Floral Colour Patterns Research Articles

Pollen, anther, stamen, and androecium mimicry.

Floral colours represent a highly diverse communication signal mainly involved in flower visitors' attraction and guidance, but also flower discrimination, filtering non-pollinators and discouraging floral antagonists. The divergent visual systems and colour preferences of flower visitors, as well as the necessity of cues for flower detection and discrimination, foster the diversity of floral colours and colour patterns. Despite the bewildering diversity of floral colour patterns, a recurrent component is a yellow UV-absorbing floral centre, and it is still not clear why this pattern is so frequent inangiosperms. The pollen, anther, stamen, and androecium mimicry (PASAM) hypothesis suggests that the system composed of the flowers possessing such yellow UV-absorbing floral reproductive structures, the flowers displaying central yellow UV-absorbing structures as floral guides, and the pollen-collecting, as well as pollen-eating, flower visitors responding to such signals constitute the world's most speciose mimicry system. In this review, we call the attention of researchers to some hypothetical PASAM systems around the globe, presenting some fascinating examples that illustrate their huge diversity. We will also present new and published data on pollen-eating and pollen-collecting pollinators' responses to PASAM structures supporting the PASAM hypothesis and will discuss how widespread these systems are around the globe. Ultimately, our goal is to promote the idea that PASAM is a plausible first approach to understanding floral colour patterns in angiosperms.

Among-years rain variation is associated with flower size, but not with signal patch size in Iris petrana.

Among-years rain variation is associated with flower size, but not with signal patch size in Iris petrana.

Conical flower cells reduce surface gloss and improve colour signal integrity for free-flying bumblebees

Colour signals of flowers facilitate detection, spontaneous preference, discrimination and flower constancy by important bee pollinators. At short distances bees orient to floral colour patterns to find a landing platform and collect nutrition, potentially improving the plants’ reproductive success when multiple flowers are visited sequentially. In addition to pigments and backscattering structures within the petals’ internal layers, the epidermal micro-structure of the petals’ surface may also influence petal reflectance properties and thus influence overall colour patterns via optical effects. Gloss, i.e., shine caused by specular reflections of incident light from smooth surfaces, may for example alter the visual appearance of surfaces including flowers. We classify the epidermal surface properties of petals from 39 species of flowering plants from 19 families by means of a cell shape index, and measure the respective surface spectral reflectance from different angles. The spontaneous behavioural preferences of free flying bumblebees (Bombus terrestris) for surfaces with different micro-textures was then tested using specially prepared casts of selected flower petals. We specifically tested how the petal colour as function of the angle of incident light, surface structure and bee approach angle influences bumblebees’ spontaneous choices for artificial flowers. We observe that bumblebees spontaneously prefer artificial flowers with conical-papillate micro-structures under both multidirectional illumination and under spotlight conditions if approaching against the direction of spotlight, suggesting conical cells help promote constant signals by removing gloss that may confound the integrity of colour signalling.

A bee's eye view of remarkable floral colour patterns in the south-west Australian biodiversity hotspot revealed by false colour photography.

Colour pattern is a key cue of bee attraction selectively driving the appeal of pollinators. It comprises the main colour of the flower with extra fine patterns, indicating a reward focal point such as nectar, nectaries, pollen, stamens and floral guides. Such advertising of floral traits guides visitation by the insects, ensuring precision in pollen gathering and deposition. The study, focused in the Southwest Australian Floristic Region, aimed to spot bee colour patterns that are usual and unusual, missing, accomplished by mimicry of pollen and anthers, and overlapping between mimic-model species in floral mimicry cases. Floral colour patterns were examined by false colour photography in 55 flower species of multiple highly diverse natural plant communities in south-west Australia. False colour photography is a method to transform a UV photograph and a colour photograph into a false colour photograph based on the trichromatic vision of bees. This method is particularly effective for rapid screening of large numbers of flowers for the presence of fine-scale bee-sensitive structures and surface roughness that are not detectable using standard spectrophotometry. Bee- and bird-pollinated flowers showed the expected but also some remarkable and unusual previously undetected floral colour pattern syndromes. Typical colour patterns include cases of pollen and flower mimicry and UV-absorbing targets. Among the atypical floral colour patterns are unusual white and UV-reflecting flowers of bee-pollinated plants, bicoloured floral guides, consistently occurring in Fabaceae spp., and flowers displaying a selective attractiveness to birds only. In the orchid genera (Diuris and Thelymitra) that employ floral mimicry of model species, we revealed a surprising mimicry phenomenon of anthers mimicked in turn by model species. The study demonstrates the applicability of 'bee view' colour imaging for deciphering pollinator cues in a biodiverse flora with potential to be applied to other eco regions. The technique provides an exciting opportunity for indexing floral traits on a biome scale to establish pollination drivers of ecological and evolutionary relevance.

Quantifying colour and spot characteristics for the ventral petals in Sinningia speciosa

This study examined the colour and spot patterns for the ventral petals of a cross line in Sinningia speciosa. The second-generation individuals of the cross line exhibited phenotypic segregation in floral colour and spot patterns. Three colour traits (colour region ratio, region of interest colour, and colour gradient) and five spot traits (spot quantity, spot density, spot area ratio, spot colour, and background colour) were defined and quantified using image processing techniques. The variation in the traits and the correlations between the traits were also investigated. The results indicated a considerable degree of variation among the traits. The proposed approach can quantify petal traits more objectively and precisely compared with conventional naked eye examination. Thus, it can be used for applications, such as new flower variety determination, that require precise trait quantification.

Two genetic changes in cis-regulatory elements caused evolution of petal spot position in Clarkia.

A major premise in evolutionary developmental biology is that regulatory changes, often involving cis-regulatory elements, are responsible for much morphological evolution. This premise is supported by recent investigations of animal development, but information is just beginning to accumulate regarding whether it also applies to the evolution of plant morphology1-4. Here, we identify the genetic differences between species in the genus Clarkia that are responsible for evolutionary change in an ecologically important element of floral colour patterns: spot position. The evolutionary shift in spot position was due to two simple genetic changes that resulted in the appearance of a transcription factor binding site mutation in the R2R3 Myb gene that changes spot formation. These genetic changes caused R2R3 Myb to be activated by a different transcription factor that is expressed in a different position in the petal. These results suggest that the regulatory rewiring paradigm is as applicable to plants as it is to animals, and support the hypothesis that cis-regulatory changes may often play a role in plant morphological evolution.

Anthocyanin biosynthesis regulation of DhMYB2 and DhbHLH1 in Dendrobium hybrids petals

Dendrobium hybrids orchid are popular throughout the world. They have various floral color and pigmentation patterns that are mainly caused by anthocyanins. It is well established that anthocyanin biosynthesis is regulated by the interplay between MYB and bHLH transcription factors (TF) in most plants. In this study, we identified one R2R3-MYB gene, DhMYB2, and one bHLH gene, DhbHLH1, from a Dendrobium hybrid. Their expression profiles were related to anthocyanin pigmentation in Dendrobium petals. Transient over-expression of these two TF genes showed that both DhMYB2 and DhbHLH1 resulted in anthocyanin production in white petals. The interaction between the two TFs was observed in vitro. In different Dendrobium hybrids petals with various pigmentations, DhMYB2 and DhbHLH1 were co-expressed with DhDFR and DhANS, which are regarded as potential regulatory targets of the two TFs. In flowers with distinct purple lips but white or yellow petals/sepals, the expression of DhbHLH1 was only related to anthocyanin accumulation in the lips. Taken together, DhMYB2 interacted with DhbHLH1 to regulate anthocyanin production in Dendrobium hybrid petals. DhbHLH1 was also responsible for the distinct anthocyanin pigmentation in lip tissues. The functional characterization of DhMYB2 and DhbHLH1 will improve understanding of anthocyanin biosynthesis modulation in Dendrobium orchids.

Insight into the evolution of the Solanaceae from the parental genomes of Petunia hybrida.

Petunia hybrida is a popular bedding plant that has a long history as a genetic model system. We report the whole-genome sequencing and assembly of inbred derivatives of its two wild parents, P. axillaris N and P. inflata S6. The assemblies include 91.3% and 90.2% coverage of their diploid genomes (1.4 Gb; 2n = 14) containing 32,928 and 36,697 protein-coding genes, respectively. The genomes reveal that the Petunia lineage has experienced at least two rounds of hexaploidization: the older gamma event, which is shared with most Eudicots, and a more recent Solanaceae event that is shared with tomato and other solanaceous species. Transcription factors involved in the shift from bee to moth pollination reside in particularly dynamic regions of the genome, which may have been key to the remarkable diversity of floral colour patterns and pollination systems. The high-quality genome sequences will enhance the value of Petunia as a model system for research on unique biological phenomena such as small RNAs, symbiosis, self-incompatibility and circadian rhythms.

A Matter of Contrast: Yellow Flower Colour Constrains Style Length in Crocus species.

Most flowers display distinct colour patterns comprising two different areas. The peripheral large-area component of floral colour patterns attracts flower visitors from some distance and the central small-area component guides flower visitors towards landing sites. Whereas the peripheral colour is largely variable among species, the central colour, produced mostly by anthers and pollen or pollen mimicking floral guides, is predominantly yellow and UV-absorbing. This holds also for yellow flowers that regularly display a UV bull’s eye pattern. Here we show that yellow-flowering Crocus species are a noticeable exception, since yellow-flowering Crocus species–being entirely UV-absorbing–exhibit low colour contrast between yellow reproductive organs and yellow tepals. The elongated yellow or orange-yellow style of Crocus flowers is a stamen-mimicking structure promoting cross-pollination by facilitating flower visitors’ contact with the apical stigma before the flower visitors are touching the anthers. Since Crocus species possess either yellow, violet or white tepals, the colour contrast between the stamen-mimicking style and the tepals varies among species. In this study comprising 106 Crocus species, it was tested whether the style length of Crocus flowers is dependent on the corolla colour. The results show that members of the genus Crocus with yellow tepals have evolved independently up to twelve times in the genus Crocus and that yellow-flowering Crocus species possess shorter styles as compared to violet- and white-flowering ones. The manipulation of flower visitors by anther-mimicking elongated styles in Crocus flowers is discussed.

Transcriptional control of floral anthocyanin pigmentation in monkeyflowers (Mimulus).

A molecular description of the control of floral pigmentation in a multi-species group displaying various flower color patterns is of great interest for understanding the molecular bases of phenotypic diversification and pollinator-mediated speciation. Through transcriptome profiling, mutant analyses and transgenic experiments, we aim to establish a 'baseline' floral anthocyanin regulation model in Mimulus lewisii and to examine the different ways of tinkering with this model in generating the diversity of floral anthocyanin patterns in other Mimulus species. We find one WD40 and one bHLH gene controlling anthocyanin pigmentation in the entire corolla of M.lewisii and two R2R3-MYB genes, PELAN and NEGAN, controlling anthocyanin production in the petal lobe and nectar guide, respectively. The autoregulation of NEGAN might be a critical property to generate anthocyanin spots. Independent losses of PELAN expression (via different mechanisms) explain two natural yellow-flowered populations of M.cardinalis (typically red-flowered). The NEGAN ortholog is the only anthocyanin-activating MYB expressed in the M.guttatus flowers. The mutant lines and transgenic tools available for M.lewisii will enable gene-by-gene replacement experiments to dissect the genetic and developmental bases of more complex floral color patterns, and to test hypotheses on phenotypic evolution in general.

Survival and reproduction of plant species in the Qinghai–Tibet Plateau

AbstractThe Qinghai–Tibet Plateau (QTP) is the highest and largest plateau in the world. It covers correspondingly wide geological, topographical, and climatic gradients, and thus hosts greater biodiversity than surrounding lowlands and other high elevation regions. Due to its extreme environmental and biological diversity, the QTP is an ideal region for studying adaptations of plant species under harsh environmental conditions at multiple evolutionary levels. Many recent ecological studies have revealed functions of distinctive morphological features of various plants in the region that improve their reproductive success. Examples include large and showy bracts, hairy inflorescences, and drooping flowers. Numerous other investigations have examined QTP plants' sexual systems, patterns of biomass allocations, and biotic interactions. This paper summarizes recent advances in understanding of morphological adaptations, plant–plant interactions, plant–pollinator interactions, floral color patterns, pollination adaptations, and resource allocation patterns of alpine plants of the QTP. The overall aim is to synthesize current knowledge of the general mechanisms of plant survival and reproduction in this fascinating region.

Floral color patterns in a tropical orchid: Are they associated with reproductive success?

AbstractWe sought to measure phenotypic selection on petal color variation in populations of Lepanthes rupestris, a diminutive epilithic orchid of Puerto Rico that exhibits a polymorphism in petal color patterns (unicolor or bicolored). We censused seven populations monthly for 20 months and noted flower production and petal color pattern. Each flower was checked for pollinarium removal (male fitness measure) and fruit production (female fitness). In all populations, plants with bicolored petals dominated and comprised 63–82% of individuals. Aside from petal color differences, the two types were indistinguishable. Flower color pattern was generally not associated with either male or female reproductive success within or among populations or over time. Environmental conditions, rainfall and humidity, may account for both temporal and spatial variation in the reproductive success observed among sites. Although we were unable to tag fitness to petal color patterns, the consistent ratio of color morphs among populations suggests that factors other than just drift are responsible for the frequencies we observed.

Stamens and mimic stamens as components of floral colour patterns

Lunau, K.: Stamens and mimic stamens as components of floral colour patterns. — Bot. Jahrb. Syst. 127: 13–41. 2007. — ISSN 0006-8152. Dedicated to Professor Dr. Gunther Osche, Freiburg im Breisgau (Germany), on the occasion of his 80th birthday; he discovered the phenomenon of mimic stamens when re-assessing floral colour pattern with his eyes newly tuned by Wicklerian thoughts about automimicry. Stamens of zoophilous plants are multifunctional floral organs which protect pollen against environmental stress, attract potential pollinators, attach pollen to the sites of secondary pollen presentation, or to the pollinator’s body surface, and provide pollen as a reward for flower-visitors. Pollen-eating flower-visitors such as syrphid flies and pollencollecting flower-visitors such as bees innately respond to visual and chemical cues of stamens and pollen prior to individual experience. Flowering plants presenting conspicuous stamens and pollen to attract pollinators incur numerous costs associated with visual exposure of pollen including investments for protection against solar radiation, pollen losses caused by wind, rain, and illegitimate flower-visitation, and pollen losses due to consumption of pollen by flower-visitors or pollinators. Many flowers use mimic stamens (e.g. staminodes, false stamens, yellow floral guides in the size and shape of anthers) and thus overcome these disadvantages. Several study cases focus on the benefits of signaling with mimic stamens in flowering plants having polymorphic flowers, distinct flowering phases, and hidden real pollen. The aim of this review is to illustrate the various cases in which flowering plants may benefit from attracting flower-visitors by means of mimic stamens.

Visual targeting of components of floral colour patterns in flower-naïve bumblebees (Bombus terrestris; Apidae)

Floral colour patterns are contrasting colour patches on flowers, a part of the signalling apparatus that was considered to display shape and colour signals used by flower-visitors to detect flowers and locate the site of floral reward. Here, we show that flower-naïve bumblebees (Bombus terrestris) spontaneously direct their approach towards the outside margin of artificial flowers, which provides contrast between these dummy flowers and the background. If no floral guides are present, the bumblebees continue to approach the margin and finally touch the marginal area of the dummy flower with the tips of their antennae. Whilst approaching dummy flowers that also have a central floral guide, the bumblebees change their direction of flight: Initially, they approach the margin, later they switch to approaching the colour guide, and finally they precisely touch the floral guide with their antennae. Variation of the shape of equally sized dummy flowers did not alter the bumblebees' preferential orientation towards the guide. Using reciprocal combinations of guide colour and surrounding colour, we showed that the approach from a distance towards the corolla and the antennal contact with the guide are elicited by the same colour parameter: spectral purity. As a consequence, the dummy flowers eliciting the greatest frequency of antennal reactions at the guide are those that combine a floral guide of high spectral purity with a corolla of less spectral purity. Our results support the hypothesis that floral guides direct bumblebees' approaches to the site of first contact with the flower, which is achieved by the tips of the antennae.

A generalised mimicry system involving angiosperm flower colour, pollen and bumblebees’ innate colour preferences

Flower colour is a major advertisement signal of zoophilous plants for pollinators. Bees, the main pollinators, exhibit innate colour preferences, which have often been attributed to only one single floral colour, though most flowers display a pattern of two or several colours. The existing studies of floral colour patterns are mostly qualitative studies. Using a model of bee colour vision we quantitatively investigate two questions: whether or not component colours of floral colour patterns may mimic pollen signals, and whether or not bumblebees exhibit innate preferences for distinct parameters of naturally existing floral colour patterns. We analysed the spectral reflectances of 162 plant species with multicoloured flowers and inflorescences, distiniguishing between inner and outer colours of floral colour patterns irrespective of the particular structures so coloured.

A new interpretation of flower guide colouration: Absorption of ultraviolet light enhances colour saturation

In melittophilous plants the colour pattern of the flowers, as perceived by bumblebees, is a gradient of centripetally increasing spectral purity. This pattern serves as a signal for innate flower recognition in naive bumblebees permitting orientation to flowers and landing on flowers. Structures which make up the total signal pattern can include the background (e.g., green leaves), corollas, and stamens or floral guides. How various colour parameters, such as dominant wavelength, intensity, and spectral purity influence the colour signal pattern of flowers is analyzed. The process of strong absorption of ultraviolet light is shown to be a mechanism for the enhancement of spectral purity in flower guides. The importance of other mechanisms is also demonstrated. The presence of a gradient of centripetally increasing spectral purity in floral colour patterns as perceived by a bumblebee's eyes is demonstrated by a comparison of the spectral reflectance in different parts of the flower and a representation of colour loci in the colour triangle.

Floral color patterns and pollinator attraction in a bog habitat

Floral color in 14 species of bog plants was examined in relation to intrafloral patterns, flower morphology, background coloration, and pollinators. There were three corolla color groups: UV-absorbing white (eight species), UV-absorbing pink–red (five species), and UV-reflecting yellow (one species). Contrast with vegetational backgrounds in UV light was minimal for species that absorbed UV, but such species exhibited contrast in the visible specrtum which enhanced long-range conspicuousness. Patterns created by contrasting colors within corollas emphasized the reproductive organs and are considered as nectar guides for short-range attraction of pollinators to food rewards. Floral color was not correlated with diameter, height, or density of corollas. Bumblebee pollinators showed preferences for pink–red stereomorphic species, whereas Diptera showed preferences for white actinomorphic inflorescences.

Ultraviolet floral patterns as functional orientation cues in hymenopterous pollination systems

Field modifications of U.V. floral patterns of Caesalpinia eriostachys Benth. and Parkinsonia aculeata L. showed that bees used U.V. designs as orientation cues. Bees exhibited an in-flight orientation ability which was apparently correlated with size, with smaller bees frequently orienting to the manipulated position of the U.V. absorbent banner petal prior to landing on the flower. The U.V. absorptive banner petal and staminal portions of the flowers of both species seemed to be the most important visual orientation guides, and except for Trigona fuscipennis and T. pectoralis, the banner petal was the more important of the two. Emasculation of the flowers may have removed the orientation cue used by these species of Trigona, which may be orienting to the flowers solely on the basis of pollen odour. Morphological and physiological changes in the floral colour patterns of both species following pollination appear to inhibit visitation by bees. The significance of U.V. floral patterns was also considered.