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.     

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.     

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.     

ON THE DARWINIAN HYPOTHESIS OF THE ADAPTIVE SIGNIFICANCE OF TRISTYLY

Darwin proposed that the function of the stamen-style polymorphism in heterostylous plants is to increase the probability of legitimate (compatible) pollinations among the floral morphs. Conspicuous pollen trimorphism in tristylous Pontederia cordata enables a test of the hypothesis. Comparison of the composition of pollen loads in naturally pollinated stigmas of intact and emasculated flowers were made at a population in Paugh Lake, Ontario, which was visited primarily by bumblebees. The magnitude of legitimate pollination was analyzed by ANOVA. In intact flowers, significant legitimate pollination was detected in the long-styled morph only. Following emasculation legitimate pollination was evident in the long- and short-styled morphs, with the mid-styled morph just short of displaying significant legitimate pollination. Similar results were obtained by chi-square analysis. It has been suggested that heterostyly may reduce mutual interference between maternal and paternal reproductive function. Two aspects of pollen-stigma interference were investigated in P. cordata. The potential importance of stigmatic or stylar clogging by incompatible pollen was examined by controlled field pollinations and measurements of seed set. The results indicate that prior application of large amounts of incompatible pollen has no significant effect on the seed set of open-pollinated inflorescences. Comparison of legitimate pollen capture in intact and emasculated flowers provided no evidence that the presence of stamens within flowers of the floral morphs interferes with the receipt of legitimate pollen. Pollen-stigma interference remains to be demonstrated in heterostylous plants.     

EXPERIMENTAL STUDIES ON THE FUNCTIONAL SIGNIFICANCE OF HETEROSTYLY

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

Gender specialization in <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae), a distylous,hummingbird-pollinated treelet

Distyly has been interpreted as a mechanism that promotes cross-pollination among conspecific plants and as one of the routes leading to the evolution of dioecy. In one of the possible evolutionary pathways, pollinators may disrupt intermorph pollen flow, and, as a consequence, floral morphs may gradually specialize as either male or female (functional dioecy). Natural patterns of pollen deposition and fruit and seed production were estimated in Palicourea demissa (Rubiaceae) and used as parameters to assess functional gender differences between floral morphs. Pollen flow was asymmetrical in P. demissa. Long-styled flowers were more effective than short-styled flowers in pollen deposition towards compatible stigmas, whereas short-styled flowers were more effective in legitimate pollen receipt. Accordingly, short-styled plants produced more fruits and viable seeds than long-styled plants. The contributions of male and female function to the potential functional gender were equivalent in both morphs. However, the realized functional gender deviated significantly from the potential functional gender in both morphs, in which short-styled plants were more successful through their female function, but long-styled plants through their male function. If pollinators disrupt the complementarities of pollen transfer between the two morphs (asymmetric pollen flow), the expression of a more profitable gender is expected in each morph. Thus, our results support the hypothesis that dioecy may evolve in distylous populations through the gradual specialization of each morph as either male or female.     

Reproductive ecology of distylous Palicourea Padifolia (Rubiaceae) in a tropical montane cloud forest. I. Hummingbirds' effectiveness as pollen vectors

The adaptiveness of distyly has been typically investigated in terms of its female function, specifically pollen receipt. However, pollen loads on stigmas can only provide moderate support for Darwin's hypothesis of the promotion of legitimate crosses. To determine the effectiveness of hummingbirds as pollen vectors between floral morphs and the consequences in terms of male (pollen transfer) and female function (pollen receipt) in Palicourea padifolia (Rubiaceae), floral visitors, their foraging modes, and temporal patterns of floral visitation were observed and documented. Differences in pollen and stigma morphology, pollen flow, rates of pollen deposition, and/or stigmatic pollen loads were then evaluated for their contribution toward differences in reproductive output between floral morphs. A pollination experiment with stuffed hummingbirds that varied in bill size was done to evaluate the contribution of bill variation toward differences between floral morphs in pollen receipt and pollen transfer and female reproductive output. Anthers of long-styled flowers contained significantly more and smaller pollen grains than those of short-styled flowers, independently of corolla and anther lengths. The shape and orientation of the stigma lobes differed between morphs and were significantly longer among short-styled flowers. Hummingbird visitation rates did not differ significantly between floral morphs, and foraging movements from focal plants towards neighboring plants were independent of floral morph. Stigmatic pollen loads under field conditions and those after controlled hummingbird visitation, along with rates of pollen accumulation through the day indicated that stigmas of short-styled flowers receive proportionately more legitimate (intermorph) pollen grains than did those of long-styled flowers. However, the species of hummingbird was marginally significant in explaining variation in pollen deposition on stigmas. Lastly, intermorph pollinations of P. padifolia resulted in significant differences in fruit production between floral morphs, independent of pollination treatment and pollinator species; short-styled flowers proportionately developed almost twice the number of fruits developed by long-styled flowers.     

Frequency-dependent variation in reproductive success in Narcissus: implications for the maintenance of stigma-height dimorphism

Negative frequency-dependent selection is a major selective force maintaining sexual polymorphisms. However, empirical demonstrations of frequency-dependent reproductive success are rare, particularly in plants. We investigate this problem by manipulating the frequencies of style morphs in a natural population of Narcissus assoanus, a self-incompatible herb with style-length dimorphism and intra-morph compatibility. We predicted that the reproductive success of morphs would vary negatively with their frequency because of the effects of morph-specific differences in sex-organ position on patterns of pollen transfer. This prediction was generally supported. The fruit and seed set of the two morphs did not differ significantly in plots with 1 : 1 morph ratios. However, short-styled plants produced significantly fewer seeds than long-styled plants in monomorphic plots, and significantly more seeds than long-styled plants in plots with 'long-biased' morph ratios. These patterns indicate that in the absence of physiological barriers to intra-morph mating, negative frequency-dependent selection contributes to the maintenance of stylar polymorphism through inter-morph pollen transfer. Our experimental results also provide insights into the mechanisms governing the biased style-morph ratios in populations of Narcissus species.     

异型花柱植物喀什补血草的传粉者功能群与花粉转移效率

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

Gender specialization and factors affecting fruit set of the wind-pollinated heterodichogamous Juglans regia

Gender expression, flowering phenology, reproductive performance and factors affecting fruit set (i.e., flowering synchrony, size and distance to the nearest pollen donor) were investigated in a cultivated population of a wind-pollinated self-compatible heterodichogamous Juglans regia (Juglandaceae). Four flowering morphs, (i.e., protandrous, protogynous, male and female) were observed. The sexual functions of the protandrous and protogynous morphs were almost synchronous; however, they were not reciprocal, and the separation of male and female flowering within most monoecious individuals was not complete. Thus, within-morph mating and geitonogamous pollination may be common. The ratio of protandrous versus protogynous morphs was biased towards the protandrous morph, but the fruit set did not differ between the morphs, suggesting that the fruit set of the protandrous morph could be partly compensated by within-morph pollination. The ratio of the female flower number or fruit number to the total male catkin length was higher in the protogynous morph than in the protandrous morph and did not vary with plant size, suggesting that gender variation was not size dependent and that the sexual function of protandrous morphs was more male biased. Fruit set depended on plant size only for protandrous morphs. The fruit set of individual plants decreased with increasing distance to the nearest pollen donor regardless of morph, possibly because of pollen limitation. The fruit set of individual plants increased with flowering synchrony, indicating that flowering synchrony could affect reproductive success.     

Evolution towards dioecy in heterostylous populations

The effective gender of a plant is defined as the proportion of the plant's genes that are transmitted through pollen (its maleness) or through ovules (its femaleness). Formulae are derived that enable the average effective gender of the morphs of a heterostylous population and the gender of individual plants to be estimated. Estimates require a knowledge of the morph ratio and the seed set that results in each morph from self-fertilization and from legitimate and illegitimate cross-fertilizations. If no illegitimate fertilizations occur, the average gender of the morphs can be estimated from their seed production alone; in this situation the average femaleness of one morph is exactly equal to the average maleness of the other morph. The average gender of long- and short-styled morphs is calculated for populations of fiveCordia species (Boraginaceae) from published data ofOpler & al. (1975). In two species, both morphs transmit their genes equally through pollen and ovules. In the other species, the long-styled morph acts predominantly (or in one species exclusively) as an ovule parent and the short-styled morph succeeds predominantly (or exclusively) as a pollen parent.The features of the evolutionary pathway from heterostyly to dioecy and the selective forces that may be responsible are outlined.     

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.     

POPULATION STRUCTURE AND MORPH-SPECIFIC FITNESS DIFFERENCES IN TRISTYLOUS LYTHRUM SALICARIA

In tristylous plant populations, style-morph frequencies are governed by an interaction between frequency-dependent selection due to disassortative mating and stochastic processes. Provided that there are no inherent fitness differences among morphs, frequency-dependent selection should result in equal morph frequencies at equilibrium. Stochastic models indicate that the short-styled morph has the highest and the long-styled morph the lowest probability of being lost from local populations as a result of random processes. We surveyed the morph composition of 82 populations of the tristylous, self-incompatible herb Lythrum salicaria in two archipelagos, one in central and one in northern Sweden, located close to the range-margin of the species. To examine whether deviations from even morph frequencies can be explained by among-morph differences in reproductive success, we quantified flower and seed production in six and three populations in the northern and southern archipelago, respectively, and we recorded segregation ratios in offspring produced in six trimorphic populations in the northern area. Seed germination and offspring growth were studied in the greenhouse. Ninety percent of the populations in the southern archipelago (N = 31) and 69% of the populations in the northern archipelago (N = 35) were trimorphic; the remaining populations were dimorphic (only populations consisting of at least three flowering plants considered). Dimorphic populations were smaller than trimorphic populations, as predicted by stochastic models. There was a striking difference in the morph composition of L. salicaria populations between the two archipelagos. In the southern archipelago, there was a slight excess of the long-styled morph and a corresponding deficiency of the short-styled morph. In contrast, the northern populations were characterized by a marked deficiency of the mid-styled morph: the average frequency of the mid-styled morph in trimorphic populations was 0.21, and nine of eleven dimorphic populations lacked the mid-styled morph. In both archipelagos, the long-styled morph (the most common morph) produced about 20% fewer seeds per fruit than the other morphs. The long-styled morph also tended to produce fewer seeds per plant. A hand-pollination experiment performed in two of the northern populations indicated that seed production per flower was pollen-limited in the long-styled morph but not in the other two morphs. Seed germination and offspring size after 24 weeks of growth did not differ among morphs. The mid-styled morph tended to have a higher representation in the offspring than in the parental generation in all six trimorphic populations studied further indicating that the deficiency of the mid-styled morph in the northern archipelago does not represent an equilibrium. Taken together, the results do not support the hypothesis that morph-specific differences in reproductive success can account for deviations from even morph frequencies in L. salicaria. It is suggested that among-morph differences in other components of fitness and historical factors may contribute to the current morph structure.     

云南草寇花序内不同部位的性分配

两性花植物花序内的性分配常存在差异,资源竞争、结构效应、交配环境(雌雄异熟、传粉者定向访花行为等)或授粉不均匀等几种假说可以解释这种现象。为验证上述假说,该研究以云南草寇两种表型(雄先熟型和雌先熟型)为材料,分析了其花序内不同部位(基部、中部和顶部)的每花花粉数、胚珠数、花粉/胚珠比、结实率和结籽率,花序内传粉者的定向访花行为,以及人工辅助授粉和去花处理对结实率和结籽率的影响。结果表明:两种表型花序内每花花粉数不随部位而变化,每花胚珠数、结实率和结籽率由基部到顶部依次降低,每花花粉/胚珠比由基部到顶部依次增加,表明顶部花存在偏雄的性分配。人工辅助授粉后,结实率、结籽率仍由基部到顶部依次降低,表明授粉不均匀假说不能解释云南草寇花序内不同部位结实率、结籽率的差异。去除基部和中部花后,顶部花人工辅助授粉条件下的结实率、结籽率与基部花人工辅助授粉条件下的结实率、结籽率无差异,表明云南草寇花序内不同部位结实率、结籽率的差异主要由资源竞争引起。雌先熟表型每花花粉数、花粉/胚珠比高于雄先熟表型,表明两种表型存在性分配差异。传粉者主要先访问云南草寇基部的花,然后向顶部移动。云南草寇花序内顶部偏雄的性分配可能是由资源竞争和传粉者定向访花造成的。     

Pollination ecology in the narrow endemic winter-flowering Primula allionii (Primulaceae)

Reduction of pollen flow can affect plant abundance and population viability and cause selection on plant mating system and floral traits. Little is known on the effect of this phenomenon in species naturally restricted to small and isolated habitats, that may have developed strategies to cope with long-term isolation and small population size. We investigated the pollination ecology of the endemic distylous winter-flowering P. allionii to verify the possible limitation of female fitness due to reduced pollinator visits. We recorded a higher production of pollen grains in long-styled morph, and a higher seed set in short-styled morph. The high intra-morph variability of sexual organ position may explain the hybridization phenomena allowing and easier intra-morph pollination. The fruit set is constant, although its winter-flowering period might decrease pollen transfer. Nevertheless, the lower competition for pollinators with neighbouring plants and the long-lasting anthesis may offset its reproductive success. Even if our results show no evidence of imminent threats, changes in plant–pollinator interactions might increase inbreeding, resulting in an increased extinction risk.     

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.     

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.     

Dimorphisms and self-incompatibility in the distylous species <Emphasis Type="Italic">Palicourea demissa</Emphasis> (Rubiaceae): possible implications for its reproductive output

Distyly has been interpreted as a mechanism that favors cross-fertilization. In this research we describe floral attributes and ancillary floral polymorphisms typically associated to heterostylous plants in Palicourea demissa (Rubiaceae), a distylous shrub of the Venezuelan Andes cloud forests. A hand-pollination experiment was done to evaluate self- and intramorph incompatibility and female reproductive output in both floral morphs. The studied population was morphologically distylous but morph differences in most ancillary floral polymorphisms and reciprocity of the sexual organ heights were found. The floral morphs were self-incompatible and did not differ in fruit set under controlled cross-pollination conditions, but at the population level they exhibited imperfect reciprocal herkogamy. Fruits and seeds of short-styled plants were larger than those of long-styled plants and fruit set was higher in short-styled plants under natural conditions, suggesting a higher reproductive potential among short-styled plants. Given the 1:1 morph ratio within the studied population, further evidence is needed to determine the influence of floral visitors and seed dispersers in the expression of heterostyly in P. demissa under natural conditions.     

The Breeding System of Oxalis squamata,a Tristylous South American Species

Oxalis squamata is a perennial endemic herb of the Chilean-Argentine Andes previously reported as a distylous species. In this paper, analysis of floral morphology, incompatibility relations and population structure show that O. squamata possesses morphological and functional tristyly. The three floral morphs differ in calyx and corolla length, style and stamen length, pollen grain size and stylar pubescence. Results of hand pollinations showed the presence of a strong incompatibility system. Legitimate pollinations with the L and M morphs as seed parents produced approximately equal numbers of seeds per pollination. The S morph produced less than half the number of seeds per legitimate pollination compared to the other two morphs. Intra- and intermorph illegitimate pollinations produce almost no seeds. Morph representation in the population was isoplethic. Fruit and seed production did not differ significantly for the three morphs in the population. Comparison of experimental and natural production of fruits and seeds suggests that seed set in the natural population may be pollen-limited for the L and M morphs.     

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.