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. 2008 May 27;105(21):7484-8.
doi: 10.1073/pnas.0800194105. Epub 2008 May 27.

The evolution of imperfect floral mimicry

Nicolas J Vereecken  1 , Florian P Schiestl
Affiliations

The evolution of imperfect floral mimicry

Nicolas J Vereecken et al. Proc Natl Acad Sci U S A. .

Abstract

The theory of mimicry predicts that selection favors signal refinement in mimics to optimally match the signals released by their specific model species. We provide here chemical and behavioral evidence that a sexually deceptive orchid benefits from its mimetic imperfection to its co-occurring and specific bee model by triggering a stronger response in male bees, which react more intensively to the similar, but novel, scent stimulus provided by the orchid.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Differentiation in odor bouquets between orchid mimics (■) and their specific female bee models (○) sampled across a combination of 13 allopatric and 2 sympatric populations in Western Europe (see Materials and Methods for details). CDF plot of biologically active odor compounds (relative proportions in percentage of the total odor blend): the analysis shows that the proportion patterns of the orchids' floral odor compounds differ significantly from the female bees' sex pheromone, irrespective of the geographic origin of the samples. The CDFs explained 71.1% of the total odor variance among samples (36.9% and 34.3%, respectively; Wilks' λ values: Wλ1 = 0.00018 and Wλ2 = 0.0016, associated P1 and P2 < 0.0001; canonical correlation values: Cc1 = 0.947; Cc2 = 0.943). Overall, 66.5% of all cross-validated samples were assigned correctly to their species/population by the two CDFs.
Fig. 2.
Fig. 2.
Differentiation in odor bouquets between orchid mimics (squares) and their specific female bee models (circles) sampled in sympatry in southern France (filled symbols) and southern Italy (open symbols). CDF plot of biologically active odor compounds (relative proportions, in percentage of the total odor blend): the analysis shows that the relative proportions of the orchids' floral odor differs significantly from the female bees' sex pheromone sampled in sympatry. The discriminatory ability of the CDFs was high, as they explained 93.9% of the total odor variance among samples (63.2% and 30.7%, respectively; Wilks' λ values: Wλ1 = 0.002 and Wλ2 = 0.038, associated P1 and P2 < 0.0001; canonical correlation values: Cc1 = 0.973; Cc2 = 0.947). Overall, 97.5% of all cross-validated samples were assigned correctly to their species/population by the two CDFs.
Fig. 3.
Fig. 3.
Differentiation between the orchid mimics and their sympatric female bee models in emission patterns of the three key odor compounds for the attraction of C. cunicularius males. (A) Mean (± SE) absolute amounts in individual odor extracts (in ng). (B) Mean (± SE) relative proportions (in percentage of the total odor blend). The analysis shows that there is no significant difference in the absolute amounts (in ng) of the three key odor compounds between sympatric orchid mimics and their female bee models, whereas the proportion patterns of these compounds (in percentage of the total odor blend) differ significantly between the models and the mimics. Mann–Whitney U test (α = 0.05), different letters on top of error bars indicate significant differences in odor emission; the numbers of samples analyzed are listed under the columns.
Fig. 4.
Fig. 4.
Differentiation between the orchid mimics (n = 130) and their female bee models (n = 261) in emission patterns of the three key odor compounds [(Z)-7-C21, (Z)-7-C23, and (Z)-7-C25] for the attraction of C. cunicularius males. Mean (± SE) relative amounts in individual odor extracts, in percentage of the total odor blend, across all 15 populations investigated for this study are shown. The analysis shows that overall there is a significant difference (Mann–Whitney U test: P < 0.01) in the mean relative amounts of the three key odor compounds between orchid mimics and their female bee models, irrespective of the geographic origin of the samples. Different letters on top of error bars indicate significant differences in odor emission; the numbers of samples analyzed are listed under the columns.
Fig. 5.
Fig. 5.
Attractiveness of odor bouquets assayed in behavioral experiments with patrolling males of C. cunicularius in the field. The tests show that orchid samples are more attractive than female bee samples; allopatric bee samples are also more attractive than sympatric bee samples. The bars show the mean (± SE) number of inspecting flights (empty bars) and contacts (filled bars) of the male bees with a dummy during 3-min behavioral bioassays. The dummies were scented with odor extracts of orchid flowers (mimic) and attractive female bees (model). We used orchid and bee samples of local (sympatric) and foreign (allopatric) populations. Mann–Whitney U test (α = 0.05), different letters on top of error bars indicate significant differences in attractiveness; the numbers of replicates are listed under the columns.
Fig. 6.
Fig. 6.
Attractiveness of manipulated odor bouquets assayed in behavioral experiments with patrolling males of C. cunicularius in the field. The bars show the mean (± SE) number of inspecting flights (empty bars) and contacts (filled bars) of the male bees with a dummy during 3-min behavioral bioassays. Extracts of virgin female bees with increased ratios of the key odor compounds [(Z)-7-C21, (Z)-7-C23, and (Z)-7-C25] are significantly more attractive than the natural, nonmanipulated samples. Mann–Whitney U test (α = 0.05), different letters on top of error bars indicate significant differences in attractiveness; the numbers of replicates are listed under the columns.
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References

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