Spatial variation in selection on corolla shape in a generalist plant is promoted by the preference patterns of its local pollinators

An adaptive role of corolla shape has been often asserted without an empirical demonstration of how natural selection acts on this trait. In generalist plants, in which flowers are visited by diverse pollinator fauna that commonly vary spatially, detecting pollinator-mediated selection on corolla shape is even more difficult. In this study, we explore the mechanisms promoting selection on corolla shape in the generalist crucifer Erysimum mediohispanicum Polatschek (Brassicaceae). We found that the main pollinators of E. mediohispanicum (large bees, small bees and bee flies) discriminate between different corolla shapes when offered artificial flowers without reward. Importantly, different pollinators prefer different shapes: bees prefer flowers with narrow petals, whereas bee flies prefer flowers with rounded overlapping petals. We also found that flowers with narrow petals (those preferred by bees) produce both more pollen and nectar than those with rounded petals. Finally, different plant populations were visited by different faunas. As a result, we found spatial variation in the selection acting on corolla shape. Selection favoured flowers with narrow petals in the populations where large or small bees are the most abundant pollinator groups. Our study suggests that pollinators, by preferring flowers with high reward, exert strong selection on the E. mediohispanicum corolla shape. The geographical variation in the pollinator-mediated selection on E. mediohispanicum corolla shape suggests that phenotypic evolution and diversification can occur in this complex floral trait even without specialization.     

Rapid response to artificial selection on flower size in Phlox

Quantitative characters are often said to evolve rather slowly, taking many generations to exhibit appreciable differences among populations. We tested this notion experimentally by performing bi-directional selection on corolla diameter of plants from a wild population of Phlox drummondii for three generations. By monitoring flower size, tube length and stigma-anther proximity of flowers, we obtained the direct and indirect responses to selection, and calculated genetic correlations, realized and narrow sense heritabilities using offspring-mother regression. Realized heritability of flower size was high (0.83), whereas genetic correlations among traits were weak or not significant. The per-generation average of the response in corolla diameter was about 5%. We found that P. drummondii has a great capacity to respond rapidly to selection, and this capacity may be in part responsible for the observed high degree of differentiation within the species. We also concluded that rapid evolution of morphological floral traits is possible.     

Heritability and correlation structure of nectar and floral morphology traits in Nicotiana alata

The heritability and genetic basis of nectar traits have been rarely studied in the field, where plants are exposed to environmental factors that could mask underlying genetic effects. Heritabilities and variance components were estimated for nectar and morphological traits of Nicotiana alata , using a partial diallel design. The main experiment was conducted in a Missouri experimental garden using a randomized block design with three plant density treatments, whereas a smaller experiment was conducted near native Brazil habitat to compare the environmental variance in traits between Missouri and Brazil. Significant heritability was detected for nectar volume and energy content, and for corolla tube length. Phenotypic correlations were significant between all traits investigated, whereas significant genetic correlations were only found between nectar volume and energy and between corolla limb width and mouth diameter. There were no significant family-by-density interactions detected in the Missouri field environment. All traits differed significantly between Missouri and Brazil environments, but significant genetic by environment (G × E) interactions between Missouri and Brazil were detected for only one trait. This study shows that nectar traits can be heritable despite considerable environmental variation.     

EVOLUTION OF FLORAL TRAITS IN A HERMAPHRODITIC PLANT: FIELD MEASUREMENTS OF HERITABILITIES AND GENETIC CORRELATIONS

Genetic variances, heritabilities, and genetic correlations of floral traits were measured in the monocarpic perennial Ipomopsis aggregata (Polemoniaceae). A paternal half-sib design was employed to generate seeds in each of four years, and seeds were planted back in the field near the parental site. The progeny were followed for up to eight years to estimate quantitative genetic parameters subject to natural levels of environmental variation over the entire life cycle. Narrow-sense heritabilities of 0.2–0.8 were detected for the morphometric traits of corolla length, corolla width, stigma position, and anther position. The proportion of time spent by the protandrous flowers in the pistillate phase (“proportion pistillate”) also exhibited detectable heritability of near 0.3. In contrast, heritability estimates for nectar reward traits were low and not significantly different from zero, due to high environmental variance between and within flowering years. The estimates of genetic parameters were combined with phenotypic selection gradients to predict evolutionary responses to selection mediated by the hummingbird pollinators. One trait, corolla width, showed the potential for a rapid response to ongoing selection through male function, as it experienced both direct selection, by influencing pollen export, and relatively high heritability. Predicted responses were lower for proportion pistillate and corolla length, even though these traits also experienced direct selection. Stigma position was expected to respond positively to indirect selection of proportion pistillate but negatively to selection of corolla length, with the net effect sensitive to variation in the selection estimates. Anther position also was not directly selected but could respond to indirect selection of genetically correlated traits.     

Association between inbreeding depression and floral traits in a generalist‐pollinated plant

Individual variation in the magnitude of inbreeding depression (ID) in plants and its association with phenotypic traits may have important consequences for mating system evolution. This association has been investigated only scarcely, and always considering traits functionally related to autogamy. Here, we explore the association between individual variation in ID and plant traits associated with pollinator attractiveness (related to plant size, corolla size and corolla shape) in two populations of Erysimum mediohispanicum (Brassicaceae). ID was calculated along the entire life cycle of the plants. In addition, we also explored the relationship between phenotypic traits and the individual levels of heterozygosity. We found significant associations between ID and corolla diameter and stalk height, being taller plants with larger corollas those undergoing a lower intensity of ID. Furthermore, we found a negative relationship between corolla diameter and heterozygosity, suggesting that plants with large flowers have purged their genetic load. Finally, we found a significant effect of corolla diameter on the intrapopulation genetic structure. All these findings suggest that plants with large flowers have secularly suffered frequent inbreeding in the study populations. Because corolla diameter is a trait frequently selected by pollinators in E. mediohispanicum, we believe that the observed relationship between this trait and ID could be mediated by pollinators, probably throughout an increasing in biparental inbreeding, geitonogamy or autogamy.     

GENETIC VARIANCE AND COVARIANCE FOR PHYSIOLOGICAL TRAITS IN LOBELIA:ARE THERE CONSTRAINTS ON ADAPTIVE EVOLUTION?

Physiological traits that control the uptake of carbon dioxide and loss of water are key determinants of plant growth and reproduction. Variation in these traits is often correlated with environmental gradients of water, light, and nutrients, suggesting that natural selection is the primary evolutionary mechanism responsible for physiological diversification. Responses to selection, however, can be constrained by the amount of standing genetic variation for physiological traits and genetic correlations between these traits. To examine the potential for constraint on adaptive evolution, we estimated the quantitative genetic basis of physiological trait variation in one population of each of two closely related species (Lobelia siphilitica and L. cardinalis). Restricted maximum likelihood analyses of greenhouse-grown half-sib families were used to estimate genetic variances and covariances for seven traits associated with carbon and water relations. We detected significant genetic variation for all traits in L. siphilitica, suggesting that carbon-gain and water-use traits could evolve in response to natural selection in this population. In particular, narrow-sense heritabilities for photosynthetic rate (A), stomatal conductance (gs), and water-use efficiency (WUE) in our L. siphilitica population were high relative to previous studies in other species. Although there was significant narrow-sense heritability for A in L. cardinalis, we detected little genetic variation for traits associated with water use (gs and WUE), suggesting that our population of this species may be unable to adapt to drier environments. Despite being tightly linked functionally, the genetic correlation between A and gs was not strong and significant in either population. Therefore, our L. siphilitica population would not be genetically constrained from evolving high A (and thus fixing more carbon for growth and reproduction) while also decreasing gs to limit water loss. However, a significant negative genetic correlation existed between WUE and plant size in L. siphilitica, suggesting that high WUE may be negatively associated with high fecundity. In contrast, our results suggest that any constraints on the evolution of photosynthetic and stomatal traits of L. cardinalis are caused primarily by a lack of genetic variation, rather than by genetic correlations between these functionally related traits.     

The search for Pleiades in trait constellations: functional integration and phenotypic selection in the complex flowers of Morrenia brachystephana (Apocynaceae)

Pollinator‐mediated natural selection on single traits, such as corolla tube or spur length, has been well documented. However, flower phenotypes are usually complex, and selection is expected to act on several traits that functionally interact rather than on a single isolated trait. Despite the fact that selection on complex phenotypes is expectedly widespread, multivariate selection modelling on such phenotypes still remains under‐explored in plants. Species of the subfamily Asclepiadoideae (Apocynaceae) provide an opportunity to study such complex flower contrivances integrated by fine‐scaled organs from disparate developmental origin. We studied the correlation structure among linear floral traits (i) by testing a priori morphological, functional or developmental hypotheses among traits and (ii) by exploring the organization of flower covariation, considering alternative expectations of modular organization or whole flower integration through conditional dependence analysis (CDA) and integration matrices. The phenotypic selection approach was applied to determine whether floral traits involved in the functioning of the pollination mechanism were affected by natural selection. Floral integration was low, suggesting that flowers are organized in more than just one correlation pleiad; our hypothetical functional correlation matrix was significantly correlated with the empirical matrix, and the CDA revealed three putative modules. Analyses of phenotypic selection showed significant linear and correlational gradients, lending support to expectations of functional interactions between floral traits. Significant correlational selection gradients found involved traits of different floral whorls, providing evidence for the existence of functional integration across developmental domains.     

The quantitative genetics of floral trait variation in Lobelia: potential constraints on adaptive evolution

Abstract Although pollinator-mediated natural selection has been measured on many floral traits and in many species, the extent to which selection is constrained from producing optimal floral phenotypes is less frequently studied. In particular, negative correlations between flower size and flower number are hypothesized to be a major constraint on the evolution of floral displays, yet few empirical studies have documented such a trade-off. To determine the potential for genetic constraints on the adaptive evolution of floral displays, I estimated the quantitative genetic basis of floral trait variation in two populations of Lobelia siphilitica . Restricted maximum likelihood (REML) analyses of greenhouse-grown half-sib families were used to estimate genetic variances and covariances for flower number and six measures of flower size. There was significant genetic variation for all seven floral traits in both populations. Flower number was negatively genetically correlated with four measures of flower size in one population and three measures in the other. When the genetic variance-covariance matrices were combined with field estimates of phenotypic selection gradients, the predicted multivariate evolutionary response was less than or opposite in sign to the selection gradient for flower number and five of six measures of flower size, suggesting genetic constraints on the evolution of these traits. More generally, my results indicate that the adaptive evolution of floral displays can be constrained by tradeoffs between flower size and number, as has been assumed by many theoretical models of floral evolution.     

金钟连翘与东北连翘种间杂交F1代表型性状遗传分析

为了阐明连翘属植物主要观赏性状和抗寒性的遗传特点,以金钟连翘品种‘Lynwood’(Forsythia intermedia‘Lynwood’)和东北连翘(F.mandschurica)杂交获得的F1代群体为研究对象,对杂交群体的花冠口直径、花裂片长度、抗寒性等12个表型性状进行测定,并对这些性状进行相关性分析和混合遗传模型分析。结果表明:金钟连翘与东北连翘杂交群体的表型性状变异丰富,各表型性状均出现大于高亲或小于低亲的超亲个体。除花裂片长度、花裂片宽度、叶片长度外,其他性状的变异度均超过15%,达到中等水平以上。各性状之间存在一定的相关性,其中抗寒性与着花密度呈极显著正相关,与花冠口直径、植株冠幅呈极显著负相关关系。混合遗传模型分析显示,花裂片长度、花裂片宽度、花裂片长宽比、叶片长度、叶片宽度、叶片长宽比、当年生枝条长度、抗寒性由微效多基因控制,花冠口直径和植株冠幅由一对加性—显性主基因控制,着花密度由两对加性—显性主基因控制,株高由两对等加性—显性主基因控制。花冠口直径、冠幅、着花密度和株高的主基因遗传力分别为76.05%、60.3%、72.22%和64.75%。研究结果为定向培育综合性状优良的连翘新品种提供了依据。     

Pollinator‐mediated selection in a specialized pollination system: matches and mismatches across populations

Most studies on pollinator‐mediated selection have been performed in generalized rather than specialized pollination systems. This situation has impeded evaluation of the extent to which selection acts on attraction or specialized key floral traits involved in the plant‐pollinator phenotypic interphase. We studied pollinator‐mediated selection in four populations of Nierembergia linariifolia, a self‐incompatible and oil‐secreting plant pollinated exclusively by oil‐collecting bees. We evaluated whether floral traits experience variable selection among populations and whether attraction and fit traits are heterogeneously selected across populations. Populations differed in every flower trait and selection was consistently observed for corolla size and flower shape, two traits involved in the first steps of the pollination process. However, we found no selection acting on mechanical‐fit traits. The observation that selection occurred upon attraction rather than mechanical‐fit traits, suggests that plants are not currently evolving fine‐tuned morphological adaptations to local pollinators and that phenotypic matching is not necessarily an expected outcome in this specialized pollination system.     

Floral stability,pollination efficiency,and experimental manipulation of the corolla phenotype in Nemophila menziesii (Hydrophyllaceae)

The present study examines the degree of phenotypic stability in vegetative and floral traits in a population of the annual, self-compatible, and protandrous Nemophila menziesii (Hydrophyllaceae) and examines whether local pollinators select for a uniform floral phenotype in this population. I found a strong positive association between flower number, leaf size, shoot length, and pedicel length, implying a general effect of overall plant vigor on this suite of traits, while characters reflecting corolla shape were phenotypically stable across a broad range of plant sizes. However, results of a manipulative experiment indicate that individual flowers maintain high pollination success despite drastic changes in the size and shape of the corolla. This suggests that pollinator-mediated selection does not directly favor floral stability in N. menziesii or that selection was weak during the course of this study. Seed number per flower was positively correlated with plant size, which varied greatly from one patch to another.     

Morph‐specific selection on floral traits in a polymorphic plant

Correlations between phenotypic traits are common in many organisms, but the relative importance of nonadaptive mechanisms and selection for the evolution and maintenance of such correlations are poorly understood. In polymorphic species, morphs may evolve quantitative differences in additional characters as a result of morph‐specific selection. The perennial rosette herb Primula farinosa is polymorphic for scape length. The short‐scaped morph is less damaged by grazers and seed predators but is more strongly pollen limited than the long‐scaped morph. We examined whether morph‐specific differences in biotic interactions are associated with differences in selection on two other traits affecting floral display (number of flowers and petal size) and on one trait likely to affect pollination efficiency (corolla tube width) in three P. farinosa populations. Differences in selection between morphs were detected in one population. In this population, selection for more flowers and larger petals was stronger in the short‐scaped than in the long‐scaped morph, and although there was selection for narrower corolla tubes in the short‐scaped morph, no statistically significant selection on corolla tube width could be detected in the long‐scaped morph. In the study populations, the short‐scaped morph produced more and larger flowers and wider corolla tubes. Current morph‐specific selection was thus only partly consistent with trait differences between morphs. The results provide evidence of morph‐specific selection on traits associated with floral display and pollination efficiency, respectively.     

Genetics of sex allocation in Mimulus (Scrophulariaceae)

Theoretical models of the evolution of resource allocation patterns to male and female function make the assumption that there are inherent trade-offs between the two. Here we use a quantitative genetic approach to quantify trade-offs between male and female function and to determine whether plant populations could readily respond to natural selection by quantifying the amount of genetic variation for pollen and ovule production. Both intra- and interspecific crossing designs were applied to two populations of the predominantly outcrossing Mimulus guttatus and two populations of the highly selfing congener, M. micranthus. The only significant correlations observed among pollen number, pollen size and ovule number were positive. Positive genetic correlations among the traits were sometimes reduced after removing the effect of flower size but still no significant negative correlations were detected. These results suggest that positive correlations between pollen and ovule production may be due to the joint positive correlation of these characters with the resource pool available for pollen and ovule production, as reflected by flower size. Heritabilities were moderate to high for ovule production but low for pollen number and pollen size and suggest that responses to selection would differ between the two traits. Crosses between the species revealed that there are additional genetic factors contributing to differences between the two species for corolla width, vs. pollen:ovule ratio. This is consistent with the hypothesis that genetic variation for resource acquisition may in part be responsible for the overall lack of a negative correlation between pollen and ovule production and provides a genetic explanation for little evidence of trade-offs between sexual functions in Mimulus.     

Natural selection on Erysimum mediohispanicum flower shape: insights into the evolution of zygomorphy

Paleontological and phylogenetic studies have shown that floral zygomorphy (bilateral symmetry) has evolved independently in several plant groups from actinomorphic (radially symmetric) ancestors as a consequence of strong selection exerted by specialized pollinators. Most studies focused on unraveling the developmental genetics of flower symmetry, but little is known about the adaptive significance of intraspecific flower shape variation under natural conditions. We provide the first evidence for natural selection favoring zygomorphy in a wild population of Erysimum mediohispanicum (Brassicaceae), a plant showing extensive continuous variation in flower shape, ranging from actinomorphic to zygomorphic flowers. By using geometric morphometric tools to describe flower shape, we demonstrate that plants bearing zygomorphic flowers received more pollinator visits and had the highest fitness, measured not only by the number of seeds produced per plant but also by the number of seeds surviving to the juvenile stage. This study provides strong evidence for the existence of significant fitness differences associated with floral shape variation in E. mediohispanicum, thus illuminating a pathway for the evolution of zygomorphy in natural populations.     

Identification of major quantitative trait loci underlying floral pollination syndrome divergence in Penstemon

Distinct floral pollination syndromes have emerged multiple times during the diversification of flowering plants. For example, in western North America, a hummingbird pollination syndrome has evolved more than 100 times, generally from within insect-pollinated lineages. The hummingbird syndrome is characterized by a suite of floral traits that attracts and facilitates pollen movement by hummingbirds, while at the same time discourages bee visitation. These floral traits generally include large nectar volume, red flower colour, elongated and narrow corolla tubes and reproductive organs that are exerted from the corolla. A handful of studies have examined the genetic architecture of hummingbird pollination syndrome evolution. These studies find that mutations of relatively large effect often explain increased nectar volume and transition to red flower colour. In addition, they suggest that adaptive suites of floral traits may often exhibit a high degree of genetic linkage, which could facilitate their fixation during pollination syndrome evolution. Here, we explore these emerging generalities by investigating the genetic basis of floral pollination syndrome divergence between two related Penstemon species with different pollination syndromes—bee-pollinated P. neomexicanus and closely related hummingbird-pollinated P. barbatus. In an F₂ mapping population derived from a cross between these two species, we characterized the effect size of genetic loci underlying floral trait divergence associated with the transition to bird pollination, as well as correlation structure of floral trait variation. We find the effect sizes of quantitative trait loci for adaptive floral traits are in line with patterns observed in previous studies, and find strong evidence that suites of floral traits are genetically linked. This linkage may be due to genetic proximity or pleiotropic effects of single causative loci. Interestingly, our data suggest that the evolution of floral traits critical for hummingbird pollination was not constrained by negative pleiotropy at loci that show co-localization for multiple traits.     

GENETIC AND ENVIRONMENTAL VARIATION IN LIFE-HISTORY TRAITS OF A MONOCARPIC PERENNIAL: A DECADE-LONG FIELD EXPERIMENT

Directional and stabilizing selection tend to deplete additive genetic variance. On the other hand, genetic variance in traits related to fitness could be retained through polygenic mutation, spatially varying selection, genotype-environment interaction, or antagonistic pleiotropy. Most estimates of genetic variance in fitness-related traits have come from laboratory studies, with few estimates of heritability made under natural conditions, particularly for longer lived organisms. Here I estimated additive genetic variance in life-history characters of a monocarpic herb, Ipomopsis aggregata, that lives for up to a decade. Experimental crosses yielded 229 full-sibships nested within 32 paternal half-sibships. More than 5000 offspring were planted as seeds into natural field sites and were followed in most cases through their entire life cycle. Survival showed substantial additive genetic variance (genetic coefficient of variation ≈ 54%). Small differences at seedling emergence were magnified over time, such that the genetic variability in survival was only detectable by tracking the success of offspring for several years starting from seed. In contrast to survival, reproductive traits such as flower number, seeds per flower, and age at flowering showed little or no genetic variability. Despite relatively high levels of additive genetic variation for some life-history characters, high environmental variance in survival resulted in very low heritabilities (0–9%) for all of these characters. Maternal effects were evident in seed mass and remained strong throughout the lengthy vegetative period. No negative genetic correlations between major components of female fitness were detected. Mean corolla width for a paternal family was, however, negatively correlated with the finite rate of increase based on female fitness. That negative correlation could help to maintain additive genetic variance in the face of strong selection through male function for wide corollas.     

Effect of selection on genetic parameters of correlated traits

Summary Changes in genetic parameters of correlated traits due to the buildup of linkage (gametic phase) disequilibrium from repeated truncation selection on a single trait are studied. After several generations of selection, an equilibrium is approached where there are no further changes in genetic parameters and limiting values are reached. Formulae are derived under an infinitesimal model for these limiting values of genetic variances and covariances, heritabilities, and genetic correlations between traits directly and indirectly selected. Changes from generation zero to the limit in all these parameters become greater as heritability of the trait under direct selection increases and, to a lesser extent, as intensity of selection increases. Change in heritability of a trait under indirect selection also increases as the absolute value of the correlation between the trait under indirect and the trait under direct selection increases. The change is maximum when the initial value of heritability is close to 0.5 and insignificant when the initital value is close to zero or one. Change in the genetic correlation between the trait under direct selection and the trait under indirect selection is maximum when its initial value is close to ±0.6 and insignificant when its initial value is close to zero or ±1. Heritability of the trait indirectly selected and genetic correlation between that trait and the trait directly selected always decrease in absolute value, whereas genetic correlation between two traits indirectly selected can either decrease or increase in absolute value. It is suggested that use be made of formulae at selection equilibrium in the prediction of correlated responses after several generations of selection.     

标准切花菊杂交F1代群体侧枝侧蕾性状的杂种优势和遗传分析

以标准切花菊〔Dendranthema morifolium(Ramat.)Tzvel.〕品种'优香'('Yuuka')为母本、品种'神马'('Jinba')为父本进行杂交,对杂交F1代群体的单株侧枝平均长度、单株侧枝数、单株侧枝数与单株叶节数的比值(R1)、主蕾直径与侧蕾直径的比值(R2)、单株侧蕾数以及主蕾与侧蕾间距离6个性状进行杂种优势和相关性分析,并利用主基因+多基因混合遗传模型检测这些性状的主基因效应.结果显示:杂交F1代群体6个侧枝侧蕾性状的变异系数为2378%~5065%,且侧枝性状的变异系数总体上高于侧蕾性状;各性状的频次均呈现连续性的正态分布趋势,说明这些性状可能属于多基因控制的数量性状.杂交F1代群体的6个侧枝侧蕾性状均在001水平上表现出显著的中亲优势,表明各性状均存在显著的杂种优势.6个性状中,单株侧枝平均长度的中亲值最大(6230 mm),R1的中亲值最小(026);单株侧枝平均长度、R2和主蕾与侧蕾间距离的中亲优势均为正值,单株侧枝数、单株侧蕾数和R1的中亲优势均为负值.6个性状的中亲优势率为-5374%~3128%,其中,单株侧枝数的中亲优势率最小,而主蕾与侧蕾间距离的中亲优势率最大.相关性分析结果显示:单株侧枝平均长度和单株侧枝数均与R1呈极显著正相关,并与R2和单株侧蕾数呈极显著负相关;R2与侧蕾数也呈极显著正相关,且二者均与主蕾与侧蕾间距离呈极显著正相关.混合遗传分析结果显示:单株侧枝平均长度、R1、R2和单株侧蕾数均受2对主基因控制,符合B-1模型,主基因表现为"加性-显性-上位性",这4个性状的遗传率分别为7707%、9672%、6438%和5307%;单株侧枝数也受2对主基因控制,符合B-2模型,主基因表现为"加性-显性",该性状的遗传率为7438%,表明这5个性状的遗传存在主基因控制效应.而主蕾与侧蕾间距离符合A-0遗传模型,说明该性状无主基因控制,易受环境影响.     

POLLINATION IN FLORAL SCENT MORPHS OF POLEMONIUM VISCOSUM: A MECHANISM FOR DISRUPTIVE SELECTION ON FLOWER SIZE

Plants of Polemonium viscosum have flowers that are either sweet or skunky in scent. The two morphs are preferentially pollinated by insects of strongly contrasting body size: bumblebee queens specialize on sweet flowers, flies on skunky ones. In this study 13 characters were examined in plant specimens from five populations to identify major components of intraspecific variation in flower and inflorescence morphology and test their correlation with floral scent. Factor analysis identified four major axes of morphological variation. The first explained 22% of the variance among specimens and correlated strongly with four flower size characters: sepal length, corolla tube length, corolla lobe width, and corolla lobe length. Floral scent morphs differed significantly in the multivariate representation of flower size defined by these characters. Sweet flowers had wider corolla lobes, longer corolla tubes, and longer sepals than skunky ones. Corolla lobe width accounted for the greatest amount of intermorph divergence. Divergence in flower size between morphs was maintained in mixed populations at four locations in alpine Colorado, with corollas of sweet flowers significantly broader or more flared than those of skunky flowers. Patterns of pollen receipt suggest that this difference is adaptive. In the sweet morph, pollination intensity and purity increased significantly with corolla flare. Conversely, in the skunky morph, corolla flare had little influence on pollination intensity and had a strong negative effect on purity. These findings suggest that selection for effective pollination should favor intraspecific divergence in flower size in Polemonium viscosum.     

Evolution of mutualism between globeflowers and their pollinating flies

Plant/seed-eater pollinators mutualisms involve a plant pollinated by an insect whose larvae develop by eating a fraction of host-plant seeds. The outcome of the interaction therefore depends on the number of ovules fertilized by adult visits and the number of seeds destroyed by larvae. Among the very few cases of such mutualisms reported so far is the globeflower-globeflower flies mutualism, which is unique in that it involves several congeneric fly species (Chiastocheta genus) coexisting within a single host-plant species, Trollius europaeus. These species exhibit contrasted oviposition behaviors resulting in a more or less beneficial outcome for the plant. We designed an adaptive dynamics model to investigate how morphological traits of globeflower could affect the evolution of oviposition in its pollinating flies. Three fly traits (flower age at oviposition, clutch size and the level of avoidance of already parasitized flowers) and one plant trait (closed or open corolla) were examined. Whatever the shape of the flower, evolutionary branching occurs between early and late ovipositing flies, driven by strong competition among larvae within a fruit. Once this branching occurred, the closed shape of the corolla is likely to offer a better protection to eggs of early but not of late ovipositing flies. The difference in egg survival results in higher competition among early larvae and thus selects for decreased clutch size in early flies. This can be seen as a first step in the evolution of a mutualistic behavior. The prediction of our model fits field observations of fly behavior, giving theoretical support to the hypothesis of fly sympatric speciation within its host plant. Moreover, flower closed globe shape can be positively selected in globeflowers as it results in a reduction of parasitism strength. This last evolution therefore leads to a stable mutualism between globeflowers and globeflower flies.