Negative frequency-dependent selection maintains a dramatic flower color polymorphism in the rewardless orchid Dactylorhiza sambucina (L.) Soo

Abstract

The orchid Dactylorhiza sambucina shows a stable and dramatic flower-color polymorphism, with both yellow- and purple-flowered individuals present in natural populations throughout the range of the species in Europe. The evolutionary significance of flower-color polymorphisms found in many rewardless orchid species has been discussed at length, but the mechanisms responsible for their maintenance remain unclear. Laboratory experiments have suggested that behavioral responses by pollinators to lack of reward availability might result in a reproductive advantage for rare-color morphs. Consequently, we performed an experiment varying the relative frequency of the two color morphs of D. sambucina to test whether rare morph advantage acted in the natural habitat of the species. We show here clear evidence from this manipulative experiment that rare-color morphs have reproductive advantage through male and female components. This is the first demonstration, to our knowledge, that negative frequency-dependent selection through pollinator preference for rare morphs can cause the maintenance of a flower-color polymorphism.

Figure 1

Two individuals of D. sambucina are shown.

Figure 2

The reflectance spectrum of the two flower colors is given as a function of the wavelength (light gray for the yellow morph and black for the purple morph). Two spectra are given for each color representing the range of the spectrum found over 10 individuals. Spectra were recorded by using a portable reflectance spectrophotometer (High Sensitivity Spectrophotometer S2000, New Electro-Optical Concepts, Beaufays, Belgium) that gave a reflectance spectrum across all of the wavelengths (including UV) to which insect pollinators are sensitive (16).

Figure 3

Relative male (a) and female (b) reproductive success of the yellow morph as a function of the relative frequency of the yellow morph in each array. Male reproductive success for a morph was quantified as the average proportion of pollinia removed from plants within each array (n = 5 to 45 plants) (●). Female reproductive success was measured as the average proportion of stigmas receiving pollen (⧫) and setting fruits (□) for plants within each array. Relative reproductive success was calculated as described in Materials and Methods. Each data point represents the mean of two arrays per frequency, with bars showing one SEM. Statistics were calculated by using all values. r is the Spearman correlation coefficient (Pearson correlation coefficients were identical). *, P < 0.05; **, P < 0.01. The slopes given are the lines fitted in regression analysis: (a) y = −0.66x + 1.452; (b1, solid line for pollinia deposition) y = −0.63x + 1.454, and (b2, broken line for fruit set) y = −0.59x + 1.354. The horizontal line corresponds to equal reproductive success between the two morphs. The intersection between regression lines and the horizontal line gives the value of predicted morph frequencies at equilibrium (represented by vertical dotted lines).