Pollination and late-acting self-incompatibility in Cyrtanthus breviflorus (Amaryllidaceae): implications for seed production

Self-incompatibility: a targeted, unexplored pre-fertilization barrier in flower crops of Asteraceae

Asteraceae (synonym as Compositae) is one of the largest angiosperm families among flowering plants comprising one-tenth of all agri-horticultural species grown across various habitats except in Antarctica. These are commercially utilized as cut and loose flowers as well as pot and bedding plants in landscape gardens due to their unique floral traits. Consequently, ineffective seed setting and presence of an intraspecific reproductive barrier known as self-incompatibility (SI) severely reduces the effectiveness of hybridization and self-fertilization by traditional crossing. There have been very few detailed studies of pollen-stigma interactions in this family. Moreover, about 63% of Aster species can barely self-fertilize due to self-incompatibility (SI). The chrysanthemum (Chrysanthemum × morifolium) is one of the most economically important ornamental plants in the Asteraceae family which hugely shows incompatibility. Reasons for the low fertility and reproductive capacity of species are still indefinite or not clear. Hence, the temporal pattern of inheritance of self-incompatibility and its effect on reproductive biology needs to be investigated further to improve the breeding efficiency. This review highlights the self-incompatible (SI) system operating in important Astraceous (ornamental) crops which are adversely affected by this mechanism along with different physiological and molecular techniques involved in breaking down self-incompatibility.

Pollen production per flower increases with floral display size across animal-pollinated flowering plants

PREMISE

The number of open flowers on a plant (i.e., floral display size) can influence plant fitness by increasing pollinator attraction. However, diminishing marginal fitness returns with increasing floral display are expected as pollinators tend to visit more flowers per plant consecutively. An extended flower visitation sequence increases the fraction of ovules disabled by self-pollination (ovule discounting) and reduces the fraction of a plant's own pollen that is exported to sire seeds in other plants (pollen discounting). Hermaphroditic species with a genetic system that prevents self-fertilization (self-incompatibility) would avoid ovule discounting and its fitness cost, whereas species without such a genetically based barrier would not. Contrarily, pollen discounting would be an unavoidable consequence of a large floral display irrespective of selfing barriers. Nevertheless, the increasing fitness costs of ovule and pollen discounting could be offset by respectively increasing ovule and pollen production per flower.

METHODS

We compiled data on floral display size and pollen and ovule production per flower for 1241 animal-pollinated, hermaphroditic angiosperm species, including data on the compatibility system for 779 species. We used phylogenetic general linear mixed models to assess the relations of pollen and ovule production to floral display size.

RESULTS

Our findings provide evidence of increasing pollen production, but not of ovule production, with increasing display size irrespective of compatibility system and even after accounting for potentially confounding effects like flower size and growth form.

CONCLUSIONS

Our comparative study supports the pollen-discount expectation of an adaptive link between per-flower pollen production and floral display across animal-pollinated angiosperms.

The influence of self-pollen deposition on female reproductive success in a self-incompatible plant, Akebia quinata

Geitonogamy is inevitable in hermaphrodite and monecious. Even for self-incompatible species, the negative effects of self-pollen are unavoidable when geitonogamous or self-mating occurs. However, the influence of self-pollen on consecutive development of flowers (e.g., fruiting and seeding) was seldom evaluated. Here, the self-incompatible monecious species, Akebia quinata, was used to estimate the influence of self-pollen deposition. We evaluated the extent of pollen limitation and geitonogamous mating under natural conditions by count of stigmatic pollen load and pollen tracking experiment. Hand pollination with different amount and combinations of self vs. cross pollen grains was applied to detect the response of fruit and seed set. The results showed that geitonogamy and pollen limitation occurred under natural conditions in A. quinata. Carpel numbers, ratio of self- and cross-pollen, and the interactive effect of ratio of self- and cross-pollen and total mixed pollen numbers, and not total pollen grain number, determined the effect of self-pollen on female reproductive success. The effect of self-pollen depended on its intensity. In general, the transfer of self-pollen significantly affected young fruit set. However, a little self-pollen together with cross-pollen did not reduce young fruit production. Although self-incompatible plants have evolved physiological mechanisms that reduce self-fertilization, our results provide new insights into the effects of self-pollen and the adaptive significance of self-incompatible monecious species.

Feeding friend and foe: ample pollen mitigates the effects of pollen theft for a gynodioecious plant, Polemonium foliosissimum (Polemoniaceae)

BACKGROUND AND AIMS

Most angiosperms rely on pollinators to transport pollen and effect fertilization. While some floral visitors are effective pollinators, others act as thieves, consuming pollen but effecting little pollination in return. The importance of pollen theft in male and female reproductive success has received little attention. Here, we examined if pollen consumption by flies altered pollen receipt and exacerbated pollen limitation for a bumblebee-pollinated plant, Polemonium foliosissimum (Polemoniaceae).

METHODS

To examine the effect of pollen-thieving flies, we took a three-pronged approach. First, we used single-visit observations to quantify pollen removal and pollen deposition by flies and bumblebees. Second, we manipulated pollen in the neighbourhood around focal plants in two years to test whether pollen reduction reduced pollen receipt. Third, we combined pollen reduction with hand-pollination to test whether pollen thieving exacerbated pollen limitation. Polemonium foliosissimum is gynodioecious in most populations in the Elk Mountains of central Colorado, USA. Thus, we also tested whether pollen theft affected hermaphrodites and females differently.

RESULTS

Flies removed significantly more pollen and deposited less pollen per visit than did bumblebees. Reduction of pollen in the neighbourhood around focal plants reduced pollen receipt in both years but only nearly significantly so in 2015. In 2016, plants were significantly pollen-limited; hand-pollination significantly increased seeds per fruit for both hermaphrodites and females. However, the reduction of pollen around focal plants did not exacerbate pollen limitation for either hermaphrodites or females.

CONCLUSIONS

Our results suggest that plants tolerate significant consumption of pollen by thieves and pollinators by producing ample pollen to feed both and fertilize available ovules. Our results demonstrate that pollen limitation in P. foliosissimum is driven by lack of effective pollinators rather than lack of pollen. Teasing out these effects elucidates the relative importance of drivers of reproductive success and thus the expected response to selection by different floral visitors.

Diel scent and nectar rhythms of an African orchid in relation to bimodal activity patterns of hawkmoth pollinators

BACKGROUND AND AIMS

The temporal dimensions of floral adaptation to pollinators are not yet well understood, partly because we lack accurate information on the diel rhythms of flower visitation for many pollinators. We investigated whether diel patterns of pollinator visitation to flowers of the African woodland orchid Bonatea polypodantha are synchronized with rhythms of floral anthesis, scent emission and nectar availability.

METHODS

Direct observations and motion-activated cameras were used to identify pollinators of B. polypodantha and to document their activity periods. The timing of pollinaria removal from flowers, emission of scent and availability of nectar was also measured.

RESULTS

We found that B. polypodantha is pollinated exclusively by short-tongued hawkmoths. Pollinaria of the orchid are affixed between the labial palps of the moths and brush over the protruding stigmatic arms. The flowers also receive visits by long-tongued hawkmoths, but these act as nectar thieves. Tracking of pollinaria removal from flowers confirmed that pollination occurs only at night. Camera footage revealed a striking crepuscular pattern of foraging by short-tongued hawkmoths with peaks of activity during the twilight periods at dusk and at dawn. In contrast, long-tongued hawkmoths were found to visit flowers throughout the night. Flowers of B. polypodantha exhibit unimodal peaks of anthesis, scent emission (dominated by nitrogenous aromatics) and nectar availability before or around dusk.

CONCLUSIONS

Flowers of B. polypodantha are pollinated exclusively by short-tongued hawkmoths, which show crepuscular foraging activity at dusk and dawn. Floral phenophases of the orchid are closely synchronized with the peak of pollinator activity at dusk.

Pollen on stigmas as proxies of pollinator competition and facilitation: complexities, caveats and future directions

BACKGROUND

Pollen transfer via animals is necessary for reproduction by ~80 % of flowering plants, and most of these plants live in multispecies communities where they can share pollinators. While diffuse plant-pollinator interactions are increasingly recognized as the rule rather than the exception, their fitness consequences cannot be deduced from flower visitation alone, so other proxies, functionally closer to seed production and amenable for use in a broad variety of diverse communities, are necessary.

SCOPE

We conceptually summarize how the study of pollen on stigmas of spent flowers can reflect key drivers and functional aspects of the plant-pollinator interaction (e.g. competition, facilitation or commensalism). We critically evaluate how variable visitation rates and other factors (pollinator pool and floral avoidance) can give rise to different relationships between heterospecific pollen and (1) conspecific pollen on the stigma and (2) conspecific tubes/grain in the style, revealing the complexity of potential interpretations. We advise on best practices for using these proxies, noting the assumptions and caveats involved in their use, and explicate what additional data are required to verify interpretation of given patterns.

CONCLUSIONS

We conclude that characterizing pollen on stigmas of spent flowers provides an attainable indirect measure of pollination interactions, but given the complex processes of pollen transfer that generate patterns of conspecific-heterospecific pollen on stigmas these cannot alone determine whether competition or facilitation are the underlying drivers. Thus, functional tests are also needed to validate these hypotheses.

Breeding systems in Cyrtanthus (Amaryllidaceae): variation in self-sterility and potential for ovule discounting

Breeding systems of plants determine their reliance on pollinators and ability to produce seeds following self-pollination. Self-sterility, where ovules that are penetrated by self-pollen tubes that do not develop into seeds, is usually considered to represent either a system of late-acting self-incompatibility or strong early inbreeding depression. Importantly, it can lead to impaired female function through ovule or seed discounting when stigmas receive mixtures of self and cross pollen, unless cross pollen is able to reach the ovary ahead of self pollen ('prepotency'). Self-sterility associated with ovule penetration by self-pollen tubes appears to be widespread among the Amaryllidaceae. We tested for self-sterility in three Cyrtanthus species - C. contractus, C. ventricosus and C. mackenii - by means of controlled hand-pollination experiments. To determine the growth rates and frequency of ovule penetration by self- versus cross-pollen tubes, we used fluorescence microscopy to examine flowers of C. contractus harvested 24, 48 and 72 h after pollination, in conjunction with a novel method of processing these images digitally. To test the potential for ovule discounting (loss of cross-fertilisation opportunities when ovules are disabled by self-pollination), we pollinated flowers of C. contractus and C. mackenii with mixtures of self- and cross pollen. We recorded full self-sterility for C. contractus and C. ventricosus, and partial self-sterility for C. mackenii. In C. contractus, we found no differences in the growth rates of self- and cross-pollen tubes, nor in the proportions of ovules penetrated by self- and cross-pollen tubes. In this species, seed set was depressed (relative to cross-pollinated controls) when flowers received a mixture of self and cross pollen, but this was not the case for C. mackenii. These results reveal variation in breeding systems among Cyrtanthus species and highlight the potential for gender conflict in self-sterile species in which ovules are penetrated and disabled by pollen tubes from self pollen.

Mixed pollen load and late-acting self-incompatibility flexibility in Adenocalymma peregrinum (Miers) L.G. Lohmann (Bignonieae: Bignoniaceae)

Mixed cross and self-pollen load on the stigma (mixed pollination) of species with late-acting self-incompatibility system (LSI) can lead to self-fertilized seed production. This "cryptic self-fertility" may allow selfed seedling development in species otherwise largely self-sterile. Our aims were to check if mixed pollinations would lead to fruit set in LSI Adenocalymma peregrinum, and test for evidence of early-acting inbreeding depression in putative selfed seeds from mixed pollinations. Experimental pollinations were carried out in a natural population. Fruit and seed set from self-, cross and mixed pollinations were analysed. Further germination tests were carried out for the seeds obtained from treatments. Our results confirm self-incompatibility, and fruit set from cross-pollinations was three-fold that from mixed pollinations. This low fruit set in mixed pollinations is most likely due to a greater number of self- than cross-fertilized ovules, which promotes LSI action and pistil abortion. Likewise, higher percentage of empty seeds in surviving fruits from mixed pollinations compared with cross-pollinations is probably due to ovule discounting caused by self-fertilization. Moreover, germinability of seeds with developed embryos was lower in fruits from mixed than from cross-pollinations, and the non-viable seeds from mixed pollinations showed one-third of the mass of those from cross-pollinations. The great number of empty seeds, lower germinability, lower mass of non-viable seeds, and higher variation in seed mass distribution in mixed pollinations, strongly suggests early-acing inbreeding depression in putative selfed seeds. In this sense, LSI and inbreeding depression acting together probably constrain self-fertilized seedling establishment in A. peregrinum.

The influence of herbivory on pre- and postzygotic stages of reproduction following open, self, and outcross pollination

PREMISE OF THE STUDY

Herbivory affects pollination success and reproductive output in plants. However, the different stages in the process from pollination to seed maturation have hardly been investigated within the context of herbivory. Herbivory might affect these stages via its effect on geitonogamous pollination and thereby the proportion of self pollen delivered to the stigma and/or via its effect on the nutritional capacity of the maternal plant.

METHODS

Plants of monocarpic Cynoglossum officinale were experimentally subjected to root herbivory and exposed to natural open pollination in combination with self and outcross hand pollination. We quantified pollen germination, pollen tube competition intensity, pollen tube attrition, fruit set, and seed initiation, abortion, and maturation.

KEY RESULTS

Although root herbivory did not affect pollen germination or pollen tube attrition, fruit set and seed initiation and maturation were negatively affected by herbivory, but for seed initiation only in the case of outcross- and open-pollinated flowers. The intensity of pollen tube competition positively affected seed initiation, but only in plants infested with the herbivore.

CONCLUSIONS

Our study demonstrates that herbivory did not affect the early stages following pollination, but significantly impacted later postpollination stages such as fruit set and seed maturation and selection based on pollen tube competition intensity on zygote development. Our findings suggest that decreased nutritional capacity of the mother plant in response to root herbivory rather than herbivory effects on pollen quality was responsible for the lower fruit and seed production in infested plants.

Comparative De Novo Transcriptome Analysis of Fertilized Ovules in Xanthoceras sorbifolium Uncovered a Pool of Genes Expressed Specifically or Preferentially in the Selfed Ovule That Are Potentially Involved in Late-Acting Self-Incompatibility

Xanthoceras sorbifolium, a tree species endemic to northern China, has high oil content in its seeds and is recognized as an important biodiesel crop. The plant is characterized by late-acting self-incompatibility (LSI). LSI was found to occur in many angiosperm species and plays an important role in reducing inbreeding and its harmful effects, as do gametophytic self-incompatibility (GSI) and sporophytic self-incompatibility (SSI). Molecular mechanisms of conventional GSI and SSI have been well characterized in several families, but no effort has been made to identify the genes involved in the LSI process. The present studies indicated that there were no significant differences in structural and histological features between the self- and cross-pollinated ovules during the early stages of ovule development until 5 days after pollination (DAP). This suggests that 5 DAP is likely to be a turning point for the development of the selfed ovules. Comparative de novo transcriptome analysis of the selfed and crossed ovules at 5 DAP identified 274 genes expressed specifically or preferentially in the selfed ovules. These genes contained a significant proportion of genes predicted to function in the biosynthesis of secondary metabolites, consistent with our histological observations in the fertilized ovules. The genes encoding signal transduction-related components, such as protein kinases and protein phosphatases, are overrepresented in the selfed ovules. X. sorbifolium selfed ovules also specifically or preferentially express many unique transcription factor (TF) genes that could potentially be involved in the novel mechanisms of LSI. We also identified 42 genes significantly up-regulated in the crossed ovules compared to the selfed ovules. The expression of all 16 genes selected from the RNA-seq data was validated using PCR in the selfed and crossed ovules. This study represents the first genome-wide identification of genes expressed in the fertilized ovules of an LSI species. The availability of a pool of specifically or preferentially expressed genes from selfed ovules for X. sorbifolium will be a valuable resource for future genetic analyses of candidate genes involved in the LSI response.

Late-acting self-incompatibility--the pariah breeding system in flowering plants

It is estimated that around half of all species of flowering plants show self-incompatibility (SI). However, the great majority of species alleged to have SI simply comply with 'the inability of a fully fertile hermaphrodite plant to produce zygotes when self-pollinated'--a definition that is neutral as to cause. Surprisingly few species have been investigated experimentally to determine whether their SI has the type of genetic control found in one of the three established mechanisms, that is, homomorphic gametophytic, homomorphic sporophytic or heteromorphic SI. Furthermore, our knowledge of the molecular basis of homomorphic SI derives from a few species in just five families--a small sample that has nevertheless revealed the existence of three different molecular mechanisms. Importantly, a sizeable cohort of species are self-sterile despite the fact that self-pollen tubes reach the ovary and in most cases penetrate ovules, a phenomenon called late-acting self-incompatibility (LSI). This review draws attention to the confusion between species that show 'self-incompatibility' and those that possess one of the 'conventional SI mechanisms' and to argue the case for recognition of LSI as having a widespread occurrence and as a mechanism that inhibits selfing and promotes outbreeding in many plant species.

Self-sterility in Camellia oleifera may be due to the prezygotic late-acting self-incompatibility

In this report, self-sterility in Camellia oleifera was explored by comparing structural and statistical characteristics following self-pollination (SP) and cross-pollination (CP). Although slightly delayed pollen germination and pollen tube growth in selfed ovaries compared to crossed ovaries was observed, there was no significant difference in the percentages of pollen that germinated and pollen tubes that grew to the base of the style. There was also no difference in morphological structure after the two pollination treatments. However, the proportions of ovule penetration and double fertilization in selfed ovules were significantly lower than in crossed ovules, indicating that a prezygotic late-acting self-incompatible mechanism may exist in C. oleifera. Callose deposition was observed in selfed abortive ovules, but not in normal. Ovules did not show differences in anatomic structure during embryonic development, whereas significant differences were observed in the final fruit and seed set. In addition, aborted ovules in selfed ovaries occurred within 35 days after SP and prior to zygote division. However, this process did not occur continuously throughout the life cycle, and no zygotes were observed in the selfed abortive ovules. These results indicated that the self-sterility in C. oleifera may be caused by prezygotic late-acting self-incompatibility (LSI).

Male interference with pollination efficiency in a hermaphroditic orchid

Hermaphroditism can lead to both intra- and intersexual conflict between male and female gender functions. However, the effect that such gender conflicts have on pollination efficiency has seldom been investigated. By artificially reducing the number of available male gametes on an individual, we quantified whether male interference with pollination efficiency occurs in the self-compatible, moth-pollinated orchid Satyrium longicauda. We partially emasculated S. longicauda inflorescences and compared pollination success and fecundity in these plants to intact controls. Pollen in both groups of plants was colour-labelled so that its dispersal by pollinators could be tracked directly in the field. Intact flowers on partially emasculated inflorescences exported more pollen and received more cross-pollen and less self-pollen than those on intact inflorescences. Proportion of fruit set per plant was similar between the two treatments; however, fruits on partially emasculated plants had proportionally more viable seeds than those on intact controls. These results provide empirical evidence that male interference with pollination efficiency can occur in a hermaphroditic plant and that such interference can compromise fecundity. The most likely mechanism for such male interference is competition for placement on the proboscis of hawkmoth pollinators. Consequently, male competition for siring success may influence the evolution of sexual systems in hermaphroditic pollinator-dependent plants.

A temporal dimension to the influence of pollen rewards on bee behaviour and fecundity in Aloe tenuior

The net effect of pollen production on fecundity in plants can range from negative – when self-pollen interferes with fecundity due to incompatibility mechanisms, to positive – when pollen availability is associated with increased pollinator visitation and fecundity due to its utilization as a reward. We investigated the responses of bees to pollen and nectar rewards, and the effects of these rewards on pollen deposition and fecundity in the hermaphroditic succulent shrub Aloe tenuior. Self-pollinated plants failed to set fruit, but their ovules were regularly penetrated by self-pollen tubes, which uniformly failed to develop into seeds as expected from ovarian self-incompatibility (or strong early inbreeding depression). Bees consistently foraged for pollen during the morning and early afternoon, but switched to nectar in the late afternoon. As a consequence of this differential foraging, we were able to test the relative contribution to fecundity of pollen- versus nectar-collecting flower visitors. We exposed emasculated and intact flowers in either the morning or late afternoon to foraging bees and showed that emasculation reduced pollen deposition by insects in the morning, but had little effect in the afternoon. Despite the potential for self-pollination to result in ovule discounting due to late-acting self-sterility, fecundity was severely reduced in artificially emasculated plants. Although there were temporal fluctuations in reward preference, most bee visits were for pollen rewards. Therefore the benefit of providing pollen that is accessible to bee foragers outweighs any potential costs to fitness in terms of gender interference in this species.

Sexual reproduction of the pentaploid, short-styled Oxalis pes-caprae allows the production of viable offspring

Reproduction is a key factor for the successful establishment and spread of introduced species. Oxalis pes-caprae is a tristylous species with a self- and morph-incompatibility sexual system that, in the invaded range of the western Mediterranean Basin, has been found to reproduce asexually because only the pentaploid, short-styled morph (5x S-morph) was introduced. The objective of this study was to test the ability of the 5x S-morph of O. pes-caprae to produce viable offspring in the absence of compatible mates, exploring the hypothesis that new morphs could have emerged by sexual reproduction events of the initially introduced morph. Pollen germination, pollen tube development, fruit and seed production, seed germination and offspring ploidy levels were analysed after controlled hand-pollinations to assess self- and morph-incompatibility and production of viable gametes by the 5x S-morph. The self-incompatibility system is still operating, but a partial breakdown in the morph-incompatibility system combined with the production of viable gametes was observed, allowing sexual reproduction of the 5x S-morph in the invaded range. The ability of the 5x S-morph to reproduce sexually may have major consequences for the dynamics of invasive populations of O. pes-caprae and could be one of the factors involved in the occurrence of new floral morphs in this invaded range.

Emasculation increases seed set in the bird-pollinated hermaphrodite Kniphofia linearifolia (Xanthorrhoeaceae): evidence for sexual conflict?

PREMISE OF THE STUDY

Hermaphroditism in plants can lead to gender conflicts, such as pollen discounting-the loss of siring opportunities following self-pollination, and ovule discounting-the loss of seed production opportunities when self-pollen tubes disable ovules through early inbreeding depression or late-acting self-incompatibility. If ovules are discounted by self-pollination, it can be predicted that emasculation (removal of self-pollen) should increase seed production, as long as pollinators are not deterred by emasculation. •

METHODS

Using the hermaphroditic Kniphofia linearifolia, which belongs to a lineage known to possess late-acting self-incompatibility, we performed hand-pollinations with either self- or cross-pollen and recorded pollen tube growth and seed production. We paired experimentally emasculated plants with non-emasculated controls and quantified pollinator visitation rates, fruit, and seed set in both groups. •

KEY RESULTS

Ovules penetrated by tubes from self-pollen uniformly failed to develop into seeds, as expected from ovarian self-incompatibility (or strong early inbreeding depression). Experimental emasculation had no effect on the rate of visitation by birds, the primary pollinators of this species, but led to increased rejection by bees, which are secondary pollinators. Despite reducing visitation by some pollinators, emasculation led to increased seed production and reduced rates of seed abortion. •

CONCLUSIONS

These results show that female performance in hermaphroditic flowers can be enhanced when flowers are emasculated, even if the overall number of pollinator visits is decreased. The most likely explanation for this effect of emasculation is that it reduces ovule discounting arising from pollinator-mediated self-pollination.

Confocal observations of late-acting self-incompatibility in Theobroma cacao L

Cocoa (Theobroma cacao) has an idiosyncratic form of late-acting self-incompatibility that operates through the non-fusion of incompatible gametes. Here, we used high-resolution confocal microscopy to define fine level changes to the embryo sac of the strongly self-incompatible cocoa genotype SCA 24 in the absence of pollination, and following compatible and incompatible pollination. All sperm nuclei had fused with the female nuclei by 48 h following compatible pollinations. However, following incompatible pollinations, we observed divergence in the behaviour of sperm nuclei following release into the embryo sac. Incomplete sperm nucleus migration occurred in approximately half of the embryo sacs, where the sperm nuclei had so far failed to reach the female gamete nuclei. Sperm nuclei reached but did not fuse with the female gamete nuclei in the residual cases. We argue that the cellular mechanisms governing sperm nucleus migration to the egg nucleus and those controlling subsequent nuclear fusion are likely to differ and should be considered independently. Accordingly, we recommend that future efforts to characterise the genetic basis of LSI in cocoa should take care to differentiate between these two events, both of which contribute to failed karyogamy. Implications of these results for continuing efforts to gain better understanding of the genetic control of LSI in cocoa are discussed.

The relative importance of reproductive assurance and automatic selection as hypotheses for the evolution of self-fertilization

BACKGROUND

The field of plant mating-system evolution has long been interested in understanding why selfing evolves from outcrossing. Many possible mechanisms drive this evolutionary trend, but most research has focused upon the transmission advantage of selfing and its ability to provide reproductive assurance when cross-pollination is uncertain. We discuss the shared conceptual framework of these ideas and their empirical support that is emerging from tests of their predictions over the last 25 years.

SCOPE

These two hypotheses are derived from the same strategic framework. The transmission advantage hypothesis involves purely gene-level selection, with reproductive assurance involving an added component of individual-level selection. Support for both of these ideas has been garnered from population-genetic tests of their predictions. Studies in natural populations often show that selfing increases seed production, but it is not clear if this benefit is sufficient to favour the evolution of selfing, and the ecological agents limiting outcross pollen are often not identified. Pollen discounting appears to be highly variable and important in systems where selfing involves multiple floral adaptations, yet seed discounting has rarely been investigated. Although reproductive assurance appears likely as a leading factor facilitating the evolution of selfing, studies must account for both seed and pollen discounting to adequately test this hypothesis.

CONCLUSIONS

The transmission advantage and reproductive assurance ideas describe components of gene transmission that favour selfing. Future work should move beyond their dichotomous presentation and focus upon understanding whether selection through pollen, seed or both explains the spread of selfing-rate modifiers in plant populations.

The presence of co-flowering species facilitates reproductive success of Pedicularis monbeigiana (Orobanchaceae) through variation in bumble-bee foraging behaviour

Background and Aims The presence of co-flowering species can alter pollinator foraging behaviour and, in turn, positively or negatively affect the reproductive success of the focal species. Such interactions were investigated between a focal species, Pedicularis monbeigiana, and a co-flowering species, Vicia dichroantha, which was mediated by behaviour alteration of the shared bumble-bee pollinator. Methods Floral display size and floral colour change of P. monbeigiana were compared between pure (P. monbeigiana only) and mixed (P. monbeigiana and V. dichroantha) plots in two populations. Pollinator visitation rates, interspecific floral switching and successive within-plant pollinator visits were recorded. In addition, supplemental pollination at plant level was performed, and the fruit set and seed set were analysed in pure and mixed plots with different densities of P. monbeigiana. Key Results Pollinator visitation rates were dramatically higher in mixed plots than in pure plots. The higher pollinator visitation rates were recorded in both low- and high-density plots. In particular, successive flower visits within an individual plant were significantly lower in mixed plots. Supplemental pollination significantly increased fruit set and seed set of individuals in pure plots, while it only marginally increased seed set per fruit of plants in mixed plots. Conclusions The presence of V. dichroantha can facilitate pollination and increase female reproductive success of P. monbeigiana via both quantity (mitigating pollinator limitation) and quality (reducing geitonogamy) effects. This study suggests that successive pollinator movements among flowers within a plant, as well as pollinator visitation rates and interspecific flower switching, may be important determinants of the direction and mechanisms of interaction between species.