Does urbanization promote floral diversification? Implications from changes in herkogamy with pollinator availability in an urban-rural area.

Although land-use changes such as urbanization have dramatically altered plant-pollinator interactions, little is known about their effects on pollen limitation and floral traits. In this study, we examined pollinator visit frequency, reproductive success, and floral trait measurements in 12 populations of the annual andromonoecious Commelina communis in an urban-rural area. Pollinator and mate availability decreased significantly with developed land area around the study site. Most urbanized populations suffered from significant pollinator-limited male and/or female reproductive success. High fruit set in urbanized populations may suggest the presence of high reproductive assurance by selfing. The stigma height and degree of herkogamy significantly decreased with increased pollinator limitation. Petal length, anther height, and/or the pollen:ovule ratio tended to be low in pollinator- and mate-limited urban populations. One urban population with high pollinator availability had flowers with higher herkogamy and stigma height compared to rural populations. These results suggest that urbanization may provide diverse selective forces that could affect the phenotypic variation in floral traits.

Fine-tuned bee-flower coevolutionary state hidden within multiple pollination interactions.

Relationships between flowers and pollinators are generally considered cases of mutualism since both agents gain benefits. Fine-tuned adaptations are usually found in the form of strict one-to-one coevolution between species. Many insect pollinators are, however, considered generalists, visiting numerous kinds of flowers, and many flower species (angiosperms) are also considered generalists, visited by many insect pollinators. We here describe a fine-tuned coevolutionary state of a flower-visiting bee that collects both nectar and pollen from an early spring flower visited by multiple pollinators. Detailed morphology of the bee proboscis is shown to be finely adjusted to the floral morphology and nectar production of the flower. Behavioral observations also confirm the precision of this mutualism. Our results suggest that a fine-tuned one-to-one coevolutionary state between a flower species and a pollinator species might be common, but frequently overlooked, in multiple flower-pollinator interactions.

Development of microsatellite markers for Isodon longitubus (Lamiaceae).

PREMISE OF THE STUDY: Microsatellite markers were developed for Isodon longitubus to study the natural hybridization of the species and its congeners. • METHODS AND RESULTS: A total of 10 primer sets were developed for I. longitubus. From the initial screening, all of 10 loci were polymorphic with five to 19 alleles per locus in the Mt. Ishizuchi population, whereas nine loci were polymorphic with two to 12 alleles per loci in the Toon population. Although one locus was monomorphic at one population, the observed and expected heterozygosity values estimated from 34 I. longitubus samples ranged from 0.273 to 1.000 and from 0.483 to 0.918, respectively. Six primer sets could amplify all three species examined in this study (I. inflexus, I. japonicus, and I. shikokianus). • CONCLUSIONS: The 10 microsatellite markers developed here will be useful in analyzing the population genetic structure of I. longitubus and in studying the natural hybridization between Isodon species.

Flower orientation enhances pollen transfer in bilaterally symmetrical flowers.

Zygomorphic flowers are usually more complex than actinomorphic flowers and are more likely to be visited by specialized pollinators. Complex zygomorphic flowers tend to be oriented horizontally. It is hypothesized that a horizontal flower orientation ensures effective pollen transfer by facilitating pollinator recognition (the recognition-facilitation hypothesis) and/or pollinator landing (the landing-control hypothesis). To examine these two hypotheses, we altered the angle of Commelina communis flowers and examined the efficiency of pollen transfer, as well as the behavior of their visitors. We exposed unmanipulated (horizontal-), upward-, and downward-facing flowers to syrphid flies (mostly Episyrphus balteatus), which are natural visitors to C. communis. The frequency of pollinator approaches and landings, as well as the amount of pollen deposited by E. balteatus, decreased for the downward-facing flowers, supporting both hypotheses. The upward-facing flowers received the same numbers of approaches and landings as the unmanipulated flowers, but experienced more illegitimate landings. In addition, the visitors failed to touch the stigmas or anthers on the upward-facing flowers, leading to reduced pollen export and receipt, and supporting the landing-control hypothesis. Collectively, our data suggested that the horizontal orientation of zygomorphic flowers enhances pollen transfer by both facilitating pollinator recognition and controlling pollinator landing position. These findings suggest that zygomorphic flowers which deviate from a horizontal orientation may have lower fitness because of decreased pollen transfer.

Bumblebee pollination and reproductive biology of Rhododendron semibarbatum (Ericaceae).

The reproductive characteristics and pollination system of Rhododendron semibarbatum were investigated at two sites in Honshu, Japan. This species is protandrous, partially self-incompatible at postzygotic stages, and requires outcrossing via pollinator visitation for effective seed production. The effective pollinators were two bumblebee species: males of Bombus ardens at Miyama, and workers of Bombus honshuensis at Agematsu. The flowers possess two staminodes ornamented with whitish hairs, which do not reflect UV light, on the filaments. Nectar was secreted continuously during the flowering period, and nectar production rate differed between the sites. Visitation by B. ardens males was more frequent and varied among and within days, whereas that by B. honshuensis workers was less frequent and constant throughout the observation period. A single visit by a B. ardens male was more effective for seed production than visitation by a B. honshuensis worker, resulting in pollen limitation in the latter case. Differences in resource requirements between the two pollinators, representing different castes, might affect their behavior, resulting in B. ardens males contributing to more effective seed production.

ALIEN BUMBLE BEE AFFECTS NATIVE PLANT REPRODUCTION THROUGH INTERACTIONS WITH NATIVE BUMBLE BEES.

The invasive alien bumble bee Bombus terrestris may hinder the reproduction of native plants that have established specialized pollination systems with native bumble bees. To test this hypothesis, we examined the visitation frequency and behavior of native and alien bumble bee species and resultant seed production in Corydalis ambigua, a native plant in Hokkaido, Japan. This species is self-incompatible: the flower has a spur and requires visitation by bumble bees for effective seed production. We compared visitation frequency as well as fruit and seed set after cross- and open pollination at five sites of C. ambigua. Four of these sites occurred near a residential district and included naturalized populations of B. terrestris, and the fifth site was located in a forested habitat with no B. terrestris. The native species B. ardens and B. hypocrita and the alien B. terrestris frequently visited C. ambigua. Bombus ardens legitimately consumed nectar, whereas B. hypocrita and B. terrestris rob nectar by perforating spurs. The legitimate pollinator B. ardens produced fruits and seeds more efficiently than the nectar robbers. At three sites, the proportion of robbed flowers per inflorescence gradually increased through the flowering period, which may be caused by the intrusion of alien B. terrestris into the native plant-pollinator interactions. At these sites, C. ambigua suffered from pollen limitation, as seed production from open pollination was lower than from cross-pollination, despite the fact that the total abundance of three bumble bees was higher than in the other two sites. Legitimate B. ardens visited fewer flowers within inflorescences with more robbed flowers, suggesting that nectar robbing may reduce the frequency of visitations by B. ardens within inflorescences, and resulting in decreased fruit set. Furthermore, reduced seed set implies a reduction in the pollination quality by B. ardens, probably due to decreases in visiting time per flower. Thus, introduction of alien B. terrestris may alter the native plant-pollinator mutualism: C. ambigua could establish a novel pollination relationship with B. terrestris because of its nonzero pollination efficiency, similar to the native robber B. hypocrita.

Temporal changes in calyx tube length of Clematis stans (Ranunculaceae): a strategy for pollination by two bumble bee species with different proboscis lengths.

We examined the adaptive significance of a temporal decrease in the calyx tube length of Clematis stans, a dioecious species pollinated by Bombus diversus (long proboscis) and B. honshuensis (short proboscis). We compared visitation frequency, pollen removal, pollen deposition, and fruit set after a single visit among three flower stages, differentiated by calyx tube length. Bombus diversus frequently visited and removed significantly more pollen from long flowers. Bombus honshuensis visited and tended to remove more pollen from short flowers. Both pollinators deposited more pollen in short flowers, resulting in higher fruit set. These results indicate that size correspondence between the proboscis and the calyx tube enhances visitation frequency and pollen removal, but not pollen deposition. Because a single visit does not fertilize all ovules of a flower, multiple visits by two bumble bee species may increase seed production and genetic diversity of offspring. By temporally changing calyx tube length, C. stans can use two bumble bee pollinators and maintain specialized relationships with each. This strategy may be adaptive when the pollinator fauna fluctuates, and is economical because it eliminates costs required to produce different types of flowers. This constitutes a novel pattern of temporal specialization in flower-pollinator relationships.

Bumblebee-pollination and temporal change of the calyx tube length in Clematis stans(Ranunculaceae).

Clematis stans is dioecious semi-arboreal, with pale purple-blue, nodding, tubulous flowers in a paniculate inflorescence. Both male and female flowers produce nectar from the base of the calyx tube during a flowering period of 3 or 4 days, and are pollinated by two bumblebee species, Bombus diversus and B. honshuensis,with different proboscis lengths. When the flowers open, four sepals constructing a calyx tube separate at the top and their respective tips gradually curl up, so that a tubular part shortens. Observations at two field sites showed that B. diversus (with a longer proboscis) most often visits the flowers with a longer calyx tube, and B. honshuensis (with a shorter proboscis) the flowers with a shorter calyx tube, i.e., later in the flowering period. By changing the calyx tube length, the flowers of C. stans accept the two bumblebee species with different proboscis length as pollinators and thus increase the chance of pollination for each flower. It was also found that the two bumblebee species prefer the male flowers to the female flowers, although the female flowers secrete more nectar as a reward than male flowers. This is likely because they visit the male flowers to collect pollen grains in addition to nectar.