The pollination biology of Burmeistera (Campanulaceae): specialization and syndromes

The floral traits of plants with specialized pollination systems both facilitate the primary pollinator and restrict other potential pollinators. To explore interactions between pollinators and floral traits of the genus Burmeistera, I filmed floral visitors and measured pollen deposition for 10 species in six cloud forest sites throughout northern Ecuador. Nine species were primarily bat-pollinated (84-100% of pollen transfer); another (B. rubrosepala) was exclusively hummingbird-pollinated. According to a principal components analysis of 11 floral measurements, flowers of B. rubrosepala were morphologically distinct. Floral traits of all species closely matched traditional ornithophilous and chiropterophilous pollination syndromes; flowers of B. rubrosepala were bright red, lacked odor, opened in the afternoon, and had narrow corolla apertures and flexible pedicels, which positioned them below the foliage. Flowers of the bat-pollinated species were dull-colored, emitted odor, opened in the evening, and had wide apertures and rigid pedicels, which positioned them beyond the foliage. Aperture width appeared most critical to restricting pollination; hummingbirds visited wide flowers without contacting the reproductive parts, and bats did not visit the narrow flowers of B. rubrosepala. Aperture width may impose an adaptive trade-off that favors the high degree of specialization in the genus. Other floral measurements were highly variable amongst bat-pollinated species, including stigma exsertion, calyx lobe morphology, and pedicel length. Because multiple species of Burmeistera often coexist, such morphological diversity may reduce pollen competition by encouraging pollinator fidelity and/or spatially partitioning pollinator's bodies.     

Pollination of <Emphasis Type="Italic">Machaerium opacum</Emphasis> (Fabaceae) by nocturnal and diurnal bees

With plants whose flowers open at night and stay open during the day, nocturnal pollinators may exploit floral resources before diurnal competitors. Moths, bats, and beetles are the most familiar nocturnal pollinators, whereas nocturnal bees as pollinators remain poorly understood. The common Cerrado tree Machaerium opacum (Fabaceae) has white and strongly scented melittophilous flowers, which first open at the night and remain open during the day and, thus, have the potential to be visited by both nocturnal and diurnal bees. We asked: (1) what is the plant’s breeding system? (2) when during the night do the flowers open? (3) what are the visual and olfactory floral cues? and (4) which nocturnal/diurnal bees visit and pollinate the flowers? We show that M. opacum is self-incompatible. Its flowers open synchronously at 03:30 h, produce nectar exclusively at night, and have an explosive mechanism of pollen presentation. The flowers have pure white petals, release strong scents during anthesis, and are pollinated by nocturnal and diurnal bees. We recorded four nocturnal and 17 diurnal species as flower visitors, with females of nocturnal species of Ptiloglossa (Colletidae) being the most abundant. After an initial pollen-releasing visit, only a minor amount of pollen remains in a flower. Several floral traits favor visits by nocturnal bees: (1) night-time flower opening, (2) nectar production at night, (3) almost complete pollen release during the first flower visit, and (4) pure white petals and strong odor production prior to sunrise, facilitating visual and olfactory detection of flowers when light is dim.     

Prezygotic barriers to gene flow between <Emphasis Type="Italic">Taraxacum ceratophorum</Emphasis> and the invasive <Emphasis Type="Italic">Taraxacum officinale</Emphasis> (Asteraceae)

Prezygotic reproductive barriers limit interspecific gene flow between congeners. Here, I examine the strength of floral isolation and interspecific pollen-pistil barriers between an invasive apomictic, Taraxacum officinale, and the indigenous sexual alpine dandelion, Taraxacum ceratophorum. Experimental arrays of either native inflorescences or a mixture of native and exotic inflorescences were used to examine insect preference and to track movement of a pollen analog. Using hand-pollinations, conspecific and heterospecific pollen germination success on native stigmas was compared. To additionally test for interspecific pollen competition, T. ceratophorum plants received one of three possible hand-pollinations: control conspecific pollination, concomitant conspecific and heterospecific pollination (mixed), or conspecific pollen followed by heterospecific pollen 15 min later (staggered). Floral isolation was negligible as no insect preference was detected. On a presence/absence basis, florets on native inflorescences received slightly less pollen analog from heterospecific donors than from conspecific donors; however, the amount of dye particles transferred from either Taraxacum species to stigmas on recipient T. ceratophorum inflorescences was equivalent. In contrast to weak floral isolation, strong pollen germination and pollen competition barriers should reduce the potential for hybridization. Heterospecific T. officinale pollen exhibited reduced germination success on T. ceratophorum stigmas in comparison to conspecific pollen. Furthermore, a significant pollen-competition effect on the percentage of hybrid offspring was detected only when T. officinale preceded T. ceratophorum pollen by 15 min. This result indicates that conspecific pollen out-competes heterospecific pollen but further suggests that biotic and abiotic factors reducing pollen accrual rates may partially remove barriers to natural hybridization.     

Pollen flow in the distylous<Emphasis Type="Italic"> Palicourea fendleri</Emphasis> (Rubiaceae): an experimental test of the Disassortative Pollen Flow Hypothesis

The Disassortative Pollen Flow Hypothesis proposed by Darwin postulates that the relative position of anthers and stigmas in distylous flowers enhances pollen flow between flowers of different morphs (legitimate pollination), in comparison to flow between flowers of the same morph (illegitimate pollination). In order to test this hypothesis, we measured pollen transport, mediated by a trained Copper-rumped Hummingbird (Amazilia tobaci), between flowers of the distylous Palicourea fendleri under laboratory conditions. In individual tests, we offered to the hummingbird a pollen donor flower and two emasculated recipient flowers in a controlled sequence. After each foraging bout, we counted the number of pollen grains transported from the donor flower to the stigmas of both recipient flowers. In agreement with Darwin's hypothesis, we found that hummingbirds transport pollen of "pin" flowers in significantly higher numbers to legitimate "thrum" stigmas, even if previously visiting a "pin" flower. However, "thrum" pollen was deposited in greater numbers on illegitimate "thrum" stigmas. We interpret this asymmetry largely as the consequence of floral morphology; pollen flow was greater between anthers and stigmas that exhibit greater spatial matching. In P. fendleri, the position of floral organs along the corolla tube does not always precisely correspond. In our experimental system, the probability that the pollinator extracts a pollen grain from the anther and the probability of self-pollination were both dependent on the type of floral morph. We discuss the relevance of the latter findings in relation to other studies of pollen flow in heterostylous species.     

Specialist <Emphasis Type="Italic">Osmia</Emphasis> bees forage indiscriminately among hybridizing <Emphasis Type="Italic">Balsamorhiza</Emphasis> floral hosts

Pollinators, even floral generalists (=polyleges), typically specialize during individual foraging bouts, infrequently switching between floral hosts. Such transient floral constancy restricts pollen flow, and thereby gene flow, to conspecific flowers in mixed plant communities. Where incipient flowering species meet, however, weak cross-fertility and often similar floral traits can yield mixed reproductive outcomes among pollinator-dependent species. In these cases, floral constancy by polyleges sometimes serves as an ethological mating barrier. More often, their foraging infidelities instead facilitate host introgression and hybridization. Many other bee species are oligolectic (taxonomic specialists for pollen). Oligoleges could be more discriminating connoisseurs than polyleges when foraging among their limited set of related floral hosts. If true, greater foraging constancy might ensue, contributing to positive assortative mating and disruptive selection, thereby facilitating speciation among their interfertile floral hosts. To test this Connoisseur Hypothesis, nesting females of two species of oligolectic Osmia bees were presented with randomized mixed arrays of flowers of two sympatric species of their pollen host, Balsamorhiza, a genus known for hybridization. In a closely spaced grid, the females of both species preferred the larger flowered B. macrophylla, evidence for discrimination. However, both species’ females showed no floral constancy whatsoever during their individual foraging bouts, switching randomly between species proportional to their floral preference. In a wider spaced array in which the bouquets reflected natural plant spacing, foraging oligolectic bees often transferred pollen surrogates (fluorescent powders) both between conspecific flowers (geitonogamy and xenogamy) and between the two Balsamorhiza species. The Connoisseur Hypothesis was therefore rejected. Foraging infidelity by these oligolectic Osmia bees will contribute to introgression and hybridization where interfertile species of Balsamorhiza meet and flower together. A literature review reveals that other plant genera whose species hybridize also attract numerous oligolectic bees, providing independent opportunities to test the generality of this conclusion.     

Fruit abortion in the Chihuahuan-Desert endemic cactus <Emphasis Type="Italic">Opuntia microdasys</Emphasis>

Resource and pollen limitation, as well as pollen/ovule incompatibility, have been proposed as causes to explain fruit abortion. To assess whether abortion in Opuntia microdasys was due to resource and/or pollen limitation and could therefore be reversed fruit set and seed set were studied using controlled pollination experiments on 60 plants that had been randomly assigned a combination of watering and fertilization treatments. On the other hand, to test whether fruit abortion was irreversible, due to pollen/ovule incompatibility, we examined the reproductive biology of the species. This included observations on floral phenology, nectar production, flower visitors, numbers of pollen grains and ovules, and self-pollination experiments. Results showed that O. microdasys is a fully self-incompatible species and its floral biology and the activity of the main pollinator allow constant deposition of incompatible pollen onto stigmas, which may contribute to fruit abortion. Reproductive success was limited by nutrients and pollen, but the fruit set increased only by 58%, compared to 47% of the control, after the experimental addition of pollen, nutrients and water. The magnitude of pollen and resource limitation suggests that similar levels of abortion will be present in good as well as in bad years. Selfing as well as incompatibility between ramets from the same clone and between closely related plants seem plausible candidates to explain the large proportion of fruit abortion, and experimental cross pollination between genotypes identified through molecular markers are necessary to fully understand the considerable abortion rate that remains unexplained after pollen and resource addition. Interestingly, the possible reason why the abortion of energetically expensive fruits has not been eliminated by natural selection is that the aborted fruits are propagules able to root and produce new plants with the same genotype of the mother. Abortion would have a dramatic effect on cross-fertilized genotypes because they result in zero fitness, but it would have a positive effect on the fitness of the maternal genotype because a clonal offspring is produced. Evidently, the exact fitness consequences to the maternal plant will depend on the differences in survival and reproduction of these different offspring types.     

Expression of a polygalacturonase enzyme in germinating pollen of <Emphasis Type="Italic">Brassica napus</Emphasis>

Penetration of pollen tubes through stigmatic tissues in Brassica napus L. may involve the release of cell wall modifying enzymes from the pollen tube tip. We examined the expression of a pectin-degrading polygalacturonase (PG) enzyme in unpollinated and early and late pollinated stigmas via immunoblotting and immuno-light microscopy using a PG polyclonal antibody. Immunoblotting analysis indicated that PG enzyme was present at low levels in unpollinated stigmas and at high levels in pollinated stigmas. The level of PG did not detectably increase between early and late pollinated stigmas. Immuno-light microscopy demonstrated that PG enzyme was present in ungerminated pollen grains, stigmatic papillae and in the tip of pollen tubes growing into the papillar wall. This latter evidence suggests that PG enzyme may play an important role in papillar cell wall penetration during pollination although other interpretations of the role of pollen PG should not be discounted. Received: 9 November 2000 / Accepted: 7 December 2000     

Diurnal and nocturnal pollination of Marginatocereus marginatus (Pachycereeae: Cactaceae) in Central Mexico

BACKGROUND AND AIMS: Chiropterophillous and ornithophillous characteristics can form part of a single reproductive strategy in plants that have flowers with diurnal and nocturnal anthesis. This broader pollination strategy can ensure seed set when pollinators are scarce or unpredictable. This appears to be true of hummingbirds, which presumably pollinate Marginatocereus marginatus, a columnar cactus with red nocturnal and diurnal flowers growing as part of dense bat-pollinated columnar cacti forests in arid regions of central Mexico. The aim of this study was to study the floral biology of M. marginatus, and evaluate the effectiveness of nocturnal vs. diurnal pollinators and the contribution of each pollinator group to overall plant fitness. METHODS: Individual flower buds were marked and followed to evaluate flower phenology and anthesis time. Flowers and nectar production were measured. An exclusion experiment was conducted to measure the relative contribution of nocturnal and diurnal pollinators to seed set. KEY RESULTS: Marginatocereus marginatus has red hermaphroditic flowers with nocturnal and diurnal anthesis. The plant cannot produce seeds by selfing and was pollinated during the day by hummingbirds and during the night by bats, demonstrating that both pollinator groups were important for plant reproduction. Strong pollen limitation was found in the absence of one of the pollinator guilds. CONCLUSIONS: Marginatocereus marginatus has an open pollination system in which both diurnal and nocturnal pollinators are needed to set seeds. This represents a fail-safe pollination system that can ensure both pollination, in a situation of low abundance of one of the pollinator groups (hummingbirds), and high competition for nocturnal pollinators with other columnar cacti that bloom synchronously with M. marginatus in the Tehuacan Valley, Mexico.     

Production and cytogenetic characterization of intertribal somatic hybrids between <Emphasis Type="Italic">Brassica napus</Emphasis> and <Emphasis Type="Italic">Isatis indigotica</Emphasis> and backcross progenies

Intertribal somatic hybrids between Brassica napus (2n = 38, AACC) and a dye and medicinal plant Isatis indigotica (2n = 14, II) were obtained by fusions of mesophyll protoplasts. From a total of 237 calli, only one symmetric hybrid (S2) and five asymmetric hybrids (As1, As4, As6, As7 and As12) were established in the field. These hybrids showed some morphological variations and had very low pollen fertility. Hybrids S2 and As1 possessed 2n = 52 (AACCII), the sum of the parental chromosomes, and As12 had 2n = 66 (possibly AACCIIII). Hybrids As4, As6 and As7 were mixoploids (2n = 48–62). Genomic in situ hybridization analysis revealed that pollen mother cells at diakinesis of As1 contained 26 bivalents comprising 19 from B. napus and 7 from I. indigotica and mainly showed the segregation 26:26 at anaphase I (AI) with 7 I. indigotica chromosomes in each polar group. Four BC₁ plants from As1 after pollinated by B. napus resembled mainly B. napus in morphology but also exhibited some characteristics from I. indigotica. These plants produced some seeds on selfing or pollination by B. napus. They had 2n = 45 (AACCI) and underwent pairing among the I. indigotica chromosomes and/or between the chromosomes of two parents at diakinesis. All hybrids mainly had the AFLP banding patterns from the addition of two parents plus some alterations. B. napus contributed chloroplast genomes in majority of the hybrids but some also had from I. indigotica. Production of B. napus–I. indigotica additions would be of considerable importance for genome analysis and breeding.     

Phylogenetic Community Context Influences Pollen Delivery to <Emphasis Type="Italic">Allium cernuum</Emphasis>

Few studies have examined how the number and identity of species in the neighbouring community influences the reproductive success of particular focal species. Pollen delivery, an important component of fitness of sexual plants, is a function of not just the floral traits of any particular individual, but of features of the population and community as it depends on pollinator abundance and preferences. Many pollinators in flowering communities will prefer patches with high floral abundance or diversity yet may exhibit lower floral constancy when more flowering species are present. Thus, pollination efficiency could increase or decrease with increased species richness and this will alter the selection pressures placed upon the floral traits (such as floral colour or reward) of any member of a particular community. Moreover, recent studies have indicated that plant-pollinator networks are phylogenetically structured (pollinators visit related plant species more than expected by chance) and this may be an important factor structuring flowering plant communities. Thus, the sheer number of species within a patch may be less important than the number of closely-related species. We investigate whether species richness or phylogenetic proximity of coflowering species influences the amount and proportion of conspecific pollen delivered to nodding onion, Allium cernuum, in fragment patches of Garry Oak meadows in South Western British Columbia, Canada. We find that pollen delivery depended upon the presence of close relatives far more than on species richness or population density, indicating a central role of the community structure on pollination in flowering plant communities. Insofar as pollen delivery relates to the relative number of seeds produced by members of the community, pollination may in turn determine the community structure of the next generation.     

Specialized pollination by fungus gnats in the introduced population of <Emphasis Type="Italic">Aspidistra elatior</Emphasis>

The monocot genus Aspidistra comprises rhizomatous perennials that are distributed in tropical to warm temperate regions of Asia. Little is known about the pollinators of almost all the species, probably due to the inconspicuous nature of Aspidistra flowers. Nevertheless, the unusual floral morphology suggests biotic pollination, since pollen grains are hidden under each flower’s stigma. Aspidistra elatior has been suspected to have a very peculiar pollination ecology. So far, pollination by mollusks, crustaceans, or collembolans has been suspected. However, a recent study showed that A. elatior is mainly pollinated by species of fungus gnats in Kuroshima Island, southern Japan, which is its natural habitat. Here, we investigated the pollination ecology of A. elatior in Shiga Prefecture, central Japan, which is the introduced population, to reveal whether fungus gnats are also the main pollinator in the introduced population. Our study confirmed fungus gnats pollination in the investigated pollination. Furthermore, the main pollinators (i.e., Cordyla sixi and Bradysia sp.) are the same in both Kuroshima and Shiga Prefecture. Therefore, A. elatior mainly depends on a narrow taxonomic group of fungus gnats for pollination. In contrast, we failed to document any terrestrial amphipods visiting the A. elatior flowers, in spite of a relatively high fruit set in natural conditions. This fact will refute the amphipod pollination hypothesis proposed by previous studies. We consider that A. elatior is pollinated by fungus gnats through fungal mimicry, due to its superficial similarity to mushroom fruiting bodies and strong, musky floral scent.     

Post-pollination biochemical changes in the floral organs of<Emphasis Type="Italic">Rhynchostylis retusa</Emphasis> (L.) Bl. and<Emphasis Type="Italic">Aerides multiflora</Emphasis> Roxb. (Orchidaceae)

If left unpollinated, the flowers ofAerides multiflora (Roxb.) andRhynchostylis retusa (L.) Bl. can remain fresh for 17 and 24 d, respectively. However, they begin to wilt at 2 to 3 days after pollination (DAP) and 3 to 4 DAP, respectively, and become senescent at 5 DAP and 7 DAP, respectively. When measured at two developmental phases — Stage 1, start of wilting and Stage 2, progression to senescence — all the floral organs from pollinated flowers had higher contents of total soluble sugars, reducing sugars, and free amino acids than those from unpollinated flowers. A corresponding increase was noted in the activities of hydrolytic enzymes, i.e., α-amylase, β-amylase, and invertase, and proteolytic enzymes (proteases) in those organs. This indicated that signals related to pollination had up-regulated those activities, leading to a breakdown of complex molecules into simpler ones for mobilization. The amounts of sugars and enzyme activity were relatively greater in the pollinated flowers ofA. multiflora compared withR. retusa, and levels were always higher in the floral lips and perianths. When inhibitors of auxin (0.25 mM TIBA) or ethylene (0.25 mM AgNO₂) were applied to the pollinated flowers, their senescence was partially prevented, thus signifying hormonal involvement in governing the pollination-induced biochemical alterations normally found in those organs.     

Post-pollination changes in the floral organs of two <Emphasis Type="Italic">Cymbidium</Emphasis> species

It was observed that the unpollinated flowers of Cymbidium pendulum (Roxb.) Sw. and C. aloifolium (L.) Sw. stayed fresh for 20 and 18 d, respectively, but attained senescence in 8 and 7 d, respectively, after pollination. The higher content of total soluble sugars, reducing sugars and free amino acids was observed in all the floral organs of pollinated flowers than in unpollinated ones. Pollination also up-regulated the activity of hydrolytic (α-amylase, β-amylase, invertase) and proteolytic enzymes (proteases) in floral organs. Amongst floral organs, the lip and perianth possessed highest contents of metabolites. Application of auxin inhibitor (0.25 μM triiodobenzoic acid) and ethylene inhibitor (0.25 μM AgNO₃) to the pollinated flowers partially prevented the process of senescence.     

Gender specialization in <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae), a distylous,hummingbird-pollinated treelet

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.     

Dioecious <Emphasis Type="Italic">Clusia nemorosa</Emphasis> achieves pollination by combining specialized and generalized floral rewards

Plants often combine multiple strategies to achieve pollen transfer. The dioecious Clusia nemorosa (Clusiaceae) produces floral resin, a specialized reward for resin-collecting bees. It also exhibits floral automimicry with female flowers mimicking male flowers in order to attract pollen-collecting insects. We observed the hourly visitation frequency and behaviour of bee visitors on two rock savanna sites in French Guiana. The major strategy in pollen transfer and the variation in visitation rates among visitor species were examined. We hypothesized that the visitation rate will vary between the floral sexes and degree of this variation will differ between the two-reward systems. We found no evidence for visits being exclusively related to resin collection, which we expected to be the principal strategy in pollen transfer. Deceit pollination appeared to have minor importance and seemed to be locally associated with the site, where demand for pollen was greater. Flower visits that probably facilitated most pollen transfer, combined resin and pollen. The pollinators involved in this system collected pollen from male flowers and resin from female flowers. The fruit set was not particularly low (44.19%) so offering different rewards by different flower genders has not constrained reproductive success in this system. Pronounced variation in visitation rates between sexes was not related to the pollination mechanisms but to the demand for the two rewards. Overall demand for pollen was greater than demand for resin. Female flowers were visited much less frequently than male flowers. Peak pollen collection occurred in the morning, i.e., as soon as the resource was available, while resin was collected throughout the day. There was a local variability in demand for the two resources because both the visitation rate and the behaviour of foraging bees varied between the two observation sites.     

The effects of nutrient addition on floral characters and pollination in two subalpine plants, <Emphasis Type="Italic">Ipomopsis aggregata</Emphasis> and <Emphasis Type="Italic">Linum lewisii</Emphasis>

The availability of soil and pollination resources are main determinants of fitness in many flowering plants, but the degree to which each is limiting and how they interact to affect plant fitness is unknown for many species. We performed resource (water and nutrients) and pollination (open and supplemental) treatments on two species of flowering plants, Ipomopsis aggregata and Linum lewisii, that differed in life-history, and we measured how resource addition affected floral characters, pollination, and reproduction (both male and female function). We separated the direct effects of resources versus indirect effects on female function via changes in pollination using a factorial experiment and path analysis. Resource addition affected I. aggregata and L. lewisii differently. Ipomopsis aggregata, a monocarp, responded to fertilization in the year of treatment application, increasing flower production, bloom duration, corolla width, nectar production, aboveground biomass, and pollen receipt relative to control plants. Fertilization also increased total seed production per plant, and hand-pollination increased seeds per fruit in I. aggregata, indicating some degree of pollen limitation of seed production. In contrast, fertilization had no effect on growth or reproductive output in the year of treatment on L. lewisii, a perennial, except that fertilization lengthened bloom duration. However, delayed effects of fertilization were seen in the year following treatment, with fertilized plants having greater aboveground biomass, seeds per fruit, and seeds per plant than control plants. In both species, there were no effects of resource addition on male function, and the direct effects of fertilization on female function were relatively stronger than the indirect effects via changes in pollination. Although we studied only two plant species, our results suggest that life-history traits may play an important role in determining the reproductive responses of plants to soil nutrient and pollen additions.     

Co-dependency between a specialist <Emphasis Type="Italic">Andrena</Emphasis> bee and its death camas host, <Emphasis Type="Italic">Toxicoscordion paniculatum</Emphasis>

Among associations of plants and their pollinating bees, mutually specialized pairings are rare. Typically, either pollen specialist (oligolectic) bees are joined by polylectic bees in a flowering species’ pollinator guild, or specialized flowers are pollinated by one or more polylectic bees. The bee Andrena astragali is a narrow oligolege, collecting pollen solely from two nearly identical species of death camas (Toxicoscordion, formerly Zigadenus). Neurotoxic alkaloids of these plants are implicated in sheep and honey bee poisoning. In this study, T. paniculatum, T. venenosum and co-flowering forbs were sampled for bees at 15 sites along a 900-km-long east–west transect across the northern Great Basin plus an altitudinal gradient in northern Utah’s Bear River Range. Only A. astragali bees were regularly seen visiting flowering panicles of these Toxicoscordion. In turn, this bee was never among the 170 bee species caught at 17 species of other prevalent co-occurring wildflowers in the same five state region (38,000 plants surveyed). Our field pollination experiments show that T. paniculatum is primarily an outcrosser dependent on pollinator visitation for most capsule and seed set. Thus, both A. astragali and two sister species of Toxicoscordion are narrowly specialized and co-dependent on each other for reproduction, illustrating a rare case of obligate mutual specialization in bee–plant interactions.     

Floral morph composition and pollen limitation in the seed set of <Emphasis Type="Italic">Nymphoides indica</Emphasis> populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.     

Evolution of flower shape in <Emphasis Type="Italic">Plantago lanceolata</Emphasis>

Plantago lanceolata produces small actinomorphic (radially symmetric), wind-pollinated flowers that have evolved from a zygomorphic, biotically pollinated ancestral state. To understand the developmental mechanisms that might underlie this change in flower shape, and associated change in pollination syndrome, we analyzed the role of CYC-like genes in P. lanceolata. Related zygomorphic species have two CYC-like genes that are expressed asymmetrically in the dorsal region of young floral meristems and in developing flowers, where they affect the rate of development of dorsal petals and stamens. Plantago has a single CYC-like gene (PlCYC) that is not expressed in early floral meristems and there is no apparent asymmetry in the pattern of PlCYC expression during later flower development. Thus, the evolution of actinomorphy in Plantago correlates with loss of dorsal-specific CYC-like gene function. PlCYC is expressed in the inflorescence stem, in pedicels, and relatively late in stamen development, suggesting a novel role for PlCYC in compacting the inflorescence and retarding stamen elongation in this wind pollinated species.