Direct evidence supporting Darwin's hypothesis of cross-pollination promoted by sex organ reciprocity

Abstract

The floral phenotype plays a main role in the attraction and fit of pollinators. Both perianth traits and the positioning of sex organs can be subjected to natural selection and determine nonrandom mating patterns in populations. In stylar-polymorphic species, the Darwinian hypothesis predicts increased mating success between individuals with sex organs at equivalent heights (i.e. with higher reciprocity). We used paternity analyses in experimental populations of a stylar-dimorphic species. By comparing the observed mating patterns with those expected under random mating, we tested the effects of sex organ reciprocity and perianth traits on mating success. We also analysed phenotypic selection on perianth traits through female and male functions. The (dis)similarity of parental perianth traits had no direct effects on the mating patterns. Sex organ reciprocity had a positive effect on mating success. Narrow floral tubes increased this effect in upper sex organs. Perianth traits showed little signs of phenotypic selection. Female and absolute fitness measures resulted in different patterns of phenotypic selection. We provide precise empirical evidence of the Darwinian hypothesis about the functioning of stylar polymorphisms, demonstrating that mating patterns are determined by sex organ reciprocity and only those perianth traits which are critical to pollinator fit.

Keywords: Narcissus papyraceus; cross-pollination; floral traits; mating patterns; phenotypic selection; sex organ reciprocity; stylar polymorphism.

Fig. 1

Short‐styled (left) and long‐styled (right) flowers of Narcissus papyraceus with indication of the floral traits measured in the parental individuals of this study: corolla width (1), corona width (2), tube width (3), corona height (4), tube length (5), upper anthers height (6), lower anthers height (7), style length (8). All floral traits were measured in both morphs.

Fig. 2

Within‐pair correlations of perianth traits of Narcissus papyraceus in each pollination environment, including null distributions (open bars) under the hypothesis of random mating and observed values (red dashed lines) of the Pearson correlation coefficient. LTPE, long‐tongued pollination environment; STPE, short‐tongued pollination environment.

Fig. 3

Distributions of the observed and expected mismatch values of maternal stigma and paternal anthers under random mating, for each sex organs level, in the experimental populations of Narcissus papyraceus at each pollination environment. The results of Wilcoxon rank‐sum test comparing observed vs expected values are provided in each plot. A normal probability curve has been fitted to the histograms to ease visualization. LTPE, long‐tongued pollination environment; STPE, short‐tongued pollination environment.

Fig. 4

Phenotypic selection on floral traits of Narcissus papyraceus calculated in experimental populations. Estimates (±SE) from (a) univariate and (b) multivariate regressions of female (λ _F), male (λ _M) and absolute (λ _A) reproductive success of individuals on floral traits. *, P < 0.05; **, P < 0.01; ***, P < 0.001; †, P < 0.1. (ns) not significant after Bonferroni correction of P‐values in multiple univariate models.