How colorful are fruits? Limited color diversity in fleshy fruits on local and global scales

Transitions between colour mechanisms affect speciation dynamics and range distributions of birds

Several ecogeographical 'rules' have been proposed to explain colour variation at broad spatial and phylogenetic scales but these rarely consider whether colours are based on pigments or structural colours. However, mechanism can have profound effects on the function and evolution of colours. Here, we combine geographic information, climate data and colour mechanism at broad phylogenetic (9,409 species) and spatial scales (global) to determine how transitions between pigmentary and structural colours influence speciation dynamics and range distributions in birds. Among structurally coloured species, we find that rapid dispersal into tropical regions drove the accumulation of iridescent species, whereas the build-up of non-iridescent species in the tropics was driven by a combination of dispersal and faster in situ evolution in the tropics. These results could be explained by pleiotropic links between colouration and dispersal behaviour or ecological factors influencing colonization success. These data elucidate geographic patterns of colouration at a global scale and provide testable hypotheses for future work on birds and other animals with structural colours.

A highly resolved nuclear phylogeny uncovers strong phylogenetic conservatism and correlated evolution of fruit color and size in Solanum L

Mutualisms between plants and fruit-eating animals were key to the radiation of angiosperms. Still, phylogenetic uncertainties limit our understanding of fleshy-fruit evolution, as in the case of Solanum, a genus with remarkable fleshy-fruit diversity, but with unresolved phylogenetic relationships. We used 1786 nuclear genes from 247 species, including 122 newly generated transcriptomes/genomes, to reconstruct the Solanum phylogeny and examine the tempo and mode of the evolution of fruit color and size. Our analysis resolved the backbone phylogeny of Solanum, providing high support for its clades. Our results pushed back the origin of Solanum to 53.1 million years ago (Ma), with most major clades diverging between 35 and 27 Ma. Evolution of Solanum fruit color and size revealed high levels of trait conservatism, where medium-sized berries that remain green when ripe are the likely ancestral form. Our analyses revealed that fruit size and color are evolutionary correlated, where dull-colored fruits are two times larger than black/purple and red fruits. We conclude that the strong phylogenetic conservatism shown in the color and size of Solanum fruits could limit the influences of fruit-eating animals on fleshy-fruit evolution. Our findings highlight the importance of phylogenetic constraints on the diversification of fleshy-fruit functional traits.

Self-assembled, disordered structural color from fruit wax bloom

Many visually guided frugivores have eyes highly adapted for blue sensitivity, which makes it perhaps surprising that blue pigmented fruits are not more common. However, some fruits are blue even though they do not contain blue pigments. We investigate dark pigmented fruits with wax blooms, like blueberries, plums, and juniper cones, and find that a structural color mechanism is responsible for their appearance. The chromatic blue-ultraviolet reflectance arises from the interaction of the randomly arranged nonspherical scatterers with light. We reproduce the structural color in the laboratory by recrystallizing wax bloom, allowing it to self-assemble to produce the blue appearance. We demonstrate that blue fruits and structurally colored fruits are not constrained to those with blue subcuticular structure or pigment. Further, convergent optical properties appear across a wide phylogenetic range despite diverse morphologies. Epicuticular waxes are elements of the future bioengineering toolbox as sustainable and biocompatible, self-assembling, self-cleaning, and self-repairing optical biomaterials.

Bird preferences for fruit size, but not color, vary in accordance with fruit traits along a tropical elevational gradient

Birds constitute one of the most important seed dispersal agents globally, especially in the tropics. The feeding preferences of frugivorous birds are, therefore, potentially of great ecological importance. A number of laboratory-based and observational studies have attempted to ascertain the preferences of certain bird species for certain fruit traits. However, little attention has been paid to community-wide preferences of frugivorous birds and the impact this may have on fruit traits on a broader scale. Here, we used artificial fruits of different colors and sizes to investigate community-wide fruit trait preferences of birds at three sites along an elevational gradient in Papua New Guinea. We recorded attack rates on artificial fruits as visible impressions made by a bird's beak during a feeding attempt. We also measured the colors and sizes of real fruits at each site, and the gape widths of frugivorous birds, allowing for comparisons between bird feeding preferences and bird and fruit traits. Regardless of elevation, red and purple fruits were universally preferred to green and attacked at similar rates to one another, despite strong elevational patterns in real fruit color. However, elevation had a significant effect on fruit size preferences. A weak, non-significant preference for large fruits was recorded at 700 m, while medium fruits were strongly preferred at 1700 m and small fruits at 2700 m. These patterns mirror those of both real fruit size and frugivorous bird gape width along the gradient, suggesting the potential for selective pressure of birds on fruit size at different elevations.

Multiple origins of lipid-based structural colors contribute to a gradient of fruit colors in Viburnum (Adoxaceae)

Structural color is poorly known in plants relative to animals. In fruits, only a handful of cases have been described, including in Viburnum tinus where the blue color results from a disordered multilayered reflector made of lipid droplets. Here, we examine the broader evolutionary context of fruit structural color across the genus Viburnum. We obtained fresh and herbarium fruit material from 30 Viburnum species spanning the phylogeny and used transmission electron microscopy, optical simulations, and ancestral state reconstruction to identify the presence/absence of photonic structures in each species, understand the mechanism producing structural color in newly identified species, relate the development of cell wall structure to reflectance in Viburnum dentatum, and describe the evolution of cell wall architecture across Viburnum. We identify at least two (possibly three) origins of blue fruit color in Viburnum in species which produce large photonic structures made of lipid droplets embedded in the cell wall and which reflect blue light. Examining the full spectrum of mechanisms producing color in pl, including structural color as well as pigments, will yield further insights into the diversity, ecology, and evolution of fruit color.

Fruit Colour and Novel Mechanisms of Genetic Regulation of Pigment Production in Tomato Fruits

Fruit colour represents a genetic trait with ecological and nutritional value. Plants mainly use colour to attract animals and favour seed dispersion. Thus, in many species, fruit colour coevolved with frugivories and their preferences. Environmental factors, however, represented other adaptive forces and further diversification was driven by domestication. All these factors cooperated in the evolution of tomato fruit, one of the most important in human nutrition. Tomato phylogenetic history showed two main steps in colour evolution: the change from green-chlorophyll to red-carotenoid pericarp, and the loss of the anthocyanic pigmentation. These events likely occurred with the onset of domestication. Then spontaneous mutations repeatedly occurred in carotenoid and phenylpropanoid pathways, leading to colour variants which often were propagated. Introgression breeding further enriched the panel of pigmentation patterns. In recent decades, the genetic determinants underneath tomato colours were identified. Novel evidence indicates that key regulatory and biosynthetic genes undergo mechanisms of gene expression regulation that are much more complex than what was imagined before: post-transcriptional mechanisms, with RNA splicing among the most common, indeed play crucial roles to fine-tune the expression of this trait in fruits and offer new substrate for the rise of genetic variables, thus providing further evolutionary flexibility to the character.

Dispersers and environment drive global variation in fruit colour syndromes

The colours of fleshy fruits play a critical role in plant dispersal by advertising ripe fruits to consumers. Fruit colours have long been classified into syndromes attributed to selection by animal dispersers, despite weak evidence for this hypothesis. Here, we test the relative importance of biotic (bird and mammal frugivory) and abiotic (wet season temperatures, growing season length and UV-B radiation) factors in determining fruit colour syndrome in 3163 species of fleshy-fruited plants. We find that both dispersers and environment are important, and they interact. In warm areas, contrastive, bird-associated fruit colours increase with relative bird frugivore prevalence, whereas in cold places these colours dominate even where mammalian dispersers are prevalent. We present near-global maps of predicted fruit colour syndrome based on our species-level model and our newly developed characterisations of relative importance of bird and mammal frugivores.

Floral Color Diversity: How Are Signals Shaped by Elevational Gradient on the Tropical-Subtropical Mountainous Island of Taiwan?

Pollinators with different vision are a key driver of flower coloration. Islands provide important insights into evolutionary processes, and previous work suggests islands may have restricted flower colors. Due to both species richness with high endemism in tropical-subtropical environments, and potentially changing pollinator distributions with altitude, we evaluated flower color diversity across the mountainous island of Taiwan in a comparative framework to understand the cause of color diversity. We sampled flower color signaling on the tropical-subtropical island of Taiwan considering altitudes from sea level to 3300 m to inform how over-dispersion, random processes or clustering may influence flower signaling. We employed a model of bee color space to plot loci from 727 species to enable direct comparisons to data sets from continental studies representing Northern and Southern Hemispheres, and also a continental mountain region. We observed that flower color diversity was similar to flowers that exist in mainland continental studies, and also showed evidence that flowers predominantly had evolved color signals that closely matched bee color preferences. At high altitudes floras tend to be phylogenetically clustered rather than over-dispersed, and their floral colors exhibited weak phylogenetic signal which is consistent with character displacement that facilitated the co-existence of related species. Overall flower color signaling on a tropical-subtropical island is mainly influenced by color preferences of key bee pollinators, a pattern consistent with continental studies.

The evolution of fruit scent: phylogenetic and developmental constraints

BACKGROUND

Fruit scent is increasingly recognized as an evolved signal whose function is to attract animal seed dispersers and facilitate plant reproduction. However, like all traits, fruit scent is likely to evolve in response to conflicting selective pressures and various constraints. Two major constraints are (i) phylogenetic constraints, in which traits are inherited from ancestors rather than adapted to current conditions and (ii) developmental constraints, if phenotypes are limited by the expression of other traits within the individual. We tested whether phylogenetic constraints play a role in fruit scent evolution by calculating the phylogenetic signal in ripe fruits of 98 species from three study sites. We then estimated the importance of developmental constraints by examining whether ripe fruits tend to emit compounds that are chemically similar to, and share biosynthetic pathways with, compounds emitted by conspecific unripe fruits from which they develop.

RESULTS

We show that closely related taxa are not more similar to each other than to very distinct taxa, thus indicating that fruit scent shows little phylogenetic signal. At the same time, although ripe and unripe fruits of the same species tend to emit different chemicals, they tend to employ chemicals originating from similar biosynthetic pathways, thus indicating that some developmental constraints determine ripe fruit scent.

CONCLUSIONS

Our results highlight the complex landscape in which fruit scent has evolved. On one hand, fruit scent evolution is not limited by common ancestry. On the other hand, the range of chemicals that can be employed in ripe fruits is probably constrained by the needs of unripe fruits.

Viburnum tinus Fruits Use Lipids to Produce Metallic Blue Structural Color

Viburnum tinus is an evergreen shrub that is native to the Mediterranean region but cultivated widely in Europe and around the world. It produces ripe metallic blue fruits throughout winter [1]. Despite its limited fleshy pulp [2], its high lipid content [3] makes it a valuable resource to the small birds [4] that act as its seed-dispersers [5]. Here, we find that the metallic blue appearance of the fruits is produced by globular lipid inclusions arranged in a disordered multilayer structure. This structure is embedded in the cell walls of the epicarp and underlaid with a dark layer of anthocyanin pigments. The presence of such large, organized lipid aggregates in plant cell walls represents a new mechanism for structural coloration and may serve as an honest signal of nutritional content.

Floral Color Diversity: How Are Signals Shaped by Elevational Gradient on the Tropical-Subtropical Mountainous Island of Taiwan?

Pollinators with different vision are a key driver of flower coloration. Islands provide important insights into evolutionary processes, and previous work suggests islands may have restricted flower colors. Due to both species richness with high endemism in tropical-subtropical environments, and potentially changing pollinator distributions with altitude, we evaluated flower color diversity across the mountainous island of Taiwan in a comparative framework to understand the cause of color diversity. We sampled flower color signaling on the tropical-subtropical island of Taiwan considering altitudes from sea level to 3300 m to inform how over-dispersion, random processes or clustering may influence flower signaling. We employed a model of bee color space to plot loci from 727 species to enable direct comparisons to data sets from continental studies representing Northern and Southern Hemispheres, and also a continental mountain region. We observed that flower color diversity was similar to flowers that exist in mainland continental studies, and also showed evidence that flowers predominantly had evolved color signals that closely matched bee color preferences. At high altitudes floras tend to be phylogenetically clustered rather than over-dispersed, and their floral colors exhibited weak phylogenetic signal which is consistent with character displacement that facilitated the co-existence of related species. Overall flower color signaling on a tropical-subtropical island is mainly influenced by color preferences of key bee pollinators, a pattern consistent with continental studies.

European mushroom assemblages are darker in cold climates

Thermal melanism theory states that dark-colored ectotherm organisms are at an advantage at low temperature due to increased warming. This theory is generally supported for ectotherm animals, however, the function of colors in the fungal kingdom is largely unknown. Here, we test whether the color lightness of mushroom assemblages is related to climate using a dataset of 3.2 million observations of 3,054 species across Europe. Consistent with the thermal melanism theory, mushroom assemblages are significantly darker in areas with cold climates. We further show differences in color phenotype between fungal lifestyles and a lifestyle differentiated response to seasonality. These results indicate a more complex ecological role of mushroom colors and suggest functions beyond thermal adaption. Because fungi play a crucial role in terrestrial carbon and nutrient cycles, understanding the links between the thermal environment, functional coloration and species' geographical distributions will be critical in predicting ecosystem responses to global warming.

Reward regulation in plant-frugivore networks requires only weak cues

Theory assumes that fair trade among mutualists requires highly reliable communication. In plant–animal mutualisms the reliability of cues that indicate reward quality is often low. Therefore, it is controversial whether communication allows animal mutualists to regulate their reward intake. Here we show that even loose relationships between fruit brightness and nutritional rewards (r² = 0.11–0.35) allow birds to regulate their nutrient intake across distinct European plant–frugivore networks. Resident, over-wintering generalist frugivores that interact with diverse plant species select bright, lipid-rich fruits, whereas migratory birds select dark, sugar- and antioxidant-rich fruits. Both nutritional strategies are consistent with previous physiological experiments suggesting that over-wintering generalists aim to maximize their energy intake, whereas migrants aim to enhance the build-up of body fat, their immune response and oxidative status during migration. Our results suggest that animal mutualists require only weak cues to regulate their reward intake according to specific nutritional strategies.

A challenge for mutualists is that partner cue reliability is often low. Here, the authors show that though fruit brightness is weakly predictive of nutritional content, the diets of birds (e.g. migrants vs. residents) are structured by fruit brightness in alignment with expected nutritional needs.

The evolution of fruit colour: phylogeny, abiotic factors and the role of mutualists

The adaptive significance of fruit colour has been investigated for over a century. While colour can fulfil various functions, the most commonly tested hypothesis is that it has evolved to increase fruit visual conspicuousness and thus promote detection and consumption by seed dispersing animals. However, fruit colour is a complex trait which is subjected to various constraints and selection pressures. As a result, the effect of animal selection on fruit colour are often difficult to identify, and several studies have failed to detect it. Here, we employ an integrative approach to examine what drives variation in fruit colour. We quantified the colour of ripe fruit and mature leaves of 97 tropical plant species from three study sites in Madagascar and Uganda. We used phylogenetically controlled models to estimate the roles of phylogeny, abiotic factors, and dispersal mode on fruit colour variation. Our results show that, independent of phylogeny and leaf coloration, mammal dispersed fruits are greener than bird dispersed fruits, while the latter are redder than the former. In addition, fruit colour does not correlate with leaf colour in the visible spectrum, but fruit reflection in the ultraviolet area of the spectrum is strongly correlated with leaf reflectance, emphasizing the role of abiotic factors in determining fruit colour. These results demonstrate that fruit colour is affected by both animal sensory ecology and abiotic factors and highlight the importance of an integrative approach which controls for the relevant confounding factors.

Interspecific visual signalling in animals and plants: a functional classification

Organisms frequently gain advantages when they engage in signalling with individuals of other species. Here, we provide a functionally structured framework of the great variety of interspecific visual signals seen in nature, and then describe the different signalling mechanisms that have evolved in response to each of these functional requirements. We propose that interspecific visual signalling can be divided into six major functional categories: anti-predator, food acquisition, anti-parasite, host acquisition, reproductive and agonistic signalling, with each function enabled by several distinct mechanisms. We support our classification by reviewing the ecological and behavioural drivers of interspecific signalling in animals and plants, principally focusing on comparative studies that address large-scale patterns of diversity. Collating diverse examples of interspecific signalling into an organized set of functional and mechanistic categories places anachronistic behavioural and morphological labels in fresh context, clarifies terminology and redirects research effort towards understanding environmental influences driving interspecific signalling in nature.This article is part of the themed issue 'Animal coloration: production, perception, function and application'.

Structural colour from helicoidal cell-wall architecture in fruits of Margaritaria nobilis

The bright and intense blue-green coloration of the fruits of Margaritaria nobilis (Phyllanthaceae) was investigated using polarization-resolved spectroscopy and transmission electron microscopy. Optical measurements of freshly collected fruits revealed a strong circularly polarized reflection of the fruit that originates from a cellulose helicoidal cell wall structure in the pericarp cells. Hyperspectral microscopy was used to capture the iridescent effect at the single-cell level.

Revisiting phylogenetic signal; strong or negligible impacts of polytomies and branch length information?

Background

Inaccurate estimates of phylogenetic signal may mislead interpretations of many ecological and evolutionary processes, and hence understanding where potential sources of uncertainty may lay has become a priority for comparative studies. Importantly, the sensitivity of phylogenetic signal indices and their associated statistical tests to incompletely resolved phylogenies and suboptimal branch-length information has been only partially investigated.

Methods

Here, we use simulations of trait evolution along phylogenetic trees to assess whether incompletely resolved phylogenies (polytomic chronograms) and phylogenies with suboptimal branch-length information (pseudo-chronograms) could produce directional biases in significance tests (p-values) associated with Blomberg et al.’s K and Pagel’s lambda (λ) statistics, two of the most widely used indices to measure and test phylogenetic signal. Specifically, we conducted pairwise comparisons between the p-values resulted from the use of “true” chronograms and their degraded counterparts (i.e. polytomic chronograms and pseudo-chronograms), and computed the frequency with which the null hypothesis of no phylogenetic signal was accepted using “true” chronograms but rejected when using their degraded counterparts (type I bias) and vice versa (type II bias).

Results

We found that the use of polytomic chronograms in combination with Blomberg et al.’s K resulted in both, clearly inflated estimates of phylogenetic signal and moderate levels of type I and II biases. More importantly, pseudo-chronograms led to high rates of type I biases. In contrast, Pagel’s λ was strongly robust to either incompletely resolved phylogenies and suboptimal branch-length information.

Conclusions

Our results suggest that pseudo-chronograms can lead to strong overestimation of phylogenetic signal when using Blomberg et al.’s K (i.e. high rates of type I biases), while polytomies may be a minor concern given other sources of uncertainty. In contrast, Pagel’s λ seems strongly robust to either incompletely resolved phylogenies and suboptimal branch-length information. Hence, Pagel’s λ may be a more appropriate alternative over Blomberg et al.’s K to measure and test phylogenetic signal in most ecologically relevant traits when phylogenetic information is incomplete.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-017-0898-y) contains supplementary material, which is available to authorized users.

Why should we investigate the morphological disparity of plant clades?

BACKGROUND

Disparity refers to the morphological variation in a sample of taxa, and is distinct from diversity or taxonomic richness. Diversity and disparity are fundamentally decoupled; many groups attain high levels of disparity early in their evolution, while diversity is still comparatively low. Diversity may subsequently increase even in the face of static or declining disparity by increasingly fine sub-division of morphological 'design' space (morphospace). Many animal clades reached high levels of disparity early in their evolution, but there have been few comparable studies of plant clades, despite their profound ecological and evolutionary importance. This study offers a prospective and some preliminary macroevolutionary analyses.

METHODS

Classical morphometric methods are most suitable when there is reasonable conservation of form, but lose traction where morphological differences become greater (e.g. in comparisons across higher taxa). Discrete character matrices offer one means to compare a greater diversity of forms. This study explores morphospaces derived from eight discrete data sets for major plant clades, and discusses their macroevolutionary implications.

KEY RESULTS

Most of the plant clades in this study show initial, high levels of disparity that approach or attain the maximum levels reached subsequently. These plant clades are characterized by an initial phase of evolution during which most regions of their empirical morphospaces are colonized. Angiosperms, palms, pines and ferns show remarkably little variation in disparity through time. Conifers furnish the most marked exception, appearing at relatively low disparity in the latest Carboniferous, before expanding incrementally with the radiation of successive, tightly clustered constituent sub-clades.

CONCLUSIONS

Many cladistic data sets can be repurposed for investigating the morphological disparity of plant clades through time, and offer insights that are complementary to more focused morphometric studies. The unique structural and ecological features of plants make them ideally suited to investigating intrinsic and extrinsic constraints on disparity.

Shape analysis of moss (Bryophyta) sporophytes: Insights into land plant evolution

PREMISE OF THE STUDY

The alternation of generations life cycle represents a key feature of land-plant evolution and has resulted in a diverse array of sporophyte forms and modifications in all groups of land plants. We test the hypothesis that evolution of sporangium (capsule) shape of the mosses-the second most diverse land-plant lineage-has been driven by differing physiological demands of life in diverse habitats. This study provides an important conceptual framework for analyzing the evolution of a single, homologous character in a continuous framework across a deep expanse of time, across all branches of the tree of life.

METHODS

We reconstruct ancestral sporangium shape and ancestral habitat on the largest phylogeny of mosses to date, and use phylogenetic generalized least squares regression to test the association between habitat and sporangium shape. In addition, we examine the association between shifts in sporangium shape and species diversification.

RESULTS

We demonstrate that sporangium shape is convergent, under natural selection, and associated with habitat type, and that many shifts in speciation rate are associated with shifts in sporangium shape.

CONCLUSIONS

Our results suggest that natural selection in different microhabitats results in the diversity of sporangium shape found in mosses, and that many increasing shifts in speciation rate result in changes in sporangium shape across their 480 million year history. Our framework provides a way to examine if diversification shifts in other land plants are also associated with massive changes in sporophyte form, among other morphological traits.

How to make a red flower: the combinatorial effect of pigments

Red flowers have evolved repeatedly across angiosperms and are frequently examined in an ecological context. However, less is known about the biochemical basis of red colouration in different taxa. In this study, we examine the spectral properties, anthocyanin composition and carotenoid expression of red flowers in the tomato family, Solanaceae, which have evolved independently multiple times across the group. Our study demonstrates that Solanaceae typically make red flowers either by the sole production of red anthocyanins or, more commonly, by the dual production of purple or blue anthocyanins and orange carotenoids. In using carotenoids to modify the effect of purple and/or blue anthocyanins, these Solanaceae species have converged on the same floral hue as those solely producing red anthocyanins, even when considering the visual system of pollinators. The use of blue anthocyanins in red flowers appears to differ from other groups, and suggests that the genetic changes underlying evolutionary shifts to red flowers may not be as predictable as previously suggested.

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.

The colour of an avifauna: A quantitative analysis of the colour of Australian birds

Animal coloration is a poorly-understood aspect of phenotypic variability. Here I expand initial studies of the colour gamut of birds by providing the first quantitative description of the colour variation of an entire avifauna: Australian landbirds (555 species). The colour of Australian birds occupies a small fraction (19%) of the entire possible colour space and colour variation is extremely uneven. Most colours are unsaturated, concentrated in the centre of colour space and based on the deposition of melanins. Other mechanisms of colour production are less common but account for larger portions of colour space and for most saturated colours. Male colours occupy 45–25% more colour space than female colours, indicating that sexual dichromatism translates into a broader range of male colours. Male-exclusive colours are often saturated, at the edge of chromatic space, and have most likely evolved for signalling. While most clades of birds occupy expected or lower-than-expected colour volumes, parrots and cockatoos (Order Psittaciformes) occupy a much larger volume than expected. This uneven distribution of colour variation across mechanisms of colour production, sexes and clades is probably shared by avifaunas in other parts of the world, but this remains to be tested with comparable data.

Signal Diversity, Sexual Selection, and Speciation

Communication is ubiquitous. Developing a framework for the diversity of signals has important consequences for understanding alternative models of sexual selection and the processes contributing to speciation. In this article we review how models of neutral evolution in the perceptual space of signal perceivers provide a first step toward constructing a framework for signal diversity. We discuss how the distinction between additive and multiplicative effects of multimodal signaling represents a second step. We then assess how signal efficiency, reliability, and the aesthetics of perceivers provide distinct mechanisms for signals to be effective, thereby partly explaining signal diversity. Understanding the relative contribution of each of these mechanisms to the effectiveness of mate choice signals unravels the relative importance of alternative models of sexual selection. It can also help to distinguish whether divergence of communication is a driver or a consequence of speciation. Throughout the review we emphasize the importance of verification and learning in repeated interactions for understanding variation in signals.

It's Not Easy Being Blue: Are There Olfactory and Visual Trade-Offs in Plant Signalling?

Understanding the signals used by plants to attract seed disperses is a pervasive quest in evolutionary and sensory biology. Fruit size, colour, and odour variation have long been discussed in the controversial context of dispersal syndromes targeting olfactory-oriented versus visually-oriented foragers. Trade-offs in signal investment could impose important physiological constraints on plants, yet have been largely ignored. Here, we measure the reflectance and volatile organic compounds of a community of Malagasy plants and our results indicate that extant plant signals may represent a trade-off between olfactory and chromatic signals. Blue pigments are the most visually-effective--blue is a colour that is visually salient to all known seed dispersing animals within the study system. Additionally, plants with blue-reflecting fruits are less odiferous than plants that reflect primarily in other regions of the colour spectrum.

The floral morphospace--a modern comparative approach to study angiosperm evolution

Morphospaces are mathematical representations used for studying the evolution of morphological diversity and for the evaluation of evolved shapes among theoretically possible ones. Although widely used in zoology, they--with few exceptions--have been disregarded in plant science and in particular in the study of broad-scale patterns of floral structure and evolution. Here we provide basic information on the morphospace approach; we review earlier morphospace applications in plant science; and as a practical example, we construct and analyze a floral morphospace. Morphospaces are usually visualized with the help of ordination methods such as principal component analysis (PCA) or nonmetric multidimensional scaling (NMDS). The results of these analyses are then coupled with disparity indices that describe the spread of taxa in the space. We discuss these methods and apply modern statistical tools to the first and only angiosperm-wide floral morphospace published by Stebbins in 1951. Despite the incompleteness of Stebbins’ original dataset, our analyses highlight major, angiosperm-wide trends in the diversity of flower morphology and thereby demonstrate the power of this previously neglected approach in plant science.

Bird fruit preferences match the frequency of fruit colours in tropical Asia

While many factors explain the colour of fleshy fruits, it is thought that black and red fruits are common in part because frugivorous birds prefer these colours. We examined this still controversial hypothesis at a tropical Asian field site, using artificial fruits, fresh fruits, four wild-caught resident frugivorous bird species, and hand-raised naïve birds from three of the same species. We demonstrate that all birds favored red artificial fruits more than yellow, blue, black and green, although the artificial black colour was found subsequently to be similar to the artificial blue colour in its spectral reflectance. Wild-caught birds preferred both black and red fleshy natural fruits, whereas hand-raised naïve birds preferred black to red natural fleshy fruits and to those of other colours. All birds avoided artificial and naturally ripe green fruits. The inter-individual variation in colour choice was low and the preferences were constant over time, supporting the hypothesis that bird colour preferences are a contributing factor driving fruit colour evolution in tropical Asia.

Distinctive convergence in Australian floral colours seen through the eyes of Australian birds

We used a colour-space model of avian vision to assess whether a distinctive bird pollination syndrome exists for floral colour among Australian angiosperms. We also used a novel phylogenetically based method to assess whether such a syndrome represents a significant degree of convergent evolution. About half of the 80 species in our sample that attract nectarivorous birds had floral colours in a small, isolated region of colour space characterized by an emphasis on long-wavelength reflection. The distinctiveness of this ‘red arm’ region was much greater when colours were modelled for violet-sensitive (VS) avian vision than for the ultraviolet-sensitive visual system. Honeyeaters (Meliphagidae) are the dominant avian nectarivores in Australia and have VS vision. Ancestral state reconstructions suggest that 31 lineages evolved into the red arm region, whereas simulations indicate that an average of five or six lineages and a maximum of 22 are likely to have entered in the absence of selection. Thus, significant evolutionary convergence on a distinctive floral colour syndrome for bird pollination has occurred in Australia, although only a subset of bird-pollinated taxa belongs to this syndrome. The visual system of honeyeaters has been the apparent driver of this convergence.

Adaptation of flower and fruit colours to multiple, distinct mutualists

Communication in plant-animal mutualisms frequently involves multiple perceivers. A fundamental uncertainty is whether and how species adapt to communicate with groups of mutualists having distinct sensory abilities. We quantified the colour conspicuousness of flowers and fruits originating from one European and two South American plant communities, using visual models of pollinators (bee and fly) and seed dispersers (bird, primate and marten). We show that flowers are more conspicuous than fruits to pollinators, and the reverse to seed dispersers. In addition, flowers are more conspicuous to pollinators than to seed dispersers and the reverse for fruits. Thus, despite marked differences in the visual systems of mutualists, flower and fruit colours have evolved to attract multiple, distinct mutualists but not unintended perceivers. We show that this adaptation is facilitated by a limited correlation between flower and fruit colours, and by the fact that colour signals as coded at the photoreceptor level are more similar within than between functional groups (pollinators and seed dispersers). Overall, these results provide the first quantitative demonstration that flower and fruit colours are adaptations allowing plants to communicate simultaneously with distinct groups of mutualists.

Analysing photonic structures in plants

The outer layers of a range of plant tissues, including flower petals, leaves and fruits, exhibit an intriguing variation of microscopic structures. Some of these structures include ordered periodic multilayers and diffraction gratings that give rise to interesting optical appearances. The colour arising from such structures is generally brighter than pigment-based colour. Here, we describe the main types of photonic structures found in plants and discuss the experimental approaches that can be used to analyse them. These experimental approaches allow identification of the physical mechanisms producing structural colours with a high degree of confidence.