Risk‐averse inflorescence departure in hummingbirds and bumble bees: could plants benefit from variable nectar volumes?

Most hermaphroditic, many-flowered plants should suffer reduced fitness from within-plant selfing (geitonogamy). Large inflorescences are most attractive to pollinators, but also promote many flower visits during a single plant visit, which may increase selfing and decrease pollen export. A plant might avoid the negative consequences of attractiveness through modification of the floral display to promote fewer flower visits, while retaining attractiveness. This report shows that increasing only the variance in nectar volume per flower results in fewer flower visits per inflorescence by wild hummingbirds ( Selasphorus rufus ) and captive bumble bees ( Bombus flavifrons ) foraging on artificial inflorescences. Inflorescences were either constant (all flowers contained the same nectar volume) or variable (half the flowers were empty, the other half contained twice as much nectar as in the constant flowers). Both types of inflorescence were simultaneously available to foragers. Risk-averse foraging behaviour was expressed as a patch departure preference: birds and bees visited fewer flowers on variable inflorescences, and this preference was expressed when resource variability could be determined only by concurrent sampling. When variance treatments were clearly labelled with colour and offered to hummingbirds, the departure effect was maintained; however, when preference was measured by inflorescence choice, birds did not consistently prefer to visit constant inflorescences. The reduced visitation lengths on variable inflorescences by both birds and bees documented in this study imply that variance in nectar production rates within inflorescences may represent an adaptive trait to avoid the costs of geitonogamy.     

Visitation rate of pollinators and nectar robbers to the flowers and inflorescences of Tabebuia aurea (Bignoniaceae): effects of floral display size and habitat fragmentation

Large floral displays favour pollinator attraction and the import and export of pollen. However, large floral displays also have negative effects, such as increased geitonogamy, pollen discounting and nectar/pollen robber attraction. The size of the floral display can be measured at different scales (e.g. the flower, inflorescence or entire plant) and variations in one of these scales may affect the behaviour of flower visitors in different ways. Moreover, the fragmentation of natural forests may affect flower visitation rates and flower visitor behaviour. In the present study, video recordings of the inflorescences of a tree species (Tabebuia aurea) from the tropical savannah of central Brazil were used to examine the effect of floral display size at the inflorescence and tree scales on the visitation rate of pollinators and nectar robbers to the inflorescence, the number of flowers approached per visit, the number of visits per flower of potential pollinators and nectar robbers, and the interaction of these variables with the degree of landscape disturbance. Nectar production was quantified with respect to flower age. Although large bees are responsible for most of the pollination, a great diversity of flower insects visit the inflorescences of T. aurea. Other bee and hummingbird species are highly active nectar robbers. Increases in inflorescence size increase the visitation rate of pollinators to inflorescences, whereas increases in the number of inflorescences on the tree decrease visitation rates to inflorescences and flowers. This effect has been strongly correlated with urban environments in which trees with the largest floral displays are observed. Pollinating bees (and nectar robbers) visit few flowers per inflorescence and concentrate visits to a fraction of available flowers, generating an overdispersed distribution of the number of visits per inflorescence and per flower. This behaviour reflects preferential visits to young flowers (including flower buds) with a greater nectar supply.     

The mating consequences of sexual segregation within inflorescences of flowering plants

Many co-sexual plants segregate female and male function among flowers on an inflorescence through dichogamy or the production of unisexual flowers. Sexual segregation may reduce self-pollination among flowers within inflorescences (geitonogamy), thereby increasing the pollen available for export to other plants. To assess these complementary roles we manipulated the simultaneously hermaphroditic (adichogamous) flowers of Eichhornia paniculata to produce ten-flowered inflorescences with either female above male flowers (female/male inflorescences) or male/female inflorescences, which competed for mating opportunities with five-flowered adichogamous inflorescences. Because of the upward movement of bumble-bees, selfing increased upward in adichogamous inflorescences (overall female selfing rate s+/-s.e.=0.320+/-0.026). Female flowers of male/female inflorescences selfed less than flowers in corresponding positions in adichogamous inflorescences so s fell to 0.135+/-0.027. In contrast, all-female flowers of female/male inflorescences selfed similarly to upper flowers on adichogamous inflorescences, elevating s (0.437+/-0.043). During 1997, male/female inflorescences sired more outcrossed seeds than female/male or adichogamous inflorescences, whereas during 1994 flowers on male/female inflorescences received fewer visits than those of adichogamous inflorescences, reducing their outcross siring success. Hence, sexual segregation limits geitonogamy and enhances outcross siring success when it does not affect pollinator behaviour, illustrating the importance of both female and male function in inflorescence design.     

克隆生长对被子植物传粉过程的影响

克隆植物与其传粉者的相互作用是植物繁殖生态学的重要研究领域之一。植物克隆生长与有性繁殖通常相伴进行, 往往产生较大的花展示与复杂的克隆空间结构, 通过传粉过程对有性繁殖过程产生影响, 共同决定植物的适合度。本文回顾了克隆生长对被子植物传粉过程影响的国内外研究进展, 从植物克隆大小、花资源空间配置、克隆构型与种群遗传结构四个方面讨论了克隆生长对传粉过程的影响及其生态学与进化生物学意义。早期研究预期, 随着克隆增大, 同株异花授粉水平增加, 因而通过增大自交率或花粉阻塞效应降低植物的适合度。但是, 后来的一些模拟与野外实验研究发现, 传粉者在同一克隆内访问的花数量并不会随克隆增大而一直增加, 访花行为也主要发生在分株内; 而且分子标记的自交率组分分析也表明自交主要发生在分株内。另一方面, 人工模型模拟以及传粉者访问行为研究表明, 当花朵数量相同时, 与所有花集中生长在同一分株上相比, 将花朵分散在多个分株上的克隆生长方式不会增加, 反而降低了同株异花授粉的发生水平。如果花序内花雌雄同熟, 花朵同时提供与接收花粉, 克隆生长会使植物接收到更高比例的异交花粉, 在提高后代质量的同时不增加同株异花授粉概率。这是从传粉生物学角度对植物克隆生长习性进化的一个全新的解释。今后, 克隆植物传粉生物学研究需要针对传粉者与克隆生长之间的相互作用建立理论模型, 探究克隆大小、克隆构型、花资源空间配置模式对传粉者访问频率和行为、花粉散布、交配格局的影响。同时, 需要在自然种群中, 尤其是克隆与非克隆的近缘类群、同一物种克隆与非克隆种群开展比较研究, 利用更高效的分子标记来研究克隆生长的生态与进化意义。     

Modes of self-fertilization in Mimulus guttatus (Scrophulariaceae): a field experiment

We inferred Lloyd's modes of selfing in a natural population of the common monkeyflower, Mimulus guttatus. Estimates were obtained using floral manipulations combined with seed counts and isozyme analyses of selfing rates. Of the 25% selfing estimated from isozyme markers, about one-half was competing, about one-third was geitonogamous, and at least one-fifth (perhaps twice this) was due to biparental inbreeding. Estimates of prior and delayed selfing were small and did not significantly differ from zero. These results were obtained using plants with the characteristic pair of open flowers at an inflorescence node. The second-opening flower showed twice the rate of selfing, presumably because of protogynous-based geitonogamy differences. Solitary-flowered plants, which have smaller flowers but no geitonogamy, showed about 50% selfing, consisting of about equal components of competing selfing and biparental inbreeding. While geitonogamy and biparental inbreeding might be unavoidable by-products of adaptations for outcrossing, competing selfing is subject to more direct natural selection and warrants adaptive explanations.     

THE EVOLUTION OF INSECT-MEDIATED SELF-POLLINATION FROM AN OUTCROSSING SYSTEM IN CALYPTRIDIUM (PORTULACACEAE)

Calyptridium monospermum (Portulacaceae) comprises two biological races visited by different groups of insects. One race, pollinated by a bumblebee, Bombus vosnesenskii (Apidae), is outcrossing, while the other race is pollinated casually by a variety of other insects whose visits result in insect-mediated self-pollination and geitonogamy within the inflorescence. Insect-mediated selfing is also the mode of pollination in the closely related species, C. umbellatum. While all plants of both species are self-compatible, insect visitation is necessary for good seed set. Selfing populations of C. monospermum exhibit many floral adaptations for insect-mediated self-pollination which parallel those of the selfing C. umbellatum, while retaining vegetative characteristics typical of C. monospermum. These floral traits involve flower color and odor, style and stamen orientation, size of stigmatic surfaces, number of pollen grains produced, density of open flowers on inflorescences, and presence or absence of protogyny. Artifical pollination experiments show that these floral adaptations are effective in increasing the probability of insect-mediated self-pollination. Insect-mediated selfing in C. umbellatum and in some populations of C. monospermum is encouraged and probably made necessary by the inconstant pollinating behavior of the insects which visit these plants, as shown by the large percentage of non-Calyptridium pollen recovered from the bodies of insects captured while visiting these plants.     

Rapid floral senescence following male function and breeding systems of some tropical orchids

No comparative study of floral senescence following male function among a range of tropical orchid genera has previously been undertaken. The timing and pattern of floral senescence and occurrence of fruit formation were studied following self-, geitonogamous and cross-pollination in 14 epiphytic and two terrestrial orchid species to determine their breeding system and assess the occurrence of floral abscission following pollinaria removal. Both pollination and pollinaria removal caused rapid floral senescence, and the pattern and timing of the floral changes were the same in all treatments. Six Dendrobium species and Pelatantheria insectifera were self-incompatible (SI) and eight other species, including one terrestrial species, were self-compatible (SC). Capsules produced from outcrossing in four SC species, Phalaenopsis cornu-cervi, Eria pubescens, Cleisostoma appendiculatum and Arundina graminifolia, were larger and heavier than those produced after selfing. Reductions in flower life span following pollinaria removal were positively correlated with flower size and longevity of unpollinated flowers but not with position in the inflorescence or nature of the breeding system. Rapid flower senescence following pollinaria removal reported here suggests that it may be widespread in tropical species. The significant association of the response with size of flowers and inflorescences among the species studied suggests that the cost of flower maintenance outweighs the benefit of remaining open for female function after pollinaria have been removed. Both SC and SI species were found among tropical orchids, but variation in capsule size following self- and cross-pollination indicates that there may be a reduction in seed production following selfing, even in SC species, and that fruit formation alone should not be taken as reliable evidence of full self-compatibility.     

Mating strategies in flowering plants: the outcrossing-selfing paradigm and beyond

Comparisons of the causes and consequences of cross- and self-fertilization have dominated research on plant mating since Darwin's seminal work on plant reproduction. Here, I provide examples of these accomplishments, but also illustrate new approaches that emphasize the role of floral design and display in pollen dispersal and fitness gain through male function. Wide variation in outcrossing rate characterizes animal-pollinated plants. In species with large floral displays, part of the selfing component of mixed mating can arise from geitonogamy and be maladaptive because of strong inbreeding depression and pollen discounting. Floral strategies that separate the benefits of floral display from the mating costs associated with geitonogamy can resolve these conflicts by reducing lost mating opportunities through male function. The results from experiments with marker genes and floral manipulations provide evidence for the function of herkogamy and dichogamy in reducing self-pollination and promoting pollen dispersal. Evidence is also presented indicating that increased selfing resulting from changes to floral design, or geitonogamy in large clones, can act as a stimulus for the evolution of dioecy. The scope of future research on mating strategies needs to be broadened to include investigations of functional links among flowers, inflorescences and plant architecture within the framework of life-history evolution.     

Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set

The fecundity of insect-pollinated plants may not be linearly related to the number of flowers produced, since floral display will influence pollinator foraging patterns. We may expect more visits to plants with more flowers, but do these large plants receive more or fewer visits per flower than small plants? Do all pollinator species respond in the same way? We would also expect foragers to move less between plants when the number of flowers per plant are large, which may reduce cross-pollination compared to plants with few flowers. We examine the relationships between numbers of inflorescence per plant, bumblebee foraging behaviour and seed set in comfrey, Symphytum officinale, a self-incompatible perennial herb. Bumblebee species differed in their response to the size of floral display. More individuals of Bombus pratorum and the nectar-robbing B.?terrestris were attracted to plants with larger floral displays, but B. pascuorum exhibited no increase in recruitment according to display size. Once attracted, all bee species visited more inflorescences per plant on plants with more inflorescences. Overall the visitation rate per inflorescence and seed set per flower was independent of the number of inflorescences per plant. Variation in seed set was not explained by the numbers of bumblebees attracted or by the number of inflorescences they visited for any bee species. However, the mean seed set per flower (1.18) was far below the maximum possible (4 per flower). We suggest that in this system seed set is not limited by pollination but by other factors, possibly nutritional resources.     

Influence of clonal growth on selfing rate in Vaccinium myrtillus L

Clonal growth, which allows the multiplication of flowering shoots of the same genet, can lead to a large floral display and may thus increase the rate of selfing through geitonogamy as a consequence of an increase in the number of successively visited flowers. The aim of the present research was to analyse the combined effect of the diversity and intermingling of clones on the rate of selfing in Vaccinium myrtillus. Four mother plants were selected within patches characterised by contrasting clonal structure (low versus high number and intermingling of clones). The selfing rate was significantly lower for plants situated within patches characterised by a high number of intermingled clones (3%) than for plants situated in patches with a low number of clones (50%). Therefore, for this species suffering from inbreeding depression, an increase in the number or the intermingling of the clones could reduce the rate and the cost of geitonogamy and allow a large floral display to attract pollinators. We also found that the main pollinators, bumblebee queens, presented a foraging behaviour favouring geitonogamy, as their successive visits to flowers were quite short (89% of flights were 40 cm or less).     

心叶凹唇姜的开花格局和雄性先熟机制

心叶凹唇姜 (Boesenbergia longiflora) 是分布于亚洲热带的一种姜科植物, 在我国滇南地区成片生长于石灰山贫瘠的岩石缝隙和路边林下的冲积土壤上。心叶凹唇姜的花寿命为两天, 第1天为严格的雄性阶段, 第2天为严格的雌性阶段, 是典型的花内雄性先熟, 一朵花的雌、雄性阶段没有重叠, 因而完全避免了自花授粉的发生。对这两种生境 (石灰山贫瘠的岩石缝隙和路边林下) 下的心叶凹唇姜的4个野外种群 (在云南省普洱市的翠云乡彩虹洞、南岛河、澜沧县惠民乡和西双版纳州绿石林森林公园) 和栽培植株的开花格局进行调查后发现, 大多数个体在同一时间内只有一个花序开花, 并且每隔一天才开一朵花, 只有10%左右的植株同时具有雄性阶段和雌性阶段的花, 这样的开花格局有效地降低了同株异花授粉发生的可能。心叶凹唇姜的平均花粉-胚珠比 (Pollen-ovuleratio) 为291, 应该是以异交为主的混合交配系统。其自然种群存在着花粉限制, 彩虹洞、南岛河和绿石林3个不同种群的平均结籽率都不到60%, 这可能与其较小的花展示有关。心叶凹唇姜为泛化传粉植物, 不同种群的有效传粉昆虫种类各不相同。在两种不同的生境条件下, 心叶凹唇姜花的大小和开花格局都没有显著的差异, 大多数个体同时只具有一种性阶段的花, 说明心叶凹唇姜这种独特的开花格局是长期进化的结果。心叶凹唇姜通过隔一天开一朵花的开花格局, 配合雄性先熟机制, 有效地降低了自交的发生。     

Differential self‐fertilization rates in response to variation in floral traits within inflorescences of Aquilegia buergeriana var. oxysepala (Ranunculaceae)

To assess variation in the proportion of self‐fertilized seeds among flowers within inflorescences and the relationship between floral traits and the rate of self‐fertilization, the proportion of self‐fertilized seeds among individual flowers was estimated using ten microsatellite markers in self‐compatible plants of Aquilegia buergeriana var. oxysepala. Within‐inflorescence variation in floral traits, such as the duration of the male and female phases, flower size, herkogamy and the number of pollen grains and ovules in two natural populations, were investigated. The first flower in an inflorescence produced more seeds and a higher proportion of self‐fertilized seeds than the second flower. The higher proportion of self‐fertilized seeds in the first flowers was accompanied by a higher number of pollen grains and ovules in the bud stage and the female phase. These results indicate that the high proportion of self‐fertilized seeds in the first flowers in an inflorescence may be due to the high number of remaining pollen grains in the female phase. This suggests that variation in floral traits within inflorescences affects seed quality and quantity among flowers within inflorescences.     

Geitonogamy in rewarding and unrewarding inflorescences: modelling pollen transfer on actual foraging sequences

Many orchid species are unusual in that they provide no nectar or pollen rewards for their pollinators. Absence of reward is expected to have a fundamental effect on pollinator visitation patterns. In particular the number of flowers visited per inflorescence is expected to be affected in both unrewarding and co-flowering rewarding species. We used arrays of artificial inflorescences, which could be either rewarding or unrewarding and were differentiated by their colour, to test how many flowers bumblebees visit in each type of inflorescence. The frequency of the two colours was varied, thus modelling the case where different frequencies of both an unrewarding and rewarding species were present in a patch. We found that bumblebees visited more flowers per rewarding inflorescence after they have experienced unrewarding or partially emptied rewarding inflorescences. We used these results to simulate pollen transfer and thus predict selfing rates on rewarding inflorescences. We found these increased when nectar depleted or when there was a greater proportion of unrewarding inflorescences in the patch. Conversely, we found that the number of flowers bumblebees visited on each unrewarding inflorescence did not significantly change through experiments. Selfing rates for unrewarding inflorescences were predicted to depend principally on the number of these inflorescences bumblebees visited rather than on the number of flowers they visit per inflorescence. This was because most visitors to orchids are supposed to be naive, and pollinators that commence foraging carrying no pollen will necessarily self any flower they pollinate on the first inflorescence they visit. Thus the average selfing rate is expected to increase as the sequence of inflorescences visited decreases in length.     

A ballistic pollen dispersal system influences pollination success and fruit‐set pattern in pollinator‐excluded environments for the endangered species Synaphea stenoloba (Proteaceae)

The critically endangered Synaphea stenoloba (Proteaceae) has numerous scentless flowers clustered in dense inflorescences and deploys a ballistic pollen ejection mechanism to release pollen. We examined the hypothesis that active pollen ejection and flowering patterns within an inflorescence influence the reproductive success (i.e. fruit formation) of individual flowers within or among inflorescences of S. stenoloba in a pollinator‐excluded environment. Our results showed that: (1) no pollen grains were observed deposited on the stigma of their own flower after the pollen ejection system was manually activated, indicating self‐pollination within an individual flower is improbable in S. stenoloba; (2) fruit set in the indoor open pollination treatment and the inflorescence‐closed pollination treatment indicated that S. stenoloba is self‐compatible and pollen ejection can potentially result in inter‐floral pollination success; (3) fruit set in the inflorescence‐closed pollination treatment was significantly lower than that of indoor open pollination, indicating within‐ and between‐flower pollination events in an inflorescence are most likely limited, with pollination between inflorescences providing the highest reproductive opportunity; and (4) analysis of the spatial distribution of cumulative fruit set on inflorescences showed that pollen could reach any flower within an inflorescence and there was no functional limitation on seed set among flowers located at various positions within the inflorescence. These data suggest that the pollen ejection mechanism in S. stenoloba can enhance inter‐plant pollination in pollinator‐excluded environments and may suggest adaptation to pollinator scarcity attributable to habitat disturbance or competition for pollinators in a diverse flora. © 2012 The Linnean Society of London, Botanical Journal of the Linnean Society, 2012, 170 , 59–68.     

Multi-cycle synchronous protandry in raceme-like inflorescences of a bumblebee-pollinated herb Aconitum grossedentatum

Multi-cycle synchronous dichogamy is expected to be a mechanism for reducing self-pollination and sexual interference. It is often found in plants with umbellate inflorescences where pollinator movement is unpredictable, but not in plants with raceme inflorescences that are pollinated by bumblebees. Plants with raceme inflorescences often acropetally open flowers, resulting in an arrangement of females at lower level and males at upper level. This is good enough to preclude geitonogamy because bees tend to move upwardly within the inflorescences. Furthermore, although the degree of segregation of sexes varies among species, their intraspecific variations within a population have rarely been examined. Here, we present a synchronous protandry in bee-pollinated Aconitum grossedentatum, which has a raceme-like inflorescence and opens flowers basipetally. To evaluate the functional significance of synchronous dichogamy in mating, we firstly observed the distribution of sex phases of open flowers. Then, we assessed the effect of each phase flower on foraging behavior by pollinators and seed-set success. The inflorescences tended to exhibit either male- or female-phase flowers at any moment early in the flowering season, but the degree of segregation of sexes declined over time within a population. The degree of the segregation did not affect bumblebee visits to flowers, but it decreased seed-set success of female-phase flowers at that time. Our results demonstrated that synchronous protandry was beneficial for pollination success in A. grossedentatum by avoiding geitonogamy. Nevertheless, we also found asynchronous protandry late in the season, suggesting that the benefits by synchronous protandry decreased over the season.

     

The influence of floral display size on selfing rates in Mimulus ringens

Pollinators often visit several flowers in sequence on plants with large floral displays. This foraging pattern is expected to influence the rate of self-fertilization in self-compatible taxa. To quantify the effects of daily floral display on pollinator movements and selfing, we experimentally manipulated flower number in four replicate (cloned) arrays of Mimulus ringens (Scrophulariaceae), each consisting of genets with unique combinations of homozygous marker genotypes. Four display classes (two, four, eight and 16 flowers) were present in each array. Pollinator visitation rate per flower and seed set per fruit were unaffected by display. However, flower number strongly influenced the frequency of within-plant pollinator movements, which increased from 13.8% of probes on two-flower displays to 77.6% of probes on 16-flower displays. The proportion of within-plant movements was significantly correlated with selfing (r = 0.993). The increase from 22.9% selfing on two-flower displays to 37.3% selfing on 16-flower displays reflects changes in the extent of geitonogamous self-pollination. We estimate that approximately half of all selfing on 16-flower displays resulted from geitonogamy. Selfing also varied dramatically among fruits within display classes. Nested ANOVA indicates that differences among flowers on two-flower ramets accounted for 45.4% of the variation in selfing, differences among genets accounted for 16.1% of the variation, and statistical and sampling error accounted for 38.5% of the variation. Differences among flowers within ramets may reflect the order of sequential floral probes on a display.     

Evolutionary and morphometric implications of morphological variation among flowers within an inflorescence: a case-study using European orchids

BACKGROUND AND AIMS: This study explores the previously largely ignored morphological variation that occurs among flowers within a single inflorescence. METHODS: Variation in four metric parameters (labellum length and width, spur length and width) that together strongly influence pollination frequency is documented within the simple racemose inflorescences of eight individuals that represent a primary hybrid and six species of European orchids. KEY RESULTS: Regression of each parameter against the location of each flower on the inflorescence, and calculation of correlation coefficients for each pair of parameters within each inflorescence, demonstrate significant decoupling of labellum and spur development, despite the fact that they are different portions of the same floral organ. Spur length and diameter are constant across inflorescences of Dactylorhiza other than the vestigial-spurred D. viridis, whereas in other genera spur length declines in parallel with labellum dimensions. These differences are likely to reflect selection pressures or developmental constraints. Strong negative deviations from the regression line for one or more parameters are evident in occasional flowers, occurring most frequently in the lowermost and uppermost one or two flowers, and so reflecting transitions in meristematic behaviour. Thus, population-level morphometric studies are best conducted on flowers taken from approximately the mid-point of the inflorescence. Moreover, in the two relatively large inflorescences where lower flowers were removed for measurement before the upper flowers had opened, labellum size increased significantly in the flowers immediately above the excisions, suggesting that excision liberated resources that were diverted into the opening buds. Repeat measurement of all flowers from one selected inflorescence demonstrated typical measurement errors of only +/- 30-80 micro m, irrespective of the size of the structure studied. If flowers are not mounted and measured immediately following excision, modest negative deviations of 30-50 micro m result from post-mounting shrinkage; this occurs less rapidly in the spur than in the thinner labellum, which should therefore be measured first. Variation in all four parameters among all the flowers of a single inflorescence is between 42 % and 107 % of that observed between a similar number of flowers sampled from a consistent location on different (but conspecific and coexisting) inflorescences. CONCLUSIONS: This result demonstrates the strong influence of epigenesis on flower morphology and further emphasizes the importance of (a) sampling from a consistent location within the inflorescences under comparison, (b) interpreting morphometric ordinations hierarchically, building from individuals to infraspecific taxa and species via populations, and (c) considering in any microevolutionary study the potentially profound effects of the cline in flower size within each inflorescence.     

Flowering patterns of long-lived Heliconia inflorescences: implications for visiting and resident nectarivores

Summary Flowering patterns of four Heliconia (Heliconiaceae) species in Trinidad, West Indies were examined for their predictability and availability to the nectarivores that rely on Heliconia floral nectar. Principal flower visitors are trapling hermit hummingbirds; inflorescences are inhabited by nectarivorous hummingbird flower mites that move between inflorescences by riding in the hummingbirds' nares. Heliconia inflorescences flower for 40–200 days, providing long-term sources of copious nectar (30–60 l per flower), but each Heliconia flower lasts only a single day. As an inflorescence ages the interval increases between open flowers within a bract; wet-season inflorescences produce open flowers more slowly than dry-season conspecifics.Estimated daily energy expenditures for hermit hummingbirds demonstrate that slow production of short-lived open flowers plus low inflorescence density preclude territorial defense of Heliconia by the hermits. Heliconia flowering patterns are viewed as a means of (i) regulating reproductive investment by the plants through staggered flower production over long periods of time, and (ii) maintaining outcrossing by necessitating a traplining visitation pattern by its hummingbird pollinators. I suggest that Heliconia exhibit a two-tiered pollination system by using hermit hummingbirds primarily for outcrossing and using hummingbird flower mites primarily for self-pollination.     

Evolution of floral display in Eichhornia paniculata (Pontederiaceae): genetic correlations between flower size and number

The evolution of floral display is thought to be constrained by trade‐offs between the size and number of flowers and inflorescences. We grew in the glasshouse 60 maternal families from each of two Brazilian populations of the annual herb, Eichhornia paniculata. We measured flower size, daily flower number, and total flower number per inflorescence, and two indices of module size, leaf area and age at flowering. We also assessed the size and number of inflorescences produced over 6 weeks. All floral traits exhibited significant heritable variation, some of which was due to genetic variation in module size. Genetic (maternal family) correlations between daily and total flower number did not differ from 1.0, indicating that display size (daily flower number) cannot evolve independently from total flower number per inflorescence. Genetic correlations between flower size and daily flower number ranged from negative to positive (r=–0.78 to +0.84), depending on population and inflorescence. Positive correlations occurred when variation in investment per inflorescence was high so that some families produced both larger and more flowers. These correlations became zero when we controlled for variation in module size. Families that flowered later produced fewer, larger inflorescences (r=–0.33, –0.85). These data support theoretical predictions regarding the combined effects of variation in resource acquisition and allocation on traits involved in trade‐offs, and they emphasize the hierarchical organization of floral displays. Our results imply that patterns of resource allocation among inflorescences influence evolutionary changes in flower size and number per inflorescence.     

Phylogenetic evidence for a flower size and number trade-off

The size and number of flowers displayed together on an inflorescence (floral display) influences pollinator attraction and pollen transfer and receipt, and is integral to plant reproductive success and fitness. Life history theory predicts that the evolution of floral display is constrained by trade-offs between the size and number of flowers and inflorescences. Indeed, a trade-off between flower size and flower number is a key assumption of models of inflorescence architecture and the evolution of floral display. Surprisingly, however, empirical evidence for the trade-off is limited. In particular, there is a lack of phylogenetic evidence for a trade-off between flower size and number. Analyses of phylogenetic independent contrasts (PICs) of 251 angiosperm species spanning 63 families yielded a significant negative correlation between flower size and flower number. At smaller phylogenetic scales, analyses of individual genera did not always find evidence of a trade-off, a result consistent with previous studies that have examined the trade-off for a single species or genus. Ours is the first study to support an angiosperm-wide trade-off between flower size and number and supports the theory that life history constraints have influenced the evolution of floral display.