Temporal variations in the linkages between plants and flower visitors and the pollination success of Primula modesta along the snowmelt gradient

Although pollination networks between plants and flower visitors are diverse and flexible, seed production of many plant species is restricted by pollen limitation. Obligate outcrossers often suffer from low pollinator activity or severe interspecific competition for pollinator acquisition among co-flowering species. This study focused on seasonal changes in plant–flower visitor linkages in an alpine ecosystem and examined whether and how this seasonality affected the seed-set of Primula modesta, a self-incompatible distylous herb having long-tubed flowers. First, we recorded the linkages between plants and flower visitors along the snowmelt gradient. Then, pollination experiment was conducted to estimate the degree of pollen limitation over the course of flowering season of P. modesta. Flower visitors were classified by their tongue length based on the morphological matching with P. modesta flowers. As the season progressed, plant–visitor linkages became more diverse and generalized, and the visitation frequency to P. modesta flowers increased. In the later part of the season, however, the seed set of P. modesta was significantly reduced due to severe pollen limitation, presumably because of increased competition for long-tongued pollinators among co-flowering species. The present study revealed that pollinator availability for specialist species may be restricted even when plant–visitor linkages are diverse and generalized as a whole. In the case of P. modesta, morphological matching and competition for pollinators might be the main factors explaining this discrepancy.     

INCOMPATIBILITY AND THE POLLEN ECONOMY OF JEPSONIA PARRYI

Jepsonia parryi (Saxifragaceae) has heterostylous flowers and is strongly self-incompatible. Pin flowers have long styles, large stigmas, short stamens, and numerous, small pollen grains with finely sculptured walls. Thrum flowers have short styles, small stigmas, long stamens, and fewer, larger pollen grains with coarsely sculptured walls. Pin plants and thrum plants occur in a 1:1 ratio in field populations. Although the insect pollinators of J. parryi transfer ample compatible pollen to pin and thrum stigmas to account for full seed production, much of the pollen deposited on stigmas is incompatible. Analysis of the pollen deposits on stigmas collected from field populations indicates that compatible “legitimate” pollination of pin and thrum flowers is essentially random and is not obviously aided by floral dimorphism. It is suggested that although heterostyly had a positive adaptive value in the past evolutionary history of Jepsonia it is no longer adaptive under the present pollination regime, although it is maintained because of its strong genetic fixity.     

青藏高原高山植物麻花艽的传粉生态学研究

对青藏高原东部麻花艽 (Gentiana straminea)海北居群的传粉生态学进行了 3a连续的观察和实验。试验表明自然去雄、人工自交和杂交处理均结实 ,而人工去雄套袋和自然套袋不结实。麻花艽自交亲合 ,但必须依赖传粉媒介才能完成授粉过程 ,不存在无融合生殖。野外捕捉到 14种访花昆虫 ,它们分别属于 2个纲 ,7个目 ,8个科。观察和分析了各种昆虫的访花行为后 ,认为苏氏熊蜂 (Bombus sushikini)是麻花艽有效而稳定的传粉者。测量表明麻花艽花蜜通道的深度和苏氏熊蜂的舌长基本吻合。苏氏熊蜂的访花频率在 10 :0 0～ 12 :0 0 ,13:0 0～ 15 :0 0和 16 :0 0～ 18:0 0时间段没有差别 ,单花的访花频率为 0 .0 0 5次 / (花· min)。和其它高山植物相比 ,青藏高原高山植物麻花艽的访花频率较高。熊蜂传粉和高频率的访花维持了麻花艽在极端寒旱的青藏高原环境下的有性生殖。此外 ,高频率的访花对于维持该地区高山植物的生殖保障具有重要的现实意义 ,但是否具有普遍性 ,还有待研究更多的高山代表类群。     

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.     

The distyly in Primula elatior (L.) Hill (Primulaceae), with a study of flowering phenology and pollen flow

Flowers of Primula elatior exhibit all the dimorphic traits typical of distyly. The difference in pollen production between pin and thrum flowers is not thought to be caused by the pollen size dimorphism. The percentage of pin pollen which is probably viable exceeds that of thrum pollen. In the population studied the morph ratio is 1:1, and plants are randomly dispersed with regard to morph type. Bumblebees are the main pollinators, and analysis of the stigmatic pollen load in naturally |xjllinated flowers shows that pin stigmas receive many more pollen grains than thrum stigmas. Intramorph pollination is considerable in pins, while thrums are subject to some differential pollination. Emasculation experiments have shown that intraflower pollination in the thrum morph is negligible. Aging of pin flowers has a reducing effect on their total pollen loads, while in thrums there is no change in total pollen load after the first days of anthesis.     

Three-dimensional reciprocity of floral morphs in wild flax (Linum suffruticosum): a new twist on heterostyly

Here, we studied the floral morphology and pollination of the distylous plant Linum suffruticosum (Linaceae) in southern Spain.We observed a previously unreported form of distyly that involved twisting and bending of styles and stamens during floral development to achieve three-dimensional reciprocity of anthers and stigmas in the long-styled (pin) and short-styled (thrum) morphs. This developmental pattern causes pin pollen to be placed on the underside of pollinating Usia flies (Bombyliidae), and thrum pollen to be placed on the top of the thorax and abdomen. The pin stigmas contact the flies on the dorsum, apparently picking up predominantly thrum pollen, and the thrum stigmas contact the flies on the ventral surface, apparently picking up predominantly pin pollen.This form of heterostyly would appear on morphological grounds to be far more efficient in dispersing pollen between compatible morphs than the typical pin-thrum system. If so, this plant fits Darwin's prediction of efficient pollen flow between heterostylous morphs more closely than anything Darwin himself reported.Molecular phylogenetic analyses indicate that this form of heterostyly evolved in a lineage that already had typical heterostyly. The analyses also indicate that there have been several independent origins of heterostyly in Linum and at least one reversal to stylar monomorphism.     

Morph-biased male sterility in the tropical distylous shrub Erythroxylum havanense (Erythroxylaceae)

This study explores whether ecological factors, such as pollinators and pollen flow, or variation in pollen and ovule development account for the observed differences (approximately twofold) in the reproductive output of pin and thrum individuals of Erythroxylum havanense. The importance of ecological factors was assessed by means of comparison of the identity of pollinators and the rates of flower visitation, and by performing controlled hand pollinations and measurements of fruit set. In addition, we described the pollen and ovule development of thrum and pin individuals. Our results indicate that pollinators of E. havanense do not distinguish between floral morphs. The differences in fruit set between pin and thrum plants held even after hand pollination and, therefore, the observed differences in reproductive output between floral morphs of E. havanense cannot be explained in terms of asymmetrical pollen flow. There were no differences in the pattern of gynoecium development between the pin and thrum morphs, however androecium development showed marked differences between the morphs, and there was a resemblance between the developmental pathways leading to male sterility of the thrum morph of E. havanense with that of species with cytoplasmic male sterility (CMS).     

Sequential variation in the components of reproductive success in the distylousJasminum fruticans (Oleaceae)

A 2-year study of three natural populations of the distylousJasminum fruticans showed that mean fruit and seed production were significantly greater in shortstyled plants (thrums) than in long-styled plants (pins). In this study, we investigated the role of four sequential factors which may differentially influence fruit and seed set in the two floral morphs: (1) differences in flowering phenology, (2) a limitation of pollen transfer towards pins, (3) a differential capacity of the two morphs to act through famale and male function and (4) differential fruit abortion in the two morphs. Fruit set was significantly influenced by differences in flowering phenology although there were no differences in flowering time between the two morphs. supplementary pollinations in a natural population significantly increased fruit set and reduced the difference in fruit set between the two morphs in relation to controls, indicating a limitation on pollen transfer which was most severe towards pin stigmas. In reciprocal crosses, seed set was significantly dependent on the paternal and maternal identity of the pin parent. There was no significant variation among thrums for their performance as male or female parent. Furthermore, individual pin plants with relatively high percentage seed set as female parents gave poor seed set as male parents and vice-versa. Whereas fruit removal had no effect on seed number in thrum plants, a greater proportion of viable seeds were produced on pin plants which were left to naturally mature their fruits than on pins which had fruits artificially removed, suggesting the occurrence of selective fruit abortion in pins but not in thrums. The initially greater maternal fitness of thrums due to their greater success as pollen recipients may thus be opposed by increased viable seed set in the pins due to factors acting after the pollination stage. The relative reproductive success of floral morphs in the distylousJ. fruticans is thus differentially influenced by ecological factors occurring at different stages of the reproductive process.     

Importance of Queen Bumble Bees as Pollinators Facilitating Inter-morph Crossing in Primula sieboldii

Abstract Flower-visiting insects, the distribution of pollen on an insect's body, and fruit and seed sets of Primula sieboldiiE. Morren were investigated in a maritime deciduous forest habitat of the Hidaka region in Hokkaido. Queens of Bombus diversus tersatus Smith (Hymenoptera: Apidae), Bombus schrencki albidopleuralis Skorikov, and Bombus deuteronymus deuteronymus Schulz were observed to visit the flowers of the species. Bumblebee claw marks were found on the flower petals of 68% of the flowering ramets of the species. Scanning-electronmicroscopy revealed that pin and thrum pollens of P. sieboldii were deposited in different positions along the proboscis of a B. diversus tersatus queen (n=38400 grains). On the glossa, all the observed pollen was of the pin type, while more than half of the pollen grains observed on the upper part of the proboscis, galea, stipes, and maxillary palpus were of the thrum type. In the field, high fruit and seed sets were recorded for both pin and thrum morphs, indicating the occurrence of sufficient legitimate pollination during the flowering season. In addition to the effectiveness of queen bumblebees as pollinators for P. sieboldii suggested in this study, phenological and morphological matches, i.e., early spring flowering during the queen bee emergence season and the similar lengths of the corolla tube and the bee proboscis, also support the inference that queen bumble bees play an important role in legitimate pollination of P. sieboldii.     

Pollen Flow,Fecundity, and the Adaptive Significance of Heterostyly in Palicourea padifolia (Rubiaceae)1

Palicourea padifolia is a distlous shrub visited primarily by hummingbirds and bees in mid-elevation rain forests in Costa Rica. At Las Cruces Biological Station, the population of P. padifolia is composed of equal numbers of pin and thrum plants and morphs are randomly distributed. Like the majority of distylous species, P. padifolia exhibits a self and intramorph-incompatible mating system. The analysis of stigmatic pollen loads obtained from emasculated flowers shows that pollen flow between floral morphs is disassortative, which supports the Darwinian hypothesis concerning the adaptive significance of heterostyly. Pins experience greater disassortative pollination than thrums. Pins also set more seed than thrums, suggesting that the higher frequency of compatible pollen deposited on pin stigmas results in greater fecundity; alternatively, greater seed set in pins could be due to the differential allocation of resources to male and female function between morphs. Overall, seed set for the Las Cruces popularion of P. padifolia is much lower than the potential maximum. Factors which could contribute to seed set limitation include the frequency and/or spatial pattern of pollinator visits, pollen availability, and resource availability.     

Reproductive conflicts ofPalicourea padifolia (Rubiaceae) a distylous shrub of a tropical cloud forest in Mexico

We studied a population of the distylousPalicourea padifolia (Rubiaceae) in a cloud forest remnant near Xalapa City, Veracruz, México to explore possible asymmetries between floral morphs in the attractiveness to pollinators, seed dispersers, nectar robbers, floral parasites, and herbivores. We first assessed heterostyly and reciprocal herkogamy by measuring floral attributes such as corolla length (buds and open flowers), style and anther heights, stigma and stamen lengths and the distance between the anther tip to the stigma lobe. We then estimated floral and fruit attributes such as flower size, anther height, number and size of pollen grains, fruit size, seed size, nectar production, and flower and fruit standing crops to assess differences between floral morphs in attracting and effectively using mutualistic pollinators and seed dispersers. Also, floral parasitism and nectar robbing were assessed in this study as a measure of flower attractiveness to antagonists. The system seems to conform well to classical heterostyly (e.g. reciprocal stamen/style lengths, pollen and anther dimorphism, intramorph incompatibility) yet, there were several tantalizing differences observed between pin and thrum morphs. Thrum flowers have longer corollas and larger but fewer pollen grains than pin flowers. Both morphs produced the same total number of inflorescences, developed the same number of buds, and opened the same number of flowers per inflorescence during the flowering season. Nectar production and sugar concentration were similar between floral morphs but the reward was not offered symmetrically to floral visitors throughout the day. Nectar concentration was higher in pin flowers in the afternoon. The numbers of developing, fully developed, and ripe fruits were the same between floral morphs, however, fruits and seeds were larger than those of thrums. The incidence of fly larvae was higher among thrum flowers and damage by nectar robbing was the same between floral morphs. Fruit abortion patterns of flowers manually pollinated suggest intra-morph sterility (self and intramorph incompatibility). There were no differences between morphs in fruit and seed set per flower following legitimate pollination although thrums were more leaky than the pins (intramorph compatibility).     

Self-compatibility in distylousHedyotis salzmannii (Rubiaceae)

Hedyotis salzmannii (Rubiaceae) is distylous. Experimental pollination revealed that both morphs are self-compatible. However, the thrum morph proved to be more self-compatible. Hand cross-pollination between different plants of the same floral form produced less seed-sets per pollination than hand cross-pollination between pin and thrum or thrum and pin pollination. Two species ofHymenoptera, oneApidae and oneHalictidae, and two species ofDiptera (Syrphidae) were observed as pollinators.     

Pollination biology and the impact of floral display, pollen donors, and distyly on seed production in Arcytophyllum lavarum (Rubiaceae)

In animal-pollinated plants, two factors affecting pollen flow and seed production are changes in floral display and the availability of compatible mates. Changes in floral display may affect the number of pollinator visits and the availability of compatible mates will affect the probability of legitimate pollination and seed production. Distyly is a floral polymorphism where long-styled (pin) and short-styled (thrum) floral morphs occur among different individuals. Distylous plants frequently exhibit self and intra-morph incompatibility. Therefore changes in morph abundance directly affect the arrival of compatible pollen to the stigmas. Floral morph by itself may also affect female reproductive success because floral morphs may display differences in seed production. We explored the effects of floral display, availability of neighboring compatible mates, and floral morph on seed production in the distylous herb ARCYTOPHYLLUM LAVARUM. We found that floral display does not affect the mean number of seeds produced per flower. There is also no effect of the proportion of neighboring legitimate pollen donors on seed production in pin or thrum flowers. However, floral morphs differed in their female reproductive success and the thrum morph produced more seeds. Hand pollination experiments suggest that differences in seed production between morphs are the result of pollen limitation. Future research will elucidate if the higher seed production in thrum flowers is a consequence of higher availability of pollen donors in the population, or higher efficiency of the pin morph as pollen donor.     

The dimorphic heterostyly inAconogonon campanulatum (Polygonaceae)

Heterostyly is clearly confirmed inAconogonon campanulatum. This distylous species is dimorphic for tepals, styles, stigma surface, stamens, pollen grain size, and pollen sexine ornamentation. The floral shape is campanulate and thrum flowers are slightly larger than pin flowers. Small solitary bees were observed as flower visitors and probably effected pollination. The possible evolution of dioecy via heterostyly within the genusAconogonon is discussed.     

Intrinsic and extrinsic factors acting on the reproductive process in alpine‐snowbed plants: roles of phenology,biological interaction,and breeding system

Pollinator activity and competition for pollinators lead to quantitative and qualitative pollen limitations on seed production and affect the reproductive success of plant species, depending on their breeding system (e.g., self‐compatibility and heterospecific compatibility) and genetic load (e.g., inbreeding depression and hybrid inviability). In alpine ecosystems, snowmelt regimes determine the distribution and phenology of plant communities. Plant species growing widely along a snowmelt gradient often grow with different species among local populations. Their pollinators also vary in their abundance, activity, and behavior during the season. These variations may modify plant–pollinator and plant–plant interactions. We integrated a series of our studies on the alpine dwarf shrub, Phyllodoce aleutica (Ericaceae), to elucidate the full set of intrinsic (species‐specific breeding system) and extrinsic factors (snow condition, pollinator activity, and interspecific competition) acting on their reproductive process. Seasonality of pollinator activity led to quantitative pollen limitation in the early‐blooming populations, whereas in the late‐blooming populations, high pollinator activity ensured pollination service, but interspecific competition for pollinators led to qualitative and quantitative pollen limitation in less competitive species. However, negative effects of illegitimate pollen receipt on seed‐set success might be reduced when cryptic incompatibility systems (i.e., outcross pollen grains took priority over self‐ and heterospecific pollen grains) could effectively prevent ovule and seed discounting. Our studies highlight the importance of species‐specific responses of plant reproduction to changing pollinator availability along environmental gradients to understand the general features of pollination networks in alpine ecosystems.     

The effectiveness of heterostyly in preventing illegitimate pollination in dish-shaped flowers

Heterostyly is thought to have evolved because it confers efficiency of cross-pollination. While it can be an alternative to an incompatibility mechanism in tube-shaped flowers, its effectiveness in dish-shaped flowers, such as those of Fagopyrum esculentum, has not been demonstrated. Stigmatic pollen loads were examined over the daily course of pollination in a planted field of this crop species. Natural pollination by bees resulted in 23% of the pollen on pin flowers and 69% on thrum flowers being legitimate. Both flower morphs therefore received the two pollen types in about the same proportions. Pin pollen constituted 74% of the pollen delivered to stigmata, even though the two morphs are present in equal numbers. Pin flowers collected about 75 pollen, and thrum flowers about 25 pollen by the end of the day of anthesis. The net result was about 15 compatible pollen per flower on both floral morphs. These results indicate that heterostyly is associated with excess pin-pollen delivery rather than excess legitimate-pollen delivery in F. esculentum.     

An unusual homostyle inHedyotis caerulea (Rubiaceae)

Hedyotis caerulea, a spring flowering herb widely distributed in eastern North America, has distylous flowers that differ by a number of morphological and physiological traits. The presence of a strong incompatibility system is indicated by the fact that intramorph crosses or self-pollinations produced little or no seed and intermorph crosses produced copious seed. An unusual homostyle was located that had most floral characters intermediate between pin and thrum flowers, although its pollen size was that of pin. The homostyle was only moderately self-compatible, and its pollen did not behave as pin pollen on thrum stigmas. It appears that, despite their intermediate position and morphology, the homostyle stigmas were fundamentally pin. Although the exceptional rarity of the homostyle suggests that it is an inadaptive failure, workers should watch for its further occurrence since additional study of it may provide insights into the genetic and physiological basis of heterostyly in theRubiaceae.     

Cryptic self‐incompatibility and distyly in Hedyotis acutangula Champ. (Rubiaceae)

Distyly, floral polymorphism frequently associated with reciprocal herkogamy, self‐ and intramorph incompatibility and secondary dimorphism, constitutes an important sexual system in the Rubiaceae. Here we report an unusual kind of distyly associated with self‐ and/or intramorph compatibility in a perennial herb, Hedyotis acutangula. Floral morphology, ancillary dimorphisms and compatibility of the two morphs were studied. H. acutangula did not exhibit precise reciprocal herkogamy, but this did not affect the equality of floral morphs in the population, as usually found in distylous plants. Both pin and thrum pollen retained relatively high viability for 8 h. The pollen to ovule ratio was 72.5 in pin flowers and 54.4 in thrum flowers. Pistils of pin flowers remained receptive for longer than those of thrum flowers. No apparent difference in the germination rate of pin and thrum pollen grains was observed when cultured in vitro, although growth of thrum pollen tubes was much faster than that of pin pollen tubes. Artificial pollination revealed that pollen tube growth in legitimate intermorph crosses was faster than in either intramorph crosses or self‐pollination, suggesting the occurrence of cryptic self‐incompatibility in this species. Cryptic self‐incompatibility functioned differently in the two morphs, with pollen tube growth rates after legitimate and illegitimate pollination much more highly differentiated in pin flowers than in thrum flowers. No fruit was produced in emasculated netted flowers, suggesting the absence of apomixis. Our results indicate that H. acutangula is distylous, with a cryptic self‐incompatibility breeding system.     

Fluorescence and scanning electron microscopic study on self-incompatibility in distylous Averrhoa carambola L.

Various combinations of intermorph, selfing and intramorph pollinations were carried out in Averrhoa carambola and the pollinated pistils were observed under fluorescence and scanning electron microscopes in time-course experiments. In both compatible and incompatible pollinations, similar behavior of pollen germination and penetration was observed in the first 4 h after pollination. In compatible intermorph pollination, pollen tubes were found at the base of the transmitting tract of the style at 8 h and 24 h after pollination in both the pin and thrum morphs. With thrum flowers, selfing resulted in pollen tubes being uniformly arrested at the junction between the stigmatic and stylar tissues. Penetration of pollen tubes into the upper portion of style was observed in thrum intramorph pollination and, when the second member was treated as the gynoecial tissue under the same pollination, penetration of tubes was further enhanced. Pin flowers, on selfing, resulted in pollen tube penetration farther down the style than was the case with thrum selfing. Intramorph pollination of pin morph behaved in a similar manner to selfing and was not affected by genotypes. Beside the stamen-style dimorphism, the receptive surface of the cob stigma was larger in pin than thrum flowers. While pin pollen was round, thrum pollen was oblong in shape with pin to thrum ratio on the polar axis being 1.2 and on the equatorial axis 0.8. The stigma of pin morph belonged to the dry type, while that of the thrum resembled the wet type.     

Effects of population size on reproductive success of the endangered and endemic species Primula merrilliana

The reproductive ability related to the population size of the endangered and endemic species Primula merrilliana Schltr.was investigated.In 26 populations observed,only four contain more than 500 flowering individuals,whereas most of them(53.8%) consist of less than 100 flowering individuals.Though the ratio of pin and thrum plants keeps its balance at 1:1 for all populations,the frequency of pin and thrum flowers was significantly biased in most small populations.Population size strongly affected reproductive success; plants in small populations produced significantly fewer flowers and fruits per plant and fewer seeds per fruit,and therefore fewer seeds per plant.The floral density was another important factor that influenced the reproductive success of P.merrilliana,because four main reproductive success parameters (fruits per plant,seeds per fruit,seeds per plant,and the proportion of flowers setting fruit) were all positively correlated with floral density.The size of plants and the number of leaves per plant (measure of habitat quality) were not influenced by the variation of population size,suggesting that the reduced fecundity in small populations may not be a consequence of lower habitat quality,inbreeding depression and pollen limitation as a result of less attractiveness in small populations are therefore likely explanations for these patterns.