Geographical variation in flower colour of a food-deceptive orchid reflects local pollinator preferences.

Abstract

Background and aims: Intraspecific variation in floral traits can reflect shifts among different pollinator niches. We investigated whether a geographical mosaic of pollinator niches can explain divergence of flower colour and morphology among populations of Disa porrecta, a food-deceptive orchid in South Africa.

Methods: We used field observations and pollen load analysis to establish pollinators of D. porrecta and measured spectral reflectance, morphology and scent emissions in populations. We used reciprocal translocation experiments and pairs of artificial inflorescences to establish pollinator foraging preferences. A phylogenetic analysis was used to assess relationships among populations of D. porrecta and related taxa.

Results: We documented two colour forms of D. porrecta that differ in pollinator assemblages. The orange-flowered form in the east of the distribution is pollinated solely by a wide-ranging nymphalid butterfly species. The pink-flowered form in the westernmost part of the distribution is pollinated primarily by a long-proboscid fly species. The orchid's floral spectral reflectance and morphology closely matches that of most nectar plants used by these pollinators, implying a potential role for mimicry. In reciprocal translocations, butterflies showed strong preference for the orange-flowered form of the orchid at sites in the east, while both long-proboscid flies and butterflies showed a preference for the pink-flowered form at sites in the west. These preferences were also evident in experiments involving pairs of artificial inflorescences that varied only in colour. Phylogenetic analysis showed that the two colour forms of Disa porrecta share an immediate common ancestor, but the direction of the colour shift could not be reliably reconstructed.

Conclusion: This study identifies a role for a geographical mosaic of nectar plants and pollinators in the divergence of floral traits in D. porrecta. Differences in flower colour among populations of this orchid species are strongly correlated with pollinator foraging preferences.