoths. We tethered flowers to evaluate the importance of flower positions for Manduca sexta-mediated pollinations; flower position dramatically influenced pollination. We examined the pollination success of phase-shifted flowers, silenced in circadian clock genes, NaZTL, NaLHY, and NaTOC1, by RNAi. Circadian rhythms in N. attenuata flowers are responsible for altered seed set from outcrossed pollen.

Plant Biology: Flower Orientation, Temperature Regulation and Pollinator Attraction

The reproductive performance of plants depends on the temperature of the flower. A recent study reports the mechanistic basis of flower head orientation in sunflowers and provides intriguing hints as to its functional significance.

Wind alters landing dynamics in bumblebees

Landing is an important but understudied behavior that flying animals must perform constantly. In still air, insects decelerate smoothly prior to landing by employing the relatively simple strategy of maintaining a constant rate of image expansion during their approach. However, it is unclear whether insects employ this strategy when faced with challenging flight environments. Here, we tested the effects of wind on bumblebees (Bombus impatiens) landing on flowers. We find that bees' approach paths to flowers shift from multidirectional in still air to unidirectional in wind, regardless of flower orientation. In addition, bees landing in a 3.5 m s^-1 headwind do not decelerate smoothly, but rather maintain a high flight speed until contact, resulting in higher peak decelerations upon impact. These findings suggest that wind has a strong influence on insect landing behavior and performance, with important implications for the design of micro aerial vehicles and the ecomechanics of insect flight.

Corolla chirality does not contribute to directed pollen movement in Hypericum perforatum (Hypericaceae): mirror image pinwheel flowers function as radially symmetric flowers in pollination

Corolla chirality, the pinwheel arrangement of petals within a flower, is found throughout the core eudicots. In 15 families, different chiral type flowers (i.e., right or left rotated corolla) exist on the same plant, and this condition is referred to as unfixed/enantiomorphic corolla chirality. There are no investigations on the significance of unfixed floral chirality on directed pollen movement even though analogous mirror image floral designs, for example, enantiostyly, has evolved in response to selection to direct pollinator and pollen movement. Here, we examine the role of corolla chirality on directing pollen transfer, pollinator behavior, and its potential influence on disassortative mating. We quantified pollen transfer and pollinator behavior and movement for both right and left rotated flowers in two populations of Hypericum perforatum. In addition, we quantified the number of right and left rotated flowers at the individual level. Pollinators were indifferent to corolla chirality resulting in no difference in pollen deposition between right and left flowers. Corolla chirality had no effect on pollinator and pollen movement between and within chiral morphs. Unlike other mirror image floral designs, corolla chirality appears to play no role in promoting disassortative mating in this species.

Reproductive isolation between Zaluzianskya species: the influence of volatiles and flower orientation on hawkmoth foraging choices

Floral trait differences between related species may play a key role in reproductive isolation imposed by pollinators. Volatile emissions can influence pollinator choice, but how they act in combination with traits such as flower orientation is rarely studied. We compared flower-opening patterns, morphology, colour, orientation and volatile emissions for two closely related species of Zaluzianskya and their natural hybrids. Hawkmoth pollinators were tested for preference between flowers of the two species, and between flowers with manipulations of volatiles or orientation. Flowers of Z. natalensis and Z. microsiphon open at night and day, respectively, but they overlap during early evening, when hawkmoths showed a strong preference for Z. natalensis. The species have similar flower size and colour, but Z. natalensis emits more floral volatiles in the evening and presents flowers vertically face-up as opposed to horizontally in Z. microsiphon, whereas natural hybrids are intermediate. Adding methyl benzoate and linalool to flowers of Z. microsiphon did not increase hawkmoth attraction, but re-orientation of flowers to face vertically increased attraction when scent cues were present, whereas re-orientation of Z. natalensis flowers to face horizontally decreased attraction. This study highlights the importance of flower orientation in imposing reproductive isolation.

Post-floral Erection of Stalks Provides Insight into the Evolution of Fruit Orientation and Its Effects on Seed Dispersal

That stalks reorient after flowering to face upwards is a common phenomenon in many flowering plants, indicating the potential importance of fruit orientation on seed dispersal. But this idea has not been subject to an empirical test. We examined this hypothesis by analysing the evolutionary correlation between fruit orientation and other characters and by investigating the effects of fruit orientation on seed dispersal. We found that 1) in a sub-alpine plant community, upward fruit orientation strongly correlates with fruits that act as seed containers, which are often of dry type and are dispersed by non-animal vectors; 2) as exemplified by the Campanulaceae s. str., fruit orientation strongly correlates with dehiscence position. Upwardly-oriented capsules dehisce at the apex, whereas pendent ones dehisce at the base, in both cases ensuring that seeds are released from an upright position; 3) in manipulation experiments on Silene chungtienensis, upward fruits (the natural state) exhibit much greater dispersal distances and more dispersive pattern than pendent ones, and have a more even distribution of dispersal direction than horizontal ones. Our results suggest that fruit orientation may have important function in seed dispersal, which may be the reason why the phenomenon that stalk erection after flowering occurs widely.

Low flower-size variation in bilaterally symmetrical flowers: Support for the pollination precision hypothesis

PREMISE OF THE STUDY

The evolutionary shift from radial to bilateral symmetry in flowers is generally associated with the evolution of low flower-size variation. This phenomenon supports the hypothesis that the lower size variation in bilateral flowers can be attributed to low pollinator diversity. In this study, we propose two other hypotheses to explain low flower-size variation in bilateral symmetrical flowers. To test the three hypotheses, we examined the relative importance of pollinator diversity, composition, and bilateral symmetry itself as selective forces on low flower-size variation.

METHODS

We examined pollinator diversity and composition and flower-size variation for 36 species in a seminatural ecosystem with high bee richness and frequent lepidopteran visitation.

KEY RESULTS

Bilateral flowers were more frequently visited than radial flowers by larger bees, but functional-group diversity of the pollinators did not differ between symmetry types. Although bilateral flowers had significantly lower flower-size variation than radial flowers, flower-size variation did not vary with pollinator diversity and composition but was instead related to bilateral symmetry.

CONCLUSIONS

Our results suggest that the lower size variation in bilateral flowers might have evolved under selection favoring the control of pollinator behavior on flowers to enhance the accurate placement of pollen on the body of the pollinator, independent of pollinator type. Because of the limited research on this issue, future work should be conducted in various types of plant-pollinator communities worldwide to further clarify the issue.

Evolutionary ecology of specialization: insights from phylogenetic analysis

In this Special feature, we assemble studies that illustrate phylogenetic approaches to studying salient questions regarding the effect of specialization on lineage diversification. The studies use an array of techniques involving a wide-ranging collection of biological systems (plants, butterflies, fish and amphibians are all represented). Their results reveal that macroevolutionary examination of specialization provides insight into the patterns of trade-offs in specialized systems; in particular, the genetic mechanisms of trade-offs appear to extend to very different aspects of life history in different groups. In turn, because a species may be a specialist from one perspective and a generalist in others, these trade-offs influence whether we perceive specialization to have effects on the evolutionary success of a lineage when we examine specialization only along a single axis. Finally, how geographical range influences speciation and extinction of specialist lineages remains a question offering much potential for further insight.

Morphological and ecological divergence of Lilium and Nomocharis within the Hengduan Mountains and Qinghai-Tibetan Plateau may result from habitat specialization and hybridization

BACKGROUND

Several previous studies have shown that some morphologically distinctive, small genera of vascular plants that are endemic to the Qinghai-Tibetan Plateau and adjacent Hengduan Mountains appear to have unexpected and complex phylogenetic relationships with their putative sisters, which are typically more widespread and more species rich. In particular, the endemic genera may form one or more poorly resolved paraphyletic clades within the sister group despite distinctive morphology. Plausible explanations for this evolutionary and biogeographic pattern include extreme habitat specialization and hybridization. One genus consistent with this pattern is Nomocharis Franchet. Nomocharis comprises 7-15 species bearing showy-flowers that are endemic to the H-D Mountains. Nomocharis has long been treated as sister to Lilium L., which is comprised of more than 120 species distributed throughout the temperate Northern Hemisphere. Although Nomocharis appears morphologically distinctive, recent molecular studies have shown that it is nested within Lilium, from which is exhibits very little sequence divergence. In this study, we have used a dated molecular phylogenetic framework to gain insight into the timing of morphological and ecological divergence in Lilium-Nomocharis and to preliminarily explore possible hybridization events. We accomplished our objectives using dated phylogenies reconstructed from nuclear internal transcribed spacers (ITS) and six chloroplast markers.

RESULTS

Our phylogenetic reconstruction revealed several Lilium species nested within a clade of Nomocharis, which evolved ca. 12 million years ago and is itself nested within the rest of Lilium. Flat/open and horizon oriented flowers are ancestral in Nomocharis. Species of Lilium nested within Nomocharis diverged from Nomocharis ca. 6.5 million years ago. These Lilium evolved recurved and campanifolium flowers as well as the nodding habit by at least 3.5 million years ago. Nomocharis and the nested Lilium species had relatively low elevation ancestors (<1000 m) and underwent diversification into new, higher elevational habitats 3.5 and 5.5 million years ago, respectively. Our phylogeny reveals signatures of hybridization including incongruence between the plastid and nuclear gene trees, geographic clustering of the maternal (i.e., plastid) lineages, and divergence ages of the nuclear gene trees consistent with speciation and secondary contact, respectively.

CONCLUSIONS

The timing of speciation and ecological and morphological evolutionary events in Nomocharis are temporally consistent with uplift in the Qinghai-Tibetan Plateau and of the Hengduan Mountains 7 and 3-4 million years ago, respectively. Thus, we speculate that the mountain building may have provided new habitats that led to specialization of morphological and ecological features in Nomocharis and the nested Lilium along ecological gradients. Additionally, we suspect that the mountain building may have led to secondary contact events that enabled hybridization in Lilium-Nomocharis. Both the habitat specialization and hybridization have probably played a role in generating the striking morphological differences between Lilium and Nomocharis.

Trends in flower symmetry evolution revealed through phylogenetic and developmental genetic advances

A striking aspect of flowering plant (angiosperm) diversity is variation in flower symmetry. From an ancestral form of radial symmetry (polysymmetry, actinomorphy), multiple evolutionary transitions have contributed to instances of non-radial forms, including bilateral symmetry (monosymmetry, zygomorphy) and asymmetry. Advances in flowering plant molecular phylogenetic research and studies of character evolution as well as detailed flower developmental genetic studies in a few model species (e.g. Antirrhinum majus, snapdragon) have provided a foundation for deep insights into flower symmetry evolution. From phylogenetic studies, we have a better understanding of where during flowering plant diversification transitions from radial to bilateral flower symmetry (and back to radial symmetry) have occurred. From developmental studies, we know that a genetic programme largely dependent on the functional action of the CYCLOIDEA gene is necessary for differentiation along the snapdragon dorsoventral flower axis. Bringing these two lines of inquiry together has provided surprising insights into both the parallel recruitment of a CYC-dependent developmental programme during independent transitions to bilateral flower symmetry, and the modifications to this programme in transitions back to radial flower symmetry, during flowering plant evolution.

Bumblebees learn polarization patterns

Foraging insect pollinators such as bees must find and identify flowers in a complex visual environment. Bees use skylight polarization patterns for navigation [1–3], a capacity mediated by the polarization-sensitive dorsal rim area (DRA) of their eye [4, 5]. While other insects use polarization sensitivity to identify appropriate habitats [6], oviposition sites, and food sources [7], to date no nonnavigational functions of polarization vision have been identified in bees. Here we investigated the ability of bumblebees (Bombus terrestris) to learn polarization patterns on artificial “flowers” in order to obtain a food reward. We show that foraging bumblebees can learn to discriminate between two differently polarized targets, but only when the target artificial “flower” is viewed from below. A context for these results is provided by polarization imaging of bee-pollinated flowers, revealing the potential for polarization patterns in real flowers. Bees may therefore have the ability to use polarization vision, possibly mediated by their polarization-sensitive DRA, both for navigation and to learn polarization patterns on flowers, the latter being the first nonnavigational function for bee polarization vision to be identified.

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Bumblebees (Bombus terrestris) learn polarization patterns on artificial “flowers”

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Polarization patterns were only learned from downward-facing, pendant “flowers”

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Polarization vision in bumblebees is not restricted to sun-compass navigation

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Polarization patterns of petals may be a component of floral signaling

Change of floral orientation within an inflorescence affects pollinator behavior and pollination efficiency in a bee-pollinated plant, Corydalis sheareri

Vertical raceme or spike inflorescences that are bee-pollinated tend to present their flowers horizontally. Horizontal presentation of flowers is hypothesized to enhance pollinator recognition and pollination precision, and it may also ensure greater consistency of pollinator movement on inflorescences. We tested the hypotheses using bee-pollinated Corydalis sheareri which has erect inflorescences consisting of flowers with horizontal orientation. We altered the orientation of individual flowers and prepared three types of inflorescences: (i) unmanipulated inflorescences with horizontal-facing flowers, (ii) inflorescences with flowers turned upward, and (iii) inflorescences with flowers turned downward. We compared number of inflorescences approached and visited, number of successive probes within an inflorescence, the direction percentage of vertical movement on inflorescences, efficiency of pollen removal and seed production per inflorescence. Deviation from horizontal orientation decreased both approaches and visits by leafcutter bees and bumble bees to inflorescences. Changes in floral orientation increased the proportion of downward movements by leafcutter bees and decreased the consistency of pollinator movement on inflorescences. In addition, pollen removal per visit and seed production per inflorescence also declined with changes of floral orientation. In conclusion, floral orientation seems more or less optimal as regards bee behavior and pollen transfer for Corydalis sheareri. A horizontal orientation may be under selection of pollinators and co-adapt with other aspects of the inflorescence and floral traits.

Bilateral flower symmetry--how, when and why?

Bilateral flower symmetry has evolved multiple times during flowering plant diversification, is associated with specialized pollination, and is hypothesized to have contributed to flowering plant species richness. The genes and genetic interactions that control bilateral symmetry are well understood in the model species Snapdragon (Antirrhinum majus). I review recent insights into the genetic control of symmetry in Snapdragon. I summarize how this foundational genetic work has been integrated with mathematical modeling approaches, which together provided new insights into the control of quantitative aspects of petal shape. Lastly, I review how evolutionary studies, stemming from knowledge of the genetic control of symmetry in Snapdragon flowers, have revealed extensive parallel recruitment of a similar genetic program during repeated evolution of bilateral symmetry.