Floral traits that reduce self‐pollination in hermaphroditic plants have usually been interpreted as mechanisms that limit the genetic consequences of self‐fertilization. However, the avoidance of sexual conflict between female and male function (self‐interference) may also represent an important selection pressure for the evolution of floral traits, particularly in self‐incompatible species. Here, we use experimental manipulations to investigate self‐interference in Narcissus assoanus, a self‐incompatible species with a stigma‐height dimorphism in which the degree of spatial separation between sex organs (herkogamy) differs strikingly between the long‐ and short‐styled morphs (hereafter L‐ and S‐morphs). We predicted that weak herkogamy in the L‐morph would cause greater self‐pollination and hence self‐interference. Experimental self‐pollination reduced seed set when it occurred prior to, or simultaneously with, cross‐pollination in the L‐morph, but only if it occurred prior to cross‐pollination in the S‐morph. In the field, autonomous self‐pollination was greater in the L‐morph than the S‐morph, but we found no evidence that self‐interference reduced maternal or paternal fitness in either morph. One‐day‐old flowers of the L‐morph have reduced stigma receptivity and hence exhibit protandry, whereas stigma receptivity and anther dehiscence are concurrent in the S‐morph. This suggests that the two style morphs have alternative strategies for reducing self‐interference: dichogamy in the L‐morph and herkogamy in the S‐morph. These results provide insight into the mechanisms that reduce sexual conflict in hermaphrodite plants and are of significance for understanding the evolution and maintenance of sexual polymorphisms.
There is growing appreciation that the ecological factors which impact on rates of pollen transfer can contribute significantly to reproductive trait evolution in plants. In heterostylous species, several studies support Darwin's claim that the reciprocal positions of stigmas and anthers enhance inter-morph mating in comparison to intra-morph mating and thus the maintenance of the polymorphism. In this study, we evaluate the relative importance of intra-morph and inter-morph pollen transfers in Narcissus assoanus, a species with dimorphic variation in style length but non-reciprocity of anther positions. This stigma-height dimorphism represents a transitional stage in theoretical models of the evolution of distyly. Seed set variation on recipient plants with donor plants of a single morph in experimental arrays in a natural population illustrate that inter-morph cross-pollination is more efficient that intra-morph cross-pollination as a result of high rates of pollen transfer from long-styled to short-styled plants. The observed rates of pollen transfer satisfy the theoretical conditions for the establishment of a stigma-height dimorphism in an ancestral monomorphic long-styled population in pollen-limited situations. These results provide experimental evidence for the Darwinian hypothesis that enhanced inter-morph cross-pollination facilitates not only the maintenance of heterostyly but also the establishment of transitional forms implicated in the evolution of this polymorphism.
The maintenance of floral polymorphisms depends on rates and efficiency of cross-pollination within and among mating types in relation to their relative frequency. Here, we examine whether stigma-anther reciprocity in style-dimorphic Narcissus assoanus (populations show either 1 : 1 ratios of long-styled [L-morph] and short-styled [S-morph] plants or significantly L-morph-biased ratios) is correlated with morph ratio variation and whether such differences affect female fitness. In a natural population we created experimental plots of S-morph maternal plants and quantified their female fertility in the presence of either L-morph donors or S-morph donors with only their lower anther levels (we emasculated upper-level anthers). We also quantified floral traits and reciprocity in the positions of stigmas and anthers in 30 natural populations across the species' geographic distribution. We found an increase in seed set on maternal S-morph plants in the presence of L-morph donors and patterns of trait variation that may contribute to enhanced reciprocity with increasing frequency of the S-morph. Subtle differences in the position of sex organs affect pollen transfer and are correlated with morph ratio variation, indicative of frequency-dependent selection.
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