Gender variation and the evolution of dioecy in Wurmbea dioica (Liliaceae)

A major obstacle for empirical tests of hypotheses concerning the evolution of dioecy in flowering plants is the limited number of species that possess both cosexual and dioecious populations. Wurmbea dioica (Liliaceae) is a diminutive, fly-pollinated geophyte native to temperate Australia. Marked geographical variation of floral traits is evident, particularly with respect to sex expression. A survey of phenotypic gender in 45 populations from Western Australia (WA), South Australia (SA), Victoria (Vic) and the Australian Capital Territory (ACT) revealed two contrasting patterns. Populations in SA, Vic, and ACT were uniformly dimorphic for gender, containing female and male plants, whereas populations in WA were either monomorphic or dimorphic. In most dimorphic populations varying numbers of male plants produced hermaphrodite flowers (male inconstancy). There was a significant negative relationship between female frequency and the proportion of inconstant male plants. Depending on region and population, male plants produced more flowers of larger size than females. In WA monomorphic populations often occurred on rich, moist soils at high density, whereas dimorphic populations were more commonly found at lower density on shallow soils in drier areas. In an area of sympatry, plants with contrasting sexual systems flowered at different times and were ecologically differentiated. The patterns of gender variation in W. dioica indicate that dioecy has evolved via the gynodioecious pathway. The spread of females in monomorphic populations may be favoured where ecological conditions result in increased selfing and inbreeding depression in hermaphrodites.     

Sex‐specific interaction between arbuscular mycorrhizal and dark septate fungi in the dioecious plant Antennaria dioica (Asteraceae)

Male and female plants of dioecious species often differ in their resource demands and this has been linked to secondary sexual dimorphism, including sex‐specific interactions with other organisms such as herbivores and pollinators. However, little is known about the interaction between dioecious plants and fungal root endophytes. Plants may be simultaneously colonised by arbuscular mycorrhizal (AM) and dark septate (DS) fungi. While it is well established that AM mutualism involves reciprocal transfer of photosynthates and mineral nutrients between roots of host plants and these fungi, the role of DS fungi remains controversial. Here, we report the temporal and spatial variation in AM and DS fungi in female, male and non‐reproductive Antennaria dioica plants in three natural populations in Finland during flowering and after seed production. Females had higher colonisation by AM fungi, but lower colonisation by DS fungi than male and non‐reproductive plants. The higher AM colonisation was observed during flowering, and this difference varied among populations. Our results suggest that females and males of A. dioica interact with AM and DS fungi differently and that this relationship is dependent on soil fertility.     

Floral trait evolution associated with shifts between insect and wind pollination in the dioecious genus Leucadendron (Proteaceae)

Transitions between animal and wind pollination have occurred in many lineages and have been linked to various floral modifications, but these have seldom been assessed in a phylogenetic framework. In the dioecious genus Leucadendron (Proteaceae), transitions from insect to wind pollination have occurred at least four times. Using analyses that controlled for relatedness among Leucadendron species, we investigated how these transitions shaped the evolution of floral structural and signaling traits, including the degree of sexual dimorphism in these traits. Pollen grains of wind‐pollinated species were found to be smaller, more numerous, and dispersed more efficiently in wind than were those of insect‐pollinated species. Wind‐pollinated species also exhibited a reduction in spectral contrast between showy subtending leaves and background foliage, reduced volatile emissions, and a greater degree of sexual dimorphism in color and scent. Uniovulate flowers and inflorescence condensation are conserved ancestral features in Leucadendron and likely served as exaptations in shifts to wind pollination. These results offer insights into the key modifications of male and female floral traits involved in transitions between insect and wind pollination.     

Sex ratio variation and spatial distribution of Siparuna grandiflora, a tropical dioecious shrub

Populations of dioecious plant species often exhibit biased sex ratios. Such biases may arise as a result of sex-based differences in life history traits, or as a result of spatial segregation of the sexes. Of these, sex-based differentiation in life history traits is likely to be the most common cause of bias. In dioecious species, selection can act upon the sexes in a somewhat independent way, leading to differentiation and evolution toward sex-specific ecological optima. I examined sex ratio variation and spatial distribution of the tropical dioecious shrub Siparuna grandiflora to determine whether populations exhibited a biased sex ratio, and if so, whether the bias could be explained in terms of non-random spatial distribution or sex-based differentiation in life history traits. Sex ratio bias was tested using contingency tables, a logistic regression approach was utilized to examine variation in life history traits, and spatial distributions were analyzed using Ripley's K, a second-order neighborhood analysis. I found that although populations of S. grandiflora have a male-biased sex ratio within and among years, there was no evidence of spatial segregation of the sexes. Rather, the sex ratio bias was shown to result primarily from sex-based differentiation in life history traits; males reproduce at a smaller size and more frequently than females. The sexes also differ in the relationship between plant size and reproductive frequency. Light availability was shown to affect reproductive activity in both sexes, though among infrequently flowering plants, females require higher light levels than males to flower. The results of this study demonstrate that ecologically significant sex-based differentiation has evolved in S. grandiflora. Received: 30 July 1997 / Accepted: 16 December 1997     

The effects of floral display on pollinator visitation vary among populations ofPhacelia linearis (Hydrophyllaceae)

Summary Individual plants in gynodioecious populations ofPhacelia linearis (Hydrophyllaceae) vary in flower gender, flower size, and flower number. This paper reports the effects of variation in floral display on the visitation behaviour of this species' pollinators (mainly pollen-collecting solitary bees) in several natural and three experimental plant populations, and discusses the results in terms of the consequences for plant fitness. The working hypotheses were: (1) that because female plants do not produce pollen, pollen-collecting insects would visit hermaphrodite plants at a higher rate than female plants and would visit more flowers per hermaphrodite than per female; and (2) that pollinator arrival rate would increase with flower size and flower number, the two main components of visual display. These hypotheses were generally supported, but the effects of floral display on pollinator visitation varied substantially among plant populations. Hermaphrodites received significantly higher rates of pollinator arrivals and significantly higher rates of visits to flowers than did females in all experimental populations. Flower size affected arrival rate and flower visit rate positively in natural populations and in two of the three experimental populations. The flower size effect was significant only among female plants in one experimental population, and only among hermaphrodites in another. The effect of flower number on arrival rate was positive and highly significant in natural populations and in all experimental populations. In two out of three experimental populations, insects visited significantly more flowers per hermaphrodite than per female and visited more flowers on many-flowered plants than on few-flowered plants, but neither effect was detected in the third experimental population. Because seed production is not pollen-limited in this species, variation in pollinator visitation behaviour should mainly affect the male reproductive success of hermaphrodite plants. These findings suggest that pollinator-mediated natural selection for floral display inP. linearis varies in space and time.     

马先蒿属花冠形态的多样性与传粉式样的关系

马先蒿属(Pediculais)是有花植物中花冠形态多样化最为集中的属。该属主要的传粉者是熊蜂属(Bormbus)昆虫;在北美,熊蜂和蜂鸟是马先蒿植物一些种类有效的传粉者;也发现壁蜂(Osmia)为其传粉。不同的传粉机制要求某一特定的取食式样储藏和释放花粉。本文讨论了花冠类型的进化趋势与传粉式样和花粉形态的关系。传粉者的选择压力是决定花冠多样化的重要因素之一;花冠类型与传粉者和传粉行为紧密相关。马先蒿植物和传粉者的相互依赖与其花冠类型、功能和物候互相适应,但花冠类型与花粉形态两者之间似乎没有明显的一一对应关系。通过北美、日本和喜马拉雅不同地理分布马先蒿种类的比较研究表明,具有相同花冠类型的种类有着相同的传粉方式,花冠形态与传粉式样存在紧密的协同进化关系。     

Floral morphometrics and the evolution of sexual dimorphism in Lycium (Solanaceae)

Plants of Lycium californicum, L. exsertum, and L. fremontii produce flowers that are either male-sterile (female) or hermaphroditic, and populations are morphologically gynodioecious. As is commonly found in gynodioecious species, flowers on female plants are smaller than those on hermaphrodites for a number of floral traits. Floral size dimorphism has often been hypothesized to be the result of either a reduction in female flower size that allows reallocation to greater fruit and seed production, or an increase in hermaphroditic flower size due to the increased importance of pollinator attraction and pollen export for hermaphroditic flowers. We provide a test of these two alternatives by measuring 11 floral characters in eight species of Lycium and using a phylogeny to reconstruct the floral size shifts associated with the evolution of gender dimorphism. Our analyses suggest that female flowers are reduced in size relative to the ancestral condition, whereas flowers on hermaphrodites have changed only slightly in size. Female and hermaphroditic flowers have also diverged both from one another and from ancestral cosexual species in several shape characteristics. We expected sexual dimorphism to be similar among the three dimorphic taxa, as gender dimorphism evolved only a single time in the ancestor of the American dimorphic lineage. While the floral sexual dimorphism is broadly similar among the three dimorphic species, there are some species-specific differences. For example, L. exsertum has the greatest floral size dimorphism, whereas L. fremontii had the greatest size-independent dimorphism in pistil characters. To determine the degree to which phylogenetic uncertainty affected reconstruction of ancestral character states, we performed a sensitivity analysis by reconstructing ancestral character states on alternative topologies. We argue that investigations such as this one, that examine floral evolution from an explicitly phylogenetic perspective, provide new insights into the study of the evolution of floral sexual dimorphism.     

青阳参花部特征及其传粉适应性

对青阳参花（Cynanchum otophyllum）部综合特征、访花昆虫种类、访花行为及传粉过程进行了研究,结果表明,青阳参花结构复杂,两个子房基部离生、花柱联合与雄蕊形成合蕊柱,柱头表面被邻近花药的侧翼紧密包围形成5个柱头腔。青阳参的花粉形成独特的花粉块,一次传粉过程可以转运大量的花粉。东方蜜蜂（Apis cerana）是青阳参的主要传粉昆虫,其传粉包括两个过程：（1）当蜜蜂的口器或足插入着粉腺的槽口后借助蜜蜂的力量将花粉块从花上拔起;（2）当蜜蜂再次访花时将携带的花粉块插入其中一个柱头腔。花粉块里面的花粉粒住柱头腔中萌发出花粉管,然后沿着花柱道向下生长最后进入子房。在整个花期仡粉保持有相对较高的生活力,而其柱头可授性则在7天后逐渐降低。     

The dioecious, sphingophilous species Citharexylum myrianthum (Verbenaceae): Pollination and visitor diversity

Citharexylum myrianthum is an example of cryptic dioecy in morphologically perfect flowers. Flowers are small, tubular, white-colored, and crepuscular, with a sweet, pleasant scent and occur in raceme-like inflorescences. Functionally, male flowers have a less developed ovary and females have anthers lacking pollen. They are pollinated by five species of Sphingidae. The low fruit set under natural conditions, compared with the very high fruit set after hand-pollination, apart from other possible factors, suggests that pollen flow is limited mainly by pollinators. The overabundance of nectar provided by C. myrianthum flowers attracts a large number of opportunistic nectar foragers in the morning. Hummingbirds, the most frequent visitors, are nectar thieves and display territorialism, being aggressive even to Passeriformes. Despite its visitor diversity C. myrianthum is pollinated by sphingids, and in this sense this species is a specialist rather than a generalist.     

Flowering phenology,floral display and reproductive success in dioecious,Aralia nudicaulis L. (Araliaceae)

Summary Aralia nudicaulis L. is a dioecious, perennial, herbaceous plant that is commonly found in the understory vegetation throughout the boreal forest of North America. Female remets have fewer flowers per inflorescence, initiate flowering earlier, and reach peak flowering before male ramets. The consequences of the asynchrony in flowering between the sexes on pollination and seed set were examined during a two-year study. In both years there was significant variation in seed set associated with the flowering times of individual female ramets. In 1983, seed production was highest in the middle of the flowering season. In 1984, seed production was greatest in the later stages of flowering. Variation in seed set was not attributed to lack of pollination in 1983. In 1984, pollination limited seed set per flower during peak flowering. However, seed production never reached the potential five seeds per flower, suggesting that resource limitation was the most important factor affecting fecundity in both years. The asynchronous pattern of flowering is suggested to be the result of the different inflorescence sizes between the sexes.     

Cost of reproduction in the perennial herb Asphodelus albus (Liliaceae)

The cost of reproduction has been studied in two populations of the polycarpic herb Asphodelus albus under natural conditions The percentage of plants with flowers was determined in four sites and varied markedly among them The occurrence of reproduction was size-dependent, increasing flowering probability with plant size The cost of reproduction was assessed in terms of modular growth in reproductive plants relative to modular growth in vegetative ones I compared the modular growth of vegetative and reproductive plants considering two different densities m each of two populations Neither incidence of flowering nor modular growth were affected by density Flowering plants exhibited a withinramet demographic cost (in terms of modular growth) relative to non-flowering ramets in one population but not in the other This cost was greater in larger plants These results were concordant with the occurrence of flowering at both sites Both populations exhibited size-dependent patterns of allocation to reproduction, but no significant relationships were found between allocation to reproduction and cost of reproduction The data presented demonstrate differences in the cost of reproduction within a species This cost might determine whether a plant begins the reproduction, but probably have no effect on the reproductive allocation since the weight of the reproductive structures was not related to modular growth     

Floral display size in comfrey, Symphytum officinale L. (Boraginaceae): relationships with visitation by three bumblebee species and subsequent seed set

The fecundity of insect-pollinated plants may not be linearly related to the number of flowers produced, since floral display will influence pollinator foraging patterns. We may expect more visits to plants with more flowers, but do these large plants receive more or fewer visits per flower than small plants? Do all pollinator species respond in the same way? We would also expect foragers to move less between plants when the number of flowers per plant are large, which may reduce cross-pollination compared to plants with few flowers. We examine the relationships between numbers of inflorescence per plant, bumblebee foraging behaviour and seed set in comfrey, Symphytum officinale, a self-incompatible perennial herb. Bumblebee species differed in their response to the size of floral display. More individuals of Bombus pratorum and the nectar-robbing B.?terrestris were attracted to plants with larger floral displays, but B. pascuorum exhibited no increase in recruitment according to display size. Once attracted, all bee species visited more inflorescences per plant on plants with more inflorescences. Overall the visitation rate per inflorescence and seed set per flower was independent of the number of inflorescences per plant. Variation in seed set was not explained by the numbers of bumblebees attracted or by the number of inflorescences they visited for any bee species. However, the mean seed set per flower (1.18) was far below the maximum possible (4 per flower). We suggest that in this system seed set is not limited by pollination but by other factors, possibly nutritional resources.     

Differential competitive ability between sexes in the dioecious Antennaria dioica (Asteraceae)

Background and Aims

Differences in competitive ability between the sexes of dioecious plants are expected as a result of allocation trade-offs associated with sex-differential reproductive costs. However, the available data on competitive ability in dioecious plants are scarce and contradictory. In this study sexual competition was evaluated using the dioecious plant Antennaria dioica in a common garden transplantation experiment.

Methods

Male and female plants were grown for 3 years either in isolation, or in competition with a plant of the same sex or the opposite sex. Flowering phenology, sexual and asexual reproduction, plant growth, nutrient content and arbuscular mycorrhizal colonization in the roots were assessed.

Key Results

Our results showed little evidence of sexual differences in competitive ability. Both sexes suffered similarly from competition, and competitive effects were manifested in some traits related to fitness but not in others. Survival was unaffected by competition, but competing plants reduced their vegetative growth and reproductive investment compared with non-competing plants. In addition, differences in sexual competitive ability were observed in relation to flowering frequency, an important life history trait not reported in previous studies.

Conclusions

The findings indicate that female and male A. dioica plants possess similar intersexual competitive abilities which may be related to the similar costs of reproduction between sexes in this species. Nevertheless, intrasexual competition is higher in females, giving support for asymmetric niche segregation between the sexes.     

Floral sex ratios, disease and seed set in dioecious Silene dioica

1 In the dioecious, perennial herb Silene dioica , the density of pollen donors in a population is determined by overall plant density, the sex ratio and the proportion of plants infected with the anther-smut fungus Microbotryum violaceum , which results in permanent sterility of both male and female plants.
2 Pollinators ( Bombus spp.) were found to prefer male flowers and to avoid diseased flowers. This may result in an overall lower visitation frequency and increased risk for pollen limitation in populations with a low density of males or a high incidence of disease.
3 Compared with open-pollinated flowers, hand pollination resulted in a significant increase in the number of seeds produced per fruit in populations with an experimentally reduced proportion of males (25% and 50% male flowers) but not in a naturally male-dominated population (75% male flowers). Seed production per plant was increased by hand pollination only in the most female-dominated population. Because the floral sex ratio is often male-biased, resources rather than pollen availability are likely to set the upper limit for total seed production per individual in most healthy populations of S. dioica.
4 There was a negative relationship between seed set and incidence of disease across 22 populations in both years of a field study. However, there was no consistent difference between the responses of highly diseased populations (incidence 30–56%) and populations with a low disease incidence (incidence 0–8%) to hand pollination.
5 In a greenhouse experiment with cloned hand-pollinated females, the presence of spores on healthy flowers was found to reduce seed set significantly. In highly diseased populations, therefore, the frequent deposition of spores by flower visitors onto remaining healthy plants may decrease seed production below the potential level determined by resources or pollen availability.     

Intersexual differences in phenology and damage by herbivores and pathogens in dioecious Rubus chamaemorus L.

Summary Shoot development and damage by herbivores and pathogens to male, female and non-floral ramets of the dioecious, perennial herb Rubus chamaemorus were studied in the field during three consecutive years. Leaves on male ramets were usually consumed more by herbivores and attacked more by fungi than were leaves on female ramets. Male ramets unfolded their leaves later than did female ramets. In 1983, when the level of herbivory was comparatively high, ramets that were fully developed in early June were more damaged by herbivores than were ramets that unfolded their leaves later. Non-floral ramets usually showed intermediate levels of damage compared to male and female flowering ramets.Defoliation caused a greater increase in ramet mortality among females than among males in a field experiment. It is suggested that the different reproductive roles of males and females differently constrain the evolution of defense against herbivores and pathogens in dioecious plants. This may result in intersexual differences both in palatability to herbivores and susceptibility to pathogens.     

Environmental stress and the evolution of dioecy: Wurmbea dioica (Colchicaceae) in Western Australia

Stressful ecological conditions have been implicated in the evolution of separate sexes in plants. Gender dimorphic species are often found in drier habitats than their sexually monomorphic relatives, and gynodioecious populations appear closer to a dioecious state as resources, particularly water, become limiting. This pattern could result if dry conditions decrease the relative seed fitness of cosexual plants, allowing female plants to become established in monomorphic populations. We studied geographical variation in gender expression and biomass allocation among 12 monomorphic and dimorphic populations of Wurmbea dioica along a latitudinal precipitation gradient in southwestern Australia to provide insight into mechanisms by which aridity might favor transitions between sexual systems. Plants in monomorphic and dimorphic populations exhibited contrasting gender expression and patterns of biomass allocation in areas with different levels of precipitation. Among dimorphic populations, lower precipitation was associated with a higher frequency of female plants, and reduced allocation to female function by hermaphrodites during flowering. In contrast, stress conditions had no effect on female allocation at flowering in monomorphic populations. Across latitudes, unisexuals and cosexuals exhibited consistent differences in above ground traits, with cosexuals having larger leaves, taller stems and larger flowers. Although all plants were smaller under drier conditions, cosexuals decreased above ground allocation to vegetative and reproductive structures with decreasing latitude. In contrast, unisexuals increased allocation to reproduction in drier areas at the expense of below ground size. Aridity was associated with reduced flower size among all gender classes, but not with changes in flower number. These data do not support the hypothesis that resource limitation of female allocation in cosexual populations favors the establishment of gender dimorphism in W. dioica. Alternative hypotheses, involving higher selfing rates and enhanced survival of unisexuals relative to cosexuals under resource-limited conditions, are discussed as possible explanations for the origin of dioecy in W. dioica. This revised version was published online in August 2006 with corrections to the Cover Date.     

Size-dependent sex allocation in a monocarpic perennial herb, <Emphasis Type="Italic">Cardiocrinum cordatum</Emphasis> (Liliaceae)

The size-dependent sex allocation model predicts that the relative resource allocation to female function often increases with plant size in animal-pollinated plants. If size effects on reproductive success vary depending on the environmental conditions, however, the size dependency may differ among populations. We tried to detect site-specific variation in size-dependent sex allocation of a monocarpic hermaphrodite with reference to light availability. Multiple flowers and fruits were sampled from the individuals of Cardiocrinum cordatum, a monocarpic understory herb, and pollen, ovule and seed production were measured with reference to the plant size in two populations. Furthermore, frequency and foraging behavior of pollinator visitation was observed. Ovule production per flower increased with plant size in both populations, while pollen production per flower increased with size only in the population under sparse canopy. Therefore, proportional allocation to male function decreased with plant size in the population under closed canopy, but did not change in the population under sparse canopy. Pollinators usually visited only one flower per plant, indicating the negligible geitonogamous pollination in this species. Although seed production under closed canopy was lower than that under sparse canopy, seed-set rate per flower and seed mass per fruit were independent of plant size in either of the populations. Size-dependent sex allocation in this species was site-specific, suggesting that not only resource storage before reproduction (i.e., plant size) but also resource availability of environment throughout the reproductive process (i.e., light availability) affect reproductive performance in this species.     

Population size,pollinator visitation and fruit production in the deceptive orchid Calypso bulbosa

It has been proposed that in non-rewarding animal-pollinated plants the pollination intensity should decrease with increasing population size and should increase with increasing local abundance of reward-producing plants. To test these hypotheses, we examined how population size, local abundance of Salix caprea, and tree cover were related to pollen removal and fruit production in 16 populations of the deceptive, early-flowering and bumblebee-pollinated orchid Calypso bulbosa in northern Sweden in 3 consecutive years. To determine whether fruit production was limited by pollinator visitation, supplemental hand-pollinations were performed in three populations in 3 years. Finally, to examine whether increased fruit production was associated with a reduction in future flower production, vegetative growth or survival, supplemental hand-pollination was repeated for 5 years in one population. The levels of pollen export, pollen deposition, and fruit set of C. bulbosa varied considerably among years and among populations. The proportion of plants exporting pollen was negatively related to population size, and positively related to density of S. caprea and to tree cover in 1 of the 3 years. In the other 2 years, no significant relationship was detected between proportion of plants exporting pollen and the latter three variables. In no year was there a significant relationship between fruit set and population size, density of S. caprea and tree cover. There was substantial among-year variation in the extent to which fruit production was limited by insufficient pollen deposition and in the amount of weather-induced damage to flowers and developing fruits. Fruit set was consistently higher in hand-pollinated than in open-pollinated plants, but this difference was statistically significant in only one of 3 years. Supplemental hand-pollination in 5 consecutive years increased cumulative fruit production 1.8 times, but did not affect flower production, plant size, or survival. Tree cover was negatively correlated with the incidence of frost damage in 1 year. The results indicate that life-time seed production may be pollen limited in C. bulbosa, and that variation in population size and local abundance of the early-flowering, nectar-producing S. caprea can only partly explain the extensive variation in pollinator visitation among populations of this species.     

Survival costs and reproductive benefits of floral display in a sexually dimorphic dioecious shrub, Leucadendron xanthoconus

The evolutionary causes of sexual dimorphism in plants have not been as widely studied as in animals and the importance of sexual selection in causing dimorphism remains controversial. Sexual selection is most obvious when it favours the evolution of a trait which enhances mating success at the expense of decreased viability. We studied the relationship between floral display (number of inflorescences), pollinator attraction and plant survival in a dioecious shrub, Leucadendron xanthoconus. Pollinator attraction, measured as the number of insect pollinators, increased linearly with floral display in males. However, males with extravagant displays had a higher probability of dying. Our data suggest that male plants are undergoing selection on floral display for increased mating success counterbalanced by selection against plants with extravagant displays. Seed set in females did not increase with floral display, except at very low inflorescence numbers. Nor was female survival correlated with floral display. Because inflorescences are terminal in the species, selection for more inflorescences in males causes increased ramification, thinner terminal branches and smaller leaves. Thus vegetative dimorphism in this species appears to be caused by selection for extravagant floral display in males, but not females. Limits to dimorphism are imposed by survival costs of elaborate display.     

Ovule pre-emption and pollen limitation in a self-fertile perennial herb (Blandfordia grandiflora,Liliaceae)

The extent, frequency and causes of pollenlimited seed production were examined in partially selffertile populations of Blandfordia grandiflora for 2 years. Percentage seed set of open-pollinated plants (50–57%) did not differ within or between years, and was about 19% less than experimentally cross-pollinated plants (70–75%). Floral visits by honeybees did not differ through the flowering season and the number of pollen grains deposited on stigmas within 12 h of flowers opening exceeded the number of ovules per flower, indicating that the quality rather than the quantity of pollination limited seed set. Pollen limitation was caused by concurrent self- and cross-pollination and the subsequent abortion of some selfed ovules due to inbreeding depression. Natural seed set (55%) was intermediate between selfed (43%) and crossed (75%) flowers and was not increased when flowers that had been available to pollinators for 24 h were hand cross-pollinated, suggesting that ovules were already fertilized. Similarly, experimental pollination with both cross and self pollen within 24 h of flowers opening did not increase seed set relative to natural seed set, indicating that both cross- and self-fertilizations had occurred. In contrast, when selfing followed crossing by 48 h, or vice versa, seed set did not differ from crossed-only or selfed-only flowers, respectively, indicating that ovules were pre-empted by the first pollination. Collectively, these results indicate that under natural conditions self pollen pre-empts ovules, rendering them unavailable for cross-fertilization. This selfing reduces fecundity by 50%, as estimated from the natural production of cross seeds when selfing was prevented. Consequently, selection should favour floral traits, such as increased stigma-anther separation or protandry, that reduce interference between male and female functions that leads to selfing.