No abstract

The co-occurrence of plant species within a community is influenced by local deterministic or neutral processes as well as historical regional processes. Floral trait distributions of co-flowering species that share pollinators may reflect the impact of pollinator preference and constancy on their assembly within local communities. While pollinator sharing may lead to increased visitation rates for species with similar flowers, the receipt of foreign pollen via interspecific pollinator movements can decrease seed set. We investigated the pattern of community flower colour assembly as perceived by native honeybee pollinators within 24 local assemblages of co-flowering Oxalis species within the Greater Cape Floristic Region, South Africa. To explore the influence of pollinators on trait assembly, we assessed the impact of colour similarity on pollinator choices and the cost of heterospecific pollen receipt. We show that flower colour is significantly clustered within Oxalis communities and that this is not due to historical constraint, as flower colour is evolutionarily labile within Oxalis and communities are randomly structured with respect to phylogeny. Pollinator observations reveal that the likelihood of pollinators switching between co-flowering species is low and increases with flower colour similarity. Interspecific hand pollination significantly reduced seed set in the four Oxalis species we investigated, and all were dependant on pollinators for reproduction. Together these results imply that flower colour similarity carries a potential fitness cost. However, pollinators were highly flower constant, and remained so despite the extreme similarity of flower colour as perceived by honeybees. This suggests that other floral traits facilitate discrimination between similarly coloured species, thereby likely resulting in a low incidence of interspecific pollen transfer (IPT). If colour similarity promotes pollinator attraction at the community level, the observed clustering of flower colour within communities might result from indirect facilitative interactions.

Summary Sexually deceptive orchids are renowned for their olfactory mimicry of female insect sex pheromones to lure male pollinators, but the role of floral morphology remains unknown. Here, we reveal compelling new experimental evidence that morphology also matters in sexual deception. Our study exploited two morphologically distinct Chiloglottis orchids that both employ the same semiochemical (chiloglottone 1) to attract their respective primary pollinator. In these experiments, we monitored attempted copulation of pollinators with orchid labella as this likely impacts plant fitness. Reciprocal pollinator choice tests revealed significant reductions in the frequency and duration of attempted copulation when pollinators were presented with alternate orchid species that differ in floral morphology, but nevertheless exhibit identical semiochemicals. Experimentally shortening the labellum also reduced the duration of attempted copulation in one of the species. Pollinators exhibited contrasting orientations during attempted copulation and pollination, which seem to be correlated with fundamental differences in the morphological adaptations for both mechanical fit and female mimicry in these orchids. Our findings confirm the overlooked importance of floral morphology for sexually deceptive orchid pollination and indicate that pollinator behaviour could impose strong selection on specific floral traits.

Summary1. Shifts between alternative pollinator types are regarded as the main source of divergent selection underlying angiosperm floral diversification. However, pollinating species can exhibit substantial intraspecific variation, particularly between genders, in key morphological and behavioural traits determining their interactions with flowers. This potential mechanism of floral diversification remains largely unexplored. 2. The bee fly, Megapalpus capensis, is the predominant pollinator of the remarkable array of floral forms of the sexually deceptive daisy Gorteria diffusa. Flies exhibit strong gender-specific interactions with the variable insect-like spots which characterize G. diffusa inflorescences. 3. In order to explore variation in the preferences of male and female pollinators for the visual, tactile and olfactory components of these spots, and its implications for floral diversification, we used a sequence of binary choice tests where we manipulated individual spot components. 4. Male and female flies exhibited contrasting preferences for spot components with females preferring simplistic spots and avoiding UV highlights, whilst males prefer any additional visual and tactile phenotypic complexity. 5. Floral odour alone elicited significant preference in females only, indicating that, in contrast to orchids, sexual deception in G. diffusa is achieved largely through visual mimicry of female pollinators. 6. Our results clearly show that elaboration of the insect-like spots has evolved in response to male preferences and suggest that a trade-off exists between the attraction of male and female flies, which may have contributed to the divergence in floral phenotype between morphotypes of G. diffusa. 7. Pollinators exhibit gender differences in floral preferences and behaviour which is another potential source of divergent selection contributing to angiosperm floral diversification.

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 longtubed 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.