Geitonogamous Self-pollination Research Articles

Self-compatibility provides little reproductive assurance in the summer-flowering Aloe reitzii var. reitzii

Floral adaptations, notably floral design and display, enable plants to govern their mating opportunities through pollinator attraction and pollen dispersal. However, large floral displays, together with multiple simultaneously open flowers in an hermaphroditic plant, increase the potential for geitonogamous self-pollination. Many angiosperms have evolved self-incompatibility that prevents self-fertilization, instead requiring outcrossing by pollen vectors for successful seed set. We investigated the reliance of the summer-flowering Aloe reitzii var. reitzii on pollinators for reproduction and by using controlled hand-pollination experiments to determine whether Aloe reitzii var. reitzii is self-compatible. Contrary to most aloes, Aloe reitzii var. reitzii is shown here to be partially self-compatible, but reliant on pollinators to transfer pollen between flowers within the same raceme or plant, as well as to other plants. Aloe reitzii var. reitzii appears incapable of autonomous self-pollination and pollinator-mediated geitonogamy causes reduced fecundity in Aloe reitzii var. reitzii. However, hand-selfed treatments produced viable seeds capable of germinating, suggesting that the partial self-compatibility in Aloe reitzii var. reitzii does provide some reproductive advantage and may have evolved due to its restricted distribution and increasing population fragmentation – possibly due to anthropogenic change.

Experimental insights on Darwin's cross‐promotion hypothesis in tristylous purple loosestrife (Lythrum salicaria)

Darwin proposed that the reciprocal arrangement of anthers and stigmas in heterostylous plants promotes cross-pollination through pollen segregation on pollinators' bodies. The floral tube in most heterostylous species constrains the feeding posture of pollinators determining the site of contact with sex organs located at different heights within a flower. Here, we evaluate Darwin's hypothesis in tristylous Lythrum salicaria, a species with a partially tubular corolla, and examine the extent to which the location of sex organs within a flower influence compatible and incompatible pollination. We predicted that the proficiency of cross-pollination would increase for more inserted sex organs due to the restrictions imposed by the floral tube on pollinator positioning. We used experimental trimorphic and monomorphic arrays and emasculated flowers to quantify intermorph pollen transfer and capture among all sex-organ heights, and estimated the contribution of intraflower self-pollination, geitonogamous self-pollination, and intramorph outcross pollination to total intramorph pollination. As predicted, disassortative pollination varied significantly with sex-organ height and was highest for short-level organs and lowest for long-level organs. In monomorphic arrays, most intramorph pollination resulted from outcross pollination followed by intraflower and geitonogamous self-pollination. Our results provide evidence for the Darwinian hypothesis. Reciprocal herkogamy promoted varying degrees of disassortative pollination with the magnitude strongly influenced by sex-organ height within a flower.

Self-compatibility and specialisation in a fly-pollinated Acianthera (Orchidaceae: Pleurothallidiinae)

Diptera is one of the most widespread order of pollinators within Angiosperms and the more important pollinator group of the orchid subtribe Pleurothallidinae. Acianthera aphthosa (Lindl.) Pridgeon & M.W.Chase was studied in a Brazilian nature reserve, covering aspects of phenology, floral morpho-anatomy, flower histochemistry, pollinators, pollination mechanisms and reproductive system. A. aphthosa possess multicellular papillae on the inner surface of the sepals, which release a fetid dung-like fragrance. The fragrance is released by apical stomata, and attracts several fly species of the families Calliphoridae, Muscidae, Sarcophagidae and Ulidiidae. However, only one species of Acrosticta (Ulidiidae) was shown to be an effective pollinator. The upper surface of the labellum possesses a secretory tissue that apparently is related to nectar production, although the secretion is retained under the cuticle. As is common in fly-pollinated orchids, the Diptera are trapped by the flower. However, contrary to what is reported for other myophilous orchids, the pollinators of A. aphthosa are not frightened after leaving the flower and commonly visit further flowers on the same plant and inflorescence. In this case, the retention of the anther cap by the pollinarium, tend to reduce the geitonogamous self-pollination. A. aphthosa is self-compatible but pollinator-dependent. To the best of our knowledge this is the first record of complete self-compatibility in Acianthera. The low amount of seeds without embryos in self-pollinated flowers, in contrast to cross-pollinations, seems to be a consequence of inbreeding depression.

Female and male fitness consequences of clonal growth in a dwarf bamboo population with a high degree of clonal intermingling.

Although many studies have reported that clonal growth interferes with sexual reproduction as a result of geitonogamous self-pollination and inbreeding depression, the mating costs of clonal growth are expected to be reduced when genets are spatially intermingled with others. This study examined how clonal growth affects both female and male reproductive success by studying a population of a mass-flowering plant, Sasa veitchii var. hirsuta, with a high degree of clonal intermingling. In a 10 × 10 m plot, genets were discriminated based on the multilocus genotypes of 11 nuclear microsatellite loci. The relationships between genet size and the components of reproductive success were then investigated. Male siring success and female and male selfing rates were assessed using paternity analysis. A total of 111 genets were spatially well intermingled with others. In contrast to previous studies with species forming distinct monoclonal patches, seed production linearly increased with genet size. While male siring success was a decelerating function of genet size, selfing rates were relatively low and not related to genet size. The results, in conjunction with previous studies, emphasize the role of the spatial arrangement of genets on both the quantity and quality of offpsring, and suggest that an intermingled distribution of genets can reduce the mating costs of clonal growth and enhance overall fitness, particularly female fitness.

Longer visits on familiar plants?: testing a regular visitor’s tendency to probe more flowers than occasional visitors

An individual pollinator may tend to consecutively probe more flowers on a plant to which it returns at shorter intervals than other plants. In a large net cage, I let individually marked bumble bees forage on flowering heads of red clovers arranged in 37 bottles (plants), each of which was monitored by an observer to record every visit and probe for 2.5 h on each of 3 days. The data of collective visits by marked individuals revealed that the bees had their own foraging areas, in which they visited a set of plants frequently and others less often, i.e., the same individual bee repeatedly returned to certain plants as a regular visitor while sampling others as an occasional visitor. I further found that as a regular visitor, an individual bee tended to probe more flowering heads on familiar plants while probing fewer on unfamiliar plants as an occasional visitor. The mean number of consecutive probes by a bee was also positively correlated with its activity (the total number of plant visits made during the observation period). The fact that each bee behaves differently on different plants indicates that the same individual pollinator can exert different influence on the reproductive success of each plant: apparently, a pollinator likely reduces the potential for geitonogamous self-pollination when foraging as an occasional visitor. Attracting occasional visitors therefore may be beneficial for plants to avoid geitonogamy. This study thus emphasizes the importance of paying attention to pollinator individuality in pollination ecology.

Insect visitation rates and foraging patterns differ in androdioecious and hermaphrodite-only populations of Laguncularia racemosa (Combretaceae) in Florida

Abstract:Insect-pollinated Laguncularia racemosa has a variable breeding system; some populations are androdioecious, with male and hermaphroditic plants, while others lack male plants. We observed the foraging behaviours of insects in three androdioecious and three hermaphrodite-only populations of L. racemosa in Florida. In each population, insect visitation rates were estimated from 30–108 timed intervals. We recorded the number of flowers visited by 144–224 insects during foraging bouts made to 15–40 male and hermaphroditic plants. Male plants in androdioecious populations had significantly more visitors than hermaphroditic plants, increasing the number of vectors carrying pollen from male plants. Further, many insects visited few flowers during foraging bouts, which should increase outcrossing frequency. According to mathematical models, male plants benefit from these combined factors. Plants in hermaphrodite-only populations had significantly more visitors than hermaphroditic plants in androdioecious populations. Proportionately more insects visited many flowers during foraging bouts in hermaphrodite-only versus androdioecious populations. The increased likelihood of geitonogamous self-pollination could help explain the lack of male plants in hermaphrodite-only populations. Differences in pollinator assemblages and the relative abundances of several species were responsible for differences in foraging behaviours: Apis mellifera, Bombus sp., Melissodes sp., Xylocopa sp., Euodynerus sp. and a calliphorid species.

Effects of floral display size on male and female reproductive success in Mimulus ringens

The number of flowers blooming simultaneously on a plant may have profound consequences for reproductive success. Large floral displays often attract more pollinator visits, increasing outcross pollen receipt. However, pollinators frequently probe more flowers in sequence on large displays, potentially increasing self-pollination and reducing pollen export per flower. To better understand how floral display size influences male and female fitness, we manipulated display phenotypes and then used paternity analysis to quantify siring success and selfing rates. To facilitate unambiguous assignment of paternity, we established four replicate (cloned) arrays of Mimulus ringens, each consisting of genets with unique combinations of homozygous marker genotypes. In each array, we trimmed displays to two, four, eight or 16 flowers. When fruits ripened, we counted the number of seeds per fruit and assigned paternity to 1935 progeny. Siring success per flower declined sharply with increasing display size, while female success per flower did not vary with display. The rate of self-fertilization increased for large floral displays, but siring losses due to geitonogamous pollen discounting were much greater than siring gains through increased self-fertilization. As display size increased, each additional seed sired through geitonogamous self-pollination was associated with a loss of 9·7 seeds sired through outcrossing. Although total fitness increased with floral display size, the marginal return on each additional flower declined steadily as display size increased. Therefore, a plant could maximize fitness by producing small displays over a long flowering period, rather than large displays over a brief flowering period.

Synchronized protandry and hermaphroditism in a tropical secondary forest tree, Schefflera heptaphylla (Araliaceae)

Selection favoring avoidance of stigma clogging, pollen discounting, self-fertilization, and other negative effects of self-pollination can produce intricate patterns of intra- and interfloral dichogamy in plants bearing numerous flowers. Here we report an extensive study of the relationships among dichogamy, floral sex allocation (pollen-to-ovule ratios), nectar production, floral visitors, mating system, and fruit set in natural populations of Schefflera heptaphylla, a widespread paleotropical secondary forest tree that produces thousands of flowers in a blooming season. Each tree produces 15–30 sequentially blooming, paniculate, compound inflorescences. Each compound inflorescence has up to three orders of umbellets, which also bloom sequentially. While hand-pollinations showed that S. heptaphylla was capable of self-fertilization, our observations of thousands of flowers showed that strong intra- and interfloral protandry severely restricts both autogamous and geitonogamous self-pollination. All flowers were bisexual, thus the sexual system of the populations we studied was hermaphroditism. The pollen-to-ovule (P/O) ratios were characteristic of outcrossing species, and P/O ratios of flowers in the last-maturing (third order) umbellets were significantly higher than those in earlier-maturing (first and second order) umbellets. Floral visitors were primarily flies (Chrysomya sp. and Syrphinae sp.) and wasps (Vespula sp. and Eumenes sp.). Flowers produced nectar during both the male (pollen presentation) and female (stigma receptivity) stages of their development, and the volume of nectar production was higher in the female stage. Nevertheless, flowers received fewer visits in the female stage than they did in the male stage, and natural fruit set was low, especially in first and third order umbellets. Fruit set from hand cross- and self-pollinations was significantly higher than natural fruit set, indicating pollen limitation of fruit set. Schefflera heptaphylla has also been reported to be andromonoecious. Both hermaphroditism and andromonoecy are consistent with theoretical predictions for variation in sex allocation among sequentially maturing flowers in protandrous species. Further studies comparing hermaphroditic and andromonoecious populations of S. heptaphylla could elucidate the selective factors affecting sex expression, nectar production, and fruit set in species with numerous flowers displaying both intra- and interfloral dichogamy.

Reproductive biology of Monttea aphylla (Scrophulariaceae)

Monttea aphylla is an aphyllus shrub abundant in the Monte Desert, Argentina. This species presents particular floral syndromes including violet, tubular flowers with trichome elaiophores that produce oil. Oil flowers are associated with specialised bees with an oil-collecting apparatus. To better understand the reproductive biology of M. aphylla, the mating system, flowering phenology, the associated pollinator assemblage and foraging behaviour was determined at the southern-most part of its distributional range. Results were compared with those of previous studies and discussed. At this southern location M. aphylla is a self-incompatible species; it relies on pollinators for fruit production and presented a low fruit set. Flowering occurred during the spring, from October to December. M. aphylla was pollinated by three species of bees (Apidae); two generalist species (Centris brethesi and Mesonychium jenseni) and one specialist to M. aphylla (Centris vardyorum). M. jenseni is a cleptoparasite of Centris species. Centris species are oil-collecting bees and showed territorial behaviour (i.e. they always visited a restricted group of plants which they protected from other visitors), they visited several flowers/plants, which enhanced geitonogamous self-pollination and reduced pollination efficiency, and which might explain the low natural fruit set observed in M. aphylla. C. vardyorum was the most important pollinator of M. aphylla in the study area. The reproductive biology of M. aphylla differs along its distributional range. Apparently, southern populations of M. aphylla are more specialised than the northern populations, the former being pollinated by a few related pollinator species whereas at the northern location a variety of visitors were observed.

The Ecological and Evolutionary Consequences of Clonality for Plant Mating

Many flowering plants exhibit dual reproductive modes, producing both sexual and asexual offspring. The commonest form of asexual reproduction is clonal growth, in which vegetative modules (ramets) are produced by the parental genotype (genet). In plants, sexual and asexual reproduction usually occur simultaneously, and this can lead to allocation trade-offs and antagonism between reproductive modes. Our review considers the ecological and evolutionary consequences of functional interactions between clonal reproduction and pollination and mating. Clonal reproduction is commonly associated with mass flowering, restricted pollen dispersal, and geitonogamous self-pollination, processes that can result in inbreeding depression and pollen discounting. We review evidence for the correlated evolution of clonality and sexual systems, particularly self-incompatibility, and identify several floral mechanisms that function to reduce mating costs by limiting selfing and pollen discounting. We conclude by discussing the loss of sexuality in clonal plants and consider the genetic and environmental basis of sexual dysfunction.

Pollination ecology of the rare desert species Eremosparton songoricum (Fabaceae)

The pollination ecology of Eremosparton songoricum (Litv.) Vass., a rare desert species endemic to central Asia, was examined by a series of observational studies and manipulative experiments in two natural populations during 2007–2008. Results showed that the duration of flowering lasted 21 and 23 days, respectively, in Populations A and B from late May to late June in 2008. Anthesis of a single flower often lasted for 3 days. However, if pollinators were excluded, both the anthesis and stigma receptivity lasted 2 days longer. Mating-system experiment showed that E. songoricum was self-compatible and pollination success relied on pollinators. Fruit set after bagging without emasculation was 0.02 ± 0.01%. This suggested that spontaneous autogamy rarely happened. E. songoricum primarily relies on a combination of large floral display, secretion of nectar and a yellow ‘nectar guide’ on the standard to further attract pollinators, which may be the result of adaptation to a habitat with unreliable access to pollinators. A single flower produced ~0.18 μL and 0.50 μL of nectar during 2 days in Populations A and B, respectively. The peak secretion occurred at 1300–1500 hours, which overlapped with the climax of visitation of the effective pollinators at 1300–1400 hours. There were four effective pollinators in both populations, namely Colletes popovi Nosk., Megachile terminate Morawitz, Coelioxys sp. and Bembix planifrons F.Mor. The most frequent one, Colletes popovi Nosk., was observed pollinating many flowers of the same individual plant (65.8 ± 1.1%) in Population A, suggesting that geitonogamous self-pollination is unavoidable. Inbreeding depression played a role during the period from fertilisation to fruit maturation.

Pollination by flower chafer beetles in Eulophia ensata and Eulophia welwitschii (Orchidaceae)

Little is known about the pollination biology of the large (c. 230 species) African orchid genus, Eulophia. Here we report the discovery of pollination by flower chafer beetles (Cetoniinae; Scarabaeidae) in two color forms of E. ensata and in E. welwitschii. Both species have congested, capitate inflorescences, traits that are generally associated with pollination by flower chafer beetles in Eulophia and other plant genera. Pollinarium reconfiguration, including pollinarium bending and anther cap retention, in these beetle-pollinated species is slow. Such slow reconfiguration is predicted by Darwin's hypothesis to be a mechanism that limits geitonogamous self-pollination by slow moving beetles. A breeding system experiment conducted on E. welwitschii showed that this species, like most others in the genus, is self-compatible, but is dependent on pollinators for fruit set. As all Eulophia species are non-rewarding, the basis of attraction of beetles to flowers of the study species seems to be the generalized resemblance of their inflorescence in terms of flower arrangement and color to the capitula of sympatric rewarding Asteraceae that are utilized as food or rendezvous sites (or both) by flower chafer beetles.

Mechanisms and evolution of deceptive pollination in orchids

The orchid family is renowned for its enormous diversity of pollination mechanisms and unusually high occurrence of non-rewarding flowers compared to other plant families. The mechanisms of deception in orchids include generalized food deception, food-deceptive floral mimicry, brood-site imitation, shelter imitation, pseudoantagonism, rendezvous attraction and sexual deception. Generalized food deception is the most common mechanism (reported in 38 genera) followed by sexual deception (18 genera). Floral deception in orchids has been intensively studied since Darwin, but the evolution of non-rewarding flowers still presents a major puzzle for evolutionary biology. The two principal hypotheses as to how deception could increase fitness in plants are (i) reallocation of resources associated with reward production to flowering and seed production, and (ii) higher levels of cross-pollination due to pollinators visiting fewer flowers on non-rewarding plants, resulting in more outcrossed progeny and more efficient pollen export. Biologists have also tried to explain why deception is overrepresented in the orchid family. These explanations include: (i) efficient removal and deposition of pollinaria from orchid flowers in a single pollinator visit, thus obviating the need for rewards to entice multiple visits from pollinators; (ii) efficient transport of orchid pollen, thus requiring less reward-induced pollinator constancy; (iii) low-density populations in many orchids, thus limiting the learning of associations of floral phenotypes and rewards by pollinators; (iv) packaging of pollen in pollinaria with limited carry-over from flower to flower, thus increasing the risks of geitonogamous self-pollination when pollinators visit many flowers on rewarding plants. All of these general and orchid-specific hypotheses are difficult to reconcile with the well-established pattern for rewardlessness to result in low pollinator visitation rates and consequently low levels of fruit production. Arguments that deception evolves because rewards are costly are particularly problematic in that small amounts of nectar are unlikely to have a significant effect on the energy budget of orchids, and because reproduction in orchids is often severely pollen-, rather than resource-limited. Several recent experimental studies have shown that deception promotes cross-pollination, but it remains unknown whether actual outcrossing rates are generally higher in deceptive orchids. Our review of the literature shows that there is currently no evidence that deceptive orchids carry higher levels of genetic load (an indirect measure of outcrossing rate) than their rewarding counterparts. Cross-pollination does, however, result in dramatic increases in seed quality in almost all orchids and has the potential to increase pollen export (by reducing pollen discounting). We suggest that floral deception is particularly beneficial, because of its promotion of outcrossing, when pollinators are abundant, but that when pollinators are consistently rare, selection may favour a nectar reward or a shift to autopollination. Given that nectar-rewardlessness is likely to have been the ancestral condition in orchids and yet is evolutionarily labile, more attention will need to be given to explanations as to why deception constitutes an 'evolutionarily stable strategy'.

Pollination ecology of Aconitum gymnandrum (Ranunculaceae) at two sites with different altitudes

In this paper, we compared pollination characteristics of Aconitum gymnandrum at two sites respectively with elevations of 2460 and 3200 m in the northeast Qinghai-Tibetan Plateau. This species is an alpine biennial with unique systematic position and floral morphology in the tribe Delphinieae, Ranunculaceae. We found that the floral longevity and male and female phase durations in the high altitude population were significantly longer than those in the low altitude population. Seeds cannot be set through apomixis in this species. Aconitum gymnandrum is highly self-compatible, but autonomous self-pollination within the individual flower is ruled out through a combination of protandry and herkogamy during floral development, suggesting that pollen vectors were indispensable for successful seed sets of A. gymnandrum. Bumblebees are main pollinators of A. gymnandrum in both populations. Within each raceme, pollinators visited flowers at the bottom first, and then spirally moved upwards; however, 3.9% and 2.7% of the visits were downward respectively in the low altitude population and the high altitude population. In addition, 37.7% and 29.3% of the movements occurred between different racemes within the same plant; therefore geitonogamous self-pollination could not be completely avoided. The higher seed sets of artificially pollinated flowers than the intact flowers suggested the existence of pollination limitation in both populations. Visiting frequency of bumblebees at the low altitude was higher than that at the high altitude, but seed sets of the intact flowers in both populations did not differ remarkably, probably due to that the longer duration of the stigmatic receptivity in the high altitude population compensates the decrease of seed sets because of the low visiting frequency of pollinators at this site.

Heteromorphic Incompatibility and Efficiency of Pollination in Two Distylous Pentanisia Species (Rubiaceae)

Distyly has been hypothesized to promote cross-pollination by reducing intrafloral and geitonogamous self-pollination, and enhancing intermorph pollination. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. Distyly and its pollination consequences were examined in two Pentanisia species with long-tubed flowers which are pollinated almost exclusively by butterflies. Anther and stigma heights were measured to quantify reciprocal herkogamy. The type of incompatibility system was determined by observing pollen tubes and seed production following controlled hand pollination. Pollen loads on pollinators and stigmas were also examined to assess the efficiency of intermorph pollen flow. Pentanisia prunelloides and P. angustifolia exhibit reciprocal herkogamy and a host of ancillary dimorphisms, including pollen colour, exine sculpturing, stigmatic papilla shape and floral-tube pubescence. Controlled hand-pollinations revealed the presence of a strong heteromorphic incompatibility system in both species. The site of incompatibility differed between the morphs; intramorph pollen tubes were blocked in the style of the short-styled morph and on the stigmatic surface of the long-styled morph. Butterflies carried pollen from the short- and long-styled morphs primarily on their head and proboscis, respectively. Natural pollination resulted in a higher proportion of pollen transfer from long- to short-styled plants than vice versa. Nevertheless, fruit set did not differ between morphs. Both Pentanisia species are fully distylous. Reciprocal herkogamy results in pollen from the two morphs being carried on different locations on pollinators' bodies, which in turn promotes intermorph pollination. Intramorph pollination does not result in fertilization, because of an effective heteromorphic incompatibility system.

Pollination effectiveness and pollen dispersal in a Rhododendron ferrugineum (Ericaceae) population.

Many alpine plants are predominantly outcrossing, thus plant reproductive success is highly dependent on effectiveness of pollinators. How pollinators transfer pollen from one flower to another is of great interest in understanding the genetic structure in plant populations. We studied (1) the role and effectiveness of insect visitors for pollination, and (2) their contribution as pollen vectors for gene dispersal in a Rhododendron ferrugineum population. Various insect visitors were recorded, including Hymenoptera, Diptera, Coleoptera, and Lepidoptera. The most frequent and effective insects were honey bees and bumblebees. Muscid flies were considered as important pollinators, particularly due to their relatively high visitation rate. Syrphid flies, Formicidae, and Coleoptera were ineffective in transporting pollen, while the effectiveness of Lepidoptera and Empididae was negligible. A fluorescence labelling experiment revealed that pollen dispersal was restricted (0 - 2 m) in a dense R. ferrugineum stand and decreased in a leptokurtic fashion. This might lead to geitonogamous self-pollination that could explain the close relationship between individuals found in genetic studies of R. ferrugineum. However, some pollen grains may travel 40 - 45 m, which implies the occurrence of cross-pollination through the foraging activities of bumblebees and honey bees.

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.

Consequences of floral complexity for bumblebee-mediated geitonogamous self-pollination in Salvia nipponica Miq. (Labiatae).

I address how floral complexity influences geitonogamous self-pollination through manipulation of pollinator behavior in Salvia nipponica. The pivoting stamens of S. nipponica hinder nectar-collecting bumblebees from crawling into flowers, increasing the probing time per flower. I predicted that longer probing times would reduce the relative cost of moving between plants, causing bees to leave plants earlier. To test this prediction, I simplified S. nipponica flowers by removing the stamens from all open flowers within a 75-m2 quadrat. Bumblebees probed these flowers more quickly than intact flowers, but the stamen removal affected neither the frequency of flower revisitation nor the flight distance between plants. In response to the decrease in the probing time per flower, bees probed more flowers on these plants. Therefore, in S. nipponica, floral complexity reduces the opportunity for geitonogamous self-pollination. Stamen removal also increased bee visitation per flower, suggesting that this sort of complexity deters visitation. To keep complex flowers attractive, therefore, selection might increase floral rewards or longevity. Floral complexity might evolve in an integrative manner with the rest of the floral phenotype.

Floral biology and unique pollination system of root holoparasites, Balanophora kuroiwai and B. tobiracola (Balanophoraceae)

We investigated the floral and pollination biology of two monoecious root holoparasites, Balanophora kuroiwai and B. tobiracola (Balanophoraceae), in the subtropical forests of southern Japan. Both species secrete nectar from extrafloral nectaries distributed among the flowers, which is mainly consumed by ants, cockroaches, and pyralid moths. Pollen grains were found attached to the bodies of these insects. Pyralid moths of the genera Assara and Nacoleia were observed laying eggs on the inflorescences of B. kuroiwai. In both Balanophora species, pyralid larvae were found feeding on vegetative tissue without exploiting the seeds, and adults emerged from the fruited infructescences. In B. kuroiwai, we assessed pollination success under different experimental conditions by estimating the percentage of styles that had pollen tubes reaching the ovules. This revealed that: (1) the plants were at least sporophytically self-compatible; (2) they were generally pollinated within an inflorescence (geitonogamy); (3) outcrossing occurred, but the rates varied greatly among inflorescences; and (4) ants were probably responsible for the geitonogamy. While ants and flightless cockroaches were the most likely contributors to geitonogamous self-pollination, we consider pyralid moths to be the most likely cross-pollinators of Balanophora species. This is a new example of pollination mutualism involving a plant and its pollinating parasite.

CONSEQUENCES OF FLORAL COMPLEXITY FOR BUMBLEBEE–MEDIATED GEITONOGAMOUS SELF-POLLINATION IN SALVIA NIPPONICA MIQ. (LABIATAE)

I address how floral complexity influences geitonogamous self-pollination through manipulation of pollinator behavior in Salvia nipponica. The pivoting stamens of S. nipponica hinder nectar-collecting bumblebees from crawling into flowers, increasing the probing time per flower. I predicted that longer probing times would reduce the relative cost of moving between plants, causing bees to leave plants earlier. To test this prediction, I simplified S. nipponica flowers by removing the stamens from all open flowers within a 75-m2 quadrat. Bumblebees probed these flowers more quickly than intact flowers, but the stamen removal affected neither the frequency of flower revisitation nor the flight distance between plants. In response to the decrease in the probing time per flower, bees probed more flowers on these plants. Therefore, in S. nipponica, floral complexity reduces the opportunity for geitonogamous self-pollination. Stamen removal also increased bee visitation per flower, suggesting that this sort of complexity deters visitation. To keep complex flowers attractive, therefore, selection might increase floral rewards or longevity. Floral complexity might evolve in an integrative manner with the rest of the floral phenotype.