Differential ovule development following self- and cross-pollination: the basis of self-sterility in Narcissus triandrus (Amaryllidaceae)

The economy of pollen dispersal in flowering plants

Mating success of flowering plants depends strongly on the efficiencies of pollen removal from flowers and its subsequent dispersal to conspecific stigmas. We characterized the economy of pollen dispersal in flowering plants by analysing pollen fates and their correlates for 228 species. The mean percentage of pollen removed from flowers (removal efficiency) varied almost twofold according to the type of pollen-dispersal unit, from less than 45% for orchids and milkweeds with solid pollinia, to greater than 80% for species with granular monads or sectile (segmented) pollinia. The mean percentage of removed pollen reaching stigmas (pollen transfer efficiency, PTE) varied from 2.4% for species with separate monads to 27.0% for orchids with solid pollinia. These values tended to be higher in plants with single pollinator species and in those with non-grooming pollinators. Nectar production increased removal efficiency, but did not influence PTE. Among types of pollen-dispersal units, the net percentage of produced pollen that was dispersed to stigmas varied negatively with removal efficiency and positively with PTE, indicating the relative importance of the latter for overall pollen economy. These findings confirm the key importance of floral traits, particularly pollen packaging, for pollen dispersal outcomes and highlight the under-appreciated pollination efficiency of non-grooming pollinators.

Stigmatic Transcriptome Analysis of Self-Incompatible and Compatible Pollination in Corylus heterophylla Fisch. × Corylus avellana L

Self-incompatibility (SI) protects plants from inbreeding depression due to self-pollination and promotes the outcrossing process to maintain a high degree of heterozygosity during evolution. Corylus is an important woody oil and nut species that shows sporophytic SI (SSI). Yet the molecular mechanism of SI in Corylus remains largely unknown. Here we conducted self- ("Dawei" × "Dawei") and cross-pollination ("Dawei" × "Liaozhen No. 7") experiments and then performed an RNA-Seq analysis to investigate the mechanism of pollen-stigma interactions and identify those genes that may be responsible for SSI in Corylus. We uncovered 19,163 up- and 13,314 downregulated genes from the comparison of different pollination treatments. These differentially expressed genes (DEGs) were significantly enriched in plant-pathogen interaction, plant hormone signal transduction, and MAPK signaling pathway-plant. We found many notable genes potentially involved in pollen-stigma interactions and SSI mechanisms, including genes encoding receptor-like protein kinases (RLK), calcium-related genes, disease-resistance genes, and WRKY transcription factors. Four upregulated and five downregulated DEGs were consistently identified in those comparison groups involving self-incompatible pollination, suggesting they had important roles in pollen-pistil interactions. We further identified the S-locus region of the Corylus heterophylla genome based on molecular marker location. This predicted S-locus contains 38 genes, of which 8 share the same functional annotation as the S-locus genes of Corylus avellana: two PIX7 homologous genes (EVM0002129 and EVM0025536), three MIK2 homologous genes (EVM0002422, EVM0005666, and EVM0009820), one aldose 1-epimerase (EVM0002095), one 3-dehydroquinate synthase II (EVM0021283), and one At3g28850 homologous gene (EVM0016149). By characterizing the pistil process during the early postpollination phase via transcriptomic analysis, this study provides new knowledge and lays the foundation for subsequent analyses of pollen-pistil interactions.

Pollen-mediated gene flow ensures connectivity among spatially discrete sub-populations of Phalaenopsis pulcherrima, a tropical food-deceptive orchid

BACKGROUND

Gene flow in plants via pollen and seeds is asymmetrical at different geographic scales. Orchid seeds are adapted to long-distance wind dispersal but pollinium transfer is often influenced by pollinator behavior. We combined field studies with an analysis of genetic diversity among 155 physically mapped adults and 1105 F1 seedlings to evaluate the relative contribution of pollen and seed dispersal to overall gene flow among three sub-populations of the food-deceptive orchid Phalaenopsis pulcherrima on Hainan Island, China.

RESULTS

Phalaenopsis pulcherrima is self-sterile and predominantly outcrossing, resulting in high population-level genetic diversity, but plants are clumped and exhibit fine-scale genetic structuring. Even so, we detected low differentiation among sub-populations, with polynomial regression analysis suggesting gene flow via seed to be more restricted than that via pollen. Paternity analysis confirmed capsules of P. pulcherrima to each be sired by a single pollen donor, probably in part facilitated by post-pollination stigma obfuscation, with a mean pollen flow distance of 272.7 m. Despite limited sampling, we detected no loss of genetic diversity from one generation to the next.

CONCLUSIONS

Outcrossing mediated by deceptive pollination and self-sterility promote high genetic diversity in P. pulcherrima. Long-range pollinia transfer ensures connectivity among sub-populations, offsetting the risk of genetic erosion at local scales.

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.

Avoiding sexual interference: herkogamy and dichogamy in style dimorphic flowers of Narcissus broussonetii (Amaryllidaceae)

Spatial (herkogamy) or temporal (dichogamy) separation of sex organs are mechanisms considered to restrict self-pollination and promote outcrossing. Additionally, avoidance of self-interference is proposed to be the driving force for the evolution of these mechanisms, particularly in self-incompatible species. However, species with anthers and stigmas at different levels may increase the rate of imprecise pollen transfer, resulting in pollen discounting. Non-reciprocal stylar dimorphism has been considered a transitional, unstable stage towards the evolution of reciprocal style dimorphism (distyly), to simultaneously avoid interference and lack of precision. In this study we investigate the spatial and temporal separation of sex organs in a population of the style dimorphic and self-incompatible Narcissus broussonetii and their consequences in the reciprocity between the sex organs of morphs and their fecundity. First, we evaluated the relative growth of sex organs after anthesis. Then, we studied the stigma receptivity along the flower lifespan including its effect on seed production in both morphs. Finally, given the weak reciprocity between the sex organs of morphs of this species, we estimated population genetic diversity parameters in Long- and Short-styled plants to explore differences between them as a result of rates of inbreeding due to different mating strategies. We observed that Long-styled plants and Short-styled plants present different strategies to avoid sexual interference and both of them had negative consequences in the reciprocity between the sex organs of morphs. Long-styled plants exhibited a delay in stigma receptivity and a higher growth rate of the style after anthesis, while Short-styled plants presented higher herkogamy and no delay in stigma receptivity. These findings suggest that the avoidance of self-interference, in stylar dimorphic Narcissus species, seems to be more critical than improving of reciprocity between the sex organs of morphs. This might explain why reciprocal herkogamy (distyly) is rare in the genus.

Transfer of Downy Mildew Resistance from Wild Basil (Ocimum americanum) to Sweet Basil (O. basilicum)

Sweet basil (Ocimum basilicum) is susceptible to downy mildew caused by the oomycete foliar pathogen Peronospora belbahrii. No resistant varieties of sweet basil are commercially available. Here, we report on the transfer of resistance gene Pb1 from the highly resistant tetraploid wild basil O. americanum var. americanum (PI 500945, 2n = 4x = 48) to the tetraploid susceptible O. basilicum 'Sweet basil' (2n = 4x = 48). F1 progeny plants derived from the interspecific hybridization PI 500945 × Sweet basil were resistant, indicating that the gene controlling resistance (Pb1) is dominant, but sterile due to the genetic distance between the parents. Despite their sterility, F1 plants were pollinated with the susceptible parent and 115 first backcross generation to the susceptible parent (BCs1) embryos were rescued in vitro. The emerging BCs1 plants segregated, upon inoculation, 5:1 resistant/susceptible, suggesting that resistance in F1 was controlled by a pair of dominant genes (Pb1A and Pb1A'). Thirty-one partially fertile BCs1 plants were self-pollinated to obtain BCs1-F2 or were backcrossed to Sweet basil to obtain the second backcross generation to the susceptible parent (BCs2). In total, 1 BCs1-F2 and 22 BCs2 progenies were obtained. The BCs1-F2 progeny segregated 35:1 resistant/susceptible, as expected from a tetraploid parent with two dominant resistant genes. The 22 BCs2 progenies segregated 1:1 resistant/susceptible (for a BCs1 parent that carried one dominant gene for resistance) or 5:1 (for a BCs1 parent that carried two dominant genes for resistance) at a ratio of 4:1. The data suggest that a pair of dominant genes (Pb1A and Pb1A') residing on a two homeologous chromosomes is responsible for resistance of PI 500945 against P. belbahrii.

Evolutionary Dynamics of Unreduced Gametes

Unreduced gametes, which have the somatic (2n) chromosome number, are an important precursor to polyploid formation and apomixis. The product of irregularities in meiosis, 2n gametes are expected to be rare and deleterious in most natural populations, contrary to their wide taxonomic distribution and the prevalence of polyploidy. To better understand this discrepancy, we review contemporary evidence related to four aspects of 2n gamete dynamics in natural populations: (i) estimates of their frequency; (ii) their environmental and genetic determinants; (iii) adaptive and nonadaptive processes regulating their evolution; and (iv) factors regulating their union and production of polyploids in diploid populations. Aided by high-throughput methods of detection, these foci will advance our understanding of variation in 2n gametes within and among species, and their role in polyploid evolution.

Unique growth paths of heterospecific pollen tubes result in late entry into ovules in the gynoecium of Sagittaria (Alismataceae)

Pollen-pistil interactions are a fundamental process in the reproductive biology of angiosperms and play a particularly important role in maintaining incipient species that exist in sympatry. However, the majority of previous studies have focused on species with syncarpous gynoecia (fused carpels) and not those with apocarpous gynoecia (unfused carpels). In the present study, we investigated the growth of conspecific pollen tubes compared to heterospecific pollen tubes in Sagittaria species, which have apocarpous gynoecia. We conducted controlled pollinations between S. pygmaea and S. trifolia and observed the growth of conspecific and heterospecific pollen tubes under a fluorescence microscope. Heterospecific and conspecific pollen tubes arrived at locules within the ovaries near simultaneously. However, conspecific pollen tubes entered into the ovules directly, whereas heterospecific tubes passed through the carpel base and adjacent receptacle tissue, to ultimately fertilize other unfertilized ovules. This longer route taken by heterospecific pollen tubes therefore caused a delay in the time required to enter into the ovules. Furthermore, heterospecific pollen tubes displayed similar growth patterns at early and peak pollination. The growth pattern of heterospecific pollen tubes at late pollination was similar to that of conspecific pollen tubes at peak pollination. Heterospecific and conspecific pollen tubes took different routes to fertilize ovules. A delayed entry of heterospecific pollen into ovules may be a novel mechanism of conspecific pollen advantage (CPA) for apocarpous species.

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.

Reciprocal herkogamy promotes disassortative mating in a distylous species with intramorph compatibility

Mating patterns in heterostylous species with intramorph compatibility have the potential to deviate from symmetrical disassortative mating owing to ecological and reproductive factors influencing pollen dispersal. Here, we investigate potential and realized patterns of mating in distylous Luculia pinceana (Rubiaceae), a species with intramorph compatibility. Our analysis provides an opportunity to test Darwin's hypothesis that reciprocal herkogamy promotes disassortative pollen transfer. We combined measurements of sex-organ reciprocity and pollen production to predict potential pollen transfer and mating patterns in a population from SW China. Marker-based paternity analysis was then used to estimate realized patterns of disassortative and assortative mating at the individual and floral morph levels. Both potential and realized mating patterns indicated a significant component of disassortative mating, satisfying theoretical conditions for the maintenance of floral dimorphism. Levels of assortative mating (37.7%) were significantly lower than disassortative mating (62.3%), but numerous offspring resulting from intramorph mating were detected in the majority of maternal seed families in both floral morphs. Our results provide empirical support for Darwin's cross-promotion hypothesis on the function of reciprocal herkogamy, but indicate that in most heterostylous species strong diallelic incompatibility may be a general requirement for complete disassortative mating.

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).

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.

The interplay between inflorescence development and function as the crucible of architectural diversity

BACKGROUND

Most angiosperms present flowers in inflorescences, which play roles in reproduction, primarily related to pollination, beyond those served by individual flowers alone. An inflorescence's overall reproductive contribution depends primarily on the three-dimensional arrangement of the floral canopy and its dynamics during its flowering period. These features depend in turn on characteristics of the underlying branching structure (scaffold) that supports and supplies water and nutrients to the floral canopy. This scaffold is produced by developmental algorithms that are genetically specified and hormonally mediated. Thus, the extensive inflorescence diversity evident among angiosperms evolves through changes in the developmental programmes that specify scaffold characteristics, which in turn modify canopy features that promote reproductive performance in a particular pollination and mating environment. Nevertheless, developmental and ecological aspects of inflorescences have typically been studied independently, limiting comprehensive understanding of the relations between inflorescence form, reproductive function, and evolution.

SCOPE

This review fosters an integrated perspective on inflorescences by summarizing aspects of their development and pollination function that enable and guide inflorescence evolution and diversification.

CONCLUSIONS

The architecture of flowering inflorescences comprises three related components: topology (branching patterns, flower number), geometry (phyllotaxis, internode and pedicel lengths, three-dimensional flower arrangement) and phenology (flower opening rate and longevity, dichogamy). Genetic and developmental evidence reveals that these components are largely subject to quantitative control. Consequently, inflorescence evolution proceeds along a multidimensional continuum. Nevertheless, some combinations of topology, geometry and phenology are represented more commonly than others, because they serve reproductive function particularly effectively. For wind-pollinated species, these combinations often represent compromise solutions to the conflicting physical influences on pollen removal, transport and deposition. For animal-pollinated species, dominant selective influences include the conflicting benefits of large displays for attracting pollinators and of small displays for limiting among-flower self-pollination. The variety of architectural components that comprise inflorescences enable diverse resolutions of these conflicts.

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

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

The low fertility of Chinese white poplar: dynamic changes in anatomical structure, endogenous hormone concentrations, and key gene expression in the reproduction of a naturally occurring hybrid

KEY MESSAGE : We report that low fertility during intraspecific hybridization in Chinese white poplar was caused by prefertilization barriers, reduced ovules, and embryonic abortion. Hormone concentrations and gene expression patterns were also evaluated during the fertilization process. Hybrid vigor holds tremendous potential for yield increases and trait improvement; however, some hybridization combinations within Populus show very low fertility. To explore the causes of this low fertility in intraspecific hybridization of Chinese white poplar, we examined anatomical structure, hormone levels and expression of key genes in two unique crossing combinations of Populus × tomentosa "Pt02" × P. × tomentosa "LM50", and (P. × tomentosa × P. alba cv. bolleana "Ptb") × P. × tomentosa "LM50". The seed set potential in the intraspecific hybridization P. × tomentosa "Pt02" × P. × tomentosa "LM50" was quite low, which was likely caused by prefertilization barriers, reduced ovule numbers, and embryonic abortion in ovaries. During intraspecific hybridization, we found reduced indoleacetic acid (IAA) in pistils, which may cause pollen tube deformations and increased IAA in heart-stage embryos, which may affect embryo development. Gibberellin A3 (GA3) decreased from the zygote dormancy stage to globular-stage embryos, which may be caused by failure of fertilization in specific embryos. The maximum zeatin (Z) concentration was found in heart-stage embryos, but Z concentrations quickly decreased, which may affect endosperm development. Increasing concentrations of abscisic acid (ABA) during zygote dormancy and eight-cell proembryo stages likely induced abscission of the infructescence. High ABA concentrations also regulated embryo maturity. Measurement of genes expression showed that high expression of SRK and/or SLG may result in rejection of pollen by stigmatic papillae through a mechanism, reminiscent of self-incompatibility. Also, low expression of LEC1 and FUS3 may cause embryonic abortion. Identification and eventual bypassing of these barriers may allow future genetic improvement of this key woody crop species.

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.

Sterile flowers increase pollinator attraction and promote female success in the Mediterranean herb Leopoldia comosa

BACKGROUND AND AIMS

Large floral displays have opposing consequences for animal-pollinated angiosperms: they attract more pollinators but also enable elevated among-flower self-pollination (geitonogamy). The presence of sterile flowers as pollinator signals may enhance attraction while allowing displays of fewer open fertile flowers, limiting geitonogamy. The simultaneous contributions of fertile and non-fertile display components to pollinator attraction and reproductive output remain undetermined.

METHODS

The simultaneous effects of the presence of sterile flowers and fertile-flower display size in two populations of Leopoldia comosa were experimentally assessed. Pollinator behaviour, pollen removal and deposition, and fruit and seed production were compared between intact plants and plants with sterile flowers removed.

KEY RESULTS

The presence of sterile flowers almost tripled pollinator attraction, supplementing the positive effect of the number of fertile flowers on the number of bees approaching inflorescences. Although attracted bees visited more flowers on larger inflorescences, the number visited did not additionally depend on the presence of sterile flowers. The presence of sterile flowers improved all aspects of plant performance, the magnitude of plant benefit being context dependent. During weather favourable to pollinators, the presence of sterile flowers increased pollen deposition on stigmas of young flowers, but this difference was not evident in older flowers, probably because of autonomous self-pollination in poorly visited flowers. Total pollen receipt per stigma decreased with increasing fertile display size. In the population with more pollinators, the presence of sterile flowers increased fruit number but not seed set or mass, whereas in the other population sterile flowers enhanced seeds per fruit, but not fruit production. These contrasts are consistent with dissimilar cross-pollination and autonomous self-pollination, coupled with the strong predispersal inbreeding depression exhibited by L. comosa populations.

CONCLUSIONS

Sterile flowers enrich pollination quality by promoting pollen export and import, while limiting the mating costs of geitonogamy associated with large fertile displays.

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.

Unusual heterostyly: style dimorphism and self-incompatibility are not tightly associated in Lithodora and Glandora (Boraginaceae)

BACKGROUND AND AIMS

Heterostyly is a floral polymorphism characterized by the reciprocal position of stamens and stigmas in different flower morphs in a population. This reciprocal herkogamy is usually associated with an incompatibility system that prevents selfing and intra-morph fertilization, termed a heteromorphic incompatibility system. In different evolutionary models explaining heterostyly, it has been alternately argued that heteromorphic incompatibility either preceded or followed the evolution of reciprocal herkogamy. In some models, reciprocal herkogamy and incompatibility have been hypothesized to be linked together during the evolution of the heterostylous system.

METHODS

We examine the incompatibility systems in species with different stylar polymorphisms from the genera Lithodora and Glandora (Boraginaceae). We then test whether evolution towards reciprocal herkogamy is associated with the acquisition of incompatibility. To this end, a phylogeny of these genera and related species is reconstructed and the morphological and reproductive changes that occurred during the course of evolution are assessed.

KEY RESULTS

Both self-compatibility and self-incompatibility are found within the studied genera, along with different degrees of intra-morph compatibility. We report for the first time extensive variability among members of the genus Glandora and related species in terms of the presence or absence of intraspecies polymorphism and heteromorphic incompatibility. Overall, our results do not support a tight link between floral polymorphism and incompatibility systems.

CONCLUSIONS

The independent evolution of stylar polymorphism and incompatibility appears to have occurred in this group of plants. This refutes the canonical view that there is strong linkage between these reproductive traits.

Effects of pollen reward removal on fecundity in a self-incompatible hermaphrodite plant

The pollen of hermaphrodite plants is often utilised by flower-visiting animals. While pollen production has obvious benefits for plant male fitness, its consequences for plant female fitness, especially in self-incompatible hermaphrodite species, are less certain. Pollen production could either enhance seed production though increased pollinator attraction, or reduce it if ovules are discounted by deposition of self pollen, as can occur in species with late-acting self-incompatibility. To test the effects of pollen reward provision on female fitness, we artificially emasculated flowers in two populations of the succulent Aloe maculata (Asphodelaceae), which has a late-acting self-incompatibility system, over the course of its flowering period. Flowers of this species are visited by sunbirds (for nectar) and native bees (for pollen and nectar). We measured floral visitation rates, floral rejection rates, pollen deposition on stigmas and fruit and seed set in both emasculated and non-emasculated plants. We found that flowers of emasculated plants suffered reduced visitation and increased rejection (arrival without visitation) by bees, but not by sunbirds; had fewer pollen grains deposited on stigmas and showed an overall decrease in fruit set and seed set. Rates of seed abortion were, however, greatly reduced in emasculated flowers. This study shows that pollen rewards can be important for seed set, even in self-incompatible plants, which have been assumed to rely on nectar rewards for pollinator attraction. Seed abortion was, however, increased by pollen production, a result that highlights the complexity of selection on pollen production in hermaphrodite flowers.

Physiology, morphology and phenology of seed dormancy break and germination in the endemic Iberian species Narcissus hispanicus (Amaryllidaceae)

BACKGROUND AND AIMS

Only very few studies have been carried out on seed dormancy/germination in the large monocot genus Narcissus. A primary aim of this study was to determine the kind of seed dormancy in Narcissus hispanicus and relate the dormancy breaking and germination requirements to the field situation.

METHODS

Embryo growth, radicle emergence and shoot growth were studied by subjecting seeds with and without an emerged radicle to different periods of warm, cold or warm plus cold in natural temperatures outdoors and under controlled laboratory conditions.

KEY RESULTS

Mean embryo length in fresh seeds was approx. 1·31 mm, and embryos had to grow to 2·21 mm before radicle emergence. Embryos grew to full size and seeds germinated (radicles emerged) when they were warm stratified for 90 d and then incubated at cool temperatures for 30 d. However, the embryos grew only a little and no seeds germinated when they were incubated at 9/5, 10 or 15/4 °C for 30 d following a moist cold pre-treatment at 5, 9/5 or 10 °C. In the natural habitat of N. hispanicus, seeds are dispersed in late May, the embryo elongates in autumn and radicles emerge (seeds germinate) in early November; however, if the seeds are exposed to low temperatures before embryo growth is completed, they re-enter dormancy (secondary dormancy). The shoot does not emerge until March, after germinated seeds are cold stratified in winter.

CONCLUSION

Seeds of N. hispanicus have deep simple epicotyl morphophysiological dormancy (MPD), with the dormancy formula C(1b)B(root) - C(3)(epicotyl). This is the first study on seeds with simple MPD to show that embryos in advanced stages of growth can re-enter dormancy (secondary dormancy).

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

BACKGROUND AND AIMS

Animal pollination is typically an uncertain process that interacts with self-incompatibility status to determine reproductive success. Seed set is often pollen-limited, but species with late-acting self-incompatibility (SI) may be particularly vulnerable, if self-pollen deposition results in ovule discounting. Pollination is examined and the occurrence of late-acting SI and ovule discounting assessed in Cyrtanthus breviflorus.

METHODS

The pollination system was characterized by observing floral visitors and assessing nectar production and spectral reflectance of flowers. To assess late-acting SI and ovule discounting, growth of self- and cross-pollen tubes, and seed set following open pollination or hand pollination with varying proportions of self- and cross-pollen, were examined.

KEY RESULTS

Native honeybees Apis mellifera scutellata pollinated flowers as they actively collected pollen. Most flowers (≥70 %) did not contain nectar, while the rest produced minute volumes of dilute nectar. The flowers which are yellow to humans are visually conspicuous to bees with a strong contrast between UV-reflecting tepals and UV-absorbing anthers and pollen. Plants were self-incompatible, but self-rejection was late-acting and both self- and cross-pollen tubes penetrated ovules. Seed set of open-pollinated flowers was pollen-limited, despite pollen deposition exceeding ovule number by 6-fold. Open-pollinated seed set was similar to that of the cross + self-pollen treatment, but was less than that of the cross-pollen-only treatment.

CONCLUSIONS

Flowers of C. breviflorus are pollinated primarily by pollen-collecting bees and possess a late-acting SI system, previously unknown in this clade of the Amaryllidaceae. Pollinators of C. breviflorus deposit mixtures of cross- and self-pollen and, because SI is late-acting, self-pollen disables ovules, reducing female fertility. This study thus contributes to growing evidence that seed production in plants with late-acting SI systems is frequently limited by pollen quality, even when pollinators are abundant.

Patterns of style polymorphism in five species of the South African genus Nivenia (Iridaceae)

BACKGROUND AND AIMS

Heterostylous plants have been characterized by the presence of two or three discrete morphs that differ in their sex organ position within populations. This polymorphism is widely distributed among the angiosperms, but detailed studies are limited to few taxonomic groups. Although a small representation, evolutionary meaningful variations of the heterostylous syndrome have been reported when precise measurements of the sexual whorls were taken. A thorough exploration of groups where heterostyly has been reported should offer new opportunities to further testing the evolutionary hypotheses explaining heterostyly. Here, the traits defining heterostyly were explored in half of the species in Nivenia, the only genus of Iridiaceae where heterostyly has been reported.

METHODS

Detailed morphometric analysis of the flower sexual whorls and some traits considered as ancillary are supplied to determine for each population (a) the kind of stylar polymorphism, (b) the morph ratio and (c) the degree of reciprocity between sexual whorls. Also the rates of assortative (within morph) versus disassortative (between morphs) pollen transfer were estimated by analysing pollen loads on stigmas. The association between floral phenotypic integration and the reciprocity between sexual whorls was estimated; both characteristics have been quoted as dependent on the accuracy of the fit between pollinators and flowers and therefore related to the efficiency of pollen transfer.

KEY RESULTS

Different types of polymorphism, differing in their degree of reciprocity, were found in Nivenia. Effective disassortative mating appears to be common, since (a) all dimorphic populations show equal morph-ratios (isoplethy), and (b) the pollen placed on the stigmas of each morph is likely to be coming from the other (complementary) morph. The most reciprocal populations of the heterostylous species have also the highest values of phenotypical integration.

CONCLUSIONS

Stigma height dimorphism, as opposed to distyly, is proven for the first time in Nivenia. The presence of different types of polymorphism within the genus is consistent with hypotheses of the evolution of heterostyly. The role of the pollinators as the leading force of the transition seems to be apparent, since floral integration is related to reciprocity.

Self-sterility in two Cytisus species (Leguminosae, Papilionoideae) due to early-acting inbreeding depression

In most angiosperms, the endosperm develops before the embryo, but with harmony between the two structures until final seed formation. In an embryological study, we show that inbreeding depression causes disharmony in development of the two structures in two Leguminosae shrubs, Cytisus multiflorus and C. striatus. Our main objective was to test the causes of self-sterility in the two species by comparing the embryological development of the self seeds with that of cross seeds. In developing selfed seeds of C. multiflorus, the embryo reaches at most the globular stage and never forms mature seeds, while in C. striatus a few mature selfed seeds are formed. In both species, the main cause of abortion of developing selfed seeds is diminished endosperm development (low values of the ratio of endosperm to embryo), which triggers collapse of the endosperm and embryo. The results indicate that self-sterility in C. striatus is postzygotic because of strong, early inbreeding depression, while in C. multiflorus there exists a mixed pre- and postzygotic mechanism; the prezygotic mechanism causes rejection of some self-pollen tubes in the style/ovary, and the early inbreeding depression triggers abortion of fertilized ovules that escaped that action.

Floral emasculation reveals pollen quality limitation of seed output in Bulbine bulbosa (Asphodelaceae)1

Pollen commonly limits seed output in natural plant populations, but the relative contributions of pollen quantity and quality to pollen limitation remain largely unexplored. Estimates of these contributions are needed to promote better understanding of the ecological and evolutionary consequences of pollen limitation. We assessed pollen limitation and distinguished how pollen quantity and quality contributed to seed output in the predominantly outcrossing herb Bulbine bulbosa. We compared seed output of emasculated and cross-pollinated plants that could not self-pollinate with that of intact plants. Seed output was similar in emasculated and cross-pollinated plants, but it was about 50% less in intact plants. These findings indicate that the quantity of cross-pollen deposited by pollinators did not limit seed output. However, pollinators also deposited self-pollen, which reduced seed output because selfing disabled ovules, rendering them unavailable for cross-fertilization. We conclude that pollen quality rather than quantity can limit seed output under natural conditions, which supports recent proposals for greater consideration of pollen quality in the study of pollen limitation.

Stigma closure and re-opening in Oroxylum indicum (Bignoniaceae): Causes and consequences

The study of plant responses to touch, particularly the responses of leaves, stems, and roots, has a long history. By contrast, floral responses are relatively unexplored. Stigma closure is common in the Bignoniaceae, but the factors influencing it are not well understood. We investigated factors influencing stigma closure and reopening and its effects on pollen receipt in seven Oroxylum indicum trees near Hat Yai, Thailand. The effects of pressure, conspecific and heterospecific pollen, and pollen load (the amount of pollen deposited) on stigma behavior were examined in 270 flowers (of the total 430 flowers evaluated in the entire study). Pressure alone resulted in faster closure than did conspecific pollination and faster reopening than did heterospecific pollination. Stigmas never reopened after conspecific pollination. Pollen load had no effect on stigma behavior. Stigmas discriminated between conspecific and foreign pollen; they reopened only after pollination with the latter. A manipulative experiment revealed that stigma closure did not affect the number of conspecific pollen grains received. We also counted pollen tubes in styles that were either hand-supplemented with outcross conspecific pollen or open-pollinated. Pollen tube numbers were highest after light pollination (∼900 grains), indicating that interference among pollen grains may occur after pollination with very heavy loads (>6000 grains). Possible fitness consequences of these responses are discussed.

Analyses of sexual reproductive success in transgenic and/or mutant plants

The pistil, the female reproductive organ of plants, is a key player in the success of sexual plant reproduction. Ultimately, the production of fruits and seeds depends on the proper pistil development and function. Therefore, the identification and characterization of pistil expressed genes is essential for a better understanding and manipulation of the plant reproduction process. For studying the function of pistil expressed genes, transgenic and/or mutant plants for the genes of interest are used. The present article provides a review of methods already exploited to analyze sexual reproductive success. We intend to supply useful information and to guide future experiments in the study of genes affecting pistil development and function.

Natural selection on floral traits through male and female function in wild populations of the heterostylous daffodil Narcissus triandrus

Measurements of natural selection in hermaphrodite populations require the analysis of performance through both female and male sex functions. Here, we investigate selection on three floral traits: flower number, flower length, and corona width through both sex functions in natural populations of the tristylous daffodil Narcissus triandrus. Selection through female function was examined in six populations, and in two of these we also estimated male selection gradients using multilocus microsatellite genotyping of parents and offspring. We detected significant directional selection for flower number through female function, and significant stabilizing selection for corona width and flower length through male function. Variation in male reproductive success was strongly influenced by the distance between mates and was significantly higher than variation in female reproductive success in one population, a result consistent with Bateman's principle. However, variation through both sex functions was similar in the other population and there was a significant negative correlation between female and male fitness indicating sex-specific trade-offs in reproductive success. Selection on floral design in N. triandrus was stronger through male than female function probably because floral morphology plays an important role in promoting effective cross-pollen transfer in populations of this heterostylous species.

Asymmetrical mating patterns and the evolution of biased style-morph ratios in a tristylous daffodil

Non-random mating in plant populations can be influenced by numerous reproductive and demographic factors, including floral morphology and inter-plant distance. Here, we investigate patterns of outcrossed mating through male function inNarcissus triandrus, a tristylous, bee-pollinated wild daffodil from the Iberian Peninsula, to test pollen transfer models which predict that floral morphology promotes asymmetrical mating and biased morph ratios. Unlike other tristylous species,N. triandrushas an incompatibility system that permits intra-morph mating and long-level rather than mid-level stamens in the L-morph. Incomplete sex-organ reciprocity should result in significant intra-morph mating in the L-morph. We measured mating patterns in two L-biased populations – dimorphic (two style morphs) and trimorphic (three style morphs) – using multilocus genotyping and maximum-likelihood-based paternity analysis. We also examined the spatial distribution of style morphs and neutral markers to investigate the potential consequence of spatially restricted mating on morph ratios. As predicted, we detected significant amounts of intra-morph mating in the L-morph in both populations. Pollen transfer coefficients generally supported predictions based on the Darwinian hypothesis that anthers and stigmas of equivalent level promote pollinator-mediated cross-pollination in heterostylous populations. There was evidence of significant spatial aggregation of both style morphs and neutral markers in populations ofN. triandrus, probably as a result of restricted pollen and seed dispersal. Our results provide empirical support for theoretical models of pollen transfer, which indicate that the commonly observed L-biased morph ratios inNarcissusspecies result from significant intra-morph mating in the L-morph because of its atypical floral morphology.

Mating patterns and demography in the tristylous daffodil Narcissus triandrus

Mating patterns in plant populations are influenced by interactions between reproductive traits and ecological conditions, both factors that are likely to vary geographically. Narcissus triandrus, a wide-ranging heterostylous herb, exhibits populations with either two (dimorphic) or three (trimorphic) style morphs and displays substantial geographical variation in demographic attributes and floral morphology. Here, we investigate this variation to determine if demography, morphology, and mating system differ between the two sexual systems. Our surveys in Portugal and NW Spain indicated that dimorphic populations were less dense, of smaller size, and had larger plants and flowers compared to trimorphic populations. Outcrossing rates estimated using allozyme markers revealed similar outcrossing rates in dimorphic and trimorphic populations (t(m) dimorphic=0.759; t(m) trimorphic=0.710). All populations experienced significant inbreeding in progeny (mean F=0.143). In contrast, parental estimates of inbreeding were not significantly different from zero (mean F=0.062), implying that few inbred offspring survive to reproductive maturity due to inbreeding depression. Although the majority of inbreeding results from selfing, significant levels of biparental inbreeding were also detected in eight of the nine populations (mean s(s)-s(m)=0.081). Density was negatively associated with levels of selfing but positively associated with biparental inbreeding. Population size was positively associated with outcrossing but not biparental inbreeding. There were no consistent differences among the style morphs in outcrossing or biparental inbreeding indicating that the maintenance of trimorphism vs dimorphism is unlikely to be associated with inbreeding of maternal parents.

New evidence for the Darwinian hypothesis of heterostyly: breeding systems and pollinators in Narcissus sect. Apodanthi

Here we analysed the role played by breeding systems and pollinators in the evolution of heterostyly by testing whether evolution towards heterostyly is associated with style polymorphism and changes in pollinator proficiency or breeding system variation (Darwinian hypothesis). We studied pollinators, pollen-transfer efficiency, and incompatibility systems in all seven species of Narcissus sect. Apodanthi for which we also obtained chloroplast DNA (cpDNA) sequences from three spacers to infer phylogenetic relationships. Five species are self-incompatible and within-morph cross-compatible. Heterostylous (Narcissus albimarginatus) and style-dimorphic (Narcissus cuatrecasasii) species that have a high degree of reciprocity in stigma and anther height are primarily pollinated by solitary bees. The style-monomorphic species (Narcissus watieri) and the style-dimorphic species with the least stigma-anther reciprocity (Narcissus rupicola) are both self-compatible and pollinated by butterflies, moths and hover flies. Phylogenetic reconstruction of character transitions indicates that the shift from style dimorphism to distyly is associated with a shift to bee pollination. Pollination by lepidopterans and flies is associated with stable style dimorphism and monomorphism. Evolution and maintenance of style polymorphisms in this group of species are independent of incompatibility systems. Taken together, our results strongly support the pollinator-based model for evolution of heterostyly and style length polymorphisms in general.

Female reproductive success and the evolution of mating-type frequencies in tristylous populations

In tristylous populations, mating-type frequencies are governed by negative frequency-dependent selection typically resulting in equal morph ratios at equilibrium. However, Narcissus triandrus generally exhibits long-styled (L)-biased populations with a deficiency of the mid-styled (M)-morph. Here we used a pollen-transfer model and measurements of female fertility in natural populations to investigate whether these uneven morph ratios were associated with variation in female reproductive success. Our theoretical analysis demonstrated that morph ratio bias can result from maternal fitness differences among the morphs, and that these effects were magnified by asymmetrical mating. In nine out of 15 populations of N. triandrus, seed set differed significantly among the morphs, but pollen limitation occurred in only two of 11 populations investigated. Average seed set of the M-morph was positively associated with its frequency in populations. Flower size was negatively correlated with the seed set of the M-morph. Our results suggest that interactions between mating patterns and female fertility are responsible for variation in morph frequencies and loss of the M-morph from tristylous populations of N. triandrus.

Herkogamy and mating patterns in the self-compatible daffodil Narcissus longispathus

BACKGROUND AND AIMS

Floral design in self-compatible plants can influence mating patterns. This study investigated Narcissus longispathus, a self-compatible bee-pollinated species with wide variation in anther-stigma separation (herkogamy), to determine the relationship between variation in this floral trait and the relative amounts of cross- and self-fertilization.

METHODS

Anther-stigma separation was measured in the field in six populations of N. longispathus from south-eastern Spain. Variation in herkogamy during the life of individual flowers was also quantified. Multilocus outcrossing rates were estimated from plants differing in herkogamy using allozyme markers.

KEY RESULTS

Anther-stigma separation varied considerably among flowers within the six populations studied (range = 1-10 mm). This variation was nearly one order of magnitude larger than the slight, statistically non-significant developmental variation during the lifespan of individual flowers. Estimates of multilocus outcrossing rate for different herkogamy classes (t(m) range = 0.49-0.76) failed to reveal a monotonic increase with increasing herkogamy.

CONCLUSIONS

It is suggested that the lack of a positive relationship between herkogamy and outcrossing rate, a result that has not been previously documented for other species, could be mostly related to details of the foraging behaviour of pollinators.

Effects of pollen shortage and self-pollination on seed production of an endangered tree, Magnolia stellata

BACKGROUND AND AIMS

Pollen limitation is a significant determinant of seed production, and can result from both insufficient pollen quantity (pollen shortage) and quality (mainly relating to self-pollination). For animal-pollinated tree species with large floral displays, pollen limitation may be determined by a balance between increased pollen quantity due to increased attractiveness for pollinators, countered by increased self-pollination due to increased geitonogamy. The contributions of pollen shortage and self-pollination on seed production were quantitatively examined in the natural pollination of an insect-pollinated, dichogamous, endangered tree, Magnolia stellata, which has a large, showy floral display.

METHODS

Manual self- and cross-pollinations were conducted to determine the effects of selfing on seed production. The outcrossing rate was measured using microsatellite analyses of open-pollinated seeds, and the embryo mortality rate caused by self-pollination was indirectly estimated. The frequency of ovule mortality due to pollen shortage was also inferred using the embryo mortality and ovule survival rates from natural pollination.

KEY RESULTS

The average fruit set, seed set per fruit, and ovule survival rate per tree from hand cross-pollination were 1.37, 3.15, and 3.34 times higher than those from hand self-pollination, respectively, indicating that self-pollination causes inbreeding depression for fruit and seed set. The multilocus-outcrossing rate (t(m)) was intermediate, 0.632, and the primary selfing rate was 0.657. This indicates that frequent geitonogamous selfing occurs. The ovule mortality rate due to pollen shortage and the embryo mortality rate due to self-pollination were estimated to be 80.8 % and 45.9 %, respectively.

CONCLUSIONS

It is concluded that seed production of M. stellata is strongly limited by both pollen shortage and self-pollination. Inefficient beetle-pollination and the automimicry system via asynchronous flowering might be responsible for the high level of pollen shortage and frequent geitonogamy. This is despite a large, showy floral display and the dichogamous system of the species.

The evolution of polymorphic sexual systems in daffodils (Narcissus)

Narcissus, the daffodil genus, exhibits an unusual diversity of sexual systems, with populations that are monomorphic, dimorphic or trimorphic for style length. Associated with this variation are striking differences among species in floral morphology and pollination biology. This diversity provides an opportunity to investigate the evolution of mating polymorphisms, and to determine how floral morphology promotes transitions among sexual systems. Because of the absence of heteromorphic incompatibility in Narcissus, floral morphology plays a key role in governing patterns of outcrossed mating. Phylogenetic evidence indicates that stylar monomorphism is ancestral in the genus, with multiple origins of stylar polymorphism, including independent origins of stigma-height dimorphism, distyly and tristyly. Sexual polymorphisms have evolved only in lineages with narrow floral tubes that are pollinated by Lepidoptera and/or long-tongued bees. Populations of polymorphic Narcissus species are typically dominated by the long-styled morph and display imperfect reciprocity in the positions of sexual organs. These features are consequences of the unusual association between stylar polymorphism and a self-incompatibility system that permits intramorph mating.

An experimental evaluation of self-interference in Narcissus assoanus: functional and evolutionary implications

Floral traits that reduce self-pollination in hermaphroditic plants have usually been interpreted as mechanisms that limit the genetic consequences of self-fertilization. However, the avoidance of sexual conflict between female and male function (self-interference) may also represent an important selection pressure for the evolution of floral traits, particularly in self-incompatible species. Here, we use experimental manipulations to investigate self-interference in Narcissus assoanus, a self-incompatible species with a stigma-height dimorphism in which the degree of spatial separation between sex organs (herkogamy) differs strikingly between the long- and short-styled morphs (hereafter L- and S-morphs). We predicted that weak herkogamy in the L-morph would cause greater self-pollination and hence self-interference. Experimental self-pollination reduced seed set when it occurred prior to, or simultaneously with, cross-pollination in the L-morph, but only if it occurred prior to cross-pollination in the S-morph. In the field, autonomous self-pollination was greater in the L-morph than the S-morph, but we found no evidence that self-interference reduced maternal or paternal fitness in either morph. One-day-old flowers of the L-morph have reduced stigma receptivity and hence exhibit protandry, whereas stigma receptivity and anther dehiscence are concurrent in the S-morph. This suggests that the two style morphs have alternative strategies for reducing self-interference: dichogamy in the L-morph and herkogamy in the S-morph. These results provide insight into the mechanisms that reduce sexual conflict in hermaphrodite plants and are of significance for understanding the evolution and maintenance of sexual polymorphisms.

Correlated evolution of floral morphology and mating-type frequencies in a sexually polymorphic plant

In sexually polymorphic species, reproductive morphology governs mating patterns and the character of negative frequency-dependent selection. If local environmental conditions cause sexual morphs to differ between populations, then frequency-dependent selection should create corresponding geographic variation in morph frequencies. We investigate this relation with a model of morph-ratio evolution and analysis of geographic variation in the heterostylous plant Narcissus triandrus. Unlike other tristylous species, N. triandrus possesses both imperfect reciprocity among morphs in sex-organ position and a self-incompatibility system that permits outcrossing within and between morphs. We sampled 137 populations throughout the Iberian Peninsula for floral-morph ratios, and measured floral morphology in 31 populations. Morph ratios exhibited three atypical features: (1) predominance of the long-styled (L) morph; (2) absence of the mid-styled (M) morph from 17.5% of populations; and (3) a negative relation between the frequencies of the L and M morphs among populations. Morph ratios varied geographically, with decreasing frequency of the M morph from the southeast to the northwest of the species' range. Much of this variation accompanied allometric change in the positions of sex organs, especially the mid-level organs, with the M morph declining in frequency and ultimately being lost in large-flowered populations. Using multivariate multiple regression, we demonstrate that variation in floral morphology among populations predicts this geographic variation in morph frequencies. Our theoretical analysis illustrates that patterns of pollen transfer governed by imperfect sex-organ reciprocity can select for unequal equilibrium morph ratios like those observed for N. triandrus. We interpret the L-biased morph ratios and the unusual morphology of N. triandrus as a consequence of its atypical intramorph compatibility system.

Phylogenetic reconstruction of the evolution of stylar polymorphisms in Narcissus (Amaryllidaceae)

We investigated the origin of stylar polymorphisms in Narcissus, which possesses a remarkable range of stylar conditions and diverse types of floral morphology and pollination biology. Reconstruction of evolutionary change was complicated by incomplete resolution of trees inferred from two rapidly evolving chloroplast regions, but we bracketed reconstructions expected on the fully resolved plastid-based tree by considering all possible resolutions of polytomies on the shortest trees. Stigma-height dimorphism likely arose on several occasions in Narcissus and persisted across multiple speciation events. As proposed in published models, this rare type of stylar polymorphism is ancestral to distyly. While there is no evidence in Narcissus that dimorphism preceded tristyly, a rapid transition between them may explain the lack of a phylogenetic footprint for this evolutionary sequence. The single instances of distyly and tristyly in Narcissus albimarginatus and N. triandrus, respectively, are clearly not homologous, an evolutionary convergence unique to Amaryllidaceae. Floral morphology was likely an important trigger for the evolution of stylar polymorphisms: Concentrated-changes tests indicate that a long, narrow floral tube may have been associated with the emergence of stigma-height dimorphism and that this type of tube, in combination with a deep corona, likely promoted, or at least was associated with, the parallel origins of heterostyly.

The Causes of self-sterility in natural populations of the relictual angiosperm, Illicium floridanum (Illiciaceae)

BACKGROUND AND AIMS

Illicium floridanum, a species belonging to the basal extant angiosperm taxon Illiciaceae, reportedly exhibits self-incompatibility (SI). To date, the site and timing of SI within the carpel of this species remains unidentified. Thus, the objective of this research was to determine the cellular and temporal aspects of SI in I. floridanum.

METHODS

Following controlled application of cross- and self-pollen in natural populations of I. floridanum, embryo sac development and temporal aspects of stigma receptivity, as well as pollen tube growth, fertilization, and embryo and endosperm development, were investigated with the aid of light and fluorescence microscopy.

KEY RESULTS

Flowers of I. floridanum exhibited complete dichogamy whereby stigmas only supported cross- and self-pollen tube growth prior to anther dehiscence. In contrast to earlier reports of SI in this species, a prezygotic SI resulting in rejection of self-pollen tube growth at the stigma was absent and there were no significant differences between cross- versus self-pollen germination and pollen tube growth within the style and ovary during the first 5 d after pollination. Structural development of the four-celled embryo sac was not differentially influenced by pollen type as noted to occur in other angiosperms with late-acting ovarian SI. The ovule micropyle and embryo sac were penetrated equally by cross- and self-pollen tubes. In addition, there were no statistically significant differences in cross- versus self-fertilization. A resting zygote and multicellular endosperm at a variety of developmental stages was present by 30 d after application of cross- or self-pollen.

CONCLUSIONS

In the clear absence of a prezygotic SI that was previously reported to result in differential self-pollen tube growth at the stigma, self- sterility in I. floridanum is likely due to early-acting inbreeding depression, although late-acting post-zygotic ovarian SI cannot be ruled out.

Mating patterns and genetic diversity in the wild Daffodil Narcissus longispathus (Amaryllidaceae)

Despite the importance of Narcissus to ornamental horticulture, there have been no population genetic studies of wild species, many of which have narrow distributions. Here, we measure selfing rates and levels of genetic diversity at allozyme loci in six populations of Narcissus longispathus, a self-compatible daffodil endemic to a few mountain ranges in southeastern Spain. The populations were distributed among four distinct river valleys encompassing two main watersheds in the Sierra de Cazorla mountains. Selfing rates averaged 0.37 (range 0.23-0.46), resulting in significant inbreeding coefficients for the progeny (f = 0.324). In contrast, estimates of inbreeding in parental genotypes were not significantly different from zero (f = 0.001), indicating that few selfed offspring survive to maturity because of inbreeding depression. Species-wide estimates of genetic diversity for the six populations were P(s) = 0.38, H(es) = 0.119 and A(s) = 1.27 with significant genetic differentiation among populations theta = 0.15. The observed patterns of genetic differentiation among populations are likely influenced by the mating system, and a combination of local topography, watershed affinities and gene flow.