Pollination ecology of Silene acutifolia (Caryophyllaceae): floral traits variation and pollinator attraction

BACKGROUND AND AIMS: The floral display influences the composition of pollinators interacting with a plant species. Geographic and temporal variation in pollinator composition complicates the understanding of the evolutionary consequences of floral display variation. This paper analyses the relationships between Silene acutifolia, a hermaphroditic perennial herb, and its pollinators, based on field studies in the north-west of Spain. METHODS: Studies were conducted over three years (1997-1999). Firstly, the main pollinators of this species were determined for two years in one population. Secondly, pollen limitation in fruit and seed production was analysed by supplementary hand pollinations, and counting the pollen grains and tubes growing in styles for two different-sized populations. Finally, the effect of flower size and number on the rate of visitation and total seed number was examined for 15 marked plants. RESULTS AND CONCLUSIONS: The primary pollinators were long-tongued insects, including Hymenoptera, Lepidoptera and Diptera, but the composition and visitation frequencies differed between years. Pollen limitation occurred in one of the years of study. There was between-population variation in the number of pollen grains and pollen tubes found in styles, suggesting pollen limitation in one population. Overall, pollinators visited plants with more open flowers more frequently, and pollinated more flowers within these plants. Conversely, petal and calyx sizes had no effect on insect visitation. Plants with higher rates of visits produced higher number of seeds, suggesting that pollinator-mediated limitation of seed and fruit production may be important in some years.     

Intra-inflorescence variation in floral traits and reproductive success of the hermaphrodite Silene acutifolia

BACKGROUND AND AIMS: Intraspecific variation in floral components and reproductive success is often located at the intra-individual level. The arrangement of flowers within inflorescences may explain a great deal of this variation. The variation in number of ovules, fruit set, number of seeds per fruit, seed set, seed weight and seed germination is investigated at different positions within the inflorescence of Silene acutifolia. METHODS: Data were obtained in natural populations, and germination experiments were conducted in a germination chamber. KEY RESULTS: The number of ovules, fruit set, number of seeds, seed set and seed weight, showed a significant decline from early (primary) position to later (tertiary) position. The patterns of intra-inflorescence variation were consistent in different populations and years of study. Seed germination showed an opposite pattern, seeds from primary position showed the lowest germination percentages and seeds from tertiary position the highest, although the effect of position on germination was only marginally significant. There was significant among-population variation in number of ovules per flower. Fruit set also varied significantly among populations, with lower fruit set in the smaller and more isolated population. No significant among-population differences were detected in number of seeds per fruit and seed set. Seeds from the smallest and more isolated population (Arnado) were the lightest. Seed germination showed strong differences between populations, seeds from Arnado started to germinate later, and showed the lowest final germination percentages. CONCLUSIONS: Architectural effects or resource competition are the most commonly proposed hypothesis to explain these patterns. Data suggest that there is less pollen available to pollinate tertiary flowers, and that there is not enough outcross pollen in Arnado. The germination percentages suggest that there is variation in the source of pollen within inflorescences, with high probability of receiving outcross pollen in flowers from primary position, and higher probability of geitonogamous crosses in tertiary flowers.     

Effects of inbred/outbred crosses on progeny sex ratio in Silene latifolia (Caryophyllaceae)

Sex ratio polymorphism has been extensively studied in Silene latifolia, but it is neither known whether inbreeding (which is likely to occur under field conditions) affects it, nor which of the proposed mechanisms (Y degeneration, X-linked drive) is more important. Both mechanisms predict reduced pollen performance. In this study, females were crossed with pollen from related and unrelated males in single-donor and two-donor crosses, and the sex ratio of offspring (n = 866, 60 crosses), sons'in vitro pollen germination and sex ratios in parental families were scored. Flowers receiving only unrelated pollen produced a significant excess of sons. Sex ratios were not significantly correlated between generations. Sons'in vitro pollen germination was significantly negatively correlated with the 'sex-ratio phenotype' of maternal grandfathers, but not of fathers. This generation leap may be consistent with X-linked determinants because Y-linked determinants alone cannot explain it (grandfathers, fathers and sons share the same Y chromosome). Further work is required, but inbreeding and limited dispersal may lead to local accumulation of biasing factors, a process potentially countered by conditional shifts to produce more sons in pure outbred crosses.     

Inbreeding and outbreeding effects on pollen fitness and zygote survival in Silene nutans (Caryophyllaceae)

In plants, selfing and outcrossing may be affected by maternal mate choice and competition among pollen and zygotes. To evaluate this in Silene nutans, we pollinated plants with mixtures of (1) self‐ and outcross pollen and (2) pollen from within a population and from another population. Pollen fitness and zygote survival was estimated from the zygote survival and paternity of seeds. Self pollen had a lower fitness than outcross pollen, and selfed zygotes were less likely, or as likely, to develop into seeds. Hybrid zygotes survived as frequently or more than local zygotes, and pollen from one of the populations fertilized most ovules in both populations. Our results thus indicate strong maternal discrimination against selfing, whereas the success of outbreeding seems mostly affected by divergent pollen performance. The implications for the evolution of maternal mate choice are discussed.     

Mating system instability in Schiedea menziesii (Caryophyllaceae)

We investigated inbreeding depression and selfing in hermaphroditic Schiedea menziesii to assess the stability of the breeding system. A combination of high selfing rates and strong inbreeding depression suggests that the mating system is unstable. The population-level selfing rate measured in three years ranged considerably from 0.425 (SE = 0.138) to 0.704 (0.048); family measures of selfing rate varied from zero to one in all three years. Inbreeding coefficients did not differ from zero, suggesting that inbred plants do not survive to reproduction in the field. Average inbreeding depression measured in two greenhouse experiments was 0.608-0.870, with values for individual plants ranging from -0.170 to 0.940. The magnitude of inbreeding depression expressed at different life-history stages depended on experimental conditions. When plants were grown during the winter, inbreeding depression was expressed at early and late life-history stages. When plants were grown during the summer, inbreeding depression was detected for germination but not for later life-history stages. Inbreeding depression for vegetative and inflorescence biomass was also measured using field-collected seeds where cross status was assigned using genotypes determined electrophoretically. We did not detect a relation between inbreeding depression and the selfing rate at the level of the individual plant. We saw no evidence for intrafloral selfing, suggesting that the evolution of increased selfing through autogamy is unlikely, despite high selfing rates. A more likely outcome of breeding system instability is the evolution of gynodioecy, which occurs in species of Schiedea closely related to S. menziesii. Females have been detected in progeny of S. menziesii that have been raised in the greenhouse. In the absence of biotic pollen vectors, the failure of these females to establish in the natural population may result from the absence of adaptations for wind pollination.     

Variation in the intensity of inbreeding depression among successive life-cycle stages and generations in gynodioecious Silene vulgaris (Caryophyllaceae)

Inbreeding depression is one of the hypotheses explaining the maintenance of females within gynodioecious plant populations. However, the measurement of fitness components in selfed and outcrossed progeny depends on life-cycle stage and the history of inbreeding. Comparative data indicate that strong inbreeding depression is more likely to occur at later life-cycle stages. We used hermaphrodite individuals of Silene vulgaris originating from three populations located in different valleys in the Swiss Alps to investigate the effect of two generations of self- and cross-fertilization on fitness components among successive stages of the life cycle in a glasshouse experiment. We detected significant inbreeding depression for most life-cycle stages including: the number of viable and aborted seeds per fruit, probability of germination, above ground biomass, probability of flowering, number of flowers per plant, flower size and pollen viability. Overall, the intensity of inbreeding depression increased among successive stages of the life cycle and cumulative inbreeding depression was significantly stronger in the first generation (delta approximately 0.5) compared with the second generation (delta approximately 0.35). We found no evidence for synergistic epistasis in our experiment. Our finding of more intense inbreeding depression during later stages of the life cycle may help to explain the maintenance of females in gynodioecious populations of S. vulgaris because purging of genetic load is less likely to occur.     

Breeding system in a population of Trigonella balansae (Leguminosae)

BACKGROUND AND AIMS: Although some taxonomic studies in the genus Trigonella have been conducted, there has been no concerted effort to study the breeding system. This paper examines the floral structure and pollination system in a population of T. balansae, an annual pasture legume. METHODS: Floral morphology, hand and vector pollination, stigma receptivity, pollen tube growth, using scanning electron and fluorescence microscopy, were conducted. KEY RESULTS: Measurements of floral structure from before to after anthesis indicates an inability for T. balansae to self-pollinate and a requirement for an external vector to effectively transfer pollen from the anthers onto the stigmas of this species. Seed set can be obtained by hand or honeybee manipulation of T. balansae flowers. CONCLUSIONS: Trigonella balansae is a self-compatible species, but which requires vectors such as honeybees to bring about pollination.     

Inbreeding effects on progeny sex ratio and gender variation in the gynodioecious Silene vulgaris (Caryophyllaceae)

In gynodioecious species, sex expression is generally determined through cytoplasmic male sterility genes interacting with nuclear restorers of the male function. With dominant restorers, there may be an excess of females in the progeny of self-fertilized compared with cross-fertilized hermaphrodites. Moreover, the effect of inbreeding on late stages of the life cycle remains poorly explored. Here, we used hermaphrodites of the gynodioecious Silene vulgaris originating from three populations located in different valleys in the Alps to investigate the effects of two generations of self- and cross-fertilization on sex ratio and gender variation. We detected an increase in females in the progeny of selfed compared with outcrossed hermaphrodites and inbreeding depression for female and male fertility. Male fertility correlated positively with sex ratio differences between outbred and inbred progeny, suggesting that dominant restorers are likely to influence male fertility qualitatively and quantitatively in S. vulgaris. We argue that the excess of females in the progeny of selfed compared with outcrossed hermaphrodites and inbreeding depression for gamete production may contribute to the maintenance of females in gynodioecious populations of S. vulgaris because purging of the genetic load is less likely to occur.     

Gender polymorphism in flowers of Silene vulgaris (Moench) Garcke (Caryophyllaceae)

Silene vulgaris occurs in Israel as a stolonifcrous tetraploid which forms mostly sexually polymorphic populations. Fully hermaphrodite and fully male-sterile plants are linked by a range of intermediate forms which are gynomonoecious and bear hermaphrodite, male-sterile, and partially male-sterile flowers. Male-sterile flowers differ from hermaphrodite ones by having a narrower calyx, a shorter corolla, a thicker style, and a considerably larger stigmatic area with longer papillae which are more markedly bulbous basally. In individual anthers of hermaphrodite flowers, diameters of stained pollen grains showed a continuous variation from 36 to 80 μm with a modal value of 56. About 30% of pollen was found to be non-stainable. Reduction in male fertility occurs at different levels, from small or inviable pollen grains through abortion of single anthers, abortion of entire androecia, and through different degrees of gynomonoecy to totally male-sterile individuals. The proportion of stainable pollen to ovules in hermaphrodite flowers alone was found to be less than 100. From the data presented, less than a third of all ovules produced in the different kinds of flower develop into seeds. This may reflect an inadequacy of pollen, which the local population of S. vulgaris can tolerate because of its stoloniferous habit.     

Population Size,Pollination and Breeding System of Silene stockenii Chater (Caryophyllaceae), an Annual Gynodioecious Species of Southern Spain

The distribution area, phenology, sex polymorphism, floral characteristics and breeding system of Silene stockenii (Caryophyllaceae), a narrowly endemic annual species of southern Spain, were studied. Only five populations were found in a total area of 2 ha. Silene stockenii is a gynodioecious species with fully female, fully hermaphrodite and intermediate plants bearing hermaphrodite, male-sterile, and partially male-sterile flowers. Male-sterile flowers are typically smaller than hermaphrodites. Nectar production was significantly higher in hermaphrodite plants and during the female phase of hermaphrodite flowers. The red flowers appear during the spring (March-May) and are pollinated by long-tongued Bombyliidae. Hand pollinations revealed that the species is self-compatible; however, natural self-pollination is rare due to marked protandry. Hand pollination significantly increased the number of seeds per fruit and seed set, indicating limited pollination in the field. In controlled pollinations female plants of S. stockenii produced higher seed set than hermaphrodite plants, but in freely pollinated plants fruit set and seed production was similar in both morphs, indicating that pollinators do not discriminate in favour of hermaphrodite plants.     

Unstable B chromosomes in Silene maritima With. (Caryophyllaceae)

Unstable B chromosomes in Silene maritima With. (Carophyllaceae). B chromosomes have been found in 7 out of 39 populations of Silene maritima With. (Carophyllaceae) studied. These 7 populations are all from the Norfolk coast. The B chromosome is small, 0.8 μm long, telocentric and appears euchromatic. Within plants the B chromosome is highly unstable with different numbers in cells of the same root or pollen mother cells (pmcs) of an anther. The transmission of the Bs is efficient and in controlled crosses progeny plants usually have higher mean B frequencies than the B-containing parents. At metaphase I the pairing behaviour of the Bs is variable and they often appear as univalents. In general the presence of Bs in pmcs results in an increase in chiasma frequency/cell and populations with Bs have higher mean chiasma frequencies than those without Bs. Cloned plants grown under stress conditions induced by the addition of NaCl to a nutrient solution showed no difference in mean B number/cell or distribution when compared with controls grown in nutrient solution only.     

Within/between population crosses reveal genetic basis for siring success in Silene latifolia (Caryophyllaceae)

Divergence at reproductive traits can generate barriers among populations, and may result from several mechanisms, including drift, local selection and co-adaptation between the sexes. Intersexual co-adaptation can arise through sexually antagonistic co-evolution, a timely hypothesis addressed in animals but, to our knowledge, not yet in flowering plants. We investigated whether male and female population of origin affected pollen competition success, offspring fitness and sex ratio in crosses within/between six genetically differentiated populations of the white campion, Silene latifolia. Each female was crossed with pollen from one focus male from the same population, and pollen from two focus males from two distinct populations, both as single-donor and two-donor crosses against a fixed tester male with a 2-h interpollination interval (n = 288 crosses). We analysed paternity with microsatellite DNA. Male populations of origin significantly differed for siring success and in vitro pollen germination rates. In vitro pollen germination rate was heritable. Siring success also depended on sex ratio in the female family of origin, but only in between-population crosses. In some female populations, two-donor crosses produced less female-biased sex ratios compared with single-donor crosses, yet in other female populations the reverse was true. Offspring sex ratio varied with donor number, depending on the female population. Within/between population crosses did not differ significantly in seed set or offspring fitness, nor were siring success and offspring fitness significantly correlated. Altogether this suggests reproductive divergence for traits affecting pollen competition in S. latifolia.     

Numerical analysis of infraspecific variation in European Silene alba and S. dioica(Caryophyllaceae)

Infraspecific variation among European populations of Silene alba and S. dioica was analysed by non-metric multidimensional scaling, which was applied to K-dissimilarity matrices based either on 16 seed characters or on 34 seed, flower and capsule characters. The material used for analysis consisted of 167 collections of wild seed, largely obtained from European botanic gardens, and 97 sets of mature greenhouse-grown progeny. S. alba was found to show topoclinal variation, primarily along an axis from the Iberian peninsula through central Europe to Romania. This pattern is not reflected in current taxonomic treatments. N. and N.W. European S. alba is morphologically heterogeneous and has probably been introduced from various southern sources. S. dioica shows less total variation between populations than S. alba , and consists of local ecogeographic races that intergrade at their margins. The two species are completely discriminated by seed, flower and capsule characters when used in combination, but S. alba populations from the eastern end of the cline closely approach S. dioica in seed morphology. This convergence is unlikely to be an effect of recent introgression.     

Maternal sex and mate relatedness affect offspring quality in the gynodioecious Silene acaulis

In gynodioecious species, females sacrifice fitness by not producing pollen, and hence must have a fitness advantage over hermaphrodites. Because females are obligately outcrossed, they may derive a fitness advantage by avoiding selfing and inbreeding depression. However, both sexes are capable of biparental inbreeding, and there are currently few estimates of the independent effects of maternal sex and multiple levels of inbreeding on female advantage. To test these hypotheses, females and hermaphrodites from six Alaskan populations of Silene acaulis were crossed with pollen from self (hermaphrodites only), a sibling, a random plant within the same population, and a plant from a different population. Germination, survivorship and early growth revealed inbreeding depression for selfs and higher germination but reduced growth in sib-crosses, relative to outcrosses. Independent of mate relatedness, females germinated more seeds that grew faster than offspring from hermaphrodites. This indicates that inbreeding depression as well as maternal sex can influence breeding system evolution. The effect of maternal sex may be explained by higher performance of female genotypes and a greater abundance of female genotypes among the offspring of female mothers.     

Effects of fire on the demography of the endangered,geophytic herb Silene spaldingii (Caryophyllaceae)

Understanding the effects of disturbances such as fire on plant demography helps elucidate the mechanisms that cause changes in community composition. I studied the effects of spring and fall fires on Silene spaldingii, an endangered perennial herb of grasslands in northwest Montana. Individual S. spaldingii plants were mapped, and size and flowering were recorded for 1 yr prior and 5 yr subsequent to the burn treatments. Enhanced seedling recruitment (70–410%) and a 22% increase in population size were the principal effects of fire on S. spaldingii, and fall burn plots had lower recruitment than spring burn plots. These effects were apparent for 2–3 yr following the treatments. Fire had no detectable effect on the survival of adults or recruits of S. spaldingii. Silene spaldingii exhibits prolonged dormancy in which plants do not produce aboveground vegetation for one to several consecutive years. Results suggest that fire has a positive effect on the population dynamics of S. spaldingii by removing litter and creating safe sites for recruitment. Prescribed fire should be an important tool for managing populations of this rare plant.     

Isolation and characterization of highly polymorphic microsatellite loci in the bladder campion,Silene vulgaris (Caryophyllaceae)

This study reports the isolation and characterization of seven highly polymorphic microsatellite loci in Silene vulgaris (Caryophyllaceae). The loci were isolated from two libraries constructed from genomic DNA enriched for CA and GA repeats. These markers yielded nine to 40 alleles per locus (mean 22.1) in a survey of 45 individuals from a single population located in the western Swiss Alps. Average observed heterozygosity ranged from 16.2 to 77.4%. These microsatellite loci should be valuable tools for studying fine‐scale genetic structure.     

Gynomonoecy in Silene italica (Caryophyllaceae): Sexual Phenotypes in Natural Populations

Abstract: Some gynodloecious species present not only hermaphrodite and female individuals but also gynomonoecious individuals that produce both female and perfect flowers. In a study of natural populations of Silene italica , we found that gynomonoecious individuals were common in all populations (from 13 to 39%) and that their frequency was independent of the frequency of females. No relationship was found between the number of flowers a plant produced and its sex. The majority of gynomonoecious individuals showed a low proportion of female flowers but all proportions were found. Female flowers of these individuals were well-developed flowers that set fruit as often as female flowers on pure females. The percentage fruit set was characteristic of the type of flowers (around 56% for perfect flowers and 76% for female flowers) whatever the sex of the individual plant. The determination of sex is not known but data indicate that environmental conditions could be responsible for a shift between the hermaphrodite and the gynomonoecious states rather than between the female and the gynomonoecious states.     

Climate and clinal variation in seed morphology of the white campion, Silene latifolia (Caryophyllaceae)

Silene latifolia (white campion) is widespread in W Eurasia and shows clinal variation in seed morphology. Two extensive parapatric races, within which seed morphology changes gradually with geographic distance, are separated by a transition zone where the morphological gradient is steeper. A sample of seeds from across the species' native range was scored for morphological characters and analysed using detrended correspondence analysis ordination and multiple regression, in order to simplify mapping of the variation pattern and to investigate statistical relationships with climatic variables. The transition zone, as defined by ordination scores, follows the convolutions of the 0C isotherm for January mean temperature along its entire length from N Europe to the Middle East. January temperature proved to be a highly significant predictor of seed morphology even after the removal of simple spatial trends. Although the origins of comparable large-scale clines have often been interpreted in historical terms, the result reported here supports a role for climatic selection in maintaining the existing variation pattern.     

Resource limitation and pollen source (self and outcross) affecting seed production in two louseworts, Pedicularis siphonantha and P. longiflora (Orobanchaceae)

Two animal-pollinated hermaphrodite plants, Pedicularis siphonantha and P. longiflora , have been used to investigate factors limiting seed production in natural populations. To evaluate the potential seed abortion due to resources limitation, seed development has been observed and seed count conducted twice. Seed production per capsule has been compared when flowers have been removed and in a control group. Open pollination has been investigated and pollen supplementation undertaken to estimate the possibility of pollen limitation. Results show that seed abortion is frequent. Stigmatic pollen load is significantly higher than ovule number per ovary under open pollination for both species. Additional self and outcross pollen did not affect seed production. Flower removal significantly increases seed production per capsule, which indicates that seed production of the studied species is limited by available resources. To detect differences in seed production between flowers pollinated by self and outcross pollen, hand pollination of bagged flowers has also been conducted in natural populations of the two Pedicularis species. Compared with open pollination, hand-pollinating self-pollen decreases, while outcross pollen increases seed production per capsule. Such results suggest that inbreeding depression in the two self-compatible species may also result in partial seed abortion under open pollination if mixed pollen is deposited on the stigma. Our results also suggest that pollen interference plays an important role in low female fertility in the two Pedicularis species.  © 2005 The Linnean Society of London, Botanical Journal of the Linnean Society , 2005, 147 , 83–89.