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

Effects of environmental heterogeneity on phenotypic variation of the endemic plant Lilium pomponium in the Maritime and Ligurian Alps

Geographical limits of species’ distributions are assumed to be coincident with ecological margins, although this assumption might not always be true. Indeed, harsh environments such as Alpine and Mediterranean ecosystems may favour high phenotypic variability among populations, especially those in peripheral sites. Floral traits are often found to be less variable and less affected by environmental heterogeneity than vegetative traits because variation in the former may have negative effects on fitness. For this reason, it is important to quantify variation in floral traits and plant fecundity in study range limits. The objective of the study is to examine phenotypic variation and differences in reproduction in endemic Lilium pomponium in the Maritime and Ligurian Alps in relation to environmental variation across its distribution range. In this species, marginal climatic populations occur both in the peripheral and central geographical locations of the distribution range; hence, geographical and ecological gradients are not concordant. Floral trait variation is related to local environmental conditions with an array of interactions among resource availability, potential pollen limitation and population size that are differentially related to floral traits. Contrary to the general expectation, all central and peripheral populations had similar, moderate seed production with each group limited by different factors acting on different stages of the life-history strategy. Our results are in line with the idea that general expectations are confirmed only when its assumptions are met and that the differences in pollination environment along an environmental gradient may not be the main determinant of the distribution limit.

'A most complex marriage arrangement': recent advances on heterostyly and unresolved questions

Heterostylous genetic polymorphisms provide paradigmatic systems for investigating adaptation and natural selection. Populations are usually comprised of two (distyly) or three (tristyly) mating types, maintained by negative frequency-dependent selection resulting from disassortative mating. Theory predicts this mating system should result in equal style-morph ratios (isoplethy) at equilibrium. Here, I review recent advances on heterostyly, focusing on examples challenging stereotypical depictions of the polymorphism and unresolved questions. Comparative analyses indicate multiple origins of heterostyly, often within lineages. Ecological studies demonstrate that structural components of heterostyly are adaptations improving the proficiency of animal-mediated cross-pollination and reducing pollen wastage. Both neutral and selective processes cause deviations from isoplethy in heterostylous populations, and, under some ecological and demographic conditions, cause breakdown of the polymorphism, resulting in either the evolution of autogamy and mixed mating, or transitions to alternative outcrossing systems, including dioecy. Earlier ideas on the genetic architecture of the S-locus supergene governing distyly have recently been overturned by discovery that the dominant S-haplotype is a hemizygous region absent from the s-haplotype. Ecological, phylogenetic and molecular genetic data have validated some features of theoretical models on the selection of the polymorphism. Although heterostyly is the best-understood floral polymorphism in angiosperms, many unanswered questions remain.

Architectural constraints, male fertility variation and biased floral morph ratios in tristylous populations

Tristyly is a genetic polymorphism in which populations are comprised of three floral morphs (mating types) differing reciprocally in sex-organ height. Intermorph (disassortative) mating governed by a trimorphic incompatibility system should result in 1:1:1 morph ratios at equilibrium, but both deterministic and stochastic processes can cause skewed morph ratios in tristylous populations. Here, we investigate mechanisms causing morph-ratio bias in Pontederia parviflora, an emergent aquatic native to tropical America. We compared reproductive traits among morphs and surveyed 71 populations to determine patterns of morph-ratio bias. We then used simulation models of morph-frequency dynamics to test the hypothesis that morph-specific differences in pollen production and their influence on male fertility can explain patterns of morph-ratio bias. Ninety-seven percent of populations that we sampled were tristylous, but with a significant excess of the short-styled morph and a deficiency of the long-styled morph. Atypically for a tristylous species, mid-level anthers of the short-styled morph produced over twice as much pollen compared with the corresponding anthers of the long-styled morph. Our computer models incorporating this difference in male fertility resulted in morph ratios not significantly different from the average frequencies from our survey suggesting that the short-styled morph is more successful than the long-styled morph in siring ovules of the mid-styled morph. We propose that the difference in male fertility between morphs may be a non-adaptive consequence of a developmental constraint caused by the architecture of tristyly in Pontederiaceae.

Pollen and stigma size changes during the transition from tristyly to distyly in Oxalis alpina (Oxalidaceae)

Pollen and stigma size have the potential to influence male fitness of hermaphroditic plants, particularly in species presenting floral polymorphisms characterised by marked differences in these traits among floral morphs. In this study, we take advantage of the evolutionary transition from tristyly to distyly experienced by Oxalis alpina (Oxalidaceae), and examined whether modifications in the ancillary traits (pollen and stigma size) respond to allometric changes in other floral traits. Also, we tested whether these modifications are in accordance with what would be expected under the hypothesis that novel competitive scenarios (as in distylous-derived reproductive system) exert morph- and whorl-specific selective pressures to match the available stigmas. We measure pollen and stigma size in five populations of O. alpina representing the tristyly-distyly transition. A general reduction in pollen and stigma size occurred along the tristyly-distyly transition, and pollen size from the two anther levels within each morph converged to a similar size that was characterised by whorl-specific changes (increases or decreases) in pollen size of different anthers in each floral type. Overall, results from this study show that the evolution of distyly in this species is characterised not only by changes in sexual organ position and flower size, but also by morph-specific changes in pollen and stigma size. This evidence supports the importance of selection on pollen and stigma size, which increase fitness of remaining morphs following the evolution of distyly, and raises questions to explore on the functional value of pollen size in heterostylous systems under pollen competition.

Variation in style morph frequencies in tristylous Lythrum salicaria in the Iberian Peninsula: the role of geographical and demographic factors

BACKGROUND AND AIMS

The balance between stochastic forces and negative frequency-dependent selection largely determines style morph frequencies in heterostylous populations. Investigation of morph frequencies at geographical range limits can provide insights into the forces maintaining the floral polymorphism, and the factors causing biased morph ratios. Here, we investigate style morph frequencies in populations at the south-western European range limit of tristylous Lythrum salicaria, to explore the role of demographic and geographical factors influencing morph ratios in its native range.

METHODS

We measured morph composition and evenness, and the size of 96 populations, along a north to south latitudinal transect from Galicia to Andalucia, Iberian Peninsula, traversing a steep climatic gradient. To examine the potential influence of morph-specific fitness components on morph ratios, we examined reproductive traits in 19 populations.

KEY RESULTS

Most populations of L. salicaria were trimorphic (94·79 %), the majority exhibiting 1 : 1 : 1 morph ratios (68·75 %). Populations with biased morph ratios had a deficiency of the short-styled morph. Population size and morph evenness were positively associated with latitude, with smaller populations and those with less even morph ratios occurring towards the south. Greater variance in morph evenness was evident at the southern range margin. There were no consistent differences in components of reproductive fitness among style morphs, but southern populations produced less fruit and seed than more northerly populations.

CONCLUSIONS

Our results demonstrate the influence of finite population size on morph frequencies in L. salicaria. However, they also illustrate the resilience of Iberian populations to the factors causing deviations from isoplethy and morph loss, especially at the southern range limit where populations are smaller. The maintenance of tristyly in small populations of L. salicaria may be aided by the genetic connectivity of populations in agricultural landscapes resulting from gene flow through pollen and seed dispersal.

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.

Recurrent modification of floral morphology in heterantherous Solanum reveals a parallel shift in reproductive strategy

Floral morphology determines the pattern of pollen transfer within and between individuals. In hermaphroditic species, the spatial arrangement of sexual organs influences the rate of self-pollination as well as the placement of pollen in different areas of the pollinator's body. Studying the evolutionary modification of floral morphology in closely related species offers an opportunity to investigate the causes and consequences of floral variation. Here, we investigate the recurrent modification of flower morphology in three closely related pairs of taxa in Solanum section Androceras (Solanaceae), a group characterized by the presence of two morphologically distinct types of anthers in the same flower (heteranthery). We use morphometric analyses of plants grown in a common garden to characterize and compare the changes in floral morphology observed in parallel evolutionary transitions from relatively larger to smaller flowers. Our results indicate that the transition to smaller flowers is associated with a reduction in the spatial separation of anthers and stigma, changes in the allometric relationships among floral traits, shifts in pollen allocation to the two anther morphs and reduced pollen : ovule ratios. We suggest that floral modification in this group reflects parallel evolution towards increased self-fertilization and discuss potential selective scenarios that may favour this recurrent shift in floral morphology and function.

Range-wide population genetics and variation in morph ratio in style-dimorphic Narcissus papyraceus

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PREMISE OF THE STUDY

Theoretical models state that natural selection and mating patterns account for floral morph ratio in style-polymorphic plants. However, the demographic history of populations can also influence variation in morph ratios. If so, we hypothesize an association between the morph ratios and the genetic structure across populations.•

METHODS

We used nuclear microsatellites to assess genetic variation and structure in populations of Narcissus papyraceus, a style-dimorphic plant whose floral morph ratios (L-morph to S-morph) gradually vary throughout its distribution range in the southwestern Mediterranean Basin. We implemented analyses to relate the genetic features of populations with their morph ratios.•

KEY RESULTS

We found greater frequencies of the S-morph in central populations and declining frequencies toward the periphery. This geographic pattern was not associated with the genetic structure of populations. Instead, we found two distinct genetic groups, mainly separated by the Strait of Gibraltar, with a mixture of morph ratios within each one. Overall, there was a weak genetic structure. Genetic diversity was greater in central and southern dimorphic populations than in northern L-monomorphic populations.•

CONCLUSIONS

Altogether, our results do not support the hypothesis that the demographic history of populations can account for the observed geographical pattern of morph ratios in N. papyraceus. We suggest that adaptive processes shown in previous studies in the species are the main determinant of the existing variation in the morph composition of populations.

The role of short-tongued insects in floral variation across the range of a style-dimorphic plant

BACKGROUND AND AIMS

Heterostyly and related style polymorphisms are suitable model systems to evaluate the importance of functional pollinators in the maintenance of population variability. In Narcissus papyraceus different functional pollinators, incompatibility system and flower morphology have been proposed to influence the maintenance of polymorphism through their effect on disassortative mating. Here a test is done to find out if the visitation rate of long- versus short-tongued pollinators correlates with the morph ratio and if the latter is related to other flower traits of the species across its main geographic range.

METHODS

Floral traits from 34 populations in the south-west of the Iberian Peninsula and in north-west Africa were measured, perianth variation was described and a comparison was made of allometric relationships between sex organs and floral tube. Correlations between pollinator guilds, stigma-anther separation of reciprocal morphs (our proxy for disassortative mating) and morph-ratio variation were analysed. Finally, the incompatibility system of the species in the northern and southern borders of its distribution are described.

KEY RESULTS

Flowers from southern populations were significantly larger than flowers from centre and northern populations. The abundance of short-styled plants decreased gradually with increasing distance from the core region (the Strait of Gibraltar), with these disappearing only in the northern range. Although there was a significant difference in stigma-anther separation among populations, morph ratio was not associated with reciprocity or floral tube length. Long-style morph frequency increased with short-tongued pollinator visitation rate. Populations from both edges of the distribution range were self-incompatible and within- and between-morph compatible.

CONCLUSIONS

The style morph ratio changed gradually, whereas perianth trait variation showed abrupt changes with two morphotypes across the range. The positive relationship between the visitation rate of short-tongued pollinators and the decrease of the short-style morph supports our initial hypothesis. The results highlight the importance of different pollinators in determining the presence of style polymorphism.

Loss of floral polymorphism in heterostylous Luculia pinceana (Rubiaceae): a molecular phylogeographic perspective

Both deterministic and stochastic forces determine the representation and frequency of floral morphs in heterostylous plant populations. Phylogeographic analysis of molecular variation can provide information on the role of historical factors, including founder events, in affecting population morph structure. Here, we investigate geographical patterns of floral morph variation in a distylous shrub Luculia pinceana (Rubiaceae) by examining the relations between floral polymorphism and molecular (cpDNA and microsatellite) variation in 25 populations sampled throughout the distribution of the species in southwest China and adjacent countries. In 19 of the 25 populations, the frequency of floral morphs was not significantly different from the expected 1:1 ratio. The remaining populations were either L-morph biased (2) or monomorphic (4) for this form and were morphologically differentiated from the remaining populations in several floral traits, that is, corolla tube length, sex organ position and stigma-anther separation. Phylogeographic analysis supports the hypothesis that L. pinceana was initially split into west-central and eastern lineages in the Early Pleistocene (~1.982 Mya). A centrally located lineage composed of morph-biased and monomorphic populations appears to have been subsequently derived from the west-central lineage, perhaps by a founder event after the last glacial maximum. Hypotheses to explain why these populations have not returned to equilibrium morph frequencies are considered.

A geographic cline induced by negative frequency-dependent selection

Background

Establishment of geographic morph frequency clines is difficult to explain in organisms with limited gene flow. Balancing selection, such as negative frequency-dependent selection (NFDS), is instead suggested to establish a morph frequency cline on a geographic scale at least theoretically. Here we tested whether a large-scale smooth cline in morph frequency is established by NFDS in the female-dimorphic damselfly, Ischnura senegalensis, where andromorphs and gynomorphs are maintained by NFDS.

Results

We found a large-scale latitudinal cline in the morph frequency: andromorph frequency ranged from 0.05 (South) to 0.79 (North). Based on the empirical data on the numbers of eggs, the number of ovariole, abdomen length and latitude, the potential fitness of andromorphs was estimated to be lower than that of gynomorphs in the south, and higher in the north, suggesting the gene-by-environment interaction. From the morph-specific latitudinal cline in potential fitness, the frequency of andromorphs was expected to shift from 0 to 1 without NFDS, because a morph with higher potential fitness wins completely and the two morphs will switch at some point. In contrast, NFDS led to the coexistence of two morphs with different potential fitness in a certain geographic range along latitude due to rare morph advantage, and resulted in a smooth geographic cline of morph frequency.

Conclusion

Our results provide suggestive evidence that the combination of NFDS and gene-by-environment interaction, i.e., multi-selection pressure on color morphs, can explain the geographic cline in morph frequency in the current system.

Ecological genetics of sex ratios in plant populations

In many angiosperm species, populations are reproductively subdivided into distinct sexual morphs including females, males and hermaphrodites. Sexual polymorphism is maintained by frequency-dependent selection, leading to predictable sex ratios at equilibrium. Charles Darwin devoted much of his book 'The Different Forms of Flowers on Plants of the Same Species' (1877) to investigating plant sexual polymorphisms and laid the foundation for many problems addressed today by integrating theory with empirical studies of the demography and genetics of populations. Here, we summarize our recent work on the ecological and genetic mechanisms influencing variation in sex ratios and their implications for evolutionary transitions among sexual systems. We present the results of a survey of sex ratios from 126 species from 47 angiosperm families and then address two general problems using examples from diverse angiosperm taxa: (i) the mechanisms governing biased sex ratios in dioecious species; (ii) the origins and maintenance of populations composed of females, males and hermaphrodites. Several themes are emphasized, including the importance of non-equilibrium conditions, the role of life history and demography in affecting sex ratios, the value of theory for modelling the dynamics of sex ratio variation, and the utility of genetic markers for investigating evolutionary processes in sexually polymorphic plant populations.

Changes in sexual organ reciprocity and phenotypic floral integration during the tristyly-distyly transition in Oxalis alpina

Although the 6 magnitude and pattern of correlation among floral traits (phenotypic integration) is usually conceived as an adaptation for successful pollination and reproduction, studies on the evolution of plant reproductive systems have generally focused on one or a few characters. If evolutionary transitions between reproductive systems involve morphological floral adjustments, changes in the magnitude and pattern of phenotypic integration of floral traits may be expected. In this study, we focused on the evolutionary dynamics of a complex adaptive trait, the extent of reciprocity (reciprocal placement) among sexual organs in a heterostylous species, and explored the associated changes in phenotypic floral integration during the transition from tristyly to distyly. The extent of reciprocity and both the magnitude and pattern of floral integration were characterized in 12 populations of Oxalis alpina representing the tristyly-distyly gradient. Although the extent of reciprocity increased along the tristyly-distyly transition, the flower size diminished. These adjustments did not affect the magnitude, but did affect the pattern, of floral integration. *Changes in the pattern of floral integration suggested that allometric, functional and pleiotropic relationships among floral traits were affected during this evolutionary transition.

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.

Geographic variation in floral morphology and style-morph ratios in a sexually polymorphic daffodil

Narcissus triandrus is a tristylous daffodil from the Iberian Peninsula that has striking geographical variation in floral morphology and style-morph ratios. Here, we investigate the relation between this variation and ecological factors to understand mechanisms governing morph ratios. We estimated morph ratios in 124 populations throughout the range of N. triandrus and measured 13 morphological traits in 35-78 populations. Sampling of morph ratios conducted over 2-10-yr intervals demonstrated strong temporal stability. Variation in floral and vegetative traits enabled statistical prediction of morph-frequency variation among populations. Latitudinal gradients in precipitation and temperature were correlated with plant and flower size, with larger flowers in northern populations associated with bumblebee visitation and stylar dimorphism. Flowers of the L- and M-morphs differed significantly from the S-morph in several size-related characters, unlike other tristylous species. This pattern and the similarity in anther positions of the L- and M-morphs suggest that the M-morph of N. triandrus originated through genetic modifiers that shortened styles of the L-morph. Our findings support the hypothesis that geographical variation in style-morph ratios is largely governed by climatic gradients in the Iberian Peninsula, which influence the floral morphology and pollination biology of N. triandrus populations.

Population structure and genetic diversity in tristylous Narcissus triandrus: insights from microsatellite and chloroplast DNA variation

We investigated cpDNA sequence and nuclear microsatellite variation among populations of the wild daffodil Narcissus triandrus to examine the role of historical vs. contemporary forces in shaping population structure, morphological differentiation and sexual-system evolution. This wide-ranging heterostylous species of the Iberian Peninsula is largely composed of two allopatric varieties (vars. cernuus and triandrus), and populations with either stylar trimorphism or dimorphism. Dimorphic populations only occur in var. triandrus, are mainly restricted to the northwestern portion of the species range, and uniformly lack the mid-styled morph (M-morph). Chloroplast DNA (cpDNA) sequence variation revealed strong geographical structuring and evidence for a fragmentation event associated with differentiation of the two varieties. In var. triandrus, population fragmentation, restricted gene flow and isolation-by-distance were also inferred. Significant differences in genetic diversity and population structure between the two varieties likely reflect historical and contemporary differences in demography and gene flow among populations. Discordance between cpDNA markers and both microsatellite and morphological variation indicate that hybridization has occurred between the two varieties at contact zones. There were no differences in genetic diversity or population structure between dimorphic and trimorphic populations, and chloroplast haplotypes were not associated with either sexual system, indicating transitions in morph structure within each maternal lineage. M-morph frequencies were positively correlated with differentiation at microsatellite loci, indicating that the evolutionary processes influencing these neutral markers also influence alleles controlling the style morphs.

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.

Spatial ecology of mating success in a sexually polymorphic plant

The spatial context of reproduction is of crucial importance to plants because of their sessile habit. Since pollen and seed dispersal is often restricted, mating success is likely to depend on the quantity and quality of mates in local neighbourhoods. Here we use neighbourhood models to investigate the spatial ecology of pollination and mating in Narcissus assoanus, a sexually polymorphic plant with two mating morphs that differ in style length. By mapping individuals in eight populations from southwestern France, we investigated the influence of the density and morph identity of plants at different spatial scales on variation in female fertility. By using inferences on the expected patterns of pollen transfer based on floral morphology, we were able to predict the quantitative relations between local morph ratios and variation in fertility. Our analyses revealed differences in the spatial clustering of morphs and in their response to plant density and morph identity within local neighbourhoods. Mating success in N. assoanus was characterized by both density- and frequency-dependent processes, a condition that may be a general feature of the spatial ecology of plant mating.

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.

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.

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.