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Background and Aims: Plants surrounded by individuals of other co-flowering species may suffer a reproductive cost from interspecific pollen transfer (IPT). However, differences in floral architecture may reduce or eliminate IPT., Methods: A study was made of Pedicularis densispica (lousewort) and its common co-flowering species, Astragalus pastorius, to compare reproductive and pollination success of lousewort plants from pure and mixed patches. Floral architecture and pollinator behaviour on flowers of the two plants were compared along with the composition of stigmatic pollen load of the louseworts. The extent of pollen limitation of plants from pure and mixed patches was also explored through supplemental pollination with self- and outcross pollen (PLs and PLx)., Key Results: Mixed patches attracted many more nectar-searching individuals of Bombus richardsi. These bumble-bees moved frequently between flowers of the two species. However, they pollinated P. densispica with their dorsum and A. pastorius with their abdomen. This difference in handling almost completely eliminated IPT. Lousewort plants from mixed patches yielded more seeds, and seeds of higher mass and germinability, than those from pure patches. Moreover, louseworts from mixed patches had lower PLs and PLx compared with those from pure patches., Conclusions: Differences in floral architecture induced differences in pollinator behaviour that minimized IPT, such that co-flowering plants significantly enhanced quantity and quality of pollinator visits for the lousewort plants in patchy habitat. These findings add to our understanding of the mechanisms of pollination facilitation.

Background and aimsUnder a widely accepted model of pollinator-driven speciation, geographic variation in pollinator assemblage drives floral divergence and automatically causes reproductive isolation. Yet it is unclear whether divergent floral adaptation initially confers strong reproductive isolation, or whether that comes at later stages of speciation and requires other forms of reproductive isolation. This study uses a pair of recently diverged, interfertile and parapatric species in the genus Clarkia to determine whether adaptation to hawkmoths, a novel pollinator functional group, would automatically confer floral isolation upon sympatric contact.MethodsFirst, genetically based differences in floral traits between C. breweri and C. concinna that would be maintained upon migration are quantified. Then scenarios of experimental sympatry are constructed in which arrays of flowers are exposed to the novel pollinator, the hawkmoth Hyles lineata, and pollinator preference and heterospecific pollen transfer are assessed. Source populations from across the ranges of each species are used to understand how geographic variation in floral traits within species may affect floral isolation.Key resultsAlthough H. lineata has never been observed visiting C. concinna in the wild, it regularly moves between species in experimental floral arrays. Hawkmoth preference and heterospecific pollen transfer vary both among moths and among geographic source locations of C. concinna. The strength of floral isolation in this system is related to variation in flower size, especially hypanthium tube width, and nectar reward among C. concinna forms.ConclusionsAlthough C. breweri has adopted a novel hawkmoth pollination system, both ethological and mechanical floral isolation by hawkmoths are incomplete and vary according to the specific phenotype of the C. concinna source population. The results suggest that strong floral isolation is not automatically conferred by a pollinator shift and may require additional evolution of deterrent floral traits and habitat isolation that reduces the immediate spatial co-occurrence of young species.

An increased divergence in characters between species in secondary contact can be shaped by selection against competition for a common resource (ecological character displacement, ECD) or against maladapted hybridization (reproductive character displacement, RCD). These selective pressures can act between incipient species (reinforcement) or well-separated species that already completed the speciation process, but that can still hybridize and produce maladapted hybrids. Here, we investigated two well-separated sexually deceptive orchid species that, unusually, share their specific pollinator. Sympatric individuals of these species are more divergent than allopatric ones in floral characters involved in a mechanical isolating barrier, a pattern suggestive of RCD. To experimentally test this scenario, we built an artificial sympatric population with allopatric individuals. We measured flower characters, genotyped the offspring in natural and artificial sympatry and estimated fertility of hybrids. Different from naturally sympatric individuals, allopatric individuals in artificial sympatry hybridized widely. Hybrids showed lower pollination success and seed viability than parentals. Character displacement did not affect plant pollination success. These findings suggest that RCD evolved between these species to avoid hybridization and that selection on reinforcement may be very strong even in plants with highly specialized pollination. [ABSTRACT FROM AUTHOR]

This article is a Commentary on Minnaar et al., 224: 1160–1170. [ABSTRACT FROM AUTHOR]

Summary: The causative link between phenotypic divergence and reproductive isolation is an important but poorly understood part of ecological speciation. We studied the effects of floral‐tube length variation on pollen placement/receipt positions and reproductive isolation.In a population of Lapeirousia anceps (Iridaceae) with bimodal floral‐tube lengths, we labelled pollen of short‐ and long‐tubed flowers with different colour fluorescent nanoparticles (quantum dots). This enabled us to map pollen placement by long‐ and short‐tubed flowers on the only floral visitor, a long‐proboscid fly. Furthermore, it allowed us to quantify pollen movement within and between short‐ and long‐tubed flowers.Short‐ and long‐tubed flowers placed pollen on different parts of the pollinator, and long‐tubed flowers placed more pollen per visit than short‐tubed flowers. This resulted in assortative pollen receipt (most pollen received comes from the same phenotype) and strong but asymmetric reproductive isolation, where short‐tubed plants are more reproductively isolated than long‐tubed plants.These results suggest that floral‐tube length divergence can promote mechanical isolation in plants through divergence in pollen placement sites on pollinators. Consequently, in concert with other reproductive isolation mechanisms, selection for differences in floral‐tube length can play an important role in ecological speciation of plants. See also the Commentary on this article by Machhala, 224: 1005–1008. [ABSTRACT FROM AUTHOR]

Background and Aims Increasing evidence challenges the conventional perception that orchids are the most distinct example of floral diversification due to floral or prezygotic isolation. Regarding the relationship between co-flowering plants, rewarding and non-rewarding orchids in particular, few studies have investigated whether non-rewarding plants affect the pollination success of rewarding plants. Here, floral isolation and mutual effects between the rewarding orchid Galearis diantha and the non-rewarding orchid Ponerorchis chusua were investigated. Methods Flowering phenological traits were monitored by noting the opening and wilting dates of the chosen individual plants. The pollinator pool and pollinator behaviour were assessed from field observations. Key morphological traits of the flowers and pollinators were measured directly in the field. Pollinator limitation and interspecific compatibility were evaluated by hand-pollination experiments. Fruit set was surveyed in monospecific and heterospecific plots. Key Results The species had overlapping peak flowering periods. Pollinators of both species displayed a certain degree of constancy in visiting each species, but they also visited other flowers before landing on the focal orchids. A substantial difference in spur size between the species resulted in the deposition of pollen on different regions of the body of the shared pollinator. Hand-pollination experiments revealed that fruit set was strongly pollinator-limited in both species. No significant difference in fruit set was found between monospecific plots and heterospecific plots. Conclusions A combination of mechanical isolation and incomplete ethological isolation eliminates the possibility of pollen transfer between the species. These results do not support either the facilitation or competition hypothesis regarding the effect of nearby rewarding flowers on non-rewarding plants. The absence of a significant effect of non-rewarding P. chusua on rewarding G. diantha can be ascribed to low levels of overlap between the pollinator pools of two species. [ABSTRACT FROM AUTHOR]