The dilemma of being a fragrant flower: the major floral volatile attracts pollinators and florivores in the euglossine-pollinated orchid Dichaea pendula

Volatile organic compounds (VOCs) mediate both mutualistic and antagonistic plant-animal interactions; thus, the attraction of mutualists and antagonists by floral VOCs constitutes an important trade-off in the evolutionary ecology of angiosperms. Here, we evaluate the role of VOCs in mediating communication between the plant and its mutualist and antagonist floral visitors. To assess the evolutionary consequences of VOC-mediated signalling to distinct floral visitors, we studied the reproductive ecology of Dichaea pendula, assessing the effects of florivores on fruit set, the pollination efficiency of pollinators and florivores, the floral scent composition and the attractiveness of the major VOC to pollinators and florivores. The orchid depends entirely on orchid-bees for sexual reproduction, and the major florivores, the weevils, feed on corollas causing self-pollination, triggering abortion of 26.4 % of the flowers. Floral scent was composed of approximately 99 % 2-methoxy-4-vinylphenol, an unusual floral VOC attractive to pollinators and florivores. The low fruit set from natural pollination (5.6 %) compared to hand cross-pollination (45.5 %) and low level of pollinator visitation [0.02 visits (flower hour)^-1] represent the limitations to pollination. Our research found that 2-methoxy-4-vinylphenol mediates both mutualistic and antagonistic interactions, which could result in contrary evolutionary pressures on novo-emission. The scarcity of pollinators, not florivory, was the major constraint to fruit set. Our results suggest that, rather than anti-florivory adaptations, adaptations to enhance pollinator attraction and cross-pollination might be the primary drivers of the evolution of VOC emission in euglossine-pollinated flowers.

Heterospecific pollen deposition among plants sharing hummingbird pollinators in the Brazilian Atlantic Forest

Abstract Hummingbirds are the most important group of pollinating birds in the Neotropics and tend to use, concomitantly, more than one plant species as food source. Pollen may be mixed on hummingbirds' body due to the visits to different plant species; therefore, these birds may promote heterospecific pollen deposition (HPD). The hummingbirds potential to promote HPD, the occurrence of HPD and its implications in plant reproduction are scarcely known in the Atlantic Forest. We have studied the transport of pollen by three hummingbird species from the Brazilian Atlantic Forest. We have also checked the actual HPD occurrence under natural conditions in two plant species, namely Canistropsis seidelii and Psychotria nuda. Moreover, we investigated Nidularium innocentii reproductive system evaluating the effect of HPD on its reproduction by simulating a pollen mixture pollination. We found hummingbirds transporting heterospecific pollen mixtures on their bodies, which in turn were deposited onto stigmas of different species. We have also found that mixed pollen deposition had negative effect on the fitness of N. innocentii. We conclude that hummingbirds carry pollen mixtures at the same body parts, leading to potential HPD at the community level. Moreover, hummingbird-plant communities in the Atlantic Rainforest show remarkable similarities in temporal organization and interaction pattern. This suggests that HPD may be a widespread phenomena in these communities.

Pollination ecology of two species of Elleanthus (Orchidaceae): novel mechanisms and underlying adaptations to hummingbird pollination

Relationships among floral biology, floral micromorphology and pollinator behaviour in bird-pollinated orchids are important issues to understand the evolution of the huge flower diversity within Orchidaceae. We aimed to investigate floral mechanisms underlying the interaction with pollinators in two hummingbird-pollinated orchids occurring in the Atlantic forest. We assessed floral biology, nectar traits, nectary and column micromorphologies, breeding systems and pollinators. In both species, nectar is secreted by lip calli through spaces between the medial lamellar surfaces of epidermal cells. Such a form of floral nectar secretion has not been previously described. Both species present functional protandry and are self-compatible yet pollinator-dependent. Fruit set in hand-pollination experiments was more than twice that under natural conditions, evidencing pollen limitation. The absence of fruit set in interspecific crosses suggests the existence of post-pollination barriers between these sympatric co-flowering species. In Elleanthus brasiliensis, fruits resulting from cross-pollination and natural conditions were heavier than those resulting from self-pollination, suggesting advantages to cross-pollination. Hummingbirds pollinated both species, which share at least one pollinator species. Species differences in floral morphologies led to distinct pollination mechanisms. In E. brasiliensis, attachment of pollinarium to the hummingbird bill occurs through a lever apparatus formed by an appendage in the column, another novelty to our knowledge of orchid pollination. In E. crinipes, pollinarium attachment occurs by simple contact with the bill during insertion into the flower tube, which fits tightly around it. The novelties described here illustrate the overlooked richness in ecology and morphophysiology in Orchidaceae.

Bees, birds and yellow flowers: pollinator-dependent convergent evolution of UV patterns

Colour is one of the most obvious advertisements of flowers, and occurs in a huge diversity among the angiosperms. Flower colour is responsible for attraction from a distance, whereas contrasting colour patterns within flowers aid orientation of flower visitors after approaching the flowers. Due to the striking differences in colour vision systems and neural processing across animal taxa, flower colours evoke specific behavioural responses by different flower visitors. We tested whether and how yellow flowers differ in their spectral reflectance depending on the main pollinator. We focused on bees and birds and examined whether the presence or absence of the widespread UV reflectance pattern of yellow flowers predicts the main pollinator. Most bee-pollinated flowers displayed a pattern with UV-absorbing centres and UV-reflecting peripheries, whereas the majority of bird-pollinated flowers are entirely UV- absorbing. In choice experiments we found that bees did not show consistent preferences for any colour or pattern types. However, all tested bee species made their first antennal contact preferably at the UV-absorbing area of the artificial flower, irrespective of its spatial position within the flower. The appearance of UV patterns within flowers is the main difference in spectral reflectance between yellow bee- and bird-pollinated flowers, and affects the foraging behaviour of flower visitors. The results support the hypothesis that flower colours and the visual capabilities of their efficient pollinators are adapted to each other.

The generalist Inga subnuda subsp. luschnathiana (Fabaceae): negative effect of floral visitors on reproductive success?

Inga species are characterised by generalist or mixed pollination system. However, this feature does not enhance reproductive rates in species with very low fruit set under natural conditions. Some ecological and genetic factors are associated with this feature, and to test the effect of massive visits on pollination success in Inga subnuda subsp. luschnathiana, we studied the efficacy of polyads deposited on stigmas of flowers isolated from visitors and polyads exposed to visitors. The proportion of polyads fixed in stigmas decreased after exposure to visitors (24 h) in comparison to stigmas isolated from visitors (hummingbirds, bees, wasps, hawkmoths and bats), and fruit set was very low. Furthermore, nectar production, sugar composition and other floral biology traits were evaluated. Increased nectar production, sugar availability and sucrose dominance during the night indicates adaptation to nocturnal visitors and supports their role as main pollinators; although the brush-flower morphology, time of anthesis, nectar dynamics and chemical composition also allow daytime visitors. Thus the species is an important resource for a diverse group of floral visitors. We conclude that excess visits (diurnal and nocturnal) are responsible for the decrease in fixed polyads in stigmas of I. subnuda subsp. luschnathiana flowers, thus contributing, with others factors, to its low fruit set. Therefore, the generalist pollination system does not result in reproductive advantages because the low fruit set in natural conditions could be the result of a negative effect of visitors/pollinators.

Trees as huge flowers and flowers as oversized floral guides: the role of floral color change and retention of old flowers in Tibouchina pulchra

Floral color changes and retention of old flowers are frequently combined phenomena restricted to the floral guide or single flowers in few-flowered inflorescences. They are thought to increase the attractiveness over long distances and to direct nearby pollinators toward the rewarding flowers. In Tibouchina pulchra, a massively flowering tree, the whole flower changes its color during anthesis. On the first day, the flowers are white and on the next 3 days, they change to pink. This creates a new large-scale color pattern in which the white pre-changed flowers contrast against the pink post-changed ones over the entire tree. We describe the spectral characteristics of floral colors of T. pulchra and test bumblebees' response to this color pattern when viewed at different angles (simulating long and short distances). The results indicated the role of different color components in bumblebee attraction and the possible scenario in which this flower color pattern has evolved. We tested bumblebees' preference for simulated trees with 75% pink and 25% white flowers resembling the color patterns of T. pulchra, and trees with green leaves and pink flowers (control) in long-distance approach. We also compared an artificial setting with three pink flowers and one white flower (T. pulchra model) against four pink flowers with white floral guides (control) in short-distance approach. Bumblebees spontaneously preferred the simulated T. pulchra patterns in both approaches despite similar reward. Moreover, in short distances, pollinator visits to peripheral, non-rewarding flowers occurred only half as frequently in the simulated T. pulchra when compared to the control. Thefore, this exceptional floral color change and the retention of old flowers in T. pulchra favors the attraction of pollinators over long distances in a deception process while it honestly directs them toward the rewarding flowers at short distances possibly exploring their innate color preferences.

Processes entangling interactions in communities: forbidden links are more important than abundance in a hummingbird-plant network

Understanding the relative importance of multiple processes on structuring species interactions within communities is one of the major challenges in ecology. Here, we evaluated the relative importance of species abundance and forbidden links in structuring a hummingbird-plant interaction network from the Atlantic rainforest in Brazil. Our results show that models incorporating phenological overlapping and morphological matches were more accurate in predicting the observed interactions than models considering species abundance. This means that forbidden links, by imposing constraints on species interactions, play a greater role than species abundance in structuring the ecological network. We also show that using the frequency of interaction as a proxy for species abundance and network metrics to describe the detailed network structure might lead to biased conclusions regarding mechanisms generating network structure. Together, our findings suggest that species abundance can be a less important driver of species interactions in communities than previously thought.

Tree size and its relationship with flowering phenology and reproductive output in Wild Nutmeg trees

Reproductive strategies, sexual selection, and their relationship with the phenotype of individuals are topics widely studied in animals, but this information is less abundant for plants. Variability in flowering phenology among individuals has direct impact on their fitness, but how reproductive phenology is affected by the size of the individuals needs further study. We quantified the flowering intensity, length, and reproductive synchronization of two sympatric dioecious Wild Nutmeg tree species (Virola, Myristicaceae) in the Brazilian Atlantic forest, and analyzed its relationships with tree size. Two distinct strategies in flowering timing and intensity were found between species (annual versus biennial flowering), and among individuals in the annual flowering species (extended versus peak flowering). Only for the annual flowering species the reproductive output is related to tree size and large trees present proportionally higher flower coverage, and lower synchronization than smaller ones. Flowering is massive and highly synchronized in the biennial species. Sex ratios are not different from 1:1 in the two species, and in the two segregated reproductive subgroups in the biennial flowering species. The biennial flowering at individual level is a novelty among reproductive patterns in plants, separating the population in two reproductive subgroups. A proportional increase in the reproductive output with size exists only for the annual flowering species. A biennial flowering can allow resource storage favouring massive flowering for all the individuals diluting their relationship with size.

Beyond the pollination syndrome: nectar ecology and the role of diurnal and nocturnal pollinators in the reproductive success of Inga sessilis (Fabaceae)

Inga species present brush-type flower morphology allowing them to be visited by distinct groups of pollinators. Nectar features in relation to the main pollinators have seldom been studied in this genus. To test the hypothesis of floral adaptation to both diurnal and nocturnal pollinators, we studied the pollination ecology of Inga sessilis, with emphasis on the nectar secretion patterns, effects of sequential removals on nectar production, sugar composition and the role of diurnal and nocturnal pollinators in its reproductive success. Inga sessilis is self-incompatible and pollinated by hummingbirds, hawkmoths and bats. Fruit set under natural conditions is very low despite the fact that most stigmas receive polyads with sufficient pollen to fertilise all ovules in a flower. Nectar secretion starts in the bud stage and flowers continually secreting nectar for a period of 8 h. Flowers actively reabsorbed the nectar a few hours before senescence. Sugar production increased after nectar removal, especially when flowers were drained during the night. Nectar sugar composition changed over flower life span, from sucrose-dominant (just after flower opening, when hummingbirds were the main visitors) to hexose-rich (throughout the night, when bats and hawkmoths were the main visitors). Diurnal pollinators contributed less than nocturnal ones to fruit production, but the former were more constant and reliable visitors through time. Our results indicate I. sessilis has floral adaptations, beyond the morphology, that encompass both diurnal and nocturnal pollinator requirements, suggesting a complementary and mixed pollination system.

Reproductive biology and species geographical distribution in the Melastomataceae: a survey based on New World taxa

BACKGROUND AND AIMS

Apomictic plants are less dependent on pollinator services and able to occupy more diverse habitats than sexual species. However, such assumptions are based on temperate species, and comparable evaluation for species-rich Neotropical taxa is lacking. In this context, the Melastomataceae is a predominantly Neotropical angiosperm family with many apomictic species, which is common in the Campos Rupestres, endemism-rich vegetation on rocky outcrops in central Brazil. In this study, the breeding system of some Campo Rupestre Melastomataceae was evaluated, and breeding system studies for New World species were surveyed to test the hypothesis that apomixis is associated with wide distributions, whilst sexual species have more restricted areas.

METHODS

The breeding systems of 20 Campo Rupestre Melastomataceae were studied using hand pollinations and pollen-tube growth analysis. In addition, breeding system information was compiled for 124 New World species of Melastomataceae with either wide (>1000 km) or restricted distributions.

KEY RESULTS

Most (80 %) of the Campo Rupestre species studied were self-compatible. Self-incompatibility in Microlicia viminalis was associated with pollen-tube arrest in the style, as described for other Melastomataceae, but most self-incompatible species analysed showed pollen-tube growth to the ovary irrespective of pollination treatment. Apomictic species showed lower pollen viability and were less frequent among the Campo Rupestre plants. Among the New World species compiled, 43 were apomictic and 77 sexual (24 self-incompatible and 53 self-compatible). Most apomictic (86 %) and self-incompatible species (71 %) presented wide distributions, whilst restricted distributions predominate only among the self-compatible ones (53 %).

CONCLUSIONS

Self-compatibility and dependence on biotic pollination were characteristic of Campo Rupestre and narrowly distributed New World Melastomataceae species, whilst apomictics are widely distributed. This is, to a certain extent, similar to the geographical parthenogenesis pattern of temperate apomictics.

Armament imbalances: match and mismatch in plant-pollinator traits of highly specialized long-spurred orchids

Background

Some species of long-spurred orchids achieve pollination by a close association with long-tongued hawkmoths. Among them, several Habenaria species present specialized mechanisms, where pollination success depends on the attachment of pollinaria onto the heads of hawkmoths with very long proboscises. However, in the Neotropical region such moths are less abundant than their shorter-tongued relatives and are also prone to population fluctuations. Both factors may give rise to differences in pollinator-mediated selection on floral traits through time and space.

Methodology/Principal Findings

We characterized hawkmoth assemblages and estimated phenotypic selection gradients on orchid spur lengths in populations of three South American Habenaria species. We examined the match between hawkmoth proboscis and flower spur lengths to determine whether pollinators may act as selective agents on flower morphology. We found significant directional selection on spur length only in Habenaria gourlieana, where most pollinators had proboscises longer than the mean of orchid spur length.

Conclusions/Significance

Phenotypic selection is dependent on the mutual match between pollinator and flower morphologies. However, our findings indicate that pollinator-mediated selection may vary through time and space according to local variations in pollinator assemblages.

Specialization in plant-hummingbird networks is associated with species richness, contemporary precipitation and quaternary climate-change velocity

Large-scale geographical patterns of biotic specialization and the underlying drivers are poorly understood, but it is widely believed that climate plays an important role in determining specialization. As climate-driven range dynamics should diminish local adaptations and favor generalization, one hypothesis is that contemporary biotic specialization is determined by the degree of past climatic instability, primarily Quaternary climate-change velocity. Other prominent hypotheses predict that either contemporary climate or species richness affect biotic specialization. To gain insight into geographical patterns of contemporary biotic specialization and its drivers, we use network analysis to determine the degree of specialization in plant-hummingbird mutualistic networks sampled at 31 localities, spanning a wide range of climate regimes across the Americas. We found greater biotic specialization at lower latitudes, with latitude explaining 20-22% of the spatial variation in plant-hummingbird specialization. Potential drivers of specialization--contemporary climate, Quaternary climate-change velocity, and species richness--had superior explanatory power, together explaining 53-64% of the variation in specialization. Notably, our data provides empirical evidence for the hypothesized roles of species richness, contemporary precipitation and Quaternary climate-change velocity as key predictors of biotic specialization, whereas contemporary temperature and seasonality seem unimportant in determining specialization. These results suggest that both ecological and evolutionary processes at Quaternary time scales can be important in driving large-scale geographical patterns of contemporary biotic specialization, at least for co-evolved systems such as plant-hummingbird networks.

Avoidance of achromatic colours by bees provides a private niche for hummingbirds

That hummingbird-pollinated plants predominantly have red flowers has been known for decades, but well-investigated research studies are still rare. Preference tests have shown that hummingbirds do not have an innate preference for red colours. In addition, hummingbirds do not depend solely upon red flowers, because white-flowered hummingbird-pollinated plants are also common and temporarily abundant. Here we show that both white and red hummingbird-pollinated flowers differ from bee-pollinated flowers in their reflection properties for ultraviolet (UV) light. Hummingbird-pollinated red flowers are on average less UV reflective, and white hummingbird-pollinated flowers are more UV reflective than the same coloured bee-pollinated ones. In preference tests with artificial flowers, neotropical orchid bees prefer red UV-reflecting artificial flowers and white UV-nonreflecting flowers over red and white flowers with the opposite UV properties. By contrast, hummingbirds showed no preference for any colour in the same tests. Plotting floral colours and test stimuli into the honeybees' perceptual colour space suggests that the less attractive colours are achromatic for bees and therefore more difficult to detect against the background. This underlying colour preference in bees might provide hummingbirds with a private niche that is not attractive to bees.

Contrasting bee pollination in two co-occurring distylic species of Cordia (Cordiaceae, Boraginales) in the Brazilian semi-arid Caatinga: generalist in C. globosa vs. specialist in C. leucocephala

In this study we compare the reproductive biology of Cordia globosa and C. leucocephala (Cordiaceae, Boraginales; formerly referred to Boraginaceae) to understand the functioning of the floral morphs and the relations with their effective pollinators. The species are synchronopatric, distylic, and self-incompatible. Though they share melittophilous traits, the main visitor and pollinator of C. globosa was the generalist and exotic bee Apis mellifera, while the only one of C. leucocephala was the oligoletic bee Ceblurgus longipalpis. These two latter species are restricted to the Caatinga of NE Brazil, contrasting with the wide distribution of Cordia globosa. While the fruit-set for C. globosa was high, independently if the pollen donor/stigma receptor was a pin (long-styled) or thrum (short-styled) individual, in C. leucocephala the fruit-set was low and occurred only when a thrum individual was the pollen donor. This raises the possibility of this species moving towards dioecy. The high natural fruit-set of C. globosa confirms the generalist bee as its effective pollinator. The low fruit-set after manual crosses in C. leucocephala may be due to low pollen viability. Additionally, the low natural fruit-set (two times lower than after crosses) may be related with the foraging behavior of the specialist pollinator.