Heterostyly inPrimula. 2. Sites of pollen inhibition,and effects of pistil constituents on compatible and incompatible pollen-tube growth

Summary In incompatible (intramorph) pollinations of the heterostylousPrimula vulgaris, pollen germination or tube growth may be partially inhibited in several sites associated with the stigma or style. Blockage may occur, a) on the stigma surface through the failure of germination or of pollen tube penetration after germination, b) in the stigma head during the passage of the tube through the specialized transmitting tissue of the head, or c) in the transmitting tract of the style. None of the barriers is complete, and the prohibition of selfing or intramorph crossing depends upon the cumulative screening effect of one following upon the other. In both morphs, the germination of incompatible pollen on the stigma is enhanced in high ambient relative humidity, but many tubes still fail to penetrate the stigma. Those that do are retarded or blocked in their growth in the transmitting tissues of the stigma head and style. Crude extracts from the tissues of the stigma head and style show some differential effect on the growth of pollen tubesin vitro, and dialysates of extracts containing high molecular weight fractions show a consistent differential effect, those from thrum tissues retarding thrum tubes while having a lesser effect on pin tubes, and those from pin tissues retarding pin tubes while having lesser effect on thrum. It is suggested that the factors influencing tube growth are present in the intercellular secretions of the transmitting tract.     

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.     

Distyly and pollen dimorphism in<Emphasis Type="Italic"> Damnacanthus</Emphasis> (Rubiaceae)

We document distyly in Damnacanthus (Rubiaceae) and investigate pollen morphology in the genus. Within nine taxa in Japan (four species, four varieties and one form), five show distyly. Non-distylous taxa of Damnacanthus always have a long style and short stamens. We examined the pollen morphology of all taxa of Damnacanthus in Japan. The pollen grains of Damnacanthus are 3- to 5-orthocolporate or sometimes loxocolporate. Polymorphic pollen is sometimes observed even within a single flower. Pollen dimorphism is associated with distyly; pollen grains from short-styled (thrum) flowers are significantly larger than those from long-styled (pin) flowers, and the muri are smooth in pin flowers whereas the pollen grains of thrum flowers have minute granules on the top and/or on the sides. In non-distylous taxa, in which the style is longer than stamens, the muri are always provided with minute granules. Interestingly, the pollen grains with granules in the non-distylous taxa contrast with the pollen of the distylous taxa, because granulate pollen grains are found only in thrum flowers. Although Damnacanthus is a stenopalynous genus, the number of colpi is useful for delimiting taxa. Based on palynological characters, we conclude that Damnacanthus should be revised. Electronic Publication     

濒危植物安徽羽叶报春两种花型的繁育特性及其适应进化

二型花柱的维持机制和自然选择压力多年来一直是生态学和进化学研究领域的热点之一。通过实验室栽培和野外观察统计相结合的方法,对安徽羽叶报春两种花型(长柱花和短柱花)的形态特征、花粉活力、柱头可授性、花粉/胚珠比、自然授粉及结籽能力、自交亲和性等繁育特性进行了比较研究。结果表明:长、短柱花的花冠直径和裂片宽无明显差异,而花冠筒、雌蕊和雄蕊高、花粉数目及大小、P/O比均有显著差异。在自然条件下,长柱花所接受的总花粉数要明显高于短柱花的总花粉数,但所接受的异型花花粉数和平均每果结籽数两者无显著差异。长柱花和短柱花的花粉和柱头活力相似,均能在较长时间内维持较高活力,仅在开花末期显著下降。两种花型的花在自花授粉、同型异花授粉、异型花授粉条件下均能结籽,但异型花授粉的结籽数均明显高于自花授粉和同型异花授粉结籽数。在长柱花各种授粉方式中,花粉萌发率无明显差异,但异型花花粉管的生长速度明显比同型异花花粉和自花花粉的快,而在短柱花柱头上表现为异型花授粉的萌发率最高,但只要萌发后在花柱中的生长速度无显明差异。此外,综合上述结果,对二种花型花部综合征的维持机制及自然选择压力进行了探讨。     

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.     

Studies on heteromorphic self-incompatibility systems: Physiological aspects of the incompatibility system of Primula obconica

Summary In Primula obconica, a species with a heteromorphic self-incompatibility system, the distinction between compatible and incompatible pollen tubes takes place on the stigma surface in thrum flowers, self tubes growing randomly over the papillar cells. No differences were seen between self and cross tube behaviour on the pin stigma surface, but self tubes were inhibited within the stigmatic tissue with differences in tube length evident after 24 h. The stigma surface bears a proteinaceous pellicle and binds the lectin Concanavalin A. Removal of the stigma removes the incompatibility barrier in mature gynoecia. Bud pollination shows that pollen tubes cannot grow in a normal manner on immature stigmas; the random growth of tubes over the stigma surface resembles that of mature thrum selfs. Fewer compatible tubes reach the style base of young gynoecia and smaller numbers of seeds are set than in mature flowers. Pin and thrum pollen grains germinate and grow in aqueous media, thrum tubes growing longer than pin. The presence of H₃BO₄ and CaCl₂ in the growth medium promotes tube elongation and lengths equivalent to compatible styles can be obtained. The pollen grains have proteinaceous materials in their walls which diffuse out on moistening. Prolonged washing in aqueous media removes these materials but the incompatibility reaction remains unchanged. Thus the incompatibility reaction is between pollen tubes and stigmatic tissue and differs from the homomorphic, sporophytic system where pollen wall proteins elicit the incompatibility response.     

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.     

Pollen flow inPrimula veris (Primulaceae)

Observations on two natural and one artificial distylous populations ofPrimula veris in Britain revealed very high proportions of intramorph pollen on pin stigmas and moderate proportions of intermorph pollen on thrum stigmas. The average ratio of pin: thrum pollen production per flower was 2.4:1. Large amounts of pollen remained in dehisced anthers, perhaps reflecting low levels of pollinator activity. Population size and density had no clear effect on pollen flow patterns, which are asymmetrical for the two morphs.     

Effects of indiscriminate foraging by tropical hummingbirds on pollination and plant reproductive success: experiments with two tropical treelets (Rubiaceae)

Summary In cloud forest at Monteverde, Costa Rica, two common treelets (Palicourea lasiorrachis and Cephaelis elata, both Rubiaceae) depend simultaneously on one hummingbird population (Lampornis calolaema) for pollination. Both species are distylous and self-incompatible. In laboratory experiments, we examined possible effects of indiscriminate foraging by hummingbirds among flowers of both species, as observed in the field, on pollination of Palicourea. In each of 35 trials, captive L. calolaema probed 2 flowers from pin plants of Palicourea followed by 20 thrum flowers of the same species, with either 0, 2, or 10 Cephaelis flowers intervening. We assessed pollen transfer by staining and counting pin pollen tubes growing in thrum styles; counts of 0, 1, or 2 pollen tubes relate directly to seed output (0, 1, or 2 seeds per fruit, respectively). Intervening Cephaelis flowers sharply reduced pollen receipt by thrum flowers of Palicourea and reduced some aspects of pollen dispersal from pins as well, thereby curtailing maternal and paternal reproductive potential of Palicourea. Such effects of interspecific pollen loss on reproductive output may lead to strong competition among some, though not all, combinations of plant species pollinated by L. calolaema or of other plant combinations that share animal pollinators.     

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.     

Heterostyly inPrimula. 1. Fine-structural and cytochemical features of the stigma and style inPrimula vulgaris huds

Summary The stigmas of the heterostylous genusPrimula are of the dry type without a free-flowing surface secretion. The papillae of the stigma surface cells of the two morphs, in pin (stigma exserted) and thrum (stamens exserted), bear a thin proteinaceous surface pellicle, overlying a discontinuous cuticle. The vacuoles of the papillate cells contain tannins, and tannin cells extend in files through the stigma heads and form a loose sheath surrounding the pollen-tube transmitting tract in the styles. The cells of the transmitting tissue in the stigma heads have a normal complement of organelles, and abundant ribosomal endoplasmic reticulum. The intercellular spaces contain an internal secretion which reacts cytochemically for both carbohydrate and protein. The transmitting tract in the styles forms a central core surrounded by several vascular strands. The cells are elongated, and the intercellular spaces here also have a carbohydrate-protein content. In a compatible pollination, thrum pollen tubes enter the stigma by penetrating the cuticle at the tip or on the flank of the pin papilla. Pin tubes on the thrum stigma enter between adjacent papillae, penetrating the thin cuticle at the base. The tubes grow through the transmitting tracts in the intercellular material.     

甜荞pin型花与thrum型花雌雄蕊发育的比较研究

为从形态上研究甜荞(Fagopyrum esculentum)二型花发育的差异性,该文运用石蜡切片技术对甜荞pin型花和thrum型花大、小孢子发生和雌、雄配子体的发育过程进行观察比较.结果表明:(1)甜荞2种花的雌蕊、雄蕊发育的细胞学特征有很高的相似性,具直生胚珠、双珠被、厚珠心;大孢子四分体直线型排列,合点端为功能...     

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.     

The control of incompatibility in distylous Pulmonaria affinis Jordan (Boraginaceae)

In P. affinis, pin pollen is shorter on average than thrum pollen. Pins have more coronal hooks on stigmatic papillae than thrums, but gaps between papillae are relatively smaller for thrums. Pin stigmas receive more pollen than thrum stigmas. Thrum stigmas receive more (dissortive) pin pollen, but pin stigmas are assortively pollinated. Pollen only germinates when trapped below papilla coronas. On thrum stigmas, most trapped pollen is pin. Pollen germination is better on thrum stigmas than pin stigmas, and thrum stigmas show a close relationship between numbers of legitimate pollen grains, numbers of germinating grains, and numbers of pollen tubes in the style. There is no inhibition of illegitimate pollen germination. Illegitimate pollen tubes are inhibited in the style. Incompatibility operates by a combination of dissortive pollination, dissortive pollen trapping, and stylar pollen tube inhibition. All heteromorphic features differing between pins and thrums are implicated in the inhibition of within-morph fertilization in thrums.     

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.     

Pollen flow in a population of Primula vulgaris Huds.

A British population of Primula vulgaris was visited four times between 1971 and 1976 for study of the nature and functioning of distyly. The stainability and production of pollen from pins exceeded those of thrums. About 20% of pin pollen and 48% of thrum pollen was removed from anthers by insect visitors. Pins and thrums were present in a 1:1 ratio in 1971 but pins outnumbered thrums in 1976. Thrums produced more seeds per flower than pins though the number of ovules produced by each was similar. Two methods for collecting stigmas from open flowers and analyzing them for pollen loans produced somewhat different results. For pin stigmas, the pollen load consisted of 2–23% thrum pollen; for thrum stigmas, the pollen load consisted of 0–71% pin pollen with most stigmas having less than 50% pin pollen. In general, intermorph pollen flow is less than would be expected if pollen flow were random. It is probable that most intramorph pollen on stigmas is a result of self- or geitonogamous pollination. The extensive literature concerning the natural pollinators of the primrose is reviewed. Although Darwin's hypothesis concerning the functional significance of distyly in promoting intermorph pollination was never quantified, the pollen flow patterns observed in P. vulgaris are unexpectedly deviant and are similar to those patterns observed in several unrelated heterostylous species in other plant families.     

Morph-specific differences in reproductive success in the distylous Primula veris in a context of habitat fragmentation

Heterostylous self-incompatible plant species are particularly sensitive to habitat fragmentation and to disruption of pollination processes because of the need of intermorph cross-pollination for producing seeds. Heterostyly is characterized by sexual polymorphism through the occurrence of two (distyly) or three (tristyly) morph types that differ in floral traits (style length and anther position). We examined whether the long-styled (pin) and short-styled (thrum) morph types show differences in reproductive components and responses to habitat fragmentation in the distylous, self-incompatible perennial herb Primula veris. We documented reproductive components for pin and thrum individuals and their relationships with population size, plant density and morph ratio (pin frequency), in nine populations from Flanders (northern Belgium) located in fragmented habitats of the intensively used agricultural landscape. Seed abortion increased in small populations as a result of inbreeding depression. Fruit set increased with plant density. Seed set was positively related to pin proportion. Seed set was higher for pin than thrum in small populations, but lower in large populations. Two hypotheses can be considered to explain these morph-specific differences: a pollen transfer asymmetry, and a reproductive advantage for the partially self-compatible pin morph. Morph types appear to respond differently to habitat fragmentation constraints. A floral morph type showing partial self-compatibility may be favored in populations under pollination failure, because it can increase reproductive success and mating opportunities through intramorph crosses.     

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.     

Seasonal changes in pollen limitation and femaleness along the snowmelt gradient in a distylous alpine herb,Primula modesta

Flowering phenology of alpine plants is strongly determined by the timing of snowmelt, and the conditions of pollination of widely distributed plants vary greatly during their flowering season. We examined the reproductive success of the distylous alpine herb, Primula modesta, along the snowmelt gradient under natural conditions, and compared it with the result of artificial pollination experiments. In addition, the compositions and visit frequencies of pollinators to the flower of P. modesta were examined during the flowering period. The pin and thrum plants of P. modesta growing at the same site have an equal ability to produce seeds if a sufficient amount of legitimate pollen grains are deposited on the stigma surface. However, under natural conditions, their seed‐set success was often (even if not always) restricted by pollen limitation, and the functional gender of the pin and thrum plants biased to the female and male, respectively, associated with their growing sites. These variations were not ascribed to resource limitation nor biased morph ratio but to the seasonal changes in pollination situations, a replacement of pollinator types from long‐ to short‐tongued pollinators resulted in unidirectional pollen transfer from long stamens (thrum plants) to long styles (pin plants). The functional gender specialization may enhance the evolution of dioecy from heterostyly, but the severe pollen limitation may cause the breakdown of heterostyly into homostyly. To consider the evolutionary pathway of heterostylous plants, an accumulation of the empirical data is required demonstrating how phenological synchrony between plants and pollinators is decided and to what degree this relationship is stable over years, along with estimates of selection and gene flow in individual plants.