AOBPreview originally published online on October 24, 2002
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Annals of Botany 90: 691-699, 2002
© 2002 Annals of Botany Company
Breeding System in the Dichogamous Hermaphrodite Silene acutifolia (Caryophyllaceae)
1 Department of Botany, Pharmacy, University of Santiago de Compostela, E-15782 Santiago de Compostela, Spain
* For correspondence. Fax +34 981 594912, e-mail bvmbuide{at}usc.es
Received: 14 May 2002; Returned for revision: 5 August 2002; Accepted: 30 August 2002 Published electronically: 24 October 2002
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
The breeding system of the dichogamous hermaphrodite species Silene acutifolia, endemic to north-west Spain and north and central Portugal, is examined. Pollen germinability and style–stigma receptivity were analysed to determine whether protandry is a barrier to self-fertilization. By 48 h after anthesis, pollen germinability had declined to approx. 10 %. The short straight styles are not receptive when flowers first open. They gradually elongate and curve outwards, develop stigma papillae and become receptive. There is no clear separation between stigma and style: the stigma papillae appear in a line along the length of the style. Fruit set is high regardless of pollen source; however, seed set is significantly reduced after both spontaneous and facilitated autogamy. Seed set following spontaneous autogamy was 30 % (86 % in controls) in 1998 and 33 % (87 % in controls) in 1999. Seed set following facilitated autogamy was 62 % (86 % in controls) in 1998 and 67 % (89 % in controls) in 1999. Thus, separation of the male and female phases does not prevent production of seeds by self-pollination, although it does reduce the likelihood of this. Furthermore, results of the present experiments indicate that this species has no self-incompatibility mechanisms (self-compatibility index = 0·98). The selfing rate in the study population was 0·41, which is supported by the lack of self-incompatibility systems and by the incomplete protandry.
Key words: Floral biology, inbreeding depression, pollen germinability, protandry, self-fertilization, Silene acutifolia, stigmatic receptivity.
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Dichogamy is the temporal separation of anther dehiscence and stigma receptivity (whether at the single-flower or whole-plant level), and has classically been described as a mechanism for reducing self-fertilization (Faegri and van der Pijl, 1979; Lande and Schemske, 1985; for a review, see Bertin and Newman, 1993). It has been suggested that protogyny is more effective than protandry for minimizing self-fertilization because it guarantees a period during which there is no pollen available; this applies both to within-flower protandry in hermaphrodite flowers, and to flower protandry in monoecious species (Bertin, 1993; but see also Griffin et al., 2000). By contrast, in protandric dichogamy, self-pollen may often remain on the stigmas when these become receptive, permitting self-fertilization (Bertin, 1993).
In species that lack physiological self-incompatibility, the contamination of stigmas with self-pollen can lead to self-fertilization, which may reduce fitness if selfed offspring suffer inbreeding depression (Griffin et al., 2000). Therefore, in cases where inbreeding depression occurs, dichogamy may be selected to avoid the negative consequences of selfing. Inbreeding depression is considered to be the strongest selective pressure acting against self-pollination (Lande and Schemske, 1985; Holsinger, 1996; Ortiz-Barney and Ackerman, 1999). If there is no inbreeding depression or other opposing forces, genes determining self-pollination will increase in frequency in each generation (Fisher, 1941). In a given population, the frequency of self-pollination can be expected to increase if inbreeding depression is less than 50 %, and to decrease if inbreeding depression is more than 50 % (Lande and Schemske, 1985; Barrett and Kohn, 1991). However, the sensitivity to inbreeding depression may evolve in conjunction with the mating system, and prolonged selfing decreases inbreeding depression by reducing genetic load (Charlesworth and Charlesworth, 1987; Barrett and Harder, 1996; Husband and Schemske, 1996). Furthermore, selfers tend to express most inbreeding depression late in their life cycles, whereas outcrossing species tend to express substantial inbreeding depression throughout their life cycles (Barrett and Harder, 1996).
Dichogamy is not simply a mechanism to prevent self-fertilization. Lloyd and Webb (1986) suggest that selection to avoid interference between pollen presentation and stigma receptivity is widespread in plants exhibiting cross-pollination, and that this selection is fully or partially responsible for diverse floral characteristics, including dichogamy. Bertin (1993) showed that both dichogamy and monoecy are present with equal frequency in self-compatible and self-incompatible plants. As self-incompatibility is itself sufficient to avoid self-fertilization, this suggests that dichogamy is more than just a mechanism for increasing female reproductive success by avoiding self-fertilization (Harder et al., 2000).
Silene L. is a particularly interesting genus in which to study the evolution of reproductive and breeding systems. The species belonging to this genus have a wide variety of reproductive systems, including gynodioecy, gynomonoecy, hermaphroditism and dioecy. The best documented pathways to gender dimorphism are: that from cosexuality leading to gynodioecy, and often to dioecy; that from monoecy via paradioecy to dioecy; that from cosexuality via androdioecy to dioecy; that from heterostyly to dioecy; and that from duodichogamy or heterodichogamy to dioecy (Webb, 1999). There have been numerous published reports on the gynodioecious and gynomonoecious species of Silene, including S. vulgaris (Charlesworth and Laporte, 1998; McCauley, 1998; McCauley and Brock, 1998; Andersson, 1999; Taylor et al., 1999), S. uniflora (Pettersson, 1997; Runyeon and Prentice, 1997), S. stockenii (Talavera et al., 1996), S. acaulis (Shykoff, 1992; Alatalo and Molau, 1995; Maurice et al., 1998), S. italica (Maurice, 1999), as well as in dioecious species, including S. latifolia (Gross and Soule, 1981; Purrington, 1993; Grant et al., 1994; Guttman and Charlesworth, 1998; Purrington and Schmitt, 1998; Lardon et al., 1999), S. dioica (Elmqvist et al., 1993; Giles and Goudet, 1997; Carlsson-Granér et al., 1998; Hemborg and Karlsson, 1999) and S. otites (Soldaat et al., 1997). In a phylogenetic study of this genus, Desfeux et al. (1996) proposed that apart from the pathway from gynodioecy to dioecy, evolution from gynodioecy to hermaphodism has occurred. Despite the large number of studies of dimorphic species, studies of hermaphrodite species remain less common [e.g. S. douglasii (Kephart et al., 1999) and S. regia (Menges, 1995)]. In this study the breeding system and the functional significance of protandry in reducing self-fertilization were investigated in Silene acutifolia Link ex Rohrb. (Caryophyllaceae), a hermaphrodite spring-flowering species endemic to north-west Spain and north and central Portugal (Talavera, 1990; Buide and Guitián, 2002).
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
The plant and the study area
Silene acutifolia is endemic to north-west Spain and north and central Portugal (Talavera, 1990). It typically grows in rocky habitats. It is a perennial species, with a rosette of basal leaves and a variable number of fertile stems bearing dichasium-type inflorescences. The number of flowers per plant varied from one to 310 in the 91 plants analysed in the present study. Flowers are purplish-pink in colour and are hypogynous. The gynoecium has three carpels united to form a compound ovary with three styles that are short when the flowers open and that elongate gradually. The ten stamens are introrse, and when elongate the anthers group in the upper part of the flower. The ten stamens are inserted in two sets, five to the bottom of the ovary and five to the bottom of petals. Despite this distribution, dehiscence does not occur in two sets: when the flower opens usually only one or two stamens are fertile, the others being short and indehiscent; these gradually elongate and open. The fruit is a dehiscent capsule. The seeds are ornamented on the surface. In the study area, the principal pollinators are hymenopterans (Bombus and Anthophora), although the pollinator spectrum is broad and also includes dipterans and lepidopterans (M. L. Buide, unpubl. res.).This study was performed at Peares (42°26'N 07°42'W; 200 m a.s.l.) in north-west Spain. In this area, S. acutifolia grows on siliceous rocky outcrops along the sides of the Sil River Canyon. Most plants are thus inaccessible, limiting the number of individuals available for study. The study was performed between April and June 1998 and April and June 1999. In the population studied, flowering begins in April. The mean duration of individual flowering was 36 d in 1997 and 23 d in 1998 (Buide and Guitián, 2002).
Stigma receptivity
To determine the stigma length that corresponds to optimum stigma receptivity, pollen germination on stigmas was assessed, as was subsequent pollen tube growth at various stigma lengths. For one flower in each of 21 different plants, the numbers of pollen grains (n = 14), germinated pollen grains (n = 13) and pollen tubes (n = 21) were counted in the three stigma–styles of each flower. The flowers were emasculated and bagged prior to anthesis; every flower was hand-pollinated at a different developmental stage, from flowers just after anthesis, with very short styles, to flowers approaching senescence, with long, curved styles. Once hand-pollinated, the flower was bagged again, then harvested 24 h after pollination and fixed in FAA (95 % ethanol, distilled water, 37–40 % formaldehyde, glacial acetic acid; 10 : 7 : 2 : 1) to allow subsequent examination of pollen grains and pollen tubes by fluorescence microscopy. Style length was measured with the aid of a stereomicroscope, and styles were then stored in 70 % ethanol. Pistils were cleared and softened with sodium hydroxide (8 N NaOH) for about 6 h, rinsed in tap water for 1–2 h to eliminate NaOH, then stained with aniline blue previously decoloured with K2HPO4 for approx. 12 h. This method causes callose in the pollen grains and tubes to be stained and to fluoresce brightly under short-wave (UV) light (see Kearns and Inouye, 1993; Kalinganire et al., 2000). Samples were viewed through an epifluorescence photomicroscope, and the number of adherent pollen grains, the number of germinated pollen grains and the number of well-developed pollen tubes (i.e. pollen tubes extending at least halfway down the style) were counted.
In vitro pollen germination
Variation in pollen germination capacity with grain age was investigated by in vitro experiments. Five flower buds from five different individuals were bagged, and pollen was collected just after the flower opened. The flowers were then bagged again and pollen was collected 24 h later from the same anthers and again 48 h later. Pollen was collected using a closed micropipette, and was sown onto Petri dishes containing agar with 30 % sucrose. This culture medium was chosen because it gave the best results in preliminary trials with agar containing 0, 10, 30 or 50 % sucrose, with or without additional nutrients (boric acid, potassium nitrate, calcium nitrate and magnesium sulfate). Petri dishes were transported to the laboratory and the percentage of pollen that had germinated 6 h after sowing was estimated using a stereomicroscope.
Crossing systems
To analyse the effects of pollen source on final fruit and seed set, various pollination experiments were performed during spring 1998 and 1999. Because of the small number of accessible plants with sufficient flowers, we could not apply all treatments to each plant, although there was always a control of each treatment in the same plant. We marked a total of 19 plants in 1998 and 60 plants in 1999. In 1998, pollination treatments were applied as follows: (1) autonomous autogamy, in which buds were bagged and bags remained closed until the corolla withered; (2) facilitated autogamy, in which buds were bagged and the three stigmas were hand-pollinated with pollen of the same flower, before being bagged again until the corolla withered; (3) geitonogamy, in which buds were emasculated, bagged and hand-pollinated with pollen of other flowers of the same plant; and (4) xenogamy, in which buds were emasculated, bagged and hand-pollinated with pollen of flowers of different plants. For each treatment we included a control flower (no manipulation) on a different branch (to avoid possible effects of manipulation on other flowers of the same branch) of the same plant. We also set up another control to eliminate the (not expected) possibility that flowers of S. acutifolia could reproduce by agamospermy or be wind pollinated, in which flowers were emasculated and bagged without hand-pollination (pollen exclusion). In spring 1998 bagging was done with mosquito netting with a 1-mm mesh. During the bagging period, we observed that small ants (Leptothorax unifasciatus) were able to pass through the mesh, raising the possibility that our results were confounded by ant-mediated pollination. To eliminate this possibility in 1999, flowers were bagged with green gauze with a 0·2-mm mesh, which did not permit ant access. To investigate whether the mesh used for bagging in 1999 had any effect on normal development, additional experiments were performed in which previously bagged supplementary flowers were hand-pollinated, and their final fruit and seed set compared with that of controls that were hand-pollinated but not bagged. In 1999, pollen exclusion was also considered as a treatment with its own control. The remaining treatments in 1999 were as in 1998.
Each flower was bagged individually before anthesis, with the bag being closed around the pedicle using copper wire. For each bagged flower, a control flower was tagged on the same plant, in the same position in its inflorescence and at the same phenological stage. Within each treatment different plants were always used. Manual pollination was performed using hard, blue (to show collected pollen more clearly) plastic triangles folded along the median. Stigmas of flowers in all treatments were pollinated at the female phase. As variation in fruit and seed production is partly due to intrinsic factors, each treatment (including the control) was performed using flowers of the same plant, but on different branches to avoid possible effects of manipulation on other flowers of the same inflorescence.
Measures of selfing rate and inbreeding depression
The selfing rate was calculated following Charlesworth (1987, 1988):
s = (px – po)/(px – ps)
where px is the chosen indicator of reproductive performance following cross pollination, ps is that indicator following autogamy and po is that indicator following natural pollination.
Authors who have calculated the selfing rate using Charlesworth’s method have sometimes used ps based upon pollen from the same flower (Kron et al., 1993; Escaravage et al., 1997), or have used ps based upon pollen from other flowers of the same plant (Aizen and Basilio, 1995). In the present study, ps was determined using pollen from other flowers of the same plant (i.e. geitonogamy), because we consider that the lower seed set following autogamy is due to protandry, not to genetic incompatibility.
Levels of inbreeding depression (
) were determined on the basis of the relationship between the seed set of self-pollinated flowers (ws) and of cross-pollinated flowers (wc) (Charlesworth and Charlesworth, 1987; Kephart et al., 1999). As for calculation of the selfing rate, self-pollination was with pollen from other flowers of the same plant. The inbreeding depression level was calculated as:
= 1 – (ws/wc)
Self-compatibility index and self-fertilization index
The self-compatibility index (SCI) and the self-fertilization index (SFI) were calculated according to Lloyd and Schoen (1992). SCI is the mean seed set (or fruit set) for manually self-pollinated flowers divided by the mean seed set (or fruit set) for cross-pollinated flowers. As in the case of selfing rate and inbreeding depression level, self-pollination was with pollen from other flowers of the same plant; this approach has been used by other authors (e.g. Pickering, 1997), although some authors (e.g. Escaravage et al., 1997) have used pollen from the same flower.
Statistical analyses
Relationships between style–stigma length and the number of adherent grains of pollen, the number of germinated grains or the number of pollen tubes were investigated by simple linear regression. Pollen germination percentages at different times after anther dehiscence were compared using the Kruskal–Wallis non-parametric test. Fruit production was compared between treatments and controls using Fisher’s exact test. Because the seed-set data were not normally distributed, ratios were compared with corresponding controls using Wilcoxon’s test.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Stigma receptivity
The length of the stigma–style was positively and significantly related to the number of pollen grains adhering (Fig. 1A), the number of pollen grains germinating (Fig. 1B) and the number of pollen tubes in the style (Fig. 1C). Below a certain threshold length, styles do not have developed papillae and do not allow pollen grains to adhere, germinate or develop pollen tubes. In spite of the significant positive regression (Fig. 1), data above these thresholds showed high dispersion, and there was no clear linear relationship. No pollen grains were observed on style–stigmas shorter than 6·2 mm (Fig. 1A). When the flower opens, styles are short and the stigma papillae are undeveloped; pollen grains do not adhere (Fig. 2A). As the style grows and the papillae develop (Fig. 2B), adherent pollen grains are observed with increasing frequency (Fig. 2C). There is no clear delimitation between style and stigma: papillae are present over the entire surface of the stigma (Fig. 2D), and in a row running longitudinally along the style (Fig. 2C and F). No germinated pollen grains were observed on styles shorter than 8·4 mm (Fig. 1B), nor were well-developed pollen tubes observed in styles shorter than 9 mm (Fig. 1C). Once the pollen grain had germinated, pollen tubes grew and developed callose plugs (Fig. 2E and F). As the style–stigma grows it curves, eventually attaining the mature appearance shown in Fig. 2G and H.
|
|
In vitro pollen germination
Germination percentages were high in pollen from recently opened flowers (68 %); 24 h later only 42 % of pollen from these anthers germinates, and this figure falls to 10 % 48 h later (Kruskal–Wallis test P = 0·001) (Fig. 3), showing that the pollen that remains in anthers without being collected by visitors becomes inviable.
|
Crossing systems
In 1998, 43 % of the bagged and emasculated flowers that did not receive manual pollination (pollen exclusion control) produced fruit (Fig. 4A). Mean seed set for these fruits was 22 % (Fig. 5A). In these experiments, bagging was with 1-mm mesh; in 1999, using 0·2-mm mesh, none of the flowers set fruit (Fig. 4B). Neither fruit nor seed set differed significantly between bagged flowers (0·2-mm mesh) that received supplementary pollination and unbagged flowers that received supplementary pollination (bagging effects, Figs 4B and 5B). This confirms that bagging with 0·2-mm mesh did not affect reproductive success.
|
|
In general, levels of fruit set were high in both years (Fig. 4), except for pollen exclusion treatments. No significant differences were observed between autonomous autogamy and the no-manipulation controls, either in 1998 (Fisher’s exact test, P = 0·48, n = 22) or in 1999 (P = 0·69, n = 40). Similarly, fruit set did not differ between flowers subjected to facilitated autogamy and the corresponding controls: in 1998, all flowers produced fruits in both groups (n = 14), and in 1999 fruits were produced by 85 % of facilitated autogamy flowers and 95 % of control flowers (Fisher’s exact test, P = 0·61, n = 40). In addition, there were no significant differences in fruit set between flowers that received pollen from the same plant and the corresponding controls (Fisher’s exact test, P = 1·00, n = 16 in 1998; P = 0·48, n = 32 in 1999). In 1998, all flowers to which pollen from other plants was applied set fruit, as did untreated control flowers (n = 12) (Fig. 4). In 1999 similar results were obtained: fruit set was 94 % for cross-pollinated and control flowers, and 100 % for emasculated flowers (n = 45).
Among-treatment differences in seed set were more marked than among-treatment differences in fruit set, with similar patterns in both years (Fig. 5). In both years seed set was significantly lower in the autonomous autogamy treatment than in the corresponding controls (Table 1). A less marked reduction with respect to controls was also observed in the facilitated autogamy treatment. Seed set in the geitonogamy treatment was practically identical to that in the corresponding controls in both years. Likewise, there was no difference between the xenogamy treatment and corresponding controls.
|
Selfing rate, inbreeding depression, self-compatibility and self-fertility
The selfing rate was 0·41. The value of inbreeding depression, measured at the level of seed set as a result of autogamous crosses with respect to xenogamous crosses, was 0·019. The value of the SCI was 0·98, and that of the SFI was 0·36.
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Silene acutifolia is a hermaphrodite plant that shows temporal separation of pollen presentation and stigma receptivity. At the start of anthesis only one or two stamens are elongated and fertile, and the three stigma–styles are short and not receptive. Fluorescence microscopy of stigma–styles of emasculated-and-bagged flowers pollinated at different times after anthesis showed that pollen does not adhere to or germinate in styles until they grow beyond a certain size. As they grow longer styles show increasing receptivity. When anthers first dehisced, pollen germinability was high; it subsequently declined to approx. 10 % 48 h later. Similar results in pollen germinability were found in S. dioica, with a reduction to near-zero within 48 h (Bassani et al., 1994). The same was observed by Nyman (1992) in Campanula dichotoma and was interpreted as a mechanism for promoting xenogamy. In contrast, other Campanula species studied by this author showed an increase in pollen germination capacity over time, as also found by Navarro (1996, 1997) in the genus Petrocoptis, now considered as Silene (Mayol and Roselló, 1999). This separation between male and female phases is supported by the reduction in seed set following facilitated autogamy. Specifically, seed set may have been reduced in this treatment because little viable pollen remained in each flower at stigma receptibility, when the hand-pollenation was performed. Furthermore, the production of seeds by self-pollination is greatly reduced if pollinator access is prevented, as indicated by the low self-fertility index (0·36). Nonetheless, this temporal separation between the male and female phases is incomplete, as shown the fact that fruit set is not reduced and that some seeds are produced after both spontaneous autogamy and facilitated autogamy. The introrse stamens also facilitate pollen deposition on the stigmas of the same flower, as they elongate and separate outwards.
On the other hand, high levels of fruit and seed set following pollination using pollen from the same genet in geitonogamous crosses also support the fact that reduced seed set in facultative autogamy is due to protandry. Because geitonogamous treatments are self-fertile, protandry is not effective at preventing within-plant self-fertilization. The type of inflorescence also contributes to increase within plant selfing. As defined by Lloyd and Webb (1986), the inflorescence is asynchronous because not all flowers are in the same sexual phase at a given moment. This is a consequence of this species’ dichasium-type inflorescence, in which first a central flower forms and then the lateral flowers develop sequentially. Thus, some flowers within a given inflorescence will show dehiscent anthers, whereas others will, at the same time, show receptive stigmas. The same pattern occurs among different inflorescences within a single plant. This heterogeneity contributes to increasing self-pollination, and is observed in other species exhibiting protandry at the floral level (Ortega Olivencia and Devesa Alcaraz, 1993; Arizaga et al., 2000); with protandry selfing rates expected to be greater in early flowers than in later ones (Brunet and Charlesworth, 1995).
The ten stamens of S. acutifolia elongate and mature gradually, so that viable pollen remains available for several days despite the loss of pollen germinability. This gradual exposure of the anthers may be seen as a mechanism for extending the period of pollen presentation (Lloyd and Webb, 1986; Harder and Thomson, 1989). Specifically, Lloyd and Yates (1982) suggest that extension in time of the pollen presentation phase may act to increase male reproductive success by increasing the time during which visitors can collect pollen; by contrast, if pollen is presented in relatively large amounts over a relatively short period, there is a high risk that one or a few individuals will remove much of the pollen and then not transfer it to another plant (see also Thomson and Barrett, 1981; Thomson et al., 1982; Thomson et al., 1989). Wilson et al. (1994) suggested that male reproductive success might be associated with prolongation of pollen presentation when the insect visitation rate is high; this is indeed the case in S. acutifolia according to fruit and seed set data obtained for controls, and following observation of the insect visitation rate (M. L. Buide, unpubl. res.).
Dichogamy may be a mechanism to avoid interference between pollen and stigmas (Lloyd and Yates, 1982; Lloyd and Webb, 1986; Bertin, 1993; Bertin and Newman, 1993; Mahy and Jacquemart, 1999). In this model, protandry may be advantageous because it segregates pollen and stigmas in time and prevents them obstructing each other (Lloyd and Yates, 1982). In S. acutifolia, as in other protandrous species, the styles are initially erect and remain together; later they extend outwards to expose the pollination surface. This means that the pollen is first exposed without interference from the styles (Lloyd and Webb, 1986).
The high values of the self-compatibility index (0·98) indicate that this species does not have self-incompatibility mechanisms. The degree of inbreeding depression detected in seed set was very low (0·019). When values of inbreeding depression are less than 0·5, theory predicts that selfing is favoured (Charlesworth and Charlesworth, 1987), and Husband and Schemske’s (1996) review of 54 species indicated that the degree of inbreeding depression is inversely related to the frequency of autogamy. The value of the selfing rate obtained in the present study was 0·41. The low values of inbreeding depression in S. acutifolia may be because the study population has already been subjected to inbreeding, so that lethal and deleterious recessive alleles have been eliminated from the population. An alternative possibility is that the apparently low levels of inbreeding depression are attributable to the fact that only seed set was measured in the present experiment, and inbreeding depression may be manifest in other phases of the life cycle. For example, in a study of the caryophyllacean Schiedea membranacea, Culley et al. (1999) suggested that the high levels of inbreeding depression found in inflorescence biomass, versus the much lower level at earlier stages, are possibly the result of expression of many mildly deleterious alleles, not of immediately lethal alleles [see also Johnston (1992), and references therein, for examples of late expression of inbreeding depression]. In the hermaphrodite Silene douglasii, Kephart et al. (1999) found high levels of inbreeding depression for maternal families and using population means, with higher levels in early and late life stages and lowest levels for survival. Other authors have detected inbreeding depression in seed production (Husband and Schemske, 1995; Kittelson and Maron, 2000).
Silene acutifolia is a hermaphrodite species, all plants of which have only bisexual flowers. It has been noted that hermaphroditism may not be as frequent in the genus Silene as suggested by many floras, which tend not to refer to gynodioecy. Silene acutifolia is, in addition, self-compatible. One of the most common pathways hypothesized for the evolution of reproductive systems involving dioecy in flowering plants is hermaphroditism 
gynodioecy dioecy (see the different pathways in Webb, 1999). In contrast with this common view, the phylogenetic study of Desfeux et al. (1996) suggests that in Silene the ancestral reproductive system is gynodioecy, which has evolved at different times to dioecy, or to hermaphoditism. The latter is viewed by these authors as part of a trend towards autogamy. The fact that in S. acutifolia dichogamy is not a barrier for self-fertilization supports this view. This would therefore suggest that S. acutifolia has arisen relatively recently within the genus. However, further experimental studies on the breeding systems of species of the genus Silene are necessary to clarify the evolutionary history of this group.
|
ACKNOWLEDGEMENTS |
|---|
The authors thank the Xunta de Galicia for partial support of this work with a predoctoral fellowship and the USC with a postdoctoral fellowship to M.L.B.; R. Anadón (USC, Spain) for help with fluorescence microscopy; Iberdrola (especially J. S. Andrés and B. Cid), who facilitated our stay at the San Pedro Hydroelectric Station in Os Peares during fieldwork; and A. Traveset (IMEDEA, CSIC, Spain) and L. Navarro (USC, Spain) for helpful comments. We also thank S. R. Kephart for useful comments on the final manuscript. Preparation of the manuscript was partially supported by the Dirección General de Investigación (B0S2000-1122-C03-01).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
-
Aizen MA, Basilio A. 1995. Within and among flower sex-phase distribution in Alstroemeria aurea (Alstroemeriaceae). Canadian Journal of Botany 73: 1986–1994.
Alatalo JM, Molau U. 1995. Effect of altitude on the sex ratio in populations of Silene acaulis (Caryophyllaceae). Nordic Journal of Botany 15: 251–256.[Web of Science]
Andersson H. 1999. Female and hermaphrodite flowers on a chimeric gynomonoecious Silene vulgaris plant produce offspring with different genders: a case of heteroplasmic sex determination? Journal of Heredity 90: 563–565.
Arizaga S, Ezcurra E, Peters E, Ramírez de Arellano F, Vega E. 2000. Pollination ecology of Agave macroacantha (Agavaceae) in a Mexican tropical desert. I. Floral biology and pollination mechanisms. American Journal of Botany 87: 1004–1010.
Barrett SCH, Harder LD. 1996. Ecology and evolution of plant mating. Trends in Ecology and Evolution 11: 73–79.
Barrett SCH, Kohn JR. 1991. Genetic and evolutionary consequences of small population size in plants: implications for conservation. In: Falk DA, Holsinger KE, eds. Genetics and conservation of rare plants. New York: Oxford University Press, 3–30.
Bassani M, Pacini E, Franchi GG. 1994. Humidity stress responses in pollen of anemophilous and entomophilous species. Grana 33: 146–150.
Beattie AJ. 1985. The evolutionary ecology of ant-plant mutualisms. New York: Cambridge University Press.
Bertin RI. 1993. Incidence of monoecy and dichogamy in relation to self-fertilization in angiosperms. American Journal of Botany 80: 557–560.[CrossRef][Web of Science]
Bertin RI, Newman CM. 1993. Dichogamy in angiosperms. The Botanical Review 59: 112–152.[CrossRef]
Brunet J, Charlesworth D. 1995. Floral sex allocation in sequentially blooming plants. Evolution 49: 70–79.[CrossRef][Web of Science]
Buide ML, Guitián J. 2002. Flowering phenology and female reproductive success in Silene acutifolia Link ex Rohrb. Plant Ecology 163: 93–103.[CrossRef][Web of Science]
Carlsson-Granér U, Elmqvist T, Ågren J, Gardfjell H, Ingvarsson P. 1998. Floral sex ratios, disease and seed set in dioecious Silene dioica. Journal of Ecology 86: 79–91.[CrossRef]
Charlesworth D. 1988. A method for estimating outcrossing rates in natural populations of plants. Heredity 61: 469–471.[Web of Science]
Charlesworth D, Charlesworth B. 1987. Inbreeding depression and its evolutionary consequences. Annual Review of Ecology and Systematics 18: 237–268.[CrossRef][Web of Science]
Charlesworth D, Laporte V. 1998. The male-sterility polymorphism of Silene vulgaris: analysis of genetic data from two populations and comparison with Thymus vulgaris. Genetics 150: 1267–1282.
Culley TM, Weller SG, Sakai AK, Rankin AE. 1999. Inbreeding depression and selfing rates in a self-compatible, hermaphroditic species, Schiedea membranacea (Caryophyllaceae). American Journal of Botany 86: 980–987.
Darwin C. 1876. The effects of cross and self fertilization in the vegetable kingdom. London: John Murray.
Desfeux C, Maurice S, Henry JP, Lejeune B, Gouyon PH. 1996. Evolution of reproductive systems in the genus Silene. Proceedings of the Royal Society of London Series B, Biological Sciences 263: 409–414.[Medline]
Elmqvist T, Liu D, Carlsson U, Giles BE. 1993. Anther-smut infection in Silene dioica: variation in floral morphology and patterns of spore deposition. Oikos 68: 207–216.[CrossRef][Web of Science]
Escaravage N, Pornon A, Doche B, Till-Bottraud I. 1997. Breeding system in an alpine species: Rhododendron ferrugineum L. (Ericaceae) in the French northern Alps. Canadian Journal of Botany 75: 736–743.
Faegri K, van der Pijl L. 1979. The principles of pollination ecology, 3rd edn. Oxford: Pergamon Press.
Fisher RA. 1941. Average excess and average effect of a gene substitution. Annals of Eugenics 11: 53–63.
García MB, Antor RJ, Espadaller X. 1995. Ant pollination of the palaeoendemic dioecious Borderea pyrenaica (Dioscoreaceae). Plant Systematics and Evolution 198: 17–27.[CrossRef][Web of Science]
Giles BE, Goudet J. 1997. Genetic differentiation in Silene dioica metapopulations: Estimation of spatiotemporal effects in a successional plant species. American Naturalist 149: 507–526.[CrossRef][Web of Science]
Gómez JM, Zamora R, Hódar JA, García D. 1996. Experimental study of pollination by ants in Mediterranean high mountain and arid habitats. Oecologia 105: 236–242.[Web of Science]
Grant S, Hunkirchen B, Saedler H. 1994. Developmental differences between male and female flowers in the dioecious plant Silene latifolia. Plant Journal 6: 471–480.[CrossRef][Web of Science]
Griffin SR, Mavraganis K, Eckert CG. 2000. Experimental analysis of protogyny in Aquilegia canadensis (Ranunculaceae). American Journal of Botany 87: 1246–1256.
Gross KL, Soule JD. 1981. Differences in biomass allocation to reproductive and vegetative structures of male and female plants of a dioecious, perennial herb, Silene alba (Miller) Krause. American Journal of Botany 68: 801–807.[CrossRef][Web of Science]
Guttman DS, Charlesworth D. 1998. An X-linked gene with a degenerate Y-linked homologue in a dioecious plant. Nature 393: 263–266.[CrossRef][Medline]
Harder LD, Thomson JD. 1989. Evolutionary options for maximizing pollen dispersal of animal-pollinated plants. American Naturalist 133: 323–344.[CrossRef][Web of Science]
Harder LD, Barrett SCH, Cole WW. 2000. The mating consequences of sexual segregation within inflorescences of flowering plants. Proceedings of the Royal Society of London Series B, Biological Sciences 267: 315–320.[Medline]
Hemborg ÅM, Karlsson PS. 1999. Sexual differences in biomass and nutrient allocation in first-year Silene dioica plants. Oecologia 118: 453–460.[CrossRef][Web of Science]
Holsinger KE. 1996. Pollination biology and the evolution of mating systems in flowering plants. Evolutionary Biology 29: 107–149.[Web of Science]
Husband BC, Schemske DW. 1995. Magnitude and timing of inbreeding depression in a diploid population of Epilobium angustifolium (Onagraceae). Heredity 75: 206–215.[Web of Science]
Husband BC, Schemske DW. 1996. Evolution of the magnitude and timing of inbreeding depression in plants. Evolution 50: 54–70.
Johnston MO. 1992. Effects of cross and self-fertilization on progeny fitness in Lobelia cardinalis and L. siphilitica. Evolution 46: 688–702.[CrossRef][Web of Science]
Kalinganire A, Harwood CE, Slee MU, Simons AJ. 2000. Floral structure, stigma receptivity and pollen viability in relation to protandry and self-incompatibility in silky oak (Grevillea robusta A. Cunn.). Annals of Botany 86: 133–148.
Kearns CA, Inouye DW. 1993. Techniques for pollination biologists. Niwot: University Press of Colorado.
Kephart SR, Brown E, Hall J. 1999. Inbreeding depression and partial selfing: evolutionary implications of mixed-mating in a coastal endemic, Silene douglassi var. oraria (Caryophyllaceae). Heredity 82: 543–554.
Kittelson PM, Maron JL. 2000. Outcrossing rate and inbreeding depression in the perennial yellow bush lupine, Lupinus arboreus (Fabaceae). American Journal of Botany 87: 652–660.
Knight T. 1799. Experiments on the fecundation of vegetables. Philosophical Transactions of the Royal Society of London Series B, Biological Sciences 89: 195–204.
Kron P, Stewart SC, Back A. 1993. Self-compatibility, autonomous self-pollination, and insect-mediated pollination in the clonal species Iris versicolor. Canadian Journal of Botany 71: 1503–1509.
Lande R, Schemske DW. 1985. The evolution of self-fertilization and inbreeding depression in plants. I. Genetic models. Evolution 39: 24–40.[CrossRef][Web of Science]
Lardon A, Georgiev S, Aghmir A, Le Merrer G, Negrutiu I. 1999. Sexual dimorphism in white campion: complex control of carpel number is revealed by Y chromosome deletions. Genetics 151: 1173–1185.
Lloyd DG, Schoen DJ. 1992. Self- and cross-fertilization in plants. I. Functional dimensions. International Journal of Plant Sciences 153: 358–369.[CrossRef]
Lloyd DG, Webb CJ. 1986. The avoidance of interference between the presentation of pollen and stigmas in angiosperms. I. Dichogamy. New Zealand Journal of Botany 24: 135–162.[Web of Science]
Lloyd DG, Yates JM. 1982. Intrasexual selection and the segregation of pollen and stigmas in hermaphrodite plants, exemplified by Wahlenbergia albomarginata (Campanulaceae). Evolution 36: 903–913.[CrossRef][Web of Science]
McCauley DE. 1998. The genetic structure of a gynodioecious plant: nuclear and cytoplasmic genes. Evolution 52: 255–260.[CrossRef][Web of Science]
McCauley DE, Brock MT. 1998. Frequency-dependent fitness in Silene vulgaris, a gynodioecious plant. Evolution 52: 30–36.
Mahy G, Jacquemart AL. 1999. Early inbreeding depression and pollen competition in Calluna vulgaris (L.) Hull. Annals of Botany 83: 697–704.
Maurice S. 1999. Gynomonoecy in Silene italica (Caryophyllaceae): sexual phenotypes in natural populations. Plant Biology 1: 346–350.[CrossRef][Web of Science]
Maurice S, Desfeux M, Henry JP. 1998. Is Silene acaulis (Cary ophyllaceae) a trioecious species? Reproductive biology of two subspecies. Canadian Journal of Botany 76: 478–485.
Mayol M, Rosselló JA. 1999. A synopsis of Silene subgenus Petrocoptis (Caryophyllaceae). Taxon 48: 471–482.[CrossRef]
Menges ES. 1995. Factors limiting fecundity and germination in small populations of Silene regia (Caryophyllaceae), a rare hummingbird-pollinated prairie forb. American Midland Naturalist 133: 242–255.[CrossRef][Web of Science]
Navarro L. 1996. Biología reproductiva y conservación de dos endemismos del noroccidente ibérico: Petrocoptis grandiflora Rothm. y Petrocoptis viscosa Rothm. (Caryophyllaceae). PhD Thesis, University of Santiago de Compostela, Santiago, Spain.
Navarro L. 1997. Is the dichogamy of Salvia verbenaca (Lamiaceae) an effective barrier to self-fertilization? Plant Systematics and Evolution 207: 111–117.[CrossRef][Web of Science]
Nyman Y. 1992. Pollination mechanisms in six Campanula species (Campanulaceae). Plant Systematics and Evolution 181: 97–108.[CrossRef][Web of Science]
Ortega Olivencia A, Devesa Alcaraz JA. 1993. Sexual reproduction in some Scrophularia species (Scrophulariaceae) from the Iberian Peninsula and the Balearic Islands. Plant Systematics and Evolution 184: 159–174.[CrossRef]
Ortiz-Barney E, Ackerman JD. 1999. The cost of selfing in Encyclia cochleata (Orchidaceae). Plant Systematics and Evolution 219: 55–64.[CrossRef][Web of Science]
Pettersson MW. 1997. Solitary plants do as well as clumped ones in Silene uniflora (Caryophyllaceae). Ecography 20: 375–382.[CrossRef]
Pickering CM. 1997. Breeding systems of Australian Ranunculus in the alpine region. Nordic Journal of Botany 17: 613–620.
Purrington CB. 1993. Parental effects on progeny sex ratio, emergence, and flowering in Silene latifolia (Caryophyllaceae). Journal of Ecology 81: 807–811.[CrossRef]
Purrington CB, Schmitt J. 1998. Consequences of sexually dimorphic timing of emergence and flowering in Silene latifolia. Journal of Ecology 86: 397–404.[CrossRef]
Richards AJ. 1997. Plant breeding systems, 2nd edn. London: Chapman and Hall.
Runyeon H, Prentice HC. 1997. Genetic differentiation in the bladder campions, Silene vulgaris and S. uniflora (Caryophyllaceae), in Sweden. Biological Journal of the Linnean Society 61: 559–584.[CrossRef]
Shykoff JA. 1992. Sex polymorphism in Silene acaulis (Caryophyllaceae) and the possible role of sexual selection in maintaining females. American Journal of Botany 79: 138–143.[CrossRef][Web of Science]
Soldaat LL, Vetter B, Klotz S. 1997. Sex ratio in populations of Silene otites in relation to vegetation cover, population size and fungal infection. Journal of Vegetation Science 8: 697–702.
Talavera S. 1990. Silene L. In: Castroviejo S, Laínz M, López Gonzalez G, Monserrat P, Muñoz Garmendia F, Paiva J, Villar L, eds. Flora Ibérica: plantas vasculares de la Península Ibérica e Islas Baleares, vol. 2. Madrid: Real Jardín Botánico, CSIC, 313–406.
Talavera S, Arista M, Salgueiro FJ. 1996. Population size, pollination and breeding system of Silene stockenii Chater (Caryophyllaceae), an annual gynodioecious species of southern Spain. Botanica Acta 109: 333–339.[Web of Science]
Taylor DR, Trimble S, McCauley DE. 1999. Ecological genetics of gynodioecy in Silene vulgaris: relative fitness of females and hermaphrodites during the colonization process. Evolution 53: 745–751.[CrossRef][Web of Science]
Thomson JD, Barrett SCH. 1981. Temporal variation of gender in Aralia hispida Vent. (Araliaceae). Evolution 35: 1094–1107.[CrossRef][Web of Science]
Thomson JD, McKenna MA, Cruzan MB. 1989. Temporal patterns of nectar and pollen production in Aralia hispida: implications for reproductive success. Ecology 70: 1061–1068.[CrossRef][Web of Science]
Thomson JD, Maddison WP, Plowright RC. 1982. Behavior of bumble bee pollinators of Aralia hispida Vent. (Araliaceae). Oecologia 54: 326–336.[CrossRef][Web of Science]
Webb CJ. 1999. Empirical studies: evolution and maintenance of dimorphic breeding systems. In: Geber MA, Dawson TE, Delph LF, eds. Gender and sexual dimorphism in flowering plants. Berlin Heidelberg: Springer-Verlag, 61–95.
Wilson P, Thomson JD, Stanton ML, Rigney LP. 1994. Beyond floral Batemania: gender biases in selection for pollination success. American Naturalist 143: 283–296.[CrossRef][Web of Science]
CiteULike 
Connotea 
Del.icio.us What's this?
This article has been cited by other articles:
|
|
 |
|
  M{a} L. Buide Disentangling the causes of intrainflorescence variation in floral traits and fecundity in the hermaphrodite Silene acutifolia Am. J. Botany, April 1, 2008; 95(4): 490 - 497. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Lankinen, W. S. Armbruster, and L. Antonsen Delayed stigma receptivity in Collinsia heterophylla (Plantaginaceae): genetic variation and adaptive significance in relation to pollen competition, delayed self-pollination, and mating-system evolution Am. J. Botany, July 1, 2007; 94(7): 1183 - 1192. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. L. BUIDE Intra-inflorescence Variation in Floral Traits and Reproductive Success of the Hermaphrodite Silene acutifolia Ann. Bot., September 1, 2004; 94(3): 441 - 448. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||