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Dancing with Viola--what’s in it for the bees?
Relationships between insect-pollinated plants and their pollinators are very varied and sometimes complex. For the plant, there is the trade-off between the cost of attracting and/or rewarding pollinators and the need for successful pollen transfer. When pollen is the reward the problem of trade-off becomes more acute because, in addition to rewarding the pollinator, there must be enough pollen to effect gamete transfer. For the pollinator, there is a trade-off between the effort expended and the ‘value’ of the reward. All these features are apparent in the floral biology of Viola, as discussed by Freitas and Sazima (University of Camponas, Brazil, pp. 311-317). They have worked on two (V. cerasifolia and V. subdimidiata) of the four Viola species that occur in Brazil. All four belong to a single relict lineage and are regarded as primitive members of a genus that contains approx. 550 species in total. The floral morphology of Viola suggests that they are bee-pollinated, with nectar as the main reward. However, some apparently advanced Viola species provide pollen as a reward, suggesting an evolutionary trend from nectar providers to pollen providers. The two species in this study are pollinated by bees, mostly females, in the genus Anthrenoides. During visits, female bees vibrate the flowers and also move backwards and forwards inside them, behaviours that optimize pollen collection from dehiscing anthers. These features, taken with, amongst other things, a relatively low nectar output and small nectary size, suggest that these two Viola species are primarily pollen providers. However, the situation is complicated by the male bees which, during their visits to these flowers, exhibit no behaviour associated with optimizing pollen production but do take nectar. The authors’ suggestion that these species represent a stage on the evolutionary pathway from nectar provision to pollen provision is clearly very appropriate.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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