THE CONSEQUENCES OF SELF-POLLINATION IN ASCLEPIAS EXALTATA,A SELF-INCOMPATIBLE MILKWEED

We investigated the effect of self-pollination on fruit- and seed-set in Asclepias exaltata, a self-incompatible milkweed. A total of 1,380 hand-pollinations were performed on 138 flowers in each of five treatments. In each treatment, two pollinia were placed in adjacent stigmatic chambers, which transmitted pollen tubes to the same ovary. Treatments 1 and 2 involved simultaneous placement of two self-pollinia (treatment 1) or two cross-pollinia (treatment 2) into adjacent stigmatic chambers. No fruits were produced after self-pollinations, but 23% of the cross-pollinated flowers matured fruits. The three remaining experimental pollination treatments were self-pollination preceding cross-pollination by 24 hr (treatment 3), simultaneous self- and cross-pollination (treatment 4), and cross-pollination preceding self-pollination by 24 hr (treatment 5). Compared with that in flowers receiving only cross-pollen, fruit-set in treatments 3–5 was reduced 81%, 49%, and 29%, respectively. Seed-set was also significantly reduced in flowers receiving self-pollen 24 hr in advance of the cross-pollen. Using genetic markers, we observed that only 0.6% of seeds resulted from self-pollination. Our data strongly suggest that self-pollination in milkweeds not only wastes pollen but also greatly reduces the number of ovules and ovaries that might otherwise mature fruits and seeds after cross-pollination.     

‘晚大新高’梨授粉及受精过程的显微动态研究

应用荧光显微法和石蜡切片解剖法对‘晚大新高’梨授粉受精过程进行了系统观察研究。结果表明：‘晚大新高’梨自花授粉不结实;异花最佳授粉品种为‘黄花’,其次为‘翠冠’和‘丰水’。与选用‘黄花’为异花授粉品种相比,自花和异花的授粉受精过程存在明显差异,自花花粉在授粉后2h开始萌发,8h花粉管生长至离柱头约1／3处停止生长,顶端膨大呈球形,表现出自交不亲和性;异花花粉在授粉后1h开始部分萌发,8h花粉管生长至花柱中部,24h到达花柱基部并进入子房,48h进入胚囊,72h完成双受精过程。     

Stigmatic fluid aids self-pollination in Roscoea debilis (Zingiberaceae): a new delayed selfing mechanism

Background and Aims

Delayed selfing is the predominant mode of autonomous self-pollination in flowering plants. However, few delayed selfing mechanisms have been documented. This research aims to explore a new delayed selfing mechanism induced by stigmatic fluid in Roscoea debilis, a small perennial ginger.

Methods

Floral biology and flower visitors were surveyed. The capacity of autonomous selfing was evaluated by pollinator exclusion. The timing of autonomous selfing was estimated by emasculation at different flowering stages. The number of seeds produced from insect-pollination was assessed by emasculation and exposure to pollinators in the natural population. The breeding system was also tested by pollination manipulations.

Key Results

Autonomous self-pollination occurred after flowers wilted. The stigmatic fluid formed a globule on the stigma on the third day of flowering. The enlarged globule seeped into the nearby pollen grains on the fourth flowering day, thus inducing pollen germination. Pollen tubes then elongated and penetrated the stigma. Hand-selfed flowers produced as many seeds as hand-crossed flowers. There was no significant difference in seed production between pollinator-excluded flowers and hand-selfed flowers. When emasculated flowers were exposed to pollinators, they produced significantly fewer seeds than intact flowers. Visits by effective pollinators were rare.

Conclusions

This study describes a new form of delayed autonomous self-pollination. As the predominant mechanism of sexual reproduction in R. debilis, delayed self-pollination ensures reproduction when pollinators are scarce.     

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.     

The breeding system of Lycium cestroides: a Solanaceae with ovarian self-incompatibility

To determine the breeding system of Lycium cestroides,several treatments were performed: self-, cross-, and geitonogamous pollinations, autonomous self-pollination, and a control (flowers exposed to natural pollinators). Production and quality of fruits and seeds as well as pollen tube growth were evaluated for each treatment. Experimental pollinations indicate that L. cestroides is self-incompatible since fruits were obtained only under cross-, and open-pollination treatments. However, in self- and geitonogamous hand pollinations, as well as in autonomous self-pollination, pollen tubes developed successfully and reached the ovules. The speed of pollen tube growth did not differ significantly among the different hand-pollinated flowers (cross-, self-, and geitonogamous). These facts indicate the presence of an ovarian self-incompatibility system. Significant differences were observed in fruit set, fruit size, and seed number per fruit between cross-pollinated flowers and open pollination (control). These results could be explained in terms of quantity and quality of pollination in each case. Received: 2 November 2000 / Revision accepted: 26 January 2001     

Pistil anatomy and pollen tube development in Polygala vayredae Costa (Polygalaceae)

Low seed ovule ratios have been observed in natural populations of Polygala vayredae Costa, a narrowly endemic species from the oriental pre-Pyrenees. To evaluate physical and nutritional constraints and pollen tube attrition in this endemic species, stigma and style anatomy, as well as pollen tube development along the pistil were investigated using light and fluorescence microscopy. The structural morphology of the stigmatic region was also examined with scanning electron microscopy. Pollen grains that reached the stigmatic papillae came into contact with a lipid-rich exudate and germinated easily. Although a large number of pollen grains reach the stigmatic papillae, few pollen tubes were able to grow into the style towards the ovary. The style was hollow, with the stylar channel beginning a few cells below the stigmatic papillae. Initially, the stylar channel area was small compared to other levels of the style, and was surrounded by lipid-rich, highly metabolic active cells. Furthermore, lipid-rich mucilage was detected inside the stylar channel. At subsequent style levels towards the ovary, no major reserves were detected histochemically. The reduced intercellular spaces below the stigmatic papillae and the reduced area of the stylar channel at its commencement are suggested to physically constrain the number of pollen tubes that can develop. In subsequent levels of the style, the stylar channel could physically support a larger number of pollen tubes, but the lack of nutritional reserves cannot be disregarded as a cause of pollen tube attrition. Finally, the number of pollen tubes entering the ovary was greater than the number of ovules, suggesting that interactions occurring at this level play a major role in the final reproductive outcome in this species.     

Style-controlled wilting of the flower

Differences in rate of wilting in cross-, self-and unpollinated flowers of self-incompatiblePetunia hybrida L. clone W166H appeared to be significant. Wilting rate was fastest following cross-pollination and slowest in unpollinated flowers. The difference between wilting behaviour of cross- and self-pollinated flowers was not caused by rate of pollen tube growth and not by the incompatibility (recognition or rejection) reaction either. It is assumed, that, following pollination, the wilting reaction is only retarded after penetration of pollen tubes of the same genetic composition as the style (complete self-pollination). The number of viable pollen grains necessary to initiate a maximal wilting-rate of flowers following cross- and self-pollination is about 800, which means that a fifth of the stigmatic surface must be covered with living pollen grains. It is suggested that pollen tube penetration and injury of the style have a similar influence on the initiation of wilting.Wilting-rate following pollination is faster in young plants as compared with wilting in old plants. The wilting process of unpollinated and self-pollinated flowers started in the early morning and lasted till afternoon. Cross-pollinated flowers wilted independently of the hour of the day. The role of flower-wilting as a means of communication to the environment with regard to pollination of the style is discussed.     

Multiple factors contribute to outcrossing in a tropical emergent Dipterocarpus tempehes, including a new pollen-tube guidance mechanism for self-incompatibility

The self-rejection system of Dipterocarpus tempehes (Dipterocarpaceae), an emergent tree of the lowland tropical forests of Borneo, were studied by means of pollination experiments, fluorescence microscopy of pollen tubes, and monitoring of ovary maturation patterns. Fruit set was higher in cross-pollinated flowers than in control and self-pollinated flowers, indicating the existence of pollen limitation and a self-rejection system. Although the adhesion and the germination of self-pollen and the growth of self-pollen tubes were not inhibited, the proportion of cross-pollen tubes that entered the style was 1.7-2.3 times higher than that of self-pollen tubes, indicating a partial self-incompatibility that inhibits self-pollen tubes from entering the style hollow. These results suggest, for the first time, that self-incompatibility is caused by a defect of pollen-tube guidance. We also suggest a threshold effect in number of pollen tubes or late-acting self-incompatibility to fully explain the drastic and uniform selection against self-pollinated flowers before ovary swelling. After that, maternal selection and/or inbreeding depression caused the abortion and delayed maturation of self-pollinated flowers. The advantages of the self-rejection process composed of partial early-acting self-incompatibility and relatively strong delayed abortion is discussed with respect to the general-flowering phenomenon in lowland dipterocarp forests.     

Effects of self-, chase and mixed self/cross-pollinations on pistil longevity and fruit set in Ceiba species (Bombacaceae) with late-acting self-incompatibility

BACKGROUND AND AIMS: Late-acting self-incompatibility (LSI), in which selfed flowers fail to form fruits despite apparently successful growth of the pollen tubes to the ovules, is a contentious and still poorly understood phenomenon. Some studies have indicated pollen tube-pistil interactions, and major gene control. Others favour an early acting inbreeding depression explanation. METHODS: Experimental pollinations, including selfs (in a subsample of which the style was cut before pollen tubes reached the ovary), chase self/cross-pollinations, crosses, and mixed self/cross-pollinations were used to study floral/pistil longevity and effect on fruit set and seed yield in two Ceiba species known to have LSI. RESULTS: Self-pollinations, including those with a cut style, had extended floral longevity compared with unpollinated flowers. Chase pollinations in which cross-pollen was applied up to 3 h after selfing set fruits, but with reduced seed set compared with crosses. Those with cross-pollen applied at 4 and 8 h after self-pollination all failed to set fruits. Flowers subjected to 1 : 1 and 2 : 1 self/cross-pollinations all produced fruits but again with a significantly lower seed set compared with crosses. CONCLUSIONS: Extended floral longevity initiated with self-pollen tubes growing in the style indicates some kind of pollen tube-pistil interaction. Fruit set only in chase pollinations up to 3 h implies that self-pollen tubes either grow more slowly in the style or penetrate ovules more slowly on arrival at the ovary compared with cross-tubes. This agrees with previous observations indicating that the incidence of penetrated ovules is initially lower in selfed compared with crossed pistils. However, the low seed yield from mixed pollinations indicates that self- and cross-pollen tubes arrive at the ovary and penetrate ovules more or less simultaneously. Possible explanations for these discordant results are discussed.     

Expression of the ACC synthase and ACC oxidase coding genes after self-pollination and incongruous pollination of tobacco pistils

In tobacco, as in other species, ethylene is produced in response to pollination. Although tobacco is a self-compatible species, it displays unilateral incongruity with other Nicotianaplants. Incongruous pollination also results in ethylene production, but this production differs depending on the pollen used and is related to the extent to which pollen tubes grow in the tobacco style. In the investigation reported here we followed the expression of the ACC synthase- and ACC oxidase-coding genes upon pollination of tobacco pistils and compared self-pollination with incongruous pollination. The pattern of expression of these genes also correlated with pollen-tube growth, although wounding alone cannot explain the results obtained. We also examined the expression of these genes upon pollination of immature tobacco pistils, in which different pollen tubes grew indistinctly inside the tobacco style and reached the ovary at the same rate. In this situation no significant differences in gene expression could be observed between the different pollinations. Ethephon, a substance that produces ethylene, could, in some cases, minimize the arrest of incongruous pollen tubes inside the style.     

Seed set after pollination with in-vitro-matured,isolated pollen of Triticum aestivum

Summary Immature pollen of two varieties of Triticum aestivum, at the stage right after the first pollen mitosis, was isolated from individual anthers and cultured in microcultures of microliter droplets. In a specifically designed medium, some of the pollen grains developed to maturity. These were applied to excised stigmas on agar, where they produced pollen tubes. Application to flowers in vivo led to seed set. Pollen was matured in vitro from a variety that produced a different protein banding pattern on SDS-PAGE as compared to the variety that was pollinated. The protein banding in the produced seeds showed the hybrid pattern, demonstrating that the seeds were not produced by self-pollination in this in-breeding species but by pollination with the in-vitro-matured pollen.     

Reproductive biology of Cyrtopodium polyphyllum (Orchidaceae): a Cyrtopodiinae pollinated by deceit

The genus Cyrtopodium comprises about 42 species distributed from southern Florida to northern Argentina. Cyrtopodium polyphyllum occurs on rocks or in sandy soils, in restinga vegetation along the Brazilian coast. It flowers during the wet season and its inflorescences produce a high number of resupinate yellow flowers. Cyrtopodium polyphyllum offers no rewards to its pollinators, but mimics the yellow, reward-producing flowers of nearby growing Stigmaphyllon arenicola (oil) and Crotalaria vitellina (nectar) individuals. Several species of bee visit flowers of C. polyphyllum , but only two species of Centris ( Centris tarsata and Centris labrosa ) act as pollinators. Visits to flowers of C. polyphyllum were scarce and, as a consequence, low-fruit set was recorded under natural conditions. Such low-fruit production contrasts with the number of fruits each plant bears after manual pollination, suggesting deficient pollen transfer among plants. C. polyphyllum is self-compatible and has a high-fruit set in both manual self- and cross-pollinated flowers. Furthermore, fruits (2%) are formed by self-pollination assisted by rain. This facultative self-pollination mechanism is an important strategy to provide reproductive assurance to C. polyphyllum as rainfall restricts the foraging activity of its pollinating bees. Fruits derived from treatments and under natural conditions had a similar high rate of potentially viable seed. Moreover, these seeds had a low polyembryony rate, which did not exceed 5%. C. polyphyllum acts by deceit involving optical signals and exploits other yellow-flowered species within its habitat by attracting their pollinators. The low capsule production under natural conditions was expected, but its reproductive success is assured through self-pollination by rain and high seed viability.     

Adaptive plasticity of floral display size in animal-pollinated plants

Plants need not participate passively in their own mating, despite their immobility and reliance on pollen vectors. Instead, plants may respond to their recent pollination experience by adjusting the number of flowers that they display simultaneously. Such responsiveness could arise from the dependence of floral display size on the longevity of individual flowers, which varies with pollination rate in many plant species. By hand-pollinating some inflorescences, but not others, we demonstrate plasticity in display size of the orchid Satyrium longicauda. Pollination induced flower wilting, but did not affect the opening of new flowers, so that within a few days pollinated inflorescences displayed fewer flowers than unpollinated inflorescences. During subsequent exposure to intensive natural pollination, pollen removal and receipt increased proportionally with increasing display size, whereas pollen-removal failure and self-pollination accelerated. Such benefit-cost relations allow plants that adjust display size in response to the prevailing pollination rate to increase their attractiveness when pollinators are rare (large displays), or to limit mating costs when pollinators are abundant (small displays). Seen from this perspective, pollination-induced flower wilting serves the entire plant by allowing it to display the number of flowers that is appropriate for the current pollination environment.     

THE EVOLUTION OF INSECT-MEDIATED SELF-POLLINATION FROM AN OUTCROSSING SYSTEM IN CALYPTRIDIUM (PORTULACACEAE)

Calyptridium monospermum (Portulacaceae) comprises two biological races visited by different groups of insects. One race, pollinated by a bumblebee, Bombus vosnesenskii (Apidae), is outcrossing, while the other race is pollinated casually by a variety of other insects whose visits result in insect-mediated self-pollination and geitonogamy within the inflorescence. Insect-mediated selfing is also the mode of pollination in the closely related species, C. umbellatum. While all plants of both species are self-compatible, insect visitation is necessary for good seed set. Selfing populations of C. monospermum exhibit many floral adaptations for insect-mediated self-pollination which parallel those of the selfing C. umbellatum, while retaining vegetative characteristics typical of C. monospermum. These floral traits involve flower color and odor, style and stamen orientation, size of stigmatic surfaces, number of pollen grains produced, density of open flowers on inflorescences, and presence or absence of protogyny. Artifical pollination experiments show that these floral adaptations are effective in increasing the probability of insect-mediated self-pollination. Insect-mediated selfing in C. umbellatum and in some populations of C. monospermum is encouraged and probably made necessary by the inconstant pollinating behavior of the insects which visit these plants, as shown by the large percentage of non-Calyptridium pollen recovered from the bodies of insects captured while visiting these plants.     

Pistil traits and flower fate in apricot (Prunus armeniaca)

Although pollination is essential for both seed and fruit set in most angiosperms, even after an adequate pollination, only a fraction of the flowers develop into fruits. The role played by floral traits on reproductive success is well known, but the possible influence of pistil traits has been overlooked, probably because of the difficulty of non-destructive pistil examination. The aim of this work was to examine the influence of several pistil traits on reproductive success in apricot ( Prunus armeniaca ). For this purpose, in a population of individually labelled flowers, the styles were cut off once the pollen tubes had reached the ovary but prior to the achievement of fertilisation. This approach allowed relating several morphological and physiological pistil parameters in the dissected styles and stigmas to the subsequent set or abscission of the corresponding ovary that remained in the plant. Under the same pollination conditions, the flowers that finally set a fruit show a larger stigmatic area and a higher number of pollen grains, pollen tubes growing along the style and xylem vessels surrounding the transmitting tissue than flowers that abscise before the establishment of fruit set. Furthermore, starch is present in the transmitting tissue of the style in all the flowers that develop into fruits but only in half of the flowers that abscise. The examined pistil traits established prior to fertilisation are related to flower fate, suggesting that the capacity of a flower to become a fruit could be preconditioned at anthesis.     

Cleistogamy in the rare high Andean perennial herb Cryptantha capituliflora (Boraginaceae)

 Axial and apical flowers of Cryptantha capituliflora were analyzed with regard to morphology and pollen tube growth to assess the occurrence of cleistogamy. Although intermediate floral forms do occur, cleistogamous flowers were significantly smaller than chasmogamous flowers, had fewer anthers, and showed a distinctive stigmatic surface. Chasmogamous flowers can be cross-pollinated. Nevertheless, the growth of self-pollen tubes in few chasmogamous buds jointly with flower characters suggests that these flowers can probably produce fruits through autonomous selfing. The mean seed number per fruit did not differ between fruits from chasmogamous and cleistogamous flowers. Cleistogamous flowers were only observed in axial inflorescences, which are completely covered by the leaf. Other species of section Cryptantha also show the same trend, with cleistogamous flowers located in the lower half of the stems. This pattern is discussed in relation to dissimilarities in the outcrossing opportunities between flower types within the plant. Received May 22, 2002; accepted November 14, 2002 Published online: March 20, 2003     

Reproductive significance of indirect pollen-tube growth in Dalechampia (Euphorbiaceae)

Most species of Dalechampia have expanded stigmatic surfaces that extend from the stylar tip part way down the sides of the elongated styles. Pollen grains landing at the stylar tip grow tubes directly down the style to the ovary. Pollen grains landing on the lateral stigmatic surfaces, however, exhibit indirect pollen-tube growth: the tubes grow first to the stylar tip, bend 180°, and then grow to the ovary. Indirect pollen-tube growth appears to be reproductively important: more than half the arriving pollen lands on the lateral stigmatic surfaces, and virtually all of these grains produce tubes that grow indirectly to the ovary; pollen on the lateral surface fertilizes seeds about as effectively as pollen at the tip (with direct tube growth). Indirect pollen-tube growth may be the result of a correlated response to selection for expanded stigmatic surfaces in Dalechampia, or it may be an adaptation to increase the intensity of pollen competition.     

The evolution of ovule number and flower size in wind-pollinated plants

In angiosperms, ovules are "packaged" within individual flowers, and an optimal strategy should occur depending on pollination and resource conditions. In animal-pollinated species, wide variation in ovule number per flower occurs, and this contrasts with wind-pollinated plants, where most species possess uniovulate flowers. This pattern is usually explained as an adaptive response to low pollen receipt in wind-pollinated species. Here, we develop a phenotypic model for the evolution of ovule number per flower that incorporates the aerodynamics of pollen capture and a fixed resource pool for provisioning of flowers, ovules, and seeds. Our results challenge the prevailing explanation for the association between uniovulate flowers and wind pollination. We demonstrate that when flowers are small and inexpensive, as they are in wind-pollinated species, ovule number should be minimized and lower than the average number of pollen tubes per style, even under stochastic pollination and fertilization regimes. The model predicts that plants benefit from producing many small inexpensive flowers, even though some flowers capture too few pollen grains to fertilize their ovules. Wind-pollinated plants with numerous flowers distributed throughout the inflorescence, each with a single ovule or a few ovules, sample more of the airstream, and this should maximize pollen capture and seed production.     

Disruption of endosperm development: an inbreeding effect in almond (Prunus dulcis)

A homozygous self-compatible almond, originated from self-fertilization of a self-compatible genotype and producing a reasonable yield following open pollination, exhibited a very high fruit drop rate when self-pollinated. To investigate whether fruit dropping in this individual is related to an abnormal development of the embryo sac following self-fertilization, histological sections of ovaries from self and cross-pollinated flowers were observed by light microscopy. Additionally, the presence of pollen tubes in the ovary and fruit set were determined for both types of pollination. Despite pollen tubes reached the ovary after both pollinations, differences in embryo sac and endosperm development after fertilization were found. Thus, while for cross-fertilized ovules a pro-embryo and an endosperm with abundant nuclei were generally observed, most self-fertilized ovules remained in a previous developmental stage in which the embryo sac was not elongated and endosperm nuclei were absent. Although 30 days after pollination fruit set was similar for both pollination types, at 60 days it was significantly reduced for self-pollination. These results provide evidence that the high fruit drop in this genotype is the consequence of a disrupted development of the endosperm, what could be an expression of its high level of inbreeding.     

两种姜科花柱卷曲性植物柱头的位置与其可授性的关系

花柱卷曲性是一种见于姜科山姜属(Alpinia)和砂仁属(Amomum)植物中的独特的性二态现象, 具有这一性系统的植物, 其居群包括上举型和下垂型两种表型, 所有个体的雌雄性别功能在时间和空间上分离。本研究通过操控授粉和花粉管生长两项实验, 探讨花柱卷曲性植物个体两性功能的隔离方式及其适应意义。云南草蔻(Alpinia blepharocalyx)操控授粉实验表明, 自花花粉对其异交率影响不显著(P > 0.05), 但可能由于自花花粉沉降导致胚珠贴现进而引起近交衰退, 操控条件下每果结籽数显著减少(P < 0.01)。云南草蔻和红豆蔻(Alpinia galanga)花粉管生长实验都显示无论对两种表型进行怎样的授粉处理, 花粉粒萌发和花粉管生长的速率仅在柱头处于可授位置时表现出最大值, 与自交和异交授粉方式无关; 上举型植株上午花粉囊虽未裂但其内花粉已成熟。研究结果表明即使不考虑花柱运动, 山姜属植物也具有异型雌雄异熟的特性。这一结果证实了花柱卷曲运动机制是通过互补式雌雄异位和异型雌雄异熟相结合形成的花部二态性, 异型雌雄异熟促进了异交, 而花柱运动的功能可能在于避免雌雄干扰。