Annals of Botany 93: 107-113, 2004
© 2004 Annals of Botany Company
Variation in the Mating System of Vincetoxicum hirundinaria (Asclepiadaceae) in Peripherial Island Populations
Section of Ecology, Department of Biology, University of Turku, FIN-20014 Turku, Finland
* For correspondence. E-mail roosa.leimu{at}utu.fi
Received: 4 June 2003; Returned for revision: 12 September 2003; Accepted: 1 October 2003
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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• Background and Aims Self-fertility may be selected for in small and isolated plant populations of normally outcrossing species. In addition, adaptations for self-fertility are likely to arise in island populations and in populations that are located at the border of the species range. The mating system of Vincetoxicum hirundinaria (Asclepiadaceae) is examined in island populations that are located at the northern border of the species range.
• Methods Pollination experiments were conducted under glasshouse conditions with plants from four populations.
• Key Results The frequencies of self-fertile individuals were relatively high and did not differ among populations. Cross-pollination resulted in higher fruit set than self-pollination. However, fruit-set from self-pollination and cross-pollination did not differ in the self-fertile individuals. Interestingly, the proportion of aborted fruits was on average higher following cross-pollination than following self-pollination. No differences were observed in seed number or seed mass between self- and cross-pollinated fruits. Pollen tube growth following self- and cross-pollinations was indistinguishable.
• Conclusions The results of this study revealed that V. hirundinaria possess a mixed-mating system in the studied island populations. Evidence was also provided for a late-acting self-incompatibility system commonly observed in Asclepiadaceae. No clear signs of inbreeding depression were observed in the early stages of development.
Key words: Asclepiadaceae, cross-fertility, inbreeding depression, self-incompatibility, self-fertility, Vincetoxicum hirundinaria.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Cross-fertilization is advantageous since it usually results in a higher level of heterozygosity and thus superior progeny compared with self-fertilization (Lloyd and Schoen, 1992). This inbreeding depression can be caused by genetic mechanisms (Charlesworth and Charlesworth, 1987). In partial dominance deleterious recessive alleles are masked in the heterozygous condition (Charlesworth and Charlesworth, 1987; Jarne and Charlesworth, 1993). Over-dominance refers to the breakdown of the heterozygote advantage (e.g. Lande and Schemske, 1985). The partial dominance hypothesis of inbreeding depression predicts that the amount of inbreeding depression decreases with increasing self-fertilization in the presence of selection (Dudash and Fenster, 2000). The over-dominance hypothesis predicts that the amount of inbreeding depression increases with increasing self-fertilization unless selection on viability against homozygotes is asymmetrical (Charlesworth and Charlesworth, 1987; Dudash and Fenster, 2000). Inbreeding depression is probably the most important factor selecting against self-fertilization (Lande and Schemske, 1985). In simple models inbreeding is favoured if the fitness of selfed progeny is greater than half of the fitness of outcrossed progeny (Lloyd, 1979). Flowering plants have several mechanisms that have evolved to prevent self-fertilization, e.g. self-incompatibility systems, heterostyly and dichogamy.
Many plant species are capable of self-fertilization and even highly self-incompatible species may show varying levels of self-fertility (Levin, 1996; Proctor et al., 1996). Few plant species are either completely outbreeding or inbreeding (Lloyd, 1979). Instead, many species and populations contain individuals that express mixed mating systems. Selfing may be promoted in a population if ovules would otherwise remain unfertilized (Jarne and Charlesworth, 1993). Selfing can be seen as a mechanism of reproductive assurance, especially delayed selfing (Lloyd, 1979). Apart from inbreeding depression, several ecological and environmental factors, e.g. pollen limitation and lower resource costs of selfing, may cause variation in the amount of self-fertility (Antonovics and Levin, 1980; Holsinger, 1988; Luijten et al., 1999). Reduced pollinator activity (Kearns et al., 1998; Young et al., 2002) and reduced mate diversity can select for mixed-mating and self-fertility (Holsinger, 1991; Jarne and Charlesworth, 1993; Young et al., 2002). This is likely to occur in small or recently colonized populations (Jarne and Charlesworth, 1993; Schmidt-Adam et al., 2000), and in island populations (Larson and Barrett, 1998).
The floral structure of the Asclepiadaceae has lead to highly specialized insect pollination that promotes outbreeding (Queller, 1985; Wyatt and Broyles, 1994). However, varying levels of self-fertility have been observed in several species of Asclepias (Wyatt, 1976; Kephart, 1981; Bookman, 1984; Kahn and Morse, 1991; Wyatt and Broyles, 1994; Lipow et al., 1999; Lipow and Wyatt, 2000b). Moreover, two other Vincetoxicum species, V. rossicum and V. nigrum, have been shown to be self-fertile in North America where these species are aggressively invasive (N. Cappuccino, pers. comm.). Many Asclepias species have a late-acting incompatibility system in which the incompatibility reaction occurs after the formation of the zygote (Wyatt and Broyles, 1994). In the previously studied species of Asclepias, pollen tube growth following self- and cross-pollination has been found to be indistinguishable (Kahn and Morse, 1991). In addition, geitonogamous pollination is common in natural populations of Asclepiadaceae (Pleasants, 1991).
This study examines among- and within-population variation in the mating system of Vincetoxicum hirundinaria (Asclepiadaceae), and compares reproductive success resulting from self-pollination and cross-pollination. Moreover, the goal was to examine if V. hirundinaria possesses a late-acting self-incompatibility system common in many related species. Results are presented of an experimental study on self- and cross-pollinations in V. hirundinaria populations that are located in an archipelago environment on the most northern border of the species distribution area. In the archipelago environment populations may be isolated and small, which could lead to selection for reproductive assurance by self-fertility. The following major questions were asked. (1) Are there differences in reproductive success following self-pollination and cross-pollination? (2) Do populations/individuals differ in their response to these different types of pollination? (3) Does inbreeding depression occur in the early phases of seed development and, if so, is there variation among populations?
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Study species
Vincetoxicum hirundinaria (Asclepiadaceae) is a long-lived perennial herb that grows on exposed slopes and cliffs and prefers calcareous substrata. Most species of Asclepiadaceae have a late-acting self-incompatibility system (Wyatt and Broyles, 1994; Lipow and Wyatt, 2000a). However, self-incompatibility in V. hirundinaria does not appear to have been studied. The main pollinators are large flies, moths and bees. Pollen is aggregated into pollen sacs (pollinia). Each flower has five pairs of pollinia. Pollination occurs when the pollinia are inserted into the stigmatic chambers, from which the pollen tubes grow towards the ovaries. Each flower contains five stigmatic chambers and two ovaries. Three of these chambers lead to one ovary and the remaining two chambers to the other ovary (Wyatt and Broyles, 1994).
In the study area described here, flowering usually starts in the middle of June and lasts until the beginning of August. Seeds ripen in early autumn. Each pod contains approx. 20 wind-dispersed seeds. Vincetoxicum hirundinaria is highly poisonous and therefore has mainly specialist herbivores. The species has a continental Eurasian distribution, but its natural range also covers the islands and coastal areas of the middle Baltic. In Finland, it occurs rather frequently in the south-western archipelago including the Åland islands and in the south-western mainland. This study was conducted in south-western Finland during the summer of 2001.
Study populations
Four populations that are situated in the south-western Archipelago of Finland were selected for the experiment. The populations vary in their size: Ånskär consists of 2825 adult individuals, Lammasluoto 5261, Naantali 647 and Seili 83. From each of these populations 20 individuals were transplanted into a glasshouse in early May and used in the pollination experiments discussed below. The plants were embedded in soil to make watering easier and to keep the growth conditions as natural as possible. Transplantation has been found not to have any significant effect on V. hirundinaria (R. Leimu and M. Riipi, unpub. res.).
Pollination experiments
The pollination experiments were conducted in a glasshouse to prevent insect pollination. All individuals received self-pollination and within-population cross-pollination. Each individual was used as a pollen donor only once in a treatment, and the donors and recipients were randomized. From each individual, four flowers per treatment (total 320 flowers per treatment) were hand-pollinated by inserting two pairs of pollinia into the opposite stigmatic chambers of a flower with a needle. The pollinated flowers were tagged. Each treatment was conducted on a separate randomly selected stem. To measure fertilization success counts were made of the number of initiated pods 10 d after pollination. The number of developed mature pods and aborted pods were assessed 6 weeks after pollination. Ripened pods were harvested before they were completely open and seed number per pod was determined. Ten seeds per plant were weighed individually with a micro-balance and an average of these seeds was calculated and used in the analyses.
Pollen-tube growth
One flower from each of the pollination treatments from each individual was fixed in a small glass vial containing FAA (formalin 10 %^; acetic acid 5 %^; ethyl alcohol 85 %) 24 h after the pollination. An additional 70 randomly chosen flowers from both pollination treatments were collected and fixed 6 d after pollination. In related species failure in zygote division can be observed 6 d after pollination (Broyles and Wyatt, 1993). The flowers were stored in a refrigerator at 4 °C. The ovaries were removed from the flowers and placed on a microscope slide. To soften the tissues, the ovaries were kept in 1 M NaOH for 3 min, after which they were stained with 0·1 % solution of Aniline Blue. Pollen tube growth and the ovules were studied using fluorescence microscopy, and their number and length determined. Pollen tube length was classified on the basis of the position the tubes had reached in the style or ovary (1 = upper part of the stylar canal, 2 = the middle parts of the style, 3 = upper parts of the ovary, 4 = middle of the ovary, 5 = beyond the middle of the ovary). Using the flowers fixed 6 d from pollination, an examination was made as to whether the ovules were aborted or had started to develop. Ovule number was counted in all flowers. Ovaries that had no pollen tubes were excluded from the analyses. If both of the ovaries in a flower contained pollen tubes a mean value was used in the analyses.
Statistical analyses
To study the effects of pollination treatment on reproductive output (the proportion of initiated fruit per pollinated flowers, mature fruit set, the proportion of initiated pods that aborted, seed number and mass, the number of pollen tubes and pollen tube length) and to assess variation between populations, nested mixed-model ANOVAs were conducted by applying the SAS procedure MIXED, which is based on the maximum likelihood estimation (Littell et al., 1996). Individuals were used as random factors and were nested within population. Population and pollination treatment were used as fixed factors. In addition, a data set including only the self-fertile individuals was analysed as above to determine if there was variation between individuals in reproductive success between self- and cross-pollinations. Multiple comparisons were conducted by using Tukey’s test. The residuals did not deviate from normal distribution and the assumption of the homogeneity of the variances was met. The chi-square test was used to examine between-population differences in the number of individuals that produced pods following self-pollination.
To examine between-population variation in ovule number and seed/ovule ratio, one-way ANOVAs were conducted. Since pollination treatment had no effect on seed number but variation between populations was observed, treatment was ignored in the analyses of seed/ovule ratio. Population was used as the category variable. Seed/ovule ratio was log-transformed to meet the assumption of normality of residual distribution. Tukey’s test was used to conduct the multiple comparisons. The association between seed number and seed weight was tested by conducting population-wise Pearson correlation analyses.
A chi-square test was used to examine if the frequencies of developed and aborted ovules differed between self- and cross-pollinated flowers.
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RESULTS |
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Fruit set
No between-population differences were observed in the number of individuals that produced pods following self-pollination (
2 = 2·124, d.f. = 3, P = 0·547). This result should, however, be interpreted with caution given the relatively small sample size. The proportion of self-fertile individuals was 30 % in the Lammasluoto population, 36·5 % in Naantali, 47·4 % in Seili and 50 % in Ånskär. The proportion of pollinated flowers that initiated or developed mature pods was significantly higher following cross-pollination compared with self-pollination (Table 1 and Fig. 1A). No significant differences were observed between individuals or populations. The population x treatment interaction was not statistically significant (Table 1). The proportion of initiated pods that aborted was significantly higher following cross-pollination than following self-pollination (Fig. 1B). Neither population nor individual had a significant effect on fruit abortion rates (Table 1).
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In the analyses including only the self-fertile individuals, no significant differences were found between the pollination treatments or populations in initiated fruit set, mature fruit production or the proportion of aborted pods.
Seed number and seed mass
Seed number varied significantly between populations and individuals (Table 1). Individuals from Ånskär produced more seeds per pod than individuals from the other three populations (Fig. 1C). No significant differences were observed between the pollination treatments (Table 1).
Significant differences were found in seed mass between populations (Table 1). Plants from Lammasluoto and Naantali had heavier seeds than those from Ånskär (data not shown). Neither individual nor treatment had a significant effect on seed mass (Table 1). A significant negative correlation was found between seed mass and seed weight in Naantali (r = –0·47, P = 0·0004) and in Ånskär (r = –0·62, P = 0·002).
Ovule production
The number of ovules per ovary varied significantly between populations (F = 5·62, d.f. = 3, P = 0·0016). Ovule number was higher in Ånskär compared with Naantali and Seili (Fig. 2). Population also had a significant effect on seed/ovule ratio (F = 3·66, d.f. = 3, P = 0·0175). Seed/ovule ratio was higher in Ånskär compared with Naantali (Fig. 2).
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Pollen tube growth
The number of pollen tubes was higher following cross- than self-pollination (Table 1). The mean number of cross-pollen tubes was 10·7 and self-pollen tubes 8·4. Furthermore, pollen tube number varied significantly between populations (Table 1). Pollen tube number was higher in the Ånskär population compared with the Lammasluoto population (Table 2). Significant variation between individuals was also found (Table 1). No differences were observed in pollen tube length between the treatments or between populations (Table 1). The variation between individuals was not significant (Table 1).
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Ovule abortion rates
No significant differences were found in the frequencies of aborted and developed ovules between the populations in either self- or cross-pollinated flowers. The sample size in this analysis was relatively small and thus the power of the analysis is weak. When populations were pooled, the frequencies of developed and aborted ovules varied significantly between self- and cross-pollinated flowers (
2 = 18·9, d.f. = 1, P = 0·001). Of the self-pollinated ovaries 16·2 % had started to develop and of the cross-pollinated ovaries 69·0 %. |
DISCUSSION |
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Even though the results provide some evidence supporting the presence of a late-acting self-incompatibility system, the level of self-fertility is relatively high in V. hirundinaria. Furthermore, the level of self-fertility varied among individuals and also to some extent between populations. Interestingly, the self-fertile individuals had equally high fruit and seed set from self- and cross-pollinations, indicating that these individuals possess a mixed-mating system. The number of individuals that produced pods following self-pollination did not vary between populations. Overall, however, self-pollination resulted in reduced fruit set compared with cross-pollination at the population level. In V. hirundinaria selfing seems to occur mainly geitonogamously, but autogamous self-pollination is also possible and the flowers can be pollinated without pollen vectors (Leimu and Syrjänen, 2002). Variation in self-fertility has also been observed in several related species (Wyatt, 1976; Kephart, 1981; Bookman, 1984; Kahn and Morse, 1991; Wyatt and Broyles, 1994; Lipow et al., 1999; Lipow and Wyatt, 2000b). The reported levels of selfed fruit set in studied species of Asclepias vary from less than 5 % up to 29·2 % (Wyatt, 1976; Kephart, 1981). Compared with these results, self-fertility in V. hirundinaria is high. Furthermore, based on allozyme electrophoresis data conducted for a larger number of populations, it actually seems that inbreeding is common in V. hirundinaria in the south-western archipelago of Finland (R. Leimu and P. Mutikainen, unpub. res.).
One explanation for the high levels of self-fertility is that V. hirundinaria has a late-acting self-incompatibility system, which is commonly observed in Asclepiadaceae (Wyatt and Broyles, 1994), and that self-fertility is actually due to pseudo-self-fertility. This means that self-fertility in the essentially self-incompatible species is due to the expression of modifier alleles at loci other than the self-incompatibility locus (Levin, 1996). Pseudo-self-fertility has been demonstrated to occur in other species of Asclepias (Lipow et al., 1999; Lipow and Wyatt, 2000a). The fact that in V. hirundinaria pollen tube growth did not differ significantly between self- and cross-pollinations, and that many of the self-pollinated ovaries contained aborted ovules at 6 d from pollination suggest that V. hirundinaria appears to have a late-acting self-incompatibility system similar to that observed in several related species. It has been suggested that if pseudo-self-fertility occurs, cross-pollination should result in higher seed production and faster pollen tube growth than self-pollination (e.g. Townsend, 1971). This was not the case in V. hirundinaria. However, even though a late-acting incompatibility system does not affect pollen tube growth, pseudo-self-fertility might still occur. Late-acting self-incompatibility can be difficult to distinguish from early-acting inbreeding depression (Proctor et al., 1996; Lipow and Wyatt, 2000a). As an alternative explanation for incompatibility, Lipow and Wyatt (2000b) suggested that all individuals are self-compatible and early-acting inbreeding depression determines the relative success of self- and cross-pollinations.
If V. hirundinaria is incompatible and possesses a late-acting self-incompatibility system, then inbreeding depression would be expected to occur. The effects of inbreeding on offspring phenotype can be seen through reduced seed set, reductions in the number of seeds per pod and in individual seed mass, poor germination and reduced seedling growth (Charlesworth and Charlesworth, 1987). Apart from the lower overall self fruit set observed in this study, no significant differences were found in seed number or seed mass between self- and cross-pollinations. However, in order to rigorously address the question of inbreeding depression, data from later stages of the life cycle are needed. Some recent studies on rare or endemic species have also failed to find evidence of inbreeding depression in the very early stages of the life cycle (Luijten et al., 1996; Scmidt-Adam et al., 2000). In V. hirundinaria, seed and ovule number and seed mass varied between populations but this variation did not contribute to the observed variation in the mating system. A negative correlation between seed mass and seed number was found in two populations (Naantali and Ånskär). This indicates a seed number/seed mass trade-off in maternal investment in offspring. This hypothesis is also supported by the observed constant seed mass. Further, ovule number cannot completely explain the observed between-population variation in seed number since there was also variation in the seed/ovule ratio, the ratio being significantly lower in the Naantali population than in the Ånskär one. In addition, a relatively high proportion of aborted pods was observed in V. hirundinaria, which is common in Asclepiadaceae (Willson and Price, 1977; Queller, 1983, 1985). Surprisingly, the abortion rates were higher following cross-pollination compared with self-pollination. Ovule abortion was, however, higher following self-pollination compared with cross-pollination. Energetic considerations could explain the higher abortion rates of pods following cross-pollination. The higher initiation following cross-pollination could lead to increased abortion rates if plants are only capable of having a limited number of mature fruits.
Adaptations for self-fertility have been suggested to be likely in island populations since the probability of establishment following dispersal increases with self-fertility (Larson and Barrett, 1998). Self-fertility may also be selected for in small populations (Jarne and Charlesworth, 1993). Further, it has been shown that pseudo-self-fertility is more common in small populations (Byers and Meagher, 1992) and in populations that are located near the border of the species range (Lipow and Wyatt, 2000a). The populations studied here are situated at the most northern border of the distribution area of V. hirundinaria. In addition, in the archipelago environment the populations may be isolated and small, which could lead to selection for reproductive assurance. In this study, a relatively high level of self-fertility was observed in all populations irrespective of their size. The highest level of self-fertility was, however, found in the small Seili population where self-pollination resulted in a fruit set that was almost as high as cross-pollination. The better colonization ability of self-fertile individuals can be considered as an alternative explanation for the observed high levels of self-fertility in V. hirundinaria. In the Baltic land-uplifting archipelago, new sites are continuously colonized. This type of environment may have enhanced selection for selfing, which might have been strong when the islands were first colonized: in the current situation selection against selfing seems to be weak. The long life-span of V. hirundinaria, in combination with this weak selection, may act to produce populations that retain a rather high level of selfing. Moreover, many Vincetoxicum species are aggressively invasive, which can be related to high levels of inbreeding.
The main conclusion of this study is that the frequencies of self-fertile individuals are relatively high in V. hirundinaria populations despite their size. The results support the presence of a late-acting self-incompatibility system, which seems to be acting since 50–70 % of individuals did not produce pods following selfing. However, the self-fertile individuals had equally high pod and seed production following self- and cross-pollinations, and thus seem to possess a mixed-mating system. No signs of inbreeding depression were found, at least in the early stages of development. Based on these results, it is difficult to say whether the observed self-fertility in V. hirundinaria is due to pseudo-self-fertility or whether the self-fertile individuals lack the incompatibility system, and thus are true selfers.
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ACKNOWLEDGEMENTS |
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I am grateful to P. Mutikainen, V. Salonen, P. Lehtonen, K. Ruohomäki, A. Parantainen, H. Korpelainen, P. Siikamäki, K. Syrjänen, D. Levin and two anonymous reviewers for helpful comments. Archipelago Research Institute, University of Turku, provided facilities. This research was funded by the Jenny and Antti Wihuri Foundation and Finish Cultural Foundation.
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LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
-
Antonovics J, Levin DA. 1980. The ecological and genetical consequences of density-dependent regulation in plants. Annual Review of Ecology and Systematics 11: 411–452.
Bookman SS. 1984. Evidence for selective fruit production in Asclepias. Evolution 38: 72–86.
Broyles SB, Wyatt R. 1993. The consequences of self-pollination in Asclepias exaltata, a self-incompatible milkweed. American Journal of Botany 80: 41–44.
Byers DL, Meagher TR. 1992. Mate availability in small populations of plant species with homomorphic sporophytic self-incompatibility. Heredity 68: 353–359.
Charlesworth D, Charlesworth B. 1987. Inbreeding depression and its evolutionary consequences. Annual Review of Ecology and Systematics 18: 237–268.[CrossRef][ISI]
Dudash MR, Fenster CB. 2000. Inbreeding and outbreeding depression in fragmented populations. In: Young AG, Clarke GM, eds. Genetics, demography and viability of fragmented populations. Cambridge: Cambridge University Press, 35–53.
Holsinger KE. 1988. Inbreeding depression doesn’t matter: the genetic basis of mating-system evolution. Evolution 42: 1235–1244.[CrossRef]
Holsinger KE. 1991. Mass-action models of plant mating systems: the evolutionary stability of mixed mating systems. American Naturalist 138: 606–622.[CrossRef]
Jarne P, Charlesworth D. 1993. The evolution of the selfing rate in functionally hermaphrodite plants and animals. Annual Review of Ecology and Systematics 24: 441–466.[CrossRef][ISI]
Kahn AP, Morse DH. 1991. Pollinium germination and putative ovule penetration in self- and cross-pollinated common milkweed Asclepias syriaca. American Midland Naturalist 126: 61–67.
Kearns CA, Inouye D, Waser NM. 1998. Endangered mutualisms: the conservation of plant–pollinator interactions. Annual Review of Ecology and Systematics 29: 83–112.[CrossRef][ISI]
Kephart SR. 1981. Breeding systems in Asclepias incarnata L. A. syriaca L. and A. verticillata L. American Journal of Botany 68: 226–232.
Lande R, Schemske DW. 1985. The evolution of self-fertilization and inbreeding depression in plants. I. Genetic models. Evolution 39: 24–40.[CrossRef][ISI]
Larson BMH, Barrett SC. 1998. Reproductive biology of island and mainland populations of Primula mistassinica (Primulaceae) on Lake Huron shorelines. Canadian Journal of Botany 76: 1819–1827.
Leimu R, Syrjänen K. 2002. Effects of population size, seed predation and plant size on male and female reproductive success in Vincetoxicum hirundinaria (Asclepiadaceae). Oikos 98: 229–238.[CrossRef]
Levin DA. 1996. The evolutionary significance of pseudo-self-fertility. American Naturalist 148: 321–332.[CrossRef]
Lipow SR, Wyatt R. 2000a. Single gene control of postzygotic self-incompatibility in poke milkweed, Asclepias exaltata L. Genetics 154: 893–907.
Lipow SR, Wyatt R. 2000b. Towards an understanding of the mixed breeding system of swamp milkweed (Asclepias incarnata). Journal of the Torrey Botanical Society 127: 193–199.
Lipow SR, Broyles SB, Wyatt R. 1999. Population differences in self-fertility in the ‘self-incompatible’ milkweed Asclepias exaltata (Asclepiadaceae). American Journal of Botany 86: 1114–1120.
Littell RC, Milliken, GA, Stroup, WW, Wolfinger, RD. 1996. SAS system for mixed models. Cary, NC, USA: SAS Institute.
Lloyd DG. 1979. Some reproductive factors affecting the selection of self-fertilization in plants. American Naturalist 113: 67–79.[CrossRef]
Lloyd DG, Schoen DJ. 1992. Self- and cross-fertilization in plants. I. Functional dimensions. International Journal of Plant Sciences 153: 358–369.[CrossRef]
Luijten S, Oostermeijer, G, Ellis-Adam, A, den Nijs, H. 1999. Variable herkogamy and autofertility in marginal populations of Gentianella germanica in the Netherlands. Folia Geobotanica 34: 483–496.
Luijten S, Oostermeijer, G, van Leeuwen, N, den Nijs, H. 1996. Reproductive success and clonal genetic structure of rare Arnica montana (Compositae) in the Netherlands. Plant Systematics and Evolution 201: 15–39.
Pleasants JM. 1991. Evidence for short-distance dispersal of pollinia in Asclepias syriaca L. Functional Ecology 5: 75–82.
Proctor M, Yeo P, Lack A. 1996. The natural history of pollination. Portland, OR: Timber Press.
Queller DC. 1983. Sexual selection in a hermaphroditic plant. Nature 305: 706–707.[CrossRef]
Queller DC. 1985. Proximate and ultimate causes of low fruit production in Asclepias exaltata. Oikos 44: 373–381.
Schmidt-Adam G, Young, AG, Murray, BG. 2000. Low outcrossing rates and shift in pollinators in New Zealand Pohutukawa (Metrosideros excelsa; Myrticeae). American Journal of Botany 87: 1265–1271.
Townsend CE. 1971. Advances in the study of incompatibility. In: Heslop-Harrison J. ed. Pollen: development and physiology. London: Butterworth, 281–309.
Willson MF, Price PW. 1977. The evolution of inflorescence size in Asclepias (Asclepiadaceae). Evolution 31: 495–511.[CrossRef]
Wyatt R. 1976. Pollination and fruit-set in Asclepias: a reappraisal. American Journal of Botany 63: 845–851.
Wyatt R, Broyles SB. 1994. Ecology and evolution of reproduction in milkweeds. Annual Review of Ecology and Systematics 25: 423–441.[CrossRef][ISI]
Young AG, Hill, JH, Murray, BG, Peakall, R. 2002. Breeding system, genetic diversity and clonal structure in the subalpine forb Rutidosis leiolepis F. Muell. (Asteraceae). Biological Conservation 106: 71–78.[CrossRef][ISI]
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