Do sun orchids mimic buzz-pollinated plants? An experimental test of the adaptive significance of false anthers

Mimicry implies that an organism gains fitness by resembling a model species, and one example is rewardless plants that attract pollinators by resembling co-flowering species that provide rewards. While trait matching between mimic and model has been characterised in many cases of putative floral mimicry, few have demonstrated that resemblance is adaptive and dependent on model presence.

Sun orchids ( Thelymitra ) are believed to mimic flowers of buzz-pollinated rewarding plants by displaying false anthers. To test the adaptive value of the false anthers we examined whether fruit production of T. crinita and T. macrophylla was reduced when anthers were experimentally removed or obscured, and whether the reduction was stronger when putative model plants were abundant. We also assessed visual flower similarity of both orchids and their putative model plants according to bee colour perception and identified shared pollinators and whether their behaviour on T. crinita was similar to that on co-flowering rewarding plants.

Fruit production of both sun orchids was strongly reduced (60-71%) by removal or painting of false anthers but was not affected by the abundance of model plants. Sun orchid flowers closely matched flower colour of co-flowering pollen-rewarding species, and T. crinita shared pollinators with the rewarding species. Visiting bees attempted to buzz and manipulate the false anther, with a behaviour similar to that observed on model plants.

The experimental results demonstrate that the false anther is an important adaptation to pollination in sun orchids. Striking visual flower similarity and shared pollinators between orchids and models suggest that sun orchids are pollinated by bees that mistake orchids for buzz-pollinated rewarding plants. The adaptive value of the false anther did not depend on model plant abundance in the local population, indicating that the relevant spatial scale is larger, or that the effects of the model species are weak in comparison to effects of other rewarding species, i.e., that magnet effects of nectar-rewarding species are dominating.

False anthers are widespread in the genus Thelymitra , and this “mimicry trait” seems to represent an evolutionary novelty that offers unique opportunities to explore adaptations to pollination in deceptive plants.