Disruption of the distylous syndrome in Primula veris

Background and Aims Distyly is a floral polymorphism characterized by the presence of two discrete morphs with reciprocal positioning of anthers and stigmas in flowers on different plants within the same population. Although reciprocal herkogamy and associated floral traits are generally thought to be discrete and strict polymorphisms, little is known about variation in floral traits related to the distylous syndrome within and among populations of a single species. In this study, variation in floral morphology and reciprocal positioning of the sexual organs in the distylous Primula veris (cowslip) is quantified.Methods Data were collected in ten populations occurring in two contrasting habitat types (grasslands and forests), and for each population the average level of reciprocity was assessed, the strength of the self-incompatibility system was determined, and seed production under natural conditions was quantified.Results In grassland populations, flowers showed clear distyly with low and symmetric reciprocity indices at both the lower and upper level. In forests, P. veris produced larger flowers that showed strong deviations in stigma–anther separation, especially in the L-morph. This deviation was mainly driven by variation in stigma height, resulting in high and asymmetric reciprocity indices and the occurrence of several short-styled homostylous plants. Self-incompatibility was, however, strict in both habitats, and morph ratios did not differ significantly from isoplethy. The observed shift in reciprocity in forest populations was associated with a significant reduction in seed production in the L-morph.Conclusions The results indicate that populations of P. veris show habitat-specific variation in flower morphology. Deviations from perfect reciprocal positioning of stigmas and anthers also translate into reduced seed production, suggesting that small changes in sexual organ reciprocity can have far-reaching ecological and evolutionary implications.     

Pollen donation patterns in a tropical distylous shrub (PSYCHOTRIA SUERRENSIS; RUBIACEAE)

Selection on male function has been invoked to explain various floral features, including number of flowers, flower size, and flower color. Here I describe two experiments designed to examine the efficiency of distyly in promoting male floral function, as measured by successful pollen dispersal to stigmas. In both experiments, I performed emasculations to control the type of pollen locally available in a natural population of Psychotria suerrensis, a tropical shrub. In the “natural-recipients” experiment, I allowed each floral morph to donate pollen on alternate days to emasculated flowers of each morph. In the “paired-recipients” experiment, I attached paired cuttings of each morph to individual donor plants. The results of both methods were consistent. Pollen borne on low anthers (from pin plants) was transferred most efficiently to low stigmas (on thrum plants). Pollen borne on high anthers (from thrum plants) was dispersed in equal amounts to flowers of both morphs. The results suggest that distyly is only partially effective in achieving efficient pollen donation. Male function of pins is enhanced by the polymorphism, but male function of thrums is not. A supplemental pollination experiment illustrates that seed set in this species is predominantly pollen limited, reducing the importance of male function, in comparison with species where seed set is primarily resource limited.     

HETEROSTYLY,POPULATION COMPOSITION,AND POLLEN FLOW IN HEDYOTIS CAERULEA

Hedyotis caerulea is a perennial, spring-flowering herb native to eastern North America with distylous flowers that differ in a number of morphological and physiological traits. However, the pin and thrum morphs produce the same numbers of buds, flowers, and fruits per plant, although it is possible that differences in these may occur in some populations at certain times of the flowering period. The two morphs are self-incompatible and cross-compatible. Most populations contained an excess of pins over thrums (anisoplethy); less commonly pins and thrums were equally represented (isoplethy). Populations change from anisoplethy to isoplethy and in the reverse direction. The spatial distribution of pin and thrum flowers in populations was random in some populations but non-random in others. There is some indication that the two morphs in some populations have somewhat different flowering periods. Pollinators seem to be chiefly bombyliid flies and perhaps thrips, but insects were rarely observed visiting flowers. In some populations, the two morphs produced equal numbers of pollen grains per flower; in others they did not. The average pollen viability varied, but on the average there was a moderate level of pollen sterility. High numbers of pollen grains remained in dehisced anthers, probably as a result of low pollinator activity. However, between 5% and 9% of the pollen produced participated in pollination. Stigmas of most pin flowers received more pin pollen grains than thrum pollen grains; on stigmas of thrum flowers pin pollen grains outnumbered thrum pollen grains. Thus, compatible pollen flow from pins to thrums was greater than in the reverse direction. Anisoplethic and isoplethic populations had the same pollen flow patterns. A plant-by-plant examination of stigmas indicated that many stigmas bore few or no pollen grains. Seed production of the two morphs was equal. Despite the inequities in pollen flow patterns, the widespread and occasionally weedy nature of H. caerulea suggests that its breeding system must be viewed as a successful one.     

Morph-specific patterns of variation in stigma height in natural populations of distylous Jasminum fruticans

The purpose of this study was to describe morph-specific patterns of stigma–anther separation and style curling in the distylous Jasminum fruticans . We also examined whether variation in floral traits is correlated with variation in seed production. Stigma–anther separation is more variable in short-styled plants than in long-styled plants. In all populations studied, some plants bear flowers that lack any stigma–anther separation. These plants have the pollen characteristics (size and number) and compatibility relations of short-styled plants. Comparison with other distylous species illustrates that the variability of stigma–anther separation in short-styled plants is a novel finding for a distylous species. Long-styled plants have greater stigma–anther separation than short-styled plants, styles are often curled and protrude from the corolla, and anthers are placed well within the corolla tube. The frequency of long-styled plants with curled styles and the mean degree of style curling were significantly correlated with style length. Short-styled plants have larger corollas than long-styled plants and never have curled styles. Morph ratios are always 50:50 in natural populations. Mean values of each floral trait in the two morphs were significantly correlated among populations. There were no consistent differences in fecundity of the two morphs nor any correlation between floral traits and seed set for each morph in natural populations. We discuss the potential causes and significance of the two morph-specific patterns we describe; reduced stigma–anther separation in short-styled plants and the presence of curled styles in long-styled plants.     

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.

     

FLORAL VARIATION AND DISTYLY IN GUETTARDA SCABRA (RUBIACEAE)

Controversy exists as to whether the tropical shrub Guettarda scabra (Rubiaceae) is distylous. Variations in stigma and anther position and floral morphology of G. scabra were studied in a population in south Florida. Stigma and anther height have unimodal distributions, but stigma-anther separation is bimodally distributed and can be used to identify a long-styled and a short-styled morph. Stigma width varies between morphs, but anther length, pollen diameter, and stigma papillae length do not. The morphs occur in a 1:1 ratio in the two populations studied. G. scabra is self-compatible and can pollinate itself. Styles of the two morphs have similar relative growth rates in early development. Stylar growth is inhibited in the short-styled morph when buds are approximately 12 mm long. Anther height differs between morphs because of different relative growth rates and because the long-styled morph corolla tube, where the anthers are attached, stops growth before the tube of the short-styled morph. Reciprocity between morphs for average stigma and anther height falls within the range of reciprocity found in other distylous Rubiaceae. Thus G. scabra is morphologically distylous but unusual among distylous species in the variation within morphs and overlap between morphs in stigma and anther heights.     

二型花柱植物海仙花报春花部性状随地理梯度的变异

植物的花部性状在异质环境中表现出不均一的适应性进化, 其自然变异可能在时空格局上呈现一定的规律性。选择同一物种的不同地理居群进行花部表型变异分析, 能揭示花部性状随地理梯度的变异模式。海仙花报春(Primula poissonii)属于典型的二型花柱植物, 依赖昆虫传粉实现严格的型间异交。该物种广布于横断山地区亚高山-高山草甸, 其分布海拔跨度大且花部性状在种内具有较高变异, 但这些变异在不同地理梯度(海拔梯度和经纬度梯度)的特定选择因子作用下的变化规律尚不清楚。本研究选择海仙花报春16个居群, 对8个花部关键性状和二型花柱繁殖器官的互补度与海拔和经纬度的关系进行研究, 探究花部性状随地理梯度变异的模式及其潜在的选择因素。研究表明, 海仙花报春两种花型的花冠管开口大小、花药高度以及短柱花柱头高度与海拔均呈正相关, 但两种花型的花冠大小, 长柱花的花瓣长度、柱头到花冠管开口的距离, 以及短柱花的花冠管长度与海拔高度间均呈负相关, 其余性状与海拔无显著相关性。除短柱花中柱头高度以外的性状均随着纬度升高而逐渐减小。长柱花中除花药和柱头间的距离以及柱头到开口的距离外, 其余性状均随着经度的增加而减小; 短柱花的花瓣长度、花药高度以及花药和柱头间的距离随着经度的增加而变大, 其余性状均随着经度的增加而减小。繁殖器官间的互补度并不随地理环境的变化而变化。花部性状的地理变异可能受访花昆虫组成的地理变化驱动。繁殖器官间互补程度的高度保守表明非选型交配在居群二态性的维持和稳定过程中起关键作用。本研究为进一步深入开展报春花属(Primula)花部性状及其选择压力的地理变异研究奠定了基础。     

异质生境对二型花柱植物雪地报春花性状分化的影响

该研究以生长在新疆北部的二型花柱植物雪地报春(Primula nivalis)为研究材料,对7个低海拔(1 657~2 013m)森林居群和7个高海拔(2 400~2 700m)草原居群的花型比例进行调查,对花部特征进行检测,并对长、短花柱植株在不同海拔居群的花性状分化进行了比较研究,以揭示该类群植物的演化与环境的关系。结果表明:(1)在高海拔草原居群中长花柱植株比例高于短花柱植株。(2)比较草原和森林生态系统中的雪地报春居群,长花柱花形态特征间存在显著差异,但短花柱花形态特征间的差异不显著。(3)高海拔草原居群中长花柱花的花冠筒长度、雄蕊长度、雌雄蕊距离、雌雄异位程度以及雌雄蕊空间位置是导致花性状分化的主要因素。(4)在草原居群中,长花柱花的雄蕊、雌雄异位程度及花冠口至雄蕊距离的变化减少了长短花柱花间的互补式对应性。研究认为,在异质生境中长花柱花性状的变化是导致该物种长短花柱花性状分化的主要因素。     

Asymmetry in pollen flow promotes gender specialization in morphs of the distylous neotropical herb <Emphasis Type="Italic">Arcytophyllum lavarum</Emphasis> (Rubiaceae)

Distyly is a floral polymorphism, characterized by a reciprocal positioning between stigmas and anthers in different flowers, where two floral morphs, long-styled (pin) and short-styled (thrum) occur within the population. Distyly is suggested as one of the routes leading to the evolution of separate sexes in plants. In this evolutionary pathway, pollinators may disrupt the complementarity of pollen transfer between morphs. Consequently, the floral morphs gradually specialize as either male or female. A key process required for gender specialization in distylous plants is a deviation of the realized functional gender (i.e. the proportion of genes transmitted to the next generation via pollen donation and seed production) from the potential functional gender (i.e. the expected contribution of male and female function to reproductive success from the number of ovules or pollen grains produced by each morph). I selected the distylous herb Arcytophyllum lavarum (Rubiaceae) to determine if asymmetry in pollen flow promotes differences in seed production, pollen donation and a discrepancy between the potential and the realized functional genders in pin and thrum floral morphs. Pollen flow in A. lavarum is asymmetric and the pin morph is more efficient at performing cross-pollination than the thrum morph. Conversely, the thrum morph produced two times more seeds than the pin morph. Male and female contributions to the potential functional gender were equivalent in both morphs. However, the pin morph transmitted more genes through pollen donation and the thrum morph more through seed production than expected from their potential functional genders. These results support the hypothesis that if pollinators consistently promote asymmetric pollen flow between morphs over generations, it is possible that gender specialization may evolve to the extreme of dioecism from an original distylous condition.     

Stylar polymorphism, reciprocity and incompatibility systems in Nymphoides montana (Menyanthaceae) endemic to southeastern Australia

Heterostyly is a stylar polymorphism that has been shaped by the evolution of floral characters adapted for efficient pollen transfer. Different types of stylar polymorphism are described in which the discrete characterization of the exact polymorphic type (e.g., distyly vs. stigma-height dimorphism) requires detailed floral measurements (e.g., sex-organ reciprocity). In clonal and aquatic Nymphoides montana, although the presence of two floral morphs that contain styles of two lengths has been previously reported, no studies have quantitatively estimated the level of reciprocity and/or described the stylar condition. Morphological variations and incompatibility relationships were explored between the two morphs in three southeastern Australian populations. In this study, one population is characterized as stigma-height dimorphism (i.e., two morphs with discrete variation in stigma height but little variation in anther height), whereas the other two populations are typical distylous (i.e., two morphs for reciprocal stigma and anther height). Nymphoides montana is dimorphic in a wide range of ancillary characters, including corolla size, stigma size, shape and papillae morphology, and pollen size, number and exine sculpture. Following glasshouse pollinations, full incompatibility systems were observed in the distylous populations, whereas the stigma-height dimorphic population showed between-morph variation in the extent of incompatibility. Despite the variation in sex-organ reciprocity and incompatibility, other lines of evidence appear to assure the maintenance of the stylar polymorphism in the N. montana study populations. All populations are nearly isoplethic (i.e., both morphs in equal frequencies), which is indicative of balanced polymorphisms that appear to be maintained by legitimate pollen transfer between the morphs.     

Asymmetrical mating patterns and the evolution of biased style-morph ratios in a tristylous daffodil.

Non-random mating in plant populations can be influenced by numerous reproductive and demographic factors, including floral morphology and inter-plant distance. Here, we investigate patterns of outcrossed mating through male function in Narcissus triandrus, a tristylous, bee-pollinated wild daffodil from the Iberian Peninsula, to test pollen transfer models which predict that floral morphology promotes asymmetrical mating and biased morph ratios. Unlike other tristylous species, N. triandrus has an incompatibility system that permits intra-morph mating and long-level rather than mid-level stamens in the L-morph. Incomplete sex-organ reciprocity should result in significant intra-morph mating in the L-morph. We measured mating patterns in two L-biased populations--dimorphic (two style morphs) and trimorphic (three style morphs)--using multilocus genotyping and maximum-likelihood-based paternity analysis. We also examined the spatial distribution of style morphs and neutral markers to investigate the potential consequence of spatially restricted mating on morph ratios. As predicted, we detected significant amounts of intra-morph mating in the L-morph in both populations. Pollen transfer coefficients generally supported predictions based on the Darwinian hypothesis that anthers and stigmas of equivalent level promote pollinator-mediated cross-pollination in heterostylous populations. There was evidence of significant spatial aggregation of both style morphs and neutral markers in populations of N. triandrus, probably as a result of restricted pollen and seed dispersal. Our results provide empirical support for theoretical models of pollen transfer, which indicate that the commonly observed L-biased morph ratios in Narcissus species result from significant intra-morph mating in the L-morph because of its atypical floral morphology.     

FEATURES OF POLLEN FLOW IN DIMORPHIC SPECIES OF LYTHRUM SECTION EUHYSSOPIFOLIA

The North American Lythrum lineare L., L. alatum Pursh, L. curtissii Fernald, and L. californicum T. & G. have distylous flowers that differ in a number of traits. In the two species examined, pins have a stronger incompatibility system than thrums. Pins outnumber thrums in natural populations, but the ratio of the two may change radically during the course of a year, probably as a result of sexual reproduction. Exotic honeybees visiting L. californicum exhibited preferential collection of pin pollen over thrum pollen. The relative proportion of pin and thrum pollen grains transferred to stigmas is not dependent upon the proportion in which these pollen grains are produced in a population. Analyses of stigmatic pollen loads of L. lineare and L. californicum indicate very high levels of intramorph pollen flow. Although distyly is likely derived from tristyly in Lythrum, pollen flow patterns in trimorphic species seem to operate more “efficiently” than in dimorphic ones. It is probable that distyly has evolved from tristyly several times in the Lythraceae, but comparisons between pollen flow patterns in distylous species and those in a tristylous species provide no clear insights into the selective pressures that have led to this fundamental change in floral morphology and reproductive system.     

Between-year changes in functional gender expression of Palicourea padifolia (Rubiaceae), a distylous, hummingbird-pollinated shrub

BACKGROUND AND AIMS: Because distylous species have two hermaphroditic style-length floral morphs, they face two sex allocation problems: the equilibrium morph ratio and the optimal allocation to pollen and seed production in each floral morph. Gender specialization is expected among distylous species when floral morphs differ in reproductive output. However, spatio-temporal variability in female reproductive output between morphs needs to be investigated to fully understand patterns of sexual expression and gender specialization in distylous plants. Between-year variation in flower and fruit production of hummingbird-pollinated Palicourea padifolia (Rubiaceae) was examined, focusing on functional gender expression of long- and short-styled morphs and comparing their reproductive performance in five consecutive years (1998-2002). METHODS: Between-year variation in inflorescence, floral bud and fruit production was monitored and quantified. These traits were then used as parameters to determine functional gender differences between floral morphs through time. KEY RESULTS: Inflorescence production varied among years but no significant differences were found between floral morphs. Long-styled plants initiated more floral buds per inflorescence every year than short-styled plants, suggesting higher allocation to pollinator attraction and, potentially, an increase in male fitness through pollen donation. Although fruit production was similar between morphs, their functional gender shifted across years. CONCLUSIONS: The gender expression inconsistency across years is surprising because a number of floral characters and attributes that contribute to differently attracting and rewarding effective pollinators in P. padifolia suggest gender specialization. The evidence that morphs of distylous species might specialize in functional gender mostly comes from differences among populations in seed production and non-equilibrium morph ratios based on 1-year field population surveys. The results suggest that more sampling through time is needed to detect gender specialization among distylous species with a perennial habit.     

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.     

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.     

AN UNUSUAL KIND OF DISTYLY IN QUINCHAMALIUM CHILENSE (SANTALACEAE) ON VOLCÁN CASABLANCA,SOUTHERN CHILE

In mature flowers of the southern Andean parasitic herb, Quinchamalium chilense (Santalaceae), the stigmas and anthers are closer together in the pin morph than in the thrum morph. While the stigmas and anthers of the two morphs are in reciprocal positions as the flowers open, such reciprocity is lost as the result of post-anthesis allometric growth of the styles and stamens. Experimental pollinations reveal that both morphs of Q. chilense are self-compatible. Natural fruit set is higher on the pin morph. The latter is also statistically under-represented in natural populations with respect to a 1:1 ratio for pins and thrums. Pin flowers produce larger numbers of pollen grains than thrum flowers and pollen of pin flowers is smaller in size than that of thrum flowers. Higher fruit set on the pin morph is consistent with some tendency towards subdioecious breeding behavior, although an expected excess of geitonogamous pollinations on the pin morph might also be contributing to the difference in fruit set on the two morphs. The unusual floral morphology of Q. chilense could have evolved as a result of selection for larger flowers by certain pollinator groups less likely to effect geitonogamous pollination. Alternatively, extended development of post-anthesis flowers might be a reflection of selection for an array of flower sizes on individual inflorescences, producing a target effect for pollinator attraction. The evolution of such inflorescence morphology would have been facilitated by the fact that a trend in the direction of subdioecious breeding behavior accrues the same outcrossing advantages as strict distyly. Precise information on periods of stigma receptivity is required to further our understanding of the floral morphology of Q. chilense.     

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.     

REPRODUCTIVE BIOLOGY OF DISTYLOUS PARTRIDGEBERRY,MITCHELLA REPENS

Long-styled (pin) and short-styled (thrum) plants in populations of Mitchella repens from North Carolina and New York displayed strongly clumped spatial dispersion. This distribution pattern results from the tendency of these prostrate perennials to spread by production of trailing stems with adventitious roots. Both morphs, in both populations, exhibited natural fruit set exceeding 86%. This percentage was not increased by hand-pollination. Although the number of ovules per pair of ovaries was fixed at eight in each morph, numbers of seeds per fruit in naturally pollinated flowers differed significantly between pins (4.3) and thrums (6.4) in the North Carolina population. In the New York population, no significant differences were found (pins had 5.0 seeds per fruit; thrums, 4.4). Artificial pollinations in both populations showed low seed set in intramorph cross-pollinations as compared with intermorph crosses. The flowers of M. repens were visited by native species of Bombus, which visited many flowers in each patch of plants. These insects moved frequently from patch to patch, effecting intermorph cross-pollination and maintaining high levels of seed set. Movement among patches may be promoted by the dispersion of nectar rewards, with flowers about to open or just opened providing maximum amounts of nectar. Calculations of functional gender suggested that in the North Carolina population, thrums contributed more than 75% of the genes transmitted by ovule production. This contrasts strongly with previous studies of unequal sexual contributions in distylous taxa, which have demonstrated greater female contributions by pins. In the New York population, however, nearly equal contributions by pins and thrums through pollen and ovule production occurred, a situation similar to that found in most distylous species. This appears to be the first demonstration of variation in functional gender between populations of a single species.     

Floral display size and spatial distribution of potential mates affect pollen deposition and female reproductive success in distylous Pulmonaria officinalis (Boraginaceae)

In animal‐pollinated plants, both the spatial distribution of flowering individuals and the number of flowers that an individual displays affect pollen deposition rates and female reproductive success. Heterostylous species are likely to be particularly sensitive to the contingencies of spatial distribution, as they are reproductively subdivided into distinct mating groups, which usually exhibit self‐ and intra‐morph incompatibility and differ in floral morphology. In this paper, we explore the joint effects of both spatial distribution of potential mates and floral display size on morph‐specific pollen deposition rates and seed set patterns in two natural populations of Pulmonaria officinalis, a distylous species with a weak self‐incompatibility system. Both total stigmatic pollen load and the proportion of legitimate pollen decreased with increasing spatial isolation. Legitimate (intermorph) pollen transfer was, however, asymmetric and decreased more rapidly with decreasing proximity to a compatible legitimate mating partner in the S‐morph than in the L‐morph. Total stigmatic pollen loads per flower increased with increasing floral display size, indicating that large plants are disproportionately more visited than smaller individuals. However, because legitimate pollen deposition decreased with increasing floral display size, these results also suggest that larger numbers of flowers increase the degree of geitonogamous pollination. In both the L‐ and S‐morph, seed set significantly decreased with increasing isolation from a legitimate mating partner, but in the L‐morph seed set was less dependent on the spatial distribution of the S‐morph. In addition, seed set significantly increased with floral display size in the L‐morph, but not in the S‐morph. These findings indicate that the spatial distribution of potential mates and variation in floral display size may cause morph‐specific differences in pollen deposition rates and female reproductive success.     

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.