Fly pollination of Gorteria diffusa (Asteraceae), and a possible mimetic function for dark spots on the capitulum

Abstract

We investigated the functional significance of raised black spots on the ray florets of Gorteria diffusa (Asteraceae) in South Africa. Field observations showed that G. diffusa is pollinated by a small bee-fly, Megapalpus nitidus (Bombyliidae), which is strikingly similar to the raised spots that occur on some of the ray florets. Removal of the spots resulted in a significant decrease in the rate of fly visits to capitula, but did not significantly affect seed set. Replacement of the spots with simple ink spots also significantly reduced the rate of pollinator visits, suggesting that flies respond to details in the structure of the spots. Investigations using scanning electron microscopy showed that the spots of G. diffusa consist of a complex of different cell types. Differences in epidermal sculpturing may partly explain the UV reflectance pattern of these spots, which is similar to that of the flies. Male flies are strongly attracted to the spots, as well as to other flies sitting in the capitula, although female flies also visit the capitula. We conclude that the spots of G. diffusa mimic resting flies, thereby eliciting mate-seeking and aggregation responses in fly pollinators. Similar dark spots have evolved in unrelated South African Gazania, Dimorphotheca, and Pelargonium species pollinated by bee-flies.

The pollination niche and its role in the diversification and maintenance of the southern African flora

“…Experiments have established the functional significance of convergent floral traits in several specialized pollination systems in southern Africa. Floral advertising traits, which have been shown by means of manipulative experiments to be functionally important for pollinator attraction, include convergent yellow or orange coloration and dark central markings of flowers pollinated by hopliine beetles (Van Kleunen et al 2007), dark raised spots typically found on flowers pollinated by Megapalpus bee-flies (Johnson & Midgley 1997), the size and red colour of flowers pollinated by Aeropetes butterflies (Johnson & Bond 1994) and the scent of flowers pollinated by spider-hunting wasps (Shuttleworth & Johnson 2009). …”

Pollinator‐mediated selection on floral size and tube color inLinum pubescens: Can differential behavior and preference in different times of the day maintain dimorphism?

“…Johnson and Midgley (1997) found that flowers of Gorteria diffusa have spots on the ray florets, which are strikingly similar to its pollinator, a small bee‐fly, Megapalpus nitidus (Bombyliidae). These flies are attracted to the spot and try to copulate with it, leading to the conclusion that the dark spots imitate flies to attract males looking for mating (Johnson & Midgley, 1997). The wide distribution of plants with a dark‐center pattern and the recorded behavior of pollinators attracted to these patterns suggest that dark flower center has an adaptive role in pollination.…”

Floral trait variation and integration as a function of sexual deception inGorteria diffusa

“…The system is characterized by variation in floral colour and the complexity and distribution of the black anthocyanin spots at the base of the ray florets (figure 1). These spots, which are complex phenotypic structures, play an important role in the attraction of the bee fly, Megapalpus capensis Wiedeman [30]. M. capensis is the main pollinator of all forms of G. diffusa [28].…”