Pollinator shifts and the loss of style polymorphism in Narcissus papyraceus (Amaryllidaceae)

Darwin proposed that the driving force for the evolution of style polymorphisms is the promotion of cross‐pollination between style morphs, through accurate placement of pollen on the pollinator’s body. This hypothesis has received much attention, but the effect of different pollinators in the fitness of morphs remains poorly understood. Narcissus papyraceus is a style dimorphic species (long ‐L‐ and short ‐S‐ styled) with isoplethic (1 : 1) and L‐monomorphic populations, mainly visited by long‐tongued (LT) nocturnal and short‐tongued (ST) diurnal pollinators, respectively. We studied natural female fertility of morphs, and assessed the role of diurnal and nocturnal pollinators. We also quantified female fertility of the morphs in experimental populations with different morph ratio, exposed to predominately long‐ or short‐tongued pollinators. We found that with LT pollinators, both morphs were successfully pollinated in all morph ratio conditions, suggesting that these insects could be involved in maintenance of the polymorphism, although other factors may also play a role. However, with ST pollinators, S‐plants displayed less fertility than L‐plants, and mating among L‐plants was favoured, implying that the polymorphism is lost. These results underscore the role of pollinators on variations in style polymorphism.     

High herkogamy but low reciprocity characterizes isoplethic populations of Jasminum malabaricum,a species with stigma‐height dimorphism

• Studies of floral polymorphisms have focused on heterostyly, while stigma‐height dimorphism has received considerably less attention. Few studies have examined the reproductive biology of species with stigma‐height dimorphism to understand how factors influencing mate availability and pollen transfer are related to morph ratios in populations.
• Floral morphological traits, especially herkogamy and reciprocity, pollinator visitation, breeding system and spatiotemporal mate availability, are known to affect inter‐morph pollination and morph ratios in species with stigma‐height dimorphism. In this study, we investigated the presence of stigma‐height dimorphism and estimated morph ratios in four naturally occurring populations of Jasminum malabaricum. We quantified morph‐ and population‐specific differences in the abovementioned factors in these populations to understand the observed morph ratios.
• The positions of anthers and stigmas were characteristic of stigma‐height dimorphism, the first report of this polymorphism in the genus. All study populations were isoplethic, implying equal fitness of both morphs. Herkogamy was higher in the short‐styled morph, while reciprocity was higher between the long‐styled stigma and short‐styled anthers. Long‐ and short‐tongued pollinators were common floral visitors, and we observed no differences between morphs in spatiotemporal mate availability or pollinator visitation. Neither morph exhibited self‐ or heteromorphic incompatibility.
• The short‐styled stigma had lower reciprocity but likely receives sufficient inter‐morph pollen from long‐tongued pollinators, and also by avoiding self‐pollination due to higher herkogamy. These results highlight the importance of sufficient effective pollinators and floral morphological features, particularly herkogamy, in maintaining isoplethy in species with stigma‐height dimorphism.

     

CONTRIBUTION OF CRYPTIC INCOMPATIBILITY TO THE MATING SYSTEM OF EICHHORNIA PANICULA TA (PONTEDERIACEAE)

Tristylous populations of the annual aquatic Eichhornia paniculata have high levels of outcrossing and intermorph mating despite being fully self- and intramorph compatible. Experimental studies of pollen germination, ???pollen-tube growth, and pollinations with mixtures of genetically marked pollen were used to determine whether postpollination processes contribute to the observed mating patterns. Differences in pollen germination were small and did not contribute to differences in pollen siring ability. The fraction of pollen tubes first entering the ovary, however, was greater for legitimate outcross pollen than for either of the other two pollen types (self or outcross illegitimate pollen) in all recipient morphs. Moreover, legitimate pollen had higher siring success when in competition with illegitimate pollen types (self or outcross) in each recipient style morph. The ranking of pollen performance for different pollen-style combinations was the same for both the pollen-tube growth and marker-gene experiments indicating that differences in pollen-tube growth rate are the principal cause of differences in pollen siring ability. Cryptic incompatibility in E. paniculata may represent a weak heteromorphic incompatibility system because the observed patterns of pollen-tube growth parallel pollen-tube growth and seed-set patterns that occur in related species with strong trimorphic incompatibility. The ability to produce mostly outcrossed progeny when pollinators are abundant, but to reliably produce seed under a variety of environmental and demographic conditions may be favored in E. paniculata because of its colonizing life history and occurrence in ephemeral habitats. Cryptic incompatibility may be more likely to occur in species subject to wide fluctuations in population size and levels of pollinator service.     

Evolutionary maintenance of stigma-height dimorphism in Narcissus papyraceus (Amaryllidaceae)

Stigma-height dimorphism is a sexual polymorphism in which plant populations are composed of two floral morphs that differ significantly in style length but not anther position. The morphs exhibit approach and reverse herkogamy, floral designs that in most species typically occur as monomorphic conditions. We investigated the floral biology of stigma-height dimorphism in the Mediterranean geophyte Narcissus papyraceus (Amaryllidaceae) in an effort to understand the evolutionary forces maintaining stylar polymorphism. Our survey of 66 populations in Spain, Portugal, and Morocco indicated that 56% were dimorphic with the long-styled morph at an average frequency of 0.79. The remaining 44% of populations sampled were monomorphic for the long-styled morph. In dimorphic populations there was a significant positive relation between population size and the frequency of the short-styled morph. Controlled pollinations demonstrated that N. papyraceus is self-sterile with no significant differences in female fertility between intra- and intermorph crosses. Prior self-pollination reduced seed set in flowers that were subsequently cross-pollinated. Estimates of mating patterns using allozyme markers in eight populations indicated that N. papyraceus is largely outcrossing (mean t(m) = 0.81) with no significant differences between monomorphic and dimorphic populations or style morphs. Stigma-height dimorphism in N. papyraceus is maintained in populations by insect-mediated cross-pollination with biased morph ratios and stylar monomorphism likely resulting from the combined influence of the inheritance of the polymorphism, morph-specific differences in assortative mating and founder effects.     

Mating patterns and pollen dispersal in a heterodichogamous tree, Juglans mandshurica (Juglandaceae)

Mating patterns in heterodichogamous species are generally considered to be disassortative between flowering morphs, but this hypothesis has hitherto not been vigorously tested. Here, mating patterns and pollen dispersal were studied in Juglans mandshurica, a heterodichogamous wind-pollinated species that is widely distributed in northern and north-eastern China. Paternity analyses carried out on 11 microsatellite loci were used to estimate morph-specific rates of outcrossing and disassortative mating. Pollen dispersal and genetic structure were also investigated in the population under study. The mating pattern of J. mandshurica was highly outcrossing and disassortative. Pairwise values of intramorph relatedness were much higher than those of intermorph relatedness, and a low level of biparental inbreeding was detected. There was no significant difference in outcrossing and disassortative mating rates between the two morphs. The effective pollen dispersal distribution showed an excess of near-neighbor matings, and most offspring of individual trees were sired by one or two nearby trees. These results corroborate the previous suggestion that mating in heterodichogamous plant species is mainly disassortative between morphs, which not only prevents selfing but also effectively reduces intramorph inbreeding.     

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.     

Floral morph composition and pollen limitation in the seed set of <Emphasis Type="Italic">Nymphoides indica</Emphasis> populations

We investigated the floral morph composition and seed set of the populations in a heterostylous aquatic plant, Nymphoides indica O. Kuntze (Menyanthaceae). The bias of floral morph ratio in the populations was negatively correlated with the seed set. In the populations with biased floral morph ratio, the pollen on the stigma of the common morphs were dominated by pollen of the same morph, resulting in reduced seed set. The addition of pollen of the opposite morph on the stigmas of the common morph resulted in an increase of fruit and seed set rates. The intermorph pollination rate and the fruit and seed set rates of Nymphoides indica were highest when two floral morphs were close in the populations. Various insect species including Lepidoptera, Diptera and Hymenoptera visited the flowers. However, geitonogamous pollination was common because of the short flying distance of the pollinators. These results indicate that the seed set of Nymphoides indica is limited by the shortage of compatible pollens and that the success of intermorph pollination is influenced by spatial distribution of the two floral morphs. As Nymphoides indica is endangered, conservation of the populations with high fecundity is proposed by us.     

INCOMPATIBILITY IN AMSINCKIA GRANDIFLORA (BORAGINACEAE): DISTRIBUTION OF CALLOSE PLUGS AND POLLEN TUBES FOLLOWING INTER- AND INTRAMORPH CROSSES

The distribution of callose plugs and pollen tubes was investigated following inter- and intramorph crosses of Amsinckia grandiflora (Boraginaceae), a distylous species possessing cryptic self-incompatibility. Callose plug distribution provided a good indication of the distribution of pollen tubes. Compared to intramorph crosses, many more callose plugs and pollen tubes were found in basal stylar regions following intermorph crosses, indicating that differential pollen tube growth is a likely cause of cryptic self-incompatibility. The incompatibility response differed for the floral morphs: in the pin (long-styled) morph pollen tubes were most likely to cease growth in the midstylar region, while inhibition was more likely to occur in the upper stylar region of the thrum (short-styled) morph. There was no evidence of stigmatic inhibition of pollen tubes for either morph, although the incompatibility response in the Boraginaceae is normally located in the stigmatic region.     

异型花柱植物喀什补血草的传粉者功能群与花粉转移效率

异型花柱是受遗传控制的花柱多态现象, 被达尔文认为是植物通过在传粉者体表不同部位滞落花粉以促进型间花粉准确传递的一种适应。该现象虽已受到广泛关注, 但在一些花型变异较大且不稳定的传粉系统中, 不同传粉者对各花型繁殖所产生的影响仍知之甚少。该研究以分布于新疆天山南坡的一个有同长花柱共存的异型花柱植物喀什补血草(Limonium kaschgaricum)种群为研究对象, 对其花型构成及频率、传粉者及花粉转移效率等进行了调查分析。结果表明: 1)种群中除了存在雌/雄蕊长度交互对应的长(L)/短(S)花柱型花外, 还有雌/雄蕊同长的花(H型), 且各花型花的花冠口直径、花冠筒长及花粉量等参数间无差异, 但花粉纹饰和柱头乳突细胞形态具二型性。其中, H型花的花粉和柱头形态与L型花(或S型花)的一致。2)花型内和自花授粉均不亲和; 型间授粉时, 花粉和柱头形态不同的花型间亲和, 反之不亲和。3)种群内存在长/短吻两类传粉昆虫。在以短吻传粉者为主的盛花初、中期, L和H型花柱头上的异型花粉数均显著高于S型花的, 且L和S型花高位性器官间的异型花粉传递效率高于低位性器官间的; 而在以长吻传粉者为主的盛花后期, L和S型花的柱头间异型花粉数无显著差异, 且高/低位性器官间具有相同的异型花粉转移效率; 与传粉者出现时期相对应的、在花期不同阶段开放花的结实率也明显不同。4)长/短吻昆虫具明显不同的传粉功能, 短吻昆虫只能对L和H型花进行有效传粉, 且访花频率和型间花粉转移效率较低, 为低效传粉者; 而长吻昆虫对各花型均能有效传粉, 具高的访花频率和型间花粉转移效率, 为高效传粉者。因为长吻昆虫的阶段性出现所形成的不稳定传粉系统, 使低效的短吻昆虫可能会成为种群中花型变异的驱动力, 并使S型花受到更大的选择压力。H型花克服了柱头缩入的弊端, 可能会成为不稳定传粉系统下的一个替代花型而持续存在。     

Disassortative mating, sexual specialization, and the evolution of gender dimorphism in heterodichogamous Acer opalus

In sexually polymorphic species, the morphs are maintained by frequency-dependent selection through disassortative mating. In heterodichogamous populations in which disassortative mating occurs between the protandrous and protogynous morphs, a decrease in female fitness in one morph is hypothesized to drive sexual specialization in the other morph, resulting in dimorphic populations. We test these ideas in a population of the heterodichogamous species, Acer opalus . We assessed both prospective gender of individuals in terms of their allocations and actual parentage using microsatellites; we found that most matings in A. opalus occur disassortatively. We demonstrate that the protogynous morph is maintained by frequency-dependent selection, but that maintenance of males versus protandrous individuals depends on their relative siring success, which changes yearly. Seeds produced later in the reproductive season were smaller than those produced earlier; this should compromise reproduction through ovules in protandrous individuals, rendering them male biased in gender. Time-dependent gender and paternity analyses indicate that the sexual morphs are specialized in their earlier sexual functions, mediated by the seasonal decrease in seed size. Our results confirm that mating patterns are context-dependent and change seasonally, suggesting that sexual specialization can be driven by seasonal effects on fitness gained through one of the two sexual functions.     

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.     

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.     

连翘花柱二型的繁育系统特征研究

花柱二型是植物雌雄蕊交互异位的一种花多态现象。该研究以花柱二型植物连翘(Forsythia suspensa)为材料,通过检测其交配系统特征,比较两花型的胚珠产量、花粉质量及活力特性,以探讨花柱二型连翘两花型的功能性别分化及其进化意义。结果表明:连翘自交和型内异交坐果率均极低(<12.12%),主要以型间交配为主。长柱型个体的单花胚珠数量为(26.86±0.37)个,显著高于短柱型花,单花花粉干重为(0.47±0.02)mg,显著低于短柱型的(0.60±0.02)mg;短柱型连翘的花药含水量为(75.97±0.82)%,显著高于长柱型的(73.18±0.61)%。长柱型个体的花粉活力总体高于短柱型,其花粉寿命也更长。研究表明,连翘两花型的雌雄性繁殖投入存在显著分异,长柱型个体更易于接受花粉,通过种子实现其适合度,表现出偏雌的功能性别;短柱型个体更易于通过花粉实现其适合度,表现为偏雄的功能性别。     

The role of short-tongued insects in floral variation across the range of a style-dimorphic plant

Background and Aims

Heterostyly and related style polymorphisms are suitable model systems to evaluate the importance of functional pollinators in the maintenance of population variability. In Narcissus papyraceus different functional pollinators, incompatibility system and flower morphology have been proposed to influence the maintenance of polymorphism through their effect on disassortative mating. Here a test is done to find out if the visitation rate of long- versus short-tongued pollinators correlates with the morph ratio and if the latter is related to other flower traits of the species across its main geographic range.

Methods

Floral traits from 34 populations in the south-west of the Iberian Peninsula and in north-west Africa were measured, perianth variation was described and a comparison was made of allometric relationships between sex organs and floral tube. Correlations between pollinator guilds, stigma–anther separation of reciprocal morphs (our proxy for disassortative mating) and morph-ratio variation were analysed. Finally, the incompatibility system of the species in the northern and southern borders of its distribution are described.

Key Results

Flowers from southern populations were significantly larger than flowers from centre and northern populations. The abundance of short-styled plants decreased gradually with increasing distance from the core region (the Strait of Gibraltar), with these disappearing only in the northern range. Although there was a significant difference in stigma–anther separation among populations, morph ratio was not associated with reciprocity or floral tube length. Long-style morph frequency increased with short-tongued pollinator visitation rate. Populations from both edges of the distribution range were self-incompatible and within- and between-morph compatible.

Conclusions

The style morph ratio changed gradually, whereas perianth trait variation showed abrupt changes with two morphotypes across the range. The positive relationship between the visitation rate of short-tongued pollinators and the decrease of the short-style morph supports our initial hypothesis. The results highlight the importance of different pollinators in determining the presence of style polymorphism.     

茜草科二型花柱植物滇丁香传粉精确性研究

异型花柱是一种受遗传因素控制的花型多态性现象,其适应意义在于提高不同花型间传粉精确性,从而促进异交。为检验二型花柱植物滇丁香（Luculia pinceana）是如何促进异交并确定有效传粉者的类型,实验统计自然种群中长、短柱型的植株数量,测量花部特征,统计自然种群植株柱头上的型间花粉落置的数量和比例,比较型内、型间花粉的萌发率以及花粉管长度,比较不同人工授粉处理结实情况,观察传粉昆虫类型并测量虫体特征。结果表明（1）滇丁香自然居群中长、短柱型植株的数量没有显著性差异。长柱型的筒深、筒直径、开口直径、雄蕊长和柱头长均显著小于短柱型,而长柱型的叶片长和宽、雌蕊长、花药体积、花粉粒体积、柱头厚均显著大于短柱型。（2）自然种群植株柱头上型间花粉所占比例显著低于型内花粉的比例。（3）型间授粉的花粉萌发率以及花粉管长度均大于型内授粉和混合授粉。（4）型间授粉的结实率显著高于型内授粉的结实率,型内授粉具有一定的结实。（5）夜行性长喙条背天蛾（Cechenena lineosa）是滇丁香的有效传粉者,其吻长和滇丁香的花筒深相适应。本研究表明二型花柱植物滇丁香主要是通过与其花部特征相匹配的长喙天蛾实现有效的传粉,通过促进型间花粉的萌发和花粉管的伸长而有利于型间授粉结实,提高传粉精确性。     

Phenological match drives pollen‐mediated gene flow in a temporally dimorphic tree

• Variation in flowering phenology is common in natural populations, and is expected to be, together with inter‐mate distance, an important driver of effective pollen dispersal. In populations composed of plants with temporally separated sexual phases (i.e. dichogamous or heterodichogamous populations), pollen‐mediated gene flow is assumed to reflect phenological overlap between complementary sexual phases. In this study, we conducted paternity analyses to test this hypothesis in the temporally dimorphic tree Acer opalus.
• We performed spatially explicit analyses based on categorical and fractional paternity assignment, and included tree size, pair‐wise genetic relatedness and morph type as additional predictors. Because differences between morphs in flowering phenology may also influence pollination distances, we modelled separate pollen dispersal kernels for the two morphs.
• Extended phenological overlap between male and female phases (mainly associated with inter‐morph crosses) resulted in higher siring success after accounting for the effects of genetic relatedness, morph type and tree size, while reduced phenological overlap (mainly associated with intra‐morph crosses) resulted in longer pollination distances achieved. Siring success also increased in larger trees.
• Mating patterns could not be predicted by phenology alone. However, as heterogeneity in flowering phenology was the single morph‐specific predictor of siring success, it is expected to be key in maintaining the temporal dimorphism in A. opalus, by promoting not only a prevalent pattern of inter‐morph mating, but also long‐distance pollination resulting from intra‐morph mating events.

     

花柱卷曲性的维持及功能性别特化

花柱多态型的型比通过选型交配形成的负频率依赖的选择来维持。种群空间格局对于频率依赖选择所维持的花柱多态型是至关重要的。该研究使用摆放实验构建不同型比的马来良姜(Alpinia mutica)人工种群, 以评价花柱卷曲性促进选型交配的功能。使用邻域模型调查了云南草蔻(Alpinia blepharaocalyx)种群中的目标植株的坐果数、结籽数及其邻域中花序的数量和型偏以探讨在局域尺度上目标植株的雌性适合度是否受负频率依赖的选择。通过比较云南草蔻上举型和下垂型各繁殖性状的差异以及两型的功能性别的计算以评估两型功能性别可能存在的特化趋势。摆放实验结果说明花柱卷曲性形成的二型种群可以促进型间花粉传递。在局域尺度上, 云南草蔻两型倾向于同型聚集分布, 这一分布特点反映出其克隆生长特性。统计结果说明: 邻域中异型花粉供体相对丰富度对两型目标植株都无影响; 目标植株雌性适合度在邻域中不受负频率依赖选择的影响。这一结果并不支持负频率依赖的选择在该研究尺度上是影响种群型比结构的主要机制。云南草蔻两型个体在花期和果期的繁殖指标上不存在显著差异; 功能性别计算结果显示选型交配是种群内唯一的交配模式时, 下垂型功能性别偏雌性而上举型功能性别偏雄性。这一功能性别特化的趋势可能是通过对上举型的雄性功能的选择而造成的, 并可能与种群型结构相关。当功能性别计算只考虑自交而忽略近交衰退时, 种群功能性别偏向雌性。     

Pollen competition between morphs in a pollen‐color dimorphic herb and the loss of phenotypic polymorphism within populations

Flower color polymorphism is relatively uncommon in natural flowering plants, suggesting that maintenance of different color morphs within populations is difficult. To address the selective mechanisms shaping pollen‐color dimorphism, pollinator preferences and reproductive performance were studied over three years in Epimedium pubescens in which some populations had plants with either green or yellow pollen (and anthers). Visitation rate and pollen removal and receipt by the bee pollinator (Andrena emeishanica) did not differ between the two color morphs. Compared to the green morph, siring success of the yellow morph's pollen was lower, but that of mixtures of pollen from green and yellow morphs was lowest. This difference, corresponding to in vivo and ex vivo experiments on pollen performance, indicated that pollen germination, rather than tube growth, of the green morph was higher than that of the yellow morph and was seriously constrained in both morphs if a pollen competitor was present. A rare green morph may invade a yellow‐morph population, but the coexistence of pollen color variants is complicated by the reduced siring success of mixed pollinations. Potential pollen competition between morphs may have discouraged the maintenance of multiple phenotypes within populations, a cryptic mechanism of competitive exclusion.     

THE ROLE OF INBREEDING DEPRESSION AND MATING SYSTEM IN THE EVOLUTION OF HETEROSTYLY

We investigated the role of morph‐based differences in the expression of inbreeding depression in loss of the mid‐styled morph from populations of tristylous Oxalis alpina. The extent of self‐compatibility (SC) of reproductive morphs, the degree of self‐fertilization, and the magnitude of inbreeding depression were investigated in three populations of O. alpina differing in their tristylous incompatibility relationships. All three populations exhibited significant inbreeding depression. In two populations with highly modified tristylous incompatibility, manifested as increased reciprocal compatibility between short‐ and long‐styled morphs, substantial SC and self‐fertilization of mid‐styled morphs were detected, and expected to result in expression of inbreeding depression in the progeny of mid‐styled morphs in the natural populations. In contrast, significant self‐fertility of the mid‐styled morph was absent from the population with typical tristylous incompatibility, and no self‐fertilization could be detected. Although self‐fertilization and expression of inbreeding depression should result in selection against the mid‐styled morph in the later stages of the transition from tristyly to distyly, in O. alpina selection against the mid‐styled morph in the early phases of the evolution of distyly is likely due to genic selection against mid‐alleles associated with modified tristylous incompatibility, rather than expression of inbreeding depression.     

EXPERIMENTAL STUDIES ON THE FUNCTIONAL SIGNIFICANCE OF HETEROSTYLY

Heterostyly has been viewed as both an antiselfing device and a mechanism that increases the proficiency of pollen transfer between plants. We used experimental manipulation of the morph structure of garden populations of self-compatible, tristylous Eichhornia paniculata to investigate the function of floral polymorphism. Outcrossing rates (t), levels of intermorph mating (d), and morph-specific male and female reproductive success were compared in replicate trimorphic and monomorphic populations. In trimorphic populations, t and d averaged 0.81 (2 SE = 0.03) and 0.77 (2 SE = 0.03) respectively, with no difference in either parameter among morphs. Ninety-five percent of outcrossed seeds were therefore the result of intermorph fertilizations. Male reproductive success of the long-styled morph was low, especially in comparison with plants of the short-styled morph. Outcrossing rates for each morph were higher in trimorphic than monomorphic populations where t averaged 0.71 (2 SE = 0.01), 0.30 (2 SE = 0.04) and 0.43 (2 SE = 0.1) for the long-, mid-, and short-styled morphs, respectively. Seed set was lower in monomorphic populations, particularly those composed of the L morph, reflecting reduced pollen deposition. Floral polymorphism therefore increased both outcrossing rate and fecundity but the magnitude of the differences varied among morphs. If the ancestral condition in heterostylous groups resembled the L morph, as has been suggested, data from this study suggests that the selective basis for the establishment of floral polymorphism could have been increased pollen transfer rather than higher levels of outcrossing.