Ikumi Dohzono

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.

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.

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.

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.

Is Bumblebee Foraging Efficiency Mediated by Morphological Correspondence to Flowers

Preference for certain types of flowers in bee species may be an adaptation for efficient foraging, and they often prefer flowers whose shape fits their mouthparts. However, it is unclear whether such flowers are truly beneficial for them. We address this issue by experimentally measuring foraging efficiency of bumblebees, the volume of sucrose solution consumed over handling time (μL/second), using long-tongued Bombus diversus Smith and short-tongued B. honshuensis Tkalcu that visit Clematis stans Siebold et Zuccarini. The corolla tube length of C. stans decreases during a flowering period, and male flowers are longer than female flowers. Long- and short-tongued bumblebees frequently visited longer and shorter flowers, respectively. Based on these preferences, we hypothesized that bumblebee foraging efficiency is higher when visiting flowers that show a good morphological fit between the proboscis and the corolla tube. Foraging efficiency of bumblebees was estimated using flowers for which nectar quality and quantity were controlled, in an experimental enclosure. We show that 1) the foraging efficiency of B. diversus was enhanced when visiting younger, longer flowers, and that 2) the foraging efficiency of B. honshuensis was higher when visiting shorter female flowers. This suggests that morphological correspondence between insects and flowers is important for insect foraging efficiency. However, in contradiction to our prediction, 3) short-tongued bumblebees B. honshuensis sucked nectar more efficiently when visiting younger, longer flowers, and 4) there was no significant difference in the foraging efficiency of B. diversus between flower sexes. These results suggest that morphological fit between the proboscis and the corolla tube is not a sole determinant of foraging efficiency. Bumblebees may adjust their sucking behavior in response to available rewards, and competition over rewards between bumblebee species might change visitation patterns in the wild. Thus, the determinants of foraging efficiency and visitation frequency for bee pollinators may be more complex than previously thought.

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.

Isolation and characterization of microsatellite loci from Corydalis ambigua (Papaveraceae).

Primers for seven unlinked and highly polymorphic microsatellite loci were developed for Corydalis ambigua to investigate its mating system and population genetics. This species can be used to explore the impact of an alien bumblebee on plant reproduction. Genetic diversity and other population genetic parameters were estimated in two populations with and without the alien bumblebee. The number of alleles per locus ranged from 4 to 20, and observed heterozygosities ranged from 0.355 to 0.969. These markers can be applied to study mating systems and population genetics in C. ambigua.