Experimental insights on the function of ancillary pollen and stigma polymorphisms in plants with heteromorphic incompatibility

Most heterostylous plants possess a reciprocal arrangement of stigmas and anthers (reciprocal herkogamy), heteromorphic self‐incompatibility, and ancillary polymorphisms of pollen and stigmas. The topographical complementarity hypothesis proposes that ancillary polymorphisms function in the rejection of incompatible pollen thus promoting disassortative pollination. Here, we test this hypothesis by investigating patterns of pollen transfer and capture in populations of dimorphic Armeria maritima and A. pubigera and distylous Limonium vulgare (Plumbaginaceae), and by studying pollen adherence and germination patterns in A. maritima following controlled hand‐pollinations. Armeria lacks reciprocal herkogamy allowing the evaluation of the extent to which ancillary polymorphisms affect the composition of pollen loads. We compared the amounts of compatible and incompatible pollen on stigmas in natural populations and calculated the proficiencies of pollen transfer for each mating type. We detected disassortative pollination in each species, and mating types did not differ in compatible pollen capture, although cob stigmas captured more incompatible pollen. Controlled hand‐pollinations revealed the failure of incompatible pollen to adhere and germinate on stigmas. Our results provided evidence that, while structural in nature, pollen‐stigma dimorphisms are tightly associated with heteromorphic incompatibility and likely function to promote disassortative pollination, especially in the absence of reciprocal herkogamy.     

Patterns and consequences of self-pollen deposition on stigmas in heterostylous Persicaria japonica (Polygonaceae)

In order to test the "anti-interference" hypothesis for adaptive significance of reciprocal herkogamy, patterns of illegitimate pollination and effects of self-pollen on the legitimate pollination and fertility were investigated in a naturally pollinated experimental population of distylous Persicaria japonica. Pollen deposition was compared among the emasculation treatments, i.e., emasculation of a single flower from individual inflorescences, emasculation of all the flowers of individual inflorescences, and no emasculation control. In both morphs, considerable illegitimate pollination was found to occur in all the treatments, and there was no significant difference in incompatible pollen load among the treatments. Therefore, it is suggested that herkogamy of P. japonica can sufficiently reduce both intraflower and intra-inflorescence self-pollinations, but not interinflorescence geitonogamous and-or interclonal illegitimate pollination. Measurements of pollen load after the repetitive pollen addition showed that space on the stigma surface may not limit the legitimate pollination under natural pollination conditions. Seed sets after legitimate pollination following prior self-pollination did not differ from the controls without self-pollination. Therefore, there is little possibility that incompatible pollen load interferes with either pollination or fertilization by compatible pollen, suggesting that "anti-interference" is unimportant for adaptive significance of reciprocal herkogamy at least in P. japonica.     

Shortened anther-stigma distance reduces compatible pollination in two distylous Primula species

Aims Distyly has been regarded as an adaptation to improve compatible pollination between two floral morphs with reciprocal herkogamy. The hypothesis that the different positions of anthers and stigmas within flowers as well as their reciprocal position between morphs, reduce the probability of self pollination raised by Darwin has been rarely tested. In this study, we measured stigmatic pollen loads in response to reduced reciprocal herkogamy in two Primula species.Methods To see whether reciprocal herkogamy can increase compatible and/or reduce incompatible pollen deposition, thus promoting compatible pollination, we shortened the distance between anthers and stigmas within the flowers by changing the position of the corolla tube, to which the anthers were fused, i.e. reduced herkogamy in natural populations of Primula secundiflora and P. poissonii and quantified stigmatic pollen loads in the field over 2 years.Important findings In both species, stigmatic pollen loads were significantly higher in the long-styled (L-morph) than in the short-styled morph (S-morph) in both control and manipulated flowers, but percentage of compatible pollen in S-morph were higher. Flowers manipulated to halve the anther–stigma distance showed a similar pattern for 2 years: total pollen grain counts on stigmas did not differ significantly but compatible pollen grains in L- and S-morphs were significantly decreased in both species. The percentage of compatible pollen loads was decreased by 68.7% in P. secundiflora and 65.3% in P. poissonii in L-morphs, while it decreased by 30.6% and 2.9% in S-morphs, respectively. Our manipulation of the relative position of anthers and stigmas in the two distylous species indicated that a lower degree of herkogamy reduced compatible but incompatible pollen transfer was likely to increase. The higher proportion of compatible pollen in the S-morph than in the L-morph in the two Primula species could be attributed to the accessibility of two-level sexual organs, floral orientations and pollinator behaviors. This is a first attempt to manipulate intraflower herkogamy for understanding adaptation of heterostyly, shedding insights into how the reciprocal herkogamy promotes compatible pollination.     

Relationship between herkogamy,incompatibility and reciprocity with pollen–ovule ratios in Melochia (Malvaceae)

Heterostyly is a floral polymorphism that increases pollination efficiency by promoting cross-pollination and reducing pollen wastage. Efficiency in pollination has been related to plant investment in gamete production and to the pollen to ovule ratio (P/O), which has been proposed as an indication of the likelihood of enough pollen grains reaching the stigmas to result in maximum reproductive success. In heterostylous species, cross-pollination is promoted by the reciprocal position of sexual organs between morphs and a heteromorphic incompatibility system, which precludes selfing and fertilizations among plants of the same morph. Morphological features like reciprocity (between morphs) and herkogamy (within morph) together with the breeding system are thought to influence pollination quality. Therefore, a close relationship between the pollination efficiency, morphological characteristics, and incompatibility would be expected. Pollination treatments and morphological measurements were carried out to describe the breeding system, herkogamy, and reciprocity of six Melochia species. Afterward, the relation between the P/O (as a surrogate of the efficiency in pollination), and reciprocity, herkogamy and incompatibility was evaluated. Monomorphic M. nodiflora and distylous M. pyramidata are self-compatible species, whereas the rest of the species are self- and morph-incompatible. There was a positive relationship between the P/O value and the degree of herkogamy and incompatibility. However, P/O values appear to increase when higher reciprocity is found in the populations. As expected, the lower values of P/O are associated with lower levels of herkogamy and compatibility in the Melochia species studied. The relationship between the factors is discussed under different scenarios of the pollinators’ predictability.     

Heteromorphic incompatibility and efficiency of pollination in two distylous Pentanisia species (Rubiaceae)

BACKGROUND AND AIMS: Distyly has been hypothesized to promote cross-pollination by reducing intrafloral and geitonogamous self-pollination, and enhancing intermorph pollination. Distylous plants typically display both reciprocal herkogamy and a heteromorphic incompatibility system, which allows mating only between morphs. Distyly and its pollination consequences were examined in two Pentanisia species with long-tubed flowers which are pollinated almost exclusively by butterflies. METHODS: Anther and stigma heights were measured to quantify reciprocal herkogamy. The type of incompatibility system was determined by observing pollen tubes and seed production following controlled hand pollination. Pollen loads on pollinators and stigmas were also examined to assess the efficiency of intermorph pollen flow. KEY RESULTS: Pentanisia prunelloides and P. angustifolia exhibit reciprocal herkogamy and a host of ancillary dimorphisms, including pollen colour, exine sculpturing, stigmatic papilla shape and floral-tube pubescence. Controlled hand-pollinations revealed the presence of a strong heteromorphic incompatibility system in both species. The site of incompatibility differed between the morphs; intramorph pollen tubes were blocked in the style of the short-styled morph and on the stigmatic surface of the long-styled morph. Butterflies carried pollen from the short- and long-styled morphs primarily on their head and proboscis, respectively. Natural pollination resulted in a higher proportion of pollen transfer from long- to short-styled plants than vice versa. Nevertheless, fruit set did not differ between morphs. CONCLUSIONS: Both Pentanisia species are fully distylous. Reciprocal herkogamy results in pollen from the two morphs being carried on different locations on pollinators' bodies, which in turn promotes intermorph pollination. Intramorph pollination does not result in fertilization, because of an effective heteromorphic incompatibility system.     

Unravelling the stylar polymorphism in Melochia (Malvaceae): reciprocity and ancillary characters

Heterostyly is a genetic polymorphism in which plant populations possess two (distyly) or three (tristyly) morphs with flowers differing reciprocally in stigma and anther height. Sex organ deployment has been described as being highly variable among and within species of several distylous taxa belonging to different taxonomic groups. However, the number of studies considering within‐species disparities is still limited. For a better overview of the existing amount of variation that can occur within and between heterostylous species, we sampled 46 populations of six Melochia spp., a style‐polymorphic genus in Cuba. We characterized the floral morphology in all populations and described a set of ancillary characters per species. All of these Melochia spp. are distylous, except for the monomorphic M. nodiflora. The S‐morph produces fewer, larger pollen than L‐morphs, and has verrucose ornamentation. The L‐morph produces reticulate pollen and has larger stigmatic papillae than the S‐morph. The monomorphic M. nodiflora shows ancillary characteristics that are similar to the L‐morph individuals in the related species. As expected, there are differences in ancillary characters among species and also dissimilarities in reciprocity among and within species of Melochia. Our results highlight the importance of considering intraspecific variation in the morphometric characterization of heterostylous taxa. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 176 , 147–158.     

被子植物异型花柱及其进化意义

异型花柱(heterostyly)是被子植物中一种特殊的花多态现象和雌雄异位形式,包括二型花柱(distyly)和三型花柱(tristyly)2种类型.据报道,在被子植物的约31个科中有异型花柱植物.该类型植物的花部特征在避免自交、促进准确的异交传粉以及通过降低雌雄功能干扰以提高亲本适合度等方面具有重要的进化意义.该文从以下3个方面总结和分析了异型花柱及其进化意义:(1)异型花柱植物的类型和花部特征、附属多态性和种群结构:(2)异型花柱植物在被子植物中的分布、起源和演化:(3)异型花柱植物的进化适应意义.结合目前作者开展的有关工作,对异型花柱植物研究中存在的一些问题进行讨论和展望,希望能为国内工作者开展该领域的研究提供一些参考.     

被子植物异型花柱及其进化意义

异型花柱(heterostyly)是被子植物中一种特殊的花多态现象和雌雄异位形式, 包括二型花柱(distyly)和三型花柱(tristyly) 2种类型。据报道, 在被子植物的约31个科中有异型花柱植物。该类型植物的花部特征在避免自交、促进准确的异交传粉以及通过降低雌雄功能干扰以提高亲本适合度等方面具有重要的进化意义。该文从以下3个方面总结和分析了异型花柱及其进化意义: (1)异型花柱植物的类型和花部特征、附属多态性和种群结构; (2)异型花柱植物在被子植物中的分布、起源和演化; (3) 异型花柱植物的进化适应意义。结合目前作者开展的有关工作, 对异型花柱植物研究中存在的一些问题进行讨论和展望, 希望能为国内工作者开展该领域的研究提供一些参考。     

Pollen deposition rates and the functioning of distyly in the perennial Pulmonaria officinalis (Boraginaceae)

Distyly typically involves reciprocal polymorphism in stamen and style lengths and a set of associated polymorphisms of pollen and stigma characters. This flower polymorphism has been regarded as a strategy to reduce the likelihood of self- and intramorph pollination and to promote legitimate intermorph pollination. Although most distylous plants are characterized by a physiological self- and intramorph-incompatibility system, previous work on Pulmonaria affinis and other distylous Boraginaceae species have shown that self-incompatibility was not strict. In this study, we examined pollen deposition rates and the functioning of the distylous breeding system in Pulmonaria officinalis. Flowers exhibited reciprocal herkogamy and several other ancillary features of heterostyly. Controlled pollinations clearly showed weak self-incompatibility, with the LS-morph showing higher rates of seed set following self-, intramorph- and intermorph-pollinations than the SS-individuals. On the other hand, SS-pollen showed higher germination rates and pollen tube growth than that of LS-pollen. Under natural conditions, both the proficiency of legitimate pollen transfer and the proportion of deposited pollen were asymmetrical. Although SS-pollen grains showed a higher proficiency for legitimate transfer than that of LS-pollen on a per pollen basis, the ratio of legitimate to illegitimate deposited pollen on stigmas of SS-flowers markedly exceeded that of LS-flowers. The latter was explained by the higher pollen production rates of LS-individuals compared to SS-individuals. High levels of illegitimate pollen deposition on LS-stigmas, on the other hand, were attributed to the limited proboscis length of the principal pollinators and the occurrence of floral hairs at the corolla entrance of LS-flowers. This study thus indicates asymmetrical pollen transport patterns and morph-specific differences in the strength of the incompatibility system; two aspects that may affect female reproductive success and morph ratio variation under pollen limited conditions.     

Tristyly, self-compatibility and floral variation in Decodon verticillatus (Lythraceae)

Heterostyly typically involves reciprocal polymorphism in stamen and style lengths, physiological self- and intramorph-incompatibility, and a set of associated polymorphisms of pollen and stigma characters. This study examined floral morphology and compatibility relationships in the monotypic, herbaceous perennial Decodon verticillatus (Lythraceae). There have been conflicting reports on the occurrence of tristyly in the species, probably because of frequent loss of style morphsm from populations in parts of the species' range. Floral morphology was examined using material collected from natural populations throughout the range. Detailed floral measurements revealed discrete trimorphism in style length and anther positioning in three populations. Data from two dimorphic populations showed similar patterns of floral polymorphism, except that both were missing the mid-styled morph. In one dimorphic population, there was evidence for modification in the length of mid-level stamens. Measurements in three populations indicated pronounced floral variability, including high frequencies of modified phenotypes with reduced stigma-anther separation. Pollen size was only weakly differentiated among anther levels, and there were no differences in pollen production among anther levels or morphs. In contrast, stigma size and papilla length showed a strong negative correlation with style length; a pattern opposite to most heterostylous species. Experimental crosses performed under glasshouse conditions on plants from two populations showed a high degree of both self- and intramorph-compatibility. A comparative analysis of floral morphology showed that D. verticillatus is not unusual in terms of the precision and reciprocity of organ positioning compared with 13 other tristylous species.     

Stigma�Canther reciprocity, pollinators, and pollen transfer efficiency in populations of heterostylous species of Lithodora and Glandora (Boraginaceae)

According to Darwin, the reciprocal position of sexual whorls in heterostylous plants enhances disassortative pollen transfer between different floral morphs. It is believed that greater reciprocity between morphs will promote more efficient transfer of pollen. Additionally, efficient pollination will act as a selective force in achieving greater reciprocity between floral morphs. In this study we test whether variation in reciprocity of sexual organs between morphs is related to the efficiency of pollinators in transferring pollen between them. To do this, we first describe the pollinator??s array in several populations of species of the genus formerly known as Lithodora, which have different types of stylar polymorphism and degrees of reciprocity, and determine their abundance, plant visitation rate, number of flowers visited per plant and handling time in the population. We estimate the efficiency of the pollinator arrays by use of an approximation based on qualitative (location of pollen loads on different areas of insect bodies) and quantitative (plant visitation rate) measurements. Our results show a correlation between the degree of reciprocity and the efficiency of pollinators associated with the populations. These observations suggest that pollinators are a possible selective force driving the evolution of heterostyly.     

Morph-specific variation of floral traits associated with reciprocal herkogamy in natural populations of Primula vulgaris and Primula veris

Documenting the morph-specific variation of floral traits associated with reciprocal herkogamy is of special importance for revealing the functional significance of traits in the evolution and maintenance of the heterostylous syndrome. In order to describe the extent and specificity of variation, stigma height, anther height, stigma-anther separation and corolla tube length were measured on 800 flowers collected in two natural populations of Primula vulgaris and P. veris. Beside the almost complete separation of stigma heights between the two morphs, we found appreciable intermorph overlap in anther height and relatively broad range of stigma-anther separation so far reported for heterostylous species. The corolla tube length–stigma-anther separation relationship showed striking difference between the two floral morphs, which supports the hypothesis that length of the corolla tube plays a more important role in positioning the sex-organs in the appropriate distance in the short-styled than in the long-styled morph.     

Is sexual organ reciprocity related to legitimate pollen deposition in distylous Pulmonaria (Boraginaceae)?

Heterostyly, i.e. the reciprocal positioning of anthers and stigmas, is a floral polymorphism that is thought to promote disassortative (i.e. between‐morph) pollination and to maintain genetic diversity within populations. Recent research, however, has shown several cases of heterostylous plant species in which the reciprocal positioning of the sexual organs varies, which may affect the likelihood of ‘legitimate’ pollination between compatible morphs, and hence morph fitness and ultimately the stability of this floral polymorphism. In this study, we investigated floral morphology and stigmatic pollen deposition patterns in nine Pulmonaria species (Boraginaceae) across Europe to test the hypothesis that sexual organ reciprocity is related to legitimate pollen deposition. We used a recently developed adaptive accuracy concept to assess the level of reciprocity and to relate inaccuracies in positioning of sexual organs to legitimate pollen deposition measured on stigmas in the field. The nine investigated Pulmonaria species showed substantial deviations from perfect reciprocity, with total inaccuracies varying between 2.9 and 20.3% relative to the squared mean of all anther and stigma heights in the population. In almost all cases, most of the inaccuracy was generated by the high‐level organs. Total pollen deposition was always higher on stigmas of long‐styled individuals, but stigmas of short‐styled individuals received significantly more legitimate pollen than stigmas of long‐styled individuals (71.23% and 38.75% respectively). We also found a weak but significant relationship between measures of inaccuracy and the level of disassortative pollination. Under these conditions of pollen flow, the distylous mating system is predicted to persist in only eleven (27%) of all sampled populations. Overall, these results indicate that deviations from perfect reciprocal herkogamy are common in Pulmonaria and have an impact on legitimate pollen deposition.     

雌雄异位和花部行为适应意义的研究进展

雌雄异位和花部行为影响植物的传粉和交配格局。二者的适应意义一直是植物生殖生态学和进化生物学研究的热点之一。该文概述了近年来雌雄异位和花部行为适应意义的研究进展,包括对雌雄异位适应意义的最早期、最传统的认识——避免自交和当前的主流观点——避免雌雄干扰;花内性别干扰的形式及其例证;花部行为在避免雌雄功能干扰、避免自交、实现延迟自交、应对不良环境等方面的适应意义。讨论了使雌雄异位程度减小的锦葵科一些植物中的单体雄蕊柱头运动可能涉及到的适应意义。针对目前在雌雄异位和花部行为适应意义研究上存在的不足,结合国际上研究二者适应意义的发展趋势正在从前期以描述性的研究为主转变到应用现代分子生物学手段(尤其是分子标记,如SSR、SNP、AFLP等)和传统方法相结合,以及随着田间实验和控制条件能力的发展正由经典的野外调查、观察为主过渡到包括花操纵在内的人工试验和野外试验相结合,提出了今后研究中应注意的问题。有必要借用多学科的手段,以可作为研究雌雄异位和花部行为的有着研究基础的植物为材料,设计和操纵不同的对比试验,系统而深入地探讨雌雄异位和花部行为的适应意义。     

Pollen transfer patterns in a distylous plant with overlapping pollen-size distributions

In this study we quantify the relative rates and amounts of between-morph (legitimate) cross-pollination, within-morph (illegitimate) cross-pollination and self-pollination in a natural population of distylous Gaertnera vaginata (Rubiaceae) endemic to the oceanic island of La Réunion in the Indian Ocean. Since pollen size distributions of each morph overlap we develop a method, based on chi-squared comparisons of observed to expected pollen size distributions, to quantify pollen transfer proficiencies. This method, unlike previous methods, does not assume an equal chance of pollen transfer for pollen grains in the region of overlap. The probability that a single pollen grain from a short-styled plant undergoes legitimate pollen transfer to long-styled flowers (4.19×10⁻²) was more than twice that for illegitimate transfer to other short-styled plants (1.56×10⁻²). In contrast, the probability that a single pollen grain from a long-styled flower undergoes legitimate pollen transfer to short-styled flowers (1.65×10⁻²) was no greater than that of illegitimate pollen transfer to other long-styled plants (2.15×10⁻²). The transfer proficiencies of individual pollen grains are thus highly asymmetric. However, because long-styled flowers produce significantly more pollen, the ratio of legitimate to illegitimate pollen on the stigmas of short-styled flowers (1.85) markedly exceeded that for long-styled flowers (1.11). Short-styled flowers incurred significantly higher levels of self-pollination than long-styled plants, emphasizing the importance for self-pollination to be removed before comparing stigmatic pollen loads. The method we propose could be applied to other distylous species with pollen size overlap between morphs and thus allow for a more widespread study of the pollination ecology of heterostylous species.     

Pollen and stigma size changes during the transition from tristyly to distyly in Oxalis alpina (Oxalidaceae)

• Pollen and stigma size have the potential to influence male fitness of hermaphroditic plants, particularly in species presenting floral polymorphisms characterised by marked differences in these traits among floral morphs. In this study, we take advantage of the evolutionary transition from tristyly to distyly experienced by Oxalis alpina (Oxalidaceae), and examined whether modifications in the ancillary traits (pollen and stigma size) respond to allometric changes in other floral traits. Also, we tested whether these modifications are in accordance with what would be expected under the hypothesis that novel competitive scenarios (as in distylous‐derived reproductive system) exert morph‐ and whorl‐specific selective pressures to match the available stigmas.
• We measure pollen and stigma size in five populations of O. alpina representing the tristyly–distyly transition.
• A general reduction in pollen and stigma size occurred along the tristyly–distyly transition, and pollen size from the two anther levels within each morph converged to a similar size that was characterised by whorl‐specific changes (increases or decreases) in pollen size of different anthers in each floral type.
• Overall, results from this study show that the evolution of distyly in this species is characterised not only by changes in sexual organ position and flower size, but also by morph‐specific changes in pollen and stigma size. This evidence supports the importance of selection on pollen and stigma size, which increase fitness of remaining morphs following the evolution of distyly, and raises questions to explore on the functional value of pollen size in heterostylous systems under pollen competition.

     

Increased variation in sex organ positions can increase reciprocity and pollination success in heterostylous plant populations

The reciprocal position of sexual organs in complementary floral morphs is central to our understanding of heterostyly. Reciprocity indices are used to quantify the spatial match between complementary sex organs, but previous indices fail to appropriately account for intra-population variation in sex organ positions. In this study, we examine how an increase in intra-population variation in sex organ heights affects reciprocity and consequently reproductive success. We formulated a reciprocity index that incorporates this variation and asked if estimates of reciprocity can predict reproductive success in naturally occurring heterostylous populations. We developed a reciprocity index that assumed pollen transfer success equalled one for a perfectly matched stigma–anther pair, and decreased to zero with increasing mismatch. We examined the relationship between intra-population variation in organ position and reciprocity, compared previously proposed indices using simulated populations and empirical data from natural populations, and tested the ability of the indices to predict reproductive success. We observed that when differences between mean complementary sex-organ heights are small, increasing intra-population variation in heights resulted in a decrease in reciprocity. However, when this difference is larger, reciprocity increased, reached a peak, and then decreased with increasing variation. Previous indices failed to capture this behavior. Seed set was positively related to reciprocity for our index. These results challenge the current understanding that increasing variation in sex-organ heights will always decrease reciprocity in heterostylous populations. This may help explain why heterostylous systems exhibit and tolerate high amounts of intra-population variation in sex organ heights.     

Self-incompatibility and pollen limitation in the rare tristylous endemic Hugonia serrata on La R��union Island

Surveys of the incompatibility status of island floras have shown a deficit in taxa possessing homomorphic or heteromorphic incompatibility. This observation provides strong evidence for Baker??s Rule (1967) in that self-incompatibility can impede these taxa of colonizing and establishing on islands. The woody liana Hugonia serrata Lam. (Linaceae) is an extremely rare plant species endemic to the Mascarene Islands of La Réunion and Mauritius in the southern Indian Ocean. Previous research has documented the existence of tristyly in this species, thereby raising the number of known tristylous families to six. However, data were based on a very small sampling size and currently no data on the self-incompatibility system of the species are available. In this study, we investigated in detail the breeding system of H. serrata. Calculation of reciprocity and precision indices based on measurements of reproductive organs in a much larger sample size confirmed tristyly in this species. This style?Cstamen polymorphism was accompanied by several ancillary polymorphisms in stigma size, stigma color, anther size and pollen size. The rigidity of the heteromorphic incompatibility system was tested using experimental hand-pollinations. Significantly fewer pollen tubes arrived at the ovary after self- and illegitimate pollination (7%) than after legitimate pollination (12%). Seed set after self-pollination (3.6%) was also much lower than after legitimate pollination (66.7%). Comparison of seed set after legitimate pollination with natural seed set showed evidence for outcross pollen limitation. Our results indicate that despite severe pollen limitation and limited mate availability the heteromorphic incompatibility system in H. serrata is still strict after colonization of La Réunion Island. Because H. serrata is extremely long-lived, these results suggest that establishment of heterostylous species on islands may have been favored by their long life span.     

Darwin's cross-promotion hypothesis and the evolution of stylar polymorphism

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.     

Evolutionary analysis of a key floral trait in aquilegia canadensis (ranunculaceae): genetic variation in herkogamy and its effect on the mating system

The mating system of flowering plant populations evolves through selection on genetically based phenotypic variation in floral traits. The physical separation of anthers and stigmas within flowers (herkogamy) is expected to be an important target of selection to limit self-fertilization. We investigated the pattern of phenotypic and genetic variation in herkogamy and its effect of self-fertilization in a broad sample of natural populations of Aquilegia canadensis, a species that is highly selfing despite strong inbreeding depression. Within natural populations, plants exhibit substantial phenotypic variation in herkogamy caused primarily by variation in pistil length rather than stamen length. Compared to other floral traits, herkogamy is much more variable and a greater proportion of variation is distributed among rather than within individuals. We tested for a genetic component of this marked phenotypic variation by growing naturally pollinated seed families from five populations in a common greenhouse environment. For three populations, we detected a significant variation in herkogamy among families, and a positive regression between parental herkogamy measured in the field and progeny herkogamy in the greenhouse, suggesting that there is often genetic variation in herkogamy within natural populations. We estimated levels of self-fertilization for groups of flowers that differed in herkogamy and show that, as expected, herkogamy was associated with reduced selfing in 13 of 19 populations. In six of these populations, we performed floral emasculations to show that this decrease in selfing is due to decreased autogamy (within-flower selfing), the mode of selfing that herkogamy should most directly influence. Taken together, these results suggest that increased herkogamy should be selected to reduce the production of low-quality selfed seed. The combination of high selfing and substantial genetic variation for herkogamy in A. canadensis is enigmatic, and reconciling this observation will require a more integrated analysis of how herkogamy influences not only self-fertilization, but also patterns of outcross pollen import and export.