The pollination ecology of buzz-pollinated Rhexia virginica (Melastomataceae)

Harvesting pollen with vibrations: towards an integrative understanding of the proximate and ultimate reasons for buzz pollination

Buzz pollination, a type of interaction in which bees use vibrations to extract pollen from certain kinds of flowers, captures a close relationship between thousands of bee and plant species. In the last 120 years, studies of buzz pollination have contributed to our understanding of the natural history of buzz pollination, and basic properties of the vibrations produced by bees and applied to flowers in model systems. Yet, much remains to be done to establish its adaptive significance and the ecological and evolutionary dynamics of buzz pollination across diverse plant and bee systems. Here, we review for bees and plants the proximate (mechanism and ontogeny) and ultimate (adaptive significance and evolution) explanations for buzz pollination, focusing especially on integrating across these levels to synthesize and identify prominent gaps in our knowledge. Throughout, we highlight new technical and modelling approaches and the importance of considering morphology, biomechanics and behaviour in shaping our understanding of the adaptive significance of buzz pollination. We end by discussing the ecological context of buzz pollination and how a multilevel perspective can contribute to explain the proximate and evolutionary reasons for this ancient bee-plant interaction.

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 in angiosperms. 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.

Just add water: Rainfall-induced anther closure and color change in Ripariosida hermaphrodita (Malvaceae)

Anther opening has commonly been thought of as unidirectional, but reports of anthers closing in response to rainfall show this is not the case. In some species, anther closure can protect pollen from degrading or washing away, thus possibly enhancing male fitness. Similarly, although floral color is often presumed to be static, numerous floral parts may change color during blooming. These color changes primarily occur in response to pollination or aging, thus potentially increasing pollination efficiency by directing floral visitors to recently opened, unpollinated flowers. Daily observations of 364 Ripariosida hermaphrodita flowers from seven individuals showed that anthers that were purple, open, and shedding pollen became beige colored and tightly closed after rainfall. These findings were further supported by observations of plants exposed to simulated rainfall in a greenhouse and time-lapse photography of flowers misted with water. To our knowledge, our work represents the first report of anther closure in response to rain in Malvaceae and the first report of floral color change induced by rainfall.

Effects of habitat disturbance on the reproductive ecology of Arum pictum ssp. sagittifolium: from pollination to seedling recruitment

BACKGROUND AND AIMS

The loss of natural habitats may strongly affect the fitness of plants that depend on animals for reproduction. However, very little is known regarding the differential effects of habitat disturbance on the distinct phases of the reproductive cycle of plants, especially in non-rewarding species.

METHODS

We assessed the effects of habitat disturbance on the entire reproductive cycle of Arum pictum ssp. sagittifolium, a species with deceptive pollination that is endemic to the western Mediterranean Basin. For this, we performed hand-pollination and bagging experiments, evaluated the role of pollinators and dispersers on reproduction, and estimated seedling recruitment in three natural and three disturbed populations (according to their surrounding natural habitat) in Mallorca Island.

KEY RESULTS

Pollinators were sphaerocerid flies (mainly Coproica, with ~50 % of visits) and staphylinid beetles, and were required for sexual reproduction. Habitat disturbance differently affected the reproductive phases of A. pictum ssp. sagittifolium. Habitat disturbance had a positive effect on Shannon pollinator diversity (but not on pollinator richness), and total pollinator and Coproica abundance were three times higher in disturbed habitats, where overall seed production was also ~30 % higher in natural habitats. Seed production increased with Coproica abundance, but only in natural habitats. Seed dispersers of A. pictum ssp. sagittifolium were birds, mainly Sylvia atricapilla. Although habitat disturbance did not influence disperser diversity or abundance, the majority of seedlings appeared under adult plants and in natural habitats.

CONCLUSIONS

Overall recruitment was higher in natural habitats, but this effect could have been masked by only assessing pollinator and disperser numbers, as processes related to the quality of these interactions might be influencing fitness. Our study highlights the need to study different reproductive phases and their multiple components and processes to properly understand the effects of habitat disturbance on the regeneration of plant populations.

Evolution and diversity of the angiosperm anther: trends in function and development

Anther development and dehiscence is considered from an evolutionary perspective to identify drivers for differentiation, functional conservation and to identify key questions for future male reproduction research. Development of viable pollen and its timely release from the anther are essential for fertilisation of angiosperm flowers. The formation and subsequent dehiscence of the anther are under tight regulatory control, and these processes are remarkably conserved throughout the diverse families of the angiosperm clade. Anther development is a complex process, which requires timely formation and communication between the multiple somatic anther cell layers (the epidermis, endothecium, middle layer and tapetum) and the developing pollen. These layers go through regulated development and selective degeneration to facilitate the formation and ultimate release of the pollen grains. Insight into the evolution and divergence of anther development and dehiscence, especially between monocots and dicots, is driving greater understanding of the male reproductive process and increased, resilient crop yields. This review focuses on anther structure from an evolutionary perspective by highlighting their diversity across plant species. We summarise new findings that illustrate the complexities of anther development and evaluate how they challenge established models of anther form and function, and how they may help to deliver future sustainable crop yields.

Buzz-Pollination in a Tropical Montane Cloud Forest: Compositional Similarity and Plant-Pollinator Interactions

Buzz-pollinated plants are an essential source of pollen for a significant portion of local bee communities. Buzz pollination research has focused on studying the properties of bee buzzes and their implications on pollen release, morphological specialization of flowers, and the reproductive ecology of buzz-pollinated plants. In contrast, diversity patterns and ecological interactions between bees and buzz-pollinated plants have been studied less. This study analyzed the buzzing bee community of twelve tropical buzz-pollinated co-occurring plant species in a tropical montane cloud forest during the flowering periods of two consecutive years, focusing on diversity, compositional similarity, structure, and specialization (H₂´) of the network. Twenty-one bee species belonging to Apidae, Colletidae, and Halictidae were recorded, fifteen species in 2014, and eighteen in 2015. Floral display and visited flowers doubled from first to second year, although the flowering period was 2 months longer in the first year. Bee compositional similarity between plants tended to be low; however, this was due rather to a high nestedness than species replacement. Temporal bee compositional similarity was also low but variable, and different plant species showed the highest similarity between years. The number of bee visits depended significantly on the number of flowers and years. Interactions between bees and plants showed a tendency to generalization. Compared to other buzz-pollinated networks, specialization (H₂´) was similar, but diversity was low and the network small. In endangered ecosystems like the Mexican cloud forest, however, buzzing bees support biodiversity and provide an essential ecological service by pollinating dominant understory flora.

Connective appendages in Huberia bradeana (Melastomataceae) affect pollen release during buzz pollination

Floral structures, such as stamen appendages, play crucial roles in pollinator attraction, pollen release dynamics and, ultimately, the reproductive success of plants. The pollen-rewarding, bee buzz-pollinated flowers of Melastomataceae often bear conspicuous staminal appendages. Surprisingly, their functional role in the pollination process remains largely unclear. We use Huberia bradeana Bochorny & R. Goldenb. (Melastomataceae) with conspicuously elongated, twisted stamen appendages to investigate their functional role in the pollination process. We studied the effect of stamen appendages on pollinator behaviour and reproductive success by comparing manipulated flowers (appendages removed) with unmanipulated flowers. To assess bee pollinator behaviour, we measured three properties of buzzes (vibrations) produced by bees on Huberia flowers: frequency, duration and number of buzzes per flower visit. We measured male and female reproductive success by monitoring pollen release and deposition after single bee visits. Finally, we used artificial vibrations and laser vibrometry to assess how flower vibrational properties change with the removal of stamen appendages. Our results show that the absence of staminal appendages does not modify bee buzzing behaviour. Pollen release was higher in unmanipulated flowers, but stigmatic pollen loads differ only marginally between the two treatments. We also detected lower vibration amplitudes in intact flowers as compared to manipulated flowers in artificial vibration experiments. The presence of connective appendages are crucial in transmitting vibrations and assuring optimal pollen release. Therefore, we propose that the high diversity of colours, shapes and sizes of connective appendages in buzz-pollinated flowers may have evolved by selection through male fitness.

Pollen dispensing schedules in buzz-pollinated plants: experimental comparison of species with contrasting floral morphologies

PREMISE

Plants can mitigate the fitness costs associated with pollen consumption by floral visitors by optimizing pollen release rates. In buzz-pollinated plants, bees apply vibrations to remove pollen from anthers with small pores. These poricidal anthers potentially function as mechanism staggering pollen release, but this has rarely been tested across plant species differing in anther morphology.

METHODS

In Solanum Section Androceras, three pairs of buzz-pollinated species have undergone independent evolutionary shifts between large- and small-flowers, which are accompanied by replicate changes in anther morphology. We used these shifts in anther morphology to characterize the association between anther morphology and pollen dispensing schedules. We applied simulated bee-like vibrations to anthers to elicit pollen release, and compared pollen dispensing schedules across anther morphologies. We also investigated how vibration velocity affects pollen release.

RESULTS

Replicate transitions in Solanum anther morphology are associated with consistent changes in pollen dispensing schedules. We found that small-flowered taxa release their pollen at higher rates than their large-flowered counterparts. Higher vibration velocities resulted in quicker pollen dispensing and more total pollen released. Finally, both the pollen dispensing rate and the amount of pollen released in the first vibration were negatively related to anther wall area, but we did not observe any association between pore size and pollen dispensing.

CONCLUSIONS

Our results provide the first empirical demonstration that the pollen dispensing properties of poricidal anthers depend on both floral characteristics and bee vibration properties. Morphological modification of anthers could thus provide a mechanism to exploit different pollination environments.

Flower-visiting insects of genus Melastoma (Myrtales: Melastomataceae) at the Fushan Botanical Garden, Taiwan

Background

We investigated the diversity and behaviour of insects that visit flowers of four native Melastoma (Family Melastomataceae) species of Taiwan and a horticultural hybrid Melastoma species at the Fushan Botanical Garden, Taiwan biweekly from May to August 2020. Visits of flower-visiting insects were classified into seven behavioural categories, based on the insects' behaviour and positions on the flower. The data are further assigned into four insect-flower interactions, namely pollination, herbivory, commensalism and neutralism. Our goal is to provide baseline data of insect-plant interactions of Melastoma, which is a common, but understudied plant genus in the country.

New information

A total of 1,289 visits to flowers were recorded by at least 63 insect morphospecies belonging to seven orders. The number of insect species recorded per Melastoma species ranged from 9 to 39. Visiting, sonication and passing were the three most frequently recorded types of behaviour, collectively accounting for 90.2% (n = 1,240) of the total observations. Pollination was the most dominant insect-flower interaction, accounting for 70.2% of the total observations, followed by neutralism (20.0%), herbivory (6.3%) and commensalism (3.5%). Sweat bees of the genera Lasioglossum and Maculonomia (Hymenoptera: Halictidae) are considered key pollinators to Melastoma species in Fushan Botanical Garden, based on their high number of visits and sonication behaviour. Our study provides the first list of insects that visit the flowers of all Taiwan's known Melastoma species and description of their interactions with the plants.

Female and male fitness of a sexually deceptive orchid with a narrow distribution area: from phenotypic traits to spatial distribution patterns

The Orchidaceae family presents one of the most extravagant pollination mechanisms: deception. While many studies on reproductive success have been performed on food-deception orchids, less have been performed on sexually deceptive orchids. Here, we focused on Ophrys balearica P. Delforge, an endemic orchid of the Balearic Islands, to study its reproductive ecology, the spatio-temporal variation of its reproductive success and the individual (floral display and geospatial position) and population parameters (patch size, shape and density) that affect its reproductive success. We performed hand-pollination experiments, along with the recording of floral display parameters and GPS position of over 1,100 individuals from seven populations in two consecutive years. We applied, for the first time, GIS tools to analyse the effects of individual's position within the population on the reproductive success. Reproductive success was measured both in male (removed pollinia) and female (fruit set) fitness. The results confirm that this species is pollinator-dependent and mostly allogamous, but also self-compatible. This species showed high values for the cumulative inbreeding depression index and high pollen limitation. Male fitness was almost equal to female fitness between years and populations, and reproductive success exhibited huge spatio-temporal variation. Although we did not find strong correlations between floral display and reproductive success, patches with low-plant density and individuals in the external portion of the population showed significantly higher plant fitness. These findings must be considered in conservation actions for endangered orchid species, especially considering that most orchids are strongly dependent on pollinators for their species' fitness.

Buzz pollination: studying bee vibrations on flowers

Approximately 6% of flowering plant species possess flowers with anthers that open through small pores or slits. Extracting pollen from this type of specialised flower is achieved most efficiently by vibrating the anthers, a behaviour that has evolved repeatedly among bees. Here I provide a brief overview of the study of vibrations produced by bees and their effects on pollen release. I discuss how bee morphology and behaviour affect the mechanical properties of vibrations, and how floral traits may influence the transmission of those vibrations from the bee to the anther, thus mediating pollen release, and ultimately bee and plant fitness. I suggest that understanding the evolution of buzz pollination requires a study of the biomechanics of bee vibrations and their transmission on flowers.

Does body size predict the buzz-pollination frequencies used by bees?

Body size is an important trait linking pollinators and plants. Morphological matching between pollinators and plants is thought to reinforce pollinator fidelity, as the correct fit ensures that both parties benefit from the interaction. We investigated the influence of body size in a specialized pollination system (buzz-pollination) where bees vibrate flowers to release pollen concealed within poricidal stamens. Specifically, we explored how body size influences the frequency of buzz-pollination vibrations. Body size is expected to affect frequency as a result of the physical constraints it places on the indirect flight muscles that control the production of floral vibrations. Larger insects beat their wings less rapidly than smaller-bodied insects when flying, but whether similar scaling relationships exist with floral vibrations has not been widely explored. This is important because the amount of pollen ejected is determined by the frequency of the vibration and the displacement of a bee's thorax. We conducted a field study in three ecogeographic regions (alpine, desert, grassland) and recorded flight and floral vibrations from freely foraging bees from 27 species across four families. We found that floral vibration frequencies were significantly higher than flight frequencies, but never exceeded 400 Hz. Also, only flight frequencies were negatively correlated with body size. As a bee's size increased, its buzz ratio (floral frequency/flight frequency) increased such that only the largest bees were capable of generating floral vibration frequencies that exceeded double that of their flight vibrations. These results indicate size affects the capacity of bees to raise floral vibration frequencies substantially above flight frequencies. This may put smaller bees at a competitive disadvantage because even at the maximum floral vibration frequency of 400 Hz, their inability to achieve comparable thoracic displacements as larger bees would result in generating vibrations with lower amplitudes, and thus less total pollen ejected for the same foraging effort.

Light induces petal color change in Quisqualis indica (Combretaceae)

Petal color change, a common phenomenon in angiosperms, is induced by various environmental and endogenous factors. Interestingly, this phenomenon is important for attracting pollinators and further reproductive success. Quisqualis indica L. (Combretaceae) is a tropical Asian climber that undergoes sequential petal color change from white to pink to red. This color changing process is thought to be a good strategy to attract more pollinators. However, the underlying physiological and biochemical mechanisms driving this petal color change phenomenon is still underexplored. In this context, we investigated whether changes in pH, pollination, light, temperature or ethylene mediate petal color change. We found that the detected changes in petal pH were not significant enough to induce color alterations. Additionally, pollination and temperatures of 20-30 °C did not alter the rate of petal color change; however, flowers did not open when exposed to constant temperatures at 15 °C or 35 °C. Moreover, the application of ethylene inhibitor, i.e., silver thiosulphate, did not prevent color change. It is worth mentioning here that in our study we found light as a strong factor influencing the whole process of petal color change, as petals remained white under dark conditions. Altogether, the present study suggests that petal color change in Q. indica is induced by light and not by changes in petal pH, pollination, ethylene, or temperature, while extremely low or high temperatures affect flower anthesis. In summary, our findings represent the probable mechanism underlying the phenomenon of petal color change, which is important for understanding flower color evolution.

Economia de pólen favorecida pela heteranteria em Desmocelis villosa (Melastomataceae)

Resumo A heteranteria sempre despertou interesse quanto à sua funcionalidade. As flores de Desmocelis villosa (Melastomataceae), possuem estames dimorfos, sendo um grupo composto por cinco anteras amarelas e o outro por cinco anteras de cor púrpura. O presente estudo avaliou se a heteranteria promove divisão de trabalho nessa espécie. Foi realizado um experimento com remoção de anteras de cada tipo para se comparar a taxa de visitação por abelhas. Ao vibrar o conjunto de estames, as abelhas coletavam o pólen das anteras amarelas enquanto as de cor púrpura depositavam seu pólen na parte final do corpo das mesmas em uma posição próxima ao estigma favorecendo a polinização. Flores com anteras amarelas apresentaram maiores taxas de visitação que aquelas que tinham apenas anteras púrpuras. Nesta espécie a heteranteria se mostra como uma estratégia eficaz na economia do pólen utilizado com o serviço de polinização, pois embora os dois tipos de anteras apresentem alta porcentagem de grãos viáveis, as anteras amarelas apresentam significativamente menos grãos de pólen que as púrpuras, indicando que a planta estrategicamente reserva a maior parte do pólen para a reprodução sexuada.

Pollinator responses to floral colour change, nectar, and scent promote reproductive fitness in Quisqualis indica (Combretaceae)

Floral colour change is visual signals for pollinators to avoid old flowers and increase pollination efficiency. Quisqualis indica flowers change colour from white to pink to red may be associated with a shift from moth to butterfly pollination. To test this hypothesis, we investigated Q. indica populations in Southwest China. Flowers secreted nectar continuously from the evening of anthesis until the following morning, then decreased gradually with floral colour change. The scent compounds in the three floral colour stages were similar; however, the scent composition was different, and the scent emission rate decreased from the white to red stage. Dichogamy in Q. indica prevents self-pollination and interference of male and female functions. Controlled pollinations demonstrated that this species is self-incompatible and needs pollinators for seed production. Different pollinators were attracted in each floral colour stage; mainly moths at night and bees and butterflies during the day. Observations of open-pollinated inflorescences showed that white flowers had a higher fruit set than pink or red flowers, indicating the high contribution of moths to reproductive success. We concluded that the nectar and scent secretion are related to floral colour change in Q. indica, in order to attract different pollinators and promote reproductive fitness.

Comparison of pollination and defensive buzzes in bumblebees indicates species-specific and context-dependent vibrations

Bees produce vibrations in many contexts, including for defense and while foraging. Buzz pollination is a unique foraging behavior in which bees vibrate the anthers of flowers to eject pollen which is then collected and used as food. The relationships between buzzing properties and pollen release are well understood, but it is less clear to what extent buzzing vibrations vary among species, even though such information is crucial to understanding the functional relationships between bees and buzz-pollinated plants. Our goals in this study were (1) to examine whether pollination buzzes differ from those produced during defense, (2) to evaluate the similarity of buzzes between different species of bumblebees (Bombus spp.), and (3) to determine if body size affects the expression of buzzing properties. We found that relative peak amplitude, peak frequency, and duration were significantly different between species, but only relative peak amplitude differed between pollination and defensive buzzes. There were significant interactions between species and buzz type for peak frequency and duration, revealing that species differed in their patterns of expression in these buzz properties depending on the context. The only parameter affected by body size was duration, with larger bees producing shorter buzzes. Our findings suggest that although pollination and defensive buzzes differ in some properties, variability in buzz structure also exhibits a marked species-specific component. Species differences in pollination buzzes may have important implications for foraging preferences in bumblebees, especially if bees select flowers best matched to release pollen for their specific buzzing characteristics.

What's the 'buzz' about? The ecology and evolutionary significance of buzz-pollination

Many plant species have evolved floral characteristics that restrict pollen access. Some of these species are visited by insects, principally bees, which make use of vibrations to extract pollen from anthers. Buzz-pollination, as this phenomenon is generally known, is a widespread method of fertilization for thousands of species in both natural and agricultural systems. Despite its prevalence in pollination systems, the ecological and evolutionary conditions that favour the evolution of buzz-pollination are poorly known. We briefly summarize the biology of buzz-pollination and review recent studies on plant and pollinator characteristics that affect pollen removal. We suggest that buzz-pollination evolves as the result of an escalation in the competition between plants and pollen-consuming floral visitors (including pollen thieves and true pollinators) to control the rate of pollen removal from flowers.

Variação da viabilidade polínica em Tibouchina (Melastomataceae)

Resumo O gênero Tibouchina é comum no Brasil e encontrado principalmente na Floresta Atlântica, em especial na região Sudeste. No presente estudo foi avaliada a viabilidade polínica de cinco espécies de Floresta Atlântica, T. cerastifolia, T. clinopodifolia, T. gracilis, T. pulchra e T. sellowiana, utilizando-se três soluções histoquímicas: carmim acético, Alexander e cloreto de 2,3,5-trifenil-tetrazólio. Foi avaliada a variação de viabilidade entre as anteras dimórficas, entre as soluções histoquímicas e entre os indivíduos. Em T. pulchra e T. sellowiana também foi avaliada a variação entre os dois primeiros dias a partir da abertura da flor. Nenhuma espécie apresentou variação relacionada ao dimorfismo das anteras. Nas duas espécies que apresentam maior longevidade floral, a viabilidade foi semelhante no primeiro e segundo dias em T. pulchra, enquanto que em T. sellowiana as flores de segundo dia apresentaram pólen menos viável. As soluções histoquímicas empregadas revelaram taxas de viabilidade polínica distintas. Houve variação na viabilidade polínica entre os indivíduos em todas as espécies. A solução de cloreto de 2,3,5-trifenil-tetrazólio revelou a menor taxa de viabilidade em todos os indivíduos. As soluções histoquímicas empregadas, a amostragem e a longevidade floral foram fatores relacionados à variação da viabilidade polínica.

A test of the effect of floral color change on pollination effectiveness using artificial inflorescences visited by bumblebees

Floral color change has been recognized as a pollination strategy, but its relative effectiveness has been evaluated insufficiently with respect to other floral traits. In this study, effects of floral color change on the visitation pattern of bumblebees were empirically assessed using artificial flowers. Four inflorescence types were postulated as strategies of flowering behavior: type 1 has no retention of old flowers, resulting in a small display size; type 2 retains old flowers without nectar production; type 3 retains old flowers with nectar; and type 4 retains color-changed old flowers without nectar. Effects of these treatments varied depending on both the total display size (single versus multiple inflorescences) and the pattern of flower-opening. In the single inflorescence experiment, a large floral display due to the retention of old flowers (types 2-4) enhanced pollinator attraction, and the number of flower visits per stay decreased with color change (type 4), suggesting a decrease in geitonogamous pollination. Type-4 plants also reduced the foraging time of bees in comparison with type-2 plants. In the multiple inflorescence experiment, the retention of old flowers did not contribute to pollinator attraction. When flowering occurred sequentially within inflorescences, type-4 plants successfully decreased the number of visits and the foraging time in comparison with type-2 plants. In contrast, floral color change did not influence the number of visits, and it extended the foraging time when flowering occurred simultaneously within inflorescences but the opening of inflorescences progressed sequentially within a plant. Therefore, the effectiveness of floral color change is highly susceptible to the display size and flowering pattern within plants, and this may limit the versatility of the color change strategy in nature.

Fruit set of highland coffee increases with the diversity of pollinating bees

The worldwide decline of pollinators may negatively affect the fruit set of wild and cultivated plants. Here, we show that fruit set of the self-fertilizing highland coffee (Coffea arabica) is highly variable and related to bee pollination. In a comparison of 24 agroforestry systems in Indonesia, the fruit set of coffee could be predicted by the number of flower-visiting bee species, and it ranged from ca. 60% (three species) to 90% (20 species). Diversity, not abundance, explained variation in fruit set, so the collective role of a species-rich bee community was important for pollination success. Additional experiments showed that single flower visits from rare solitary species led to higher fruit set than with abundant social species. Pollinator diversity was affected by two habitat parameters indicating guild-specific nesting requirements: the diversity of social bees decreased with forest distance, whereas the diversity of solitary bees increased with light intensity of the agroforestry systems. These results give empirical evidence for a positive relationship between ecosystem functions such as pollination and biodiversity. Conservation of rainforest adjacent to adequately managed agroforestry systems could improve the yields of farmers.

Melastomeae come full circle: biogeographic reconstruction and molecular clock dating

Rhexia, with 11 species in the Coastal Plain province of North America, is the only temperate zone endemic of the tropical eudicot family Melastomataceae. It is a member of the only pantropical tribe of that family, Melastomeae. Based on the chloroplast gene ndhF, we use a fossil-calibrated molecular clock to address the question of the geographic origin and age of Rhexia. Sequences from 37 species in 21 genera representing the tribe's geographical range were analyzed together with five outgroups. To obtain better clade support, another chloroplast region, the rpl16 intron, was added for 24 of the species. Parsimony analysis of the combined data and maximum-likelihood analysis of ndhF alone indicate that the deepest split is between Rhexia plus its sister group, a small Central American genus, and all other Melastomeae. Old World Melastomeae are monophyletic and nested within New World Melastomeae. Although likelihood-ratio tests of clock and nonclock substitution models for the full or moderately pruned datasets rejected the clock, these models yielded identical topologies (for 30 taxa) with few significantly different branch lengths as assessed by a Student's t-test. Age estimates obtained were 22 million years ago (Mya) for the divergence of Rhexia from its sister group, 12 Mya for the dispersal of Melastomeae from the New World to West Africa, and 1 Mya for the diversification of Melastoma in Southeast Asia. The only other genus of Melastomeae to have reached Southeast Asia from Africa or Madagascar is Osbeckia. The age and geographic distribution of fossils, which come from Miocene sites throughout Eurasia, suggest that Melastomeae once ranged from Eurasia across Beringia to North America from whence they reached South America and subsequently Africa and Southeast Asia. Climate deterioration led to their extinction in the Northern Hemisphere, with Rhexia possibly surviving in Coastal Plain refugia.

Molecular phylogenetics of Melastomataceae and Memecylaceae: implications for character evolution

Melastomataceae are among the most abundant and diversified groups of plants throughout the tropics, but their intrafamily relationships and morphological evolution are poorly understood. Here we report the results of parsimony and maximum likelihood (ML) analyses of cpDNA sequences from the rbcL and ndhF genes and the rpl16 intron, generated for eight outgroups (Crypteroniaceae, Alzateaceae, Rhynchocalycaceae, Oliniaceae, Penaeaceae, Myrtaceae, and Onagraceae) and 54 species of melastomes. The sample represents 42 of the family's currently recognized ∼150 genera, the 13 traditional tribes, and the three subfamilies, Astronioideae, Melastomatoideae, and Memecyloideae (= Memecylaceae DC.). Parsimony and ML yield congruent topologies that place Memecylaceae as sister to Melastomataceae. Pternandra, a Southeast Asian genus of 15 species of which five were sampled, is the first- branching Melastomataceae. This placement has low bootstrap support (72%), but agrees with morphological treatments that placed Pternandra in Melastomatacaeae because of its acrodromal leaf venation, usually ranked as a tribe or subfamily. The interxylary phloem islands found in Memecylaceae and Pternandra, but not most other Melastomataceae, likely evolved in parallel because Pternandra resembles Melastomataceae in its other wood characters. A newly discovered plesiomorphic character in Pternandra, also present in Memecylaceae, is a fibrous anther endothecium. Higher Melastomataceae lack an endothecium as do the closest relatives of Melastomataceae and Memecylaceae. The next deepest split is between Astronieae, with anthers opening by slits, and all remaining Melastomataceae, which have anthers opening by pores. Within the latter, several generic groups, corresponding to traditional tribes, receive solid statistical support, but relationships among them, with one exception, are different from anything predicted on the basis of morphological data. Thus, Miconieae and Merianieae are sister groups, and both are sister to a trichotomy of Bertolonieae, Microlicieae + Melastomeae, and Dissochaeteae + Blakeeae. Sonerileae/Oxysporeae are nested within Dissochaeteae, Rhexieae within Melastomeae, and African and Asian Melastomeae within neotropical Melastomeae. These findings have profound implications for our understanding of melastome morphological evolution (and biogeography), implying, for example, that berries evolved from capsules minimally four times, stamen connectives went from dorsally enlarged to basal/ventrally enlarged, and loss of an endothecium preceded poricidal dehiscence.

Floral Display in Narcissus: Variation in Flower Size and Number at the Species, Population, and Individual Levels

Floral display (the size, number, and arrangement of open flowers) influences pollinator visitation to animal-pollinated plants and should be an important determinant of reproductive success. We examined variation in the size and number of open flowers in wild daffodils (Narcissus). Our analysis of published data on 45 taxa showed that flower number varied negatively with flower diameter among Narcissus species, which supports the widespread assumption that there is a trade-off between these traits. In contrast, field measurements indicated a positive relation between flower number and diameter within two populations of Narcissus dubius, and no relation was evident after we controlled for variation in bulb size. The discrepancy between inter- and intraspecific patterns may have occurred because variable resource levels obscure trade-offs when variation in flower size is low (e.g., within species). Size-related increases in floral tube length were half as great as corresponding increases in flower diameter, a result that is consistent with stronger stabilizing selection on tube length. Staggered flowering within N. dubius inflorescences limited the mean number of open flowers to <66% of total flower number, and slow expansion by later opening flowers resulted in significant differences in flower size throughout flowering. Although pollinators preferred large flowers, experimental reductions in flower diameter did not affect seed production. Our results illustrate how the relative importance of the factors influencing floral display can vary among levels of biological organization. Interspecific variation in flower size and number appeared to be constrained by allocation trade-offs, but intraspecific variation in both traits was more greatly influenced by plant resource status. Within plants, the size and number of open flowers reflected the relative age of individual flowers and floral longevity.

The pollination ecology of buzz-pollinated Rhexia virginica (Melastomataceae)

We examined the function of floral traits associated with buzz pollination through studies of Rhexia virginica (Melastomataceae) in the Muskoka region of Ontario, Canada. Controlled pollinations demonstrated that the species is self-compatible, but dependent on insects for pollen transfer. Bumble bees made 82 and 90% of observed insect visits to R. virginica in 1996 and 1997, respectively, and effectively buzzed flowers. Buzz pollination did not appear to be highly "specialized" since various species of bumble bee were capable of pollination, and pollen transfer efficiency appeared to be relatively low. Experimental manipulations provided little support for the hypothesis that the yellow color of melastome anthers mimics abundant pollen, thereby deceiving pollinators to visit regardless of whether most pollen has been removed. Fruit set averaged 52.6% among populations, owing largely to infrequent pollinator visits and pollen limitation. Flowers of R. virginica were infertile after a single day of anthesis, but petals were subsequently maintained for 1-2 d and stamens underwent a color change from bright yellow to red. Second-day flowers may function to increase floral display size and hence fertility, without a concomitant increase in pollen discounting. Studies of bumble bee foraging behavior and correlates of seed set provided indirect support for this hypothesis.