Trait variation along an urban-rural gradient in Asian dayflower: the contribution of phenotypic plasticity and genetic divergence.

Environmental changes associated with urbanisation can exert pressure that facilitates both adaptations and plastic responses in plants. Field surveys or common garden experiments (CGE) alone cannot differentiate between the observed phenotypic traits resulting from plastic responses versus evolutionary adaptations to urban environments. We conducted a field survey of habitat environmental factors and four vegetative traits in 12 Commelina communis populations along an urban-rural gradient in the Osaka-Kobe megacity area. We collected seedlings from six of the 12 populations, transplanted them into a greenhouse, and measured six vegetative traits. We investigated correlations between pairs of measured traits in both the field survey and CGE. Plant height and leaf area increased significantly in increasingly developed land areas (DLA) in the field survey, whereas no such variations were found in the CGE, suggesting that the observed phenotypic variation was due to a plastic response to urban eutrophication. Leaf number and specific leaf area (SLA) significantly decreased with increasing DLA in the CGE, suggesting the adaptation of these traits to urban environments. Positive correlations between plant height and leaf area were only observed in urban populations in both the field survey and CGE, indicating the evolution of this trait correlation in urban environments. It has been suggested that urban environments promote both plastic response and genetic divergence of a set of traits in native plants. Our findings suggest that low leaf number, SLA, and positive plant height-leaf area correlations have evolved. In addition, larger plant size is achieved via phenotypic plasticity in urban environments.

Horizontal orientation facilitates pollen transfer and rain damage avoidance in actinomorphic flowers of Platycodon grandiflorus.

In zoophilous plants, floral orientation evolved under both biotic and abiotic pressure to enhance pollination success. However, the adaptive significance of horizontal orientation in radially symmetrical (actinomorphic) flowers remains largely unknown, although that of bilaterally symmetrical flowers has been well studied. We experimentally altered floral angle in a population of insect-pollinated Platycodon grandiflorus flowers to examine the effects of floral orientation on pollinator behaviour, pollination success and pollen rain damage avoidance. To further investigate the potential pollen damage by rain, we obtained past precipitation records for the study area during the flowering season, and experimentally tested P. grandiflorus pollen damage by water. Horizontally oriented flowers received more pollinator visits and had pollen grains on the stigma in male and/or female phases than downward- and/or upward-oriented flowers and avoided pollen damage by rainfall better than upward-oriented flowers. A pollen germination experiment showed that approximately 30% of pollen grains burst in distilled water, indicating that pollen damage by rainfall may be significant in P. grandiflorus. Our field experiments revealed that upward-oriented flowers cannot avoid pollen damage by rainfall during the flowering period, and that both upward- and downward-oriented flowers experience pollinator limitation in female success. Therefore, horizontal flower orientation appears to be adaptive in this insect-pollinated actinomorphic species that blooms during the rainy season.

Intersexual flower differences in an andromonoecious species: small pollen-rich staminate flowers under resource limitation.

Andromonoecy, the presence of perfect and staminate flowers in the same individual, has evolved repeatedly in angiosperms. The staminate flowers are generally smaller than the perfect flowers in species that produce staminate flowers plastically when resources are limited. The smaller staminate flowers are expected to be less attractive to pollinators and have reduced size-matching with pollinators than perfect flowers. We hypothesized that these potential disadvantages of staminate versus perfect flowers facilitate the evolution of sex-specific floral morphology, such as allometric relationship between flower size and male reproductive organ. We compared six floral morphology traits, pollen production, pollinator visits and pollen removal from anthers between staminate and perfect flowers in several natural Commelina communis populations. Nectarless and zygomorphic C. communis flowers have polymorphic stamens with attracting, feeding and pollinating anthers and were visited by diverse pollinators. Staminate flowers were significantly smaller than perfect flowers, despite a large overlap in size between sexes. The lengths of pollinating stamens did not differ between staminate and perfect flowers, and staminate flowers produced significantly more pollen. We observed significantly more pollinator visits to perfect flowers than to staminate flowers. By contrast, pollen removal from pollinating stamens was significantly higher in staminate flowers than in perfect flowers. There is sexual dimorphism in flower morphology in C. communis. Staminate flowers with smaller attraction organs, similar pollinating stamens and higher pollen production assure higher pollen donor success relative to perfect flowers. Our results suggest that the morphological changes in staminate flowers enhance pollination success, even with limited resources.

Covering and shading by neighbouring plants diminish pollinator visits to and reproductive success of a forest edge specialist dwarf species.

The pollination and reproductive success of flowering plants can be negatively influenced in various ways by neighbouring heterospecific plants, such as resource competition and reproductive interference. We hypothesized that covering together with shading by neighbouring plants may reduce pollinator visits to and reproductive success of plants by reducing floral attractiveness and pollinator activity and by interrupting flower access, respectively. To test this hypothesis, we examined whether shaded and covered flowers suffered from pollinator limitation and low reproductive success in a population of the dwarf herb Lithospermum zollingeri, which co-exists with woody and herbaceous plants in anthropogenically maintained forest edge meadows. Here, shaded and covered flowers were defined as those beneath the shade of the woods and those whose front portion was covered by any vegetative part of neighbouring plants, respectively. The shaded and covered flowers were visited by significantly fewer pollinators than sunlit and open flowers in the field. However, three major pollinator species responded differently to shading and covering. Significant pollen limitation reduced seed set in covered flowers, and shaded flowers produced fewer seeds. Pollen removal from the anthers was not influenced by shading or covering. Our study demonstrates the negative effects of covering on pollinator visits and seed production. It also elucidates the negative effects of shading on reproductive success in L. zollingeri, which depends on managed semi-natural conditions. Land management abandonment, which has increased shaded and covered conditions in artificial forest edge meadows and open forest floors, might promote a rapid reduction in the populations of such dwarf plants.

Feeding experience affects web relocation and investment in web threads in an orb-web spider, Cyclosa argenteoalba.

Orb-web spiders often relocate their webs when they assess a web site as prey poor. When a spider spins its web at a new site, it may not be able to assess the prey availability at the site accurately on the first day, owing to stochastic variation in foraging success, but it is gradually able to make an assessment. Therefore, a spider's foraging behaviour may change according to how long it has been at its current web site. To test this possibility, we conducted a prey removal experiment, with the spider Cyclosa argenteoalba, to compare the response to prey deprivation of spiders that were on new sites with that of spiders that had been at a site for several days. Spiders in both groups had a higher relocation rate than the natural rate, but more spiders in the new-site group relocated their webs. Spiders thus seemed to use previous experience of prey capture at a web site to decide whether to relocate their web. The total length of silk thread in a web was greater on the second day at a new web site than on the first. We suggest that spiders minimize their investment in web threads until they are certain that the web site is prey rich. Copyright 1999 The Association for the Study of Animal Behaviour.

Variation of breeding system, floral rewards, and reproductive success in clonal Calystegia species (Convolvulaceae).

The reproductive biology of four Japanese Calystegia species (Convolvulaceae) was studied to examine the effects of clonality and population structure on reproductive success. Calystegia soldanella, C. hederacea, and C. japonica are self-incompatible, while C. sepium is self-compatible but needs pollinator services for self-pollination. The showy, bisexual flowers of Calystegia offer pollen and nectar that attract many kinds of insects such as bees and syrphid flies. Clones of C. soldanella often formed mats just above the high tide line on beaches and produced a great number of seeds. Calystegia hederacea and C. japonica were distributed as patches of separate clones and often failed to transfer pollen grains among clones. Fruit and seed sets in C. hederacea and C. japonica were not limited by pollinators but by the number of compatible pollen grains. Although C. sepium clones were also distributed in patches, high and stable fecundity was achieved by self-compatibility and pollinator attraction. We suggest that self-compatibility in C. sepium has evolved under fertilization limitation caused by geitonogamy or facilitated selfing.