Co‐flowering with congeners does not affect buzz‐pollinator specialization and pollination performance in Rhexia mariana, but does affect floral trait variance

Pollinator-mediated selection might lead to among-trait differences in the degree and pattern of floral integration and intra-flower variation. To examine the patterns of intra-flower variation in floral traits, including nectar volume, we performed a field study using the zygomorphic flowers of Aconitum japonicum ssp. subcuneatum. We investigated (1) correlations between the sizes of the left and right sepals and petals, (2) variation in floral traits among plants, within plants and within flowers, (3) effects of sexual phases on floral integration variation in floral and nectar traits, and (4) the effect of size and intra-flower variation in traits of the left and right sepals and petals on pollen removal by pollinators. Lateral sepal area, but not lower sepal area, was highly correlated between the left and right sepals. Floral traits were more integrated during the male phase than during the female phase. Nectar standing crop in male-phase flowers correlated with helmet height and lateral and lower sepal area, but in female-phase flowers it only correlated with spur length. While intra-flower variance in lateral sepal area accounted for approximately 10% of the overall variance in these traits, the variance in lower sepal area accounted for 70% of the overall variance. Lateral sepal area had a negative effect on the number of pollen grains remaining after pollinator visits. Low variance in lateral sepals within flowers and measurements of pollen removal suggest that lateral sepals play a more important role in pollen export than the other traits. Left and right sepals may be the targets of selection for symmetry in zygomorphic flowers.

The acquisition of floral nectar spurs is correlated with increased species diversity across multiple clades. We tested whether variation in nectar spurs influences reproductive isolation and, thus, can potentially promote species diversity using two species of Aquilegia, Aquilegia formosa and Aquilegia pubescens, which form narrow hybrid zones. Floral visitors strongly discriminated between the two species both in natural populations and at mixed-species arrays of individual flowers. Bees and hummingbirds visited flowers of A. formosa at a much greater rate than flowers of A. pubescens. Hawkmoths, however, nearly exclusively visited flowers of A. pubescens. We found that altering the orientation of A. pubescens flowers from upright to pendent, like the flowers of A. formosa, reduced hawkmoth visitation by an order of magnitude. In contrast, shortening the length of the nectar spurs of A. pubescens flowers to a length similar to A. formosa flowers did not affect hawkmoth visitation. However, pollen removal was significantly reduced in flowers with shortened nectar spurs. These data indicate that floral traits promote floral isolation between these species and that specific floral traits affect floral isolation via ethological isolation while others affect floral isolation via mechanical isolation.

Asymmetric reproductive isolation and interference in neriid flies: the roles of genital morphology and behaviour

Nearly forty years ago R. L. Berg proposed that plants with specialized pollination ecology evolve genetic and developmental systems that decouple floral morphology from phenotypic variation in vegetative traits. These species evolve separate floral and vegetative trait clusters, or as she termed them, "correlation pleiades." The predictions of this hypothesis have been generally supported, but only a small sample of temperate-zone herb and grass species has been tested. To further evaluate this hypothesis, especially its applicability to plants of other growth forms, we examined the patterns of phenotypic variation and covariation of floral and vegetative traits in nine species of Neotropical plants. We recognized seven specific predictions of Berg's hypothesis. Our results supported some predictions but not others. Species with specialized pollination systems usually had floral traits decoupled (weak correlation; Canna and Eichornia) or buffered (relationship with shallow proportional slope; Calathea and Canna) from variation in vegetative traits. However, the same trend was also observed in three species with unspecialized pollination systems (Echinodorus, Muntingia, and Wedelia). One species with unspecialized pollination (Croton) and one wind-pollinated species (Cyperus) showed no decoupling or buffering, as predicted. While species with specialized pollination usually showed lower coefficients of variation for floral traits than vegetative traits (as predicted), the same was also true of species with unspecialized or wind pollination (unlike our prediction). Species with specialized pollination showed less variation in floral traits than did species with unspecialized or wind pollination, as predicted. However, the same was true of the corresponding vegetative traits, which was unexpected. Also in contrast to our prediction, plants with specialized pollination systems did not exhibit tighter phenotypic integration of floral characters than did species with generalized pollination systems. We conclude that the patterns of morphological integration among floral traits and between floral and vegetative traits tend to be species specific, not easily predicted from pollination ecology, and generally more complicated than R. L. Berg envisaged.

Extreme variation in floral characters and its consequences for pollinator attraction among populations of an Andean cactus

To understand the process and mechanism of speciation, a detailed analysis of origin and demographic history of recently diverged species pairs is necessary. Here, we investigate the evolutionary history ofActaea purpurea(P.K. Hsiao) J. Compton and its closest relatives,A. japonicaThunb. andA. biternata(Siebold and Zuccarini) Prantl. We aim to estimate important parameters of the divergence event, and to lay the foundation for further investigation of the speciation mechanism of this system. Floral and vegetative traits were measured and analyzed. Genetic structure, divergence history, and historical gene flow were also inferred from the plastid and single nucleotide polymorphism data. Floral traits were divergent, and a strong match between pollinator and floral traits was revealed. Genetically the two species were also well diverged, and the time of divergence was dated to the Pleistocene. The demographic modelling results suggest thatA. purpureahad continuous limited gene flow withA. japonicaandA. biternatasince divergence. More work is now needed to confirm that floral trait divergence was selected by pollinators, as well as to understand how pollinator isolation acts in conjunction with other reproductive barriers to reduce gene flow between the two species.

Different populations of plant species can adapt to their local pollinators and diverge in floral traits accordingly. Floral traits are subject to pollinator-driven natural selection to enhance plant reproductive success. Studies on temperate plant systems have shown pollinator-driven selection results in floral trait variation along elevational gradients, but studies in tropical systems are lacking. We analyzed floral traits and pollinator assemblages in the Neotropical bee-pollinated taxon Costus guanaiensis var. tarmicus across four sites along a steep elevational gradient in Peru. We found variations in floral traits of size, color, and reward, and in the pollinator assemblage along the elevational gradient. We examined our results considering two hypotheses, (1) local adaptation to different bee assemblages, and (2) the early stages of an evolutionary shift to a new pollinator functional group (hummingbirds). We found some evidence consistent with the adaptation of C. guanaiensis var. tarmicus to the local bee fauna along the studied elevational gradient. Corolla width across sites was associated with bee thorax width of the local most frequent pollinator. However, we could not rule out the possibility of the beginning of a bee-to-hummingbird pollination shift in the highest-studied site. Our study is one of the few geographic-scale analyses of floral trait and pollinator assemblage variation in tropical plant species. Our results broaden our understanding of plant-pollinator interactions beyond temperate systems by showing substantial intraspecific divergence in both floral traits and pollinator assemblages across geographic space in a tropical plant species.

Introduction The remarkable diversity of the angiosperms is often attributed to their specialized reproductive associations with diverse pollinating vectors (Crepet 1984; Eriksson and Bremer 1992; Grimaldi 1999). The appeal of this argument lies in the premise that specializing on particular pollinating groups can directly result in reproductive isolation from related plant species, even in sympatry, thus generating diversity (Grant 1949; Stebbins 1970). This pollinator shift model is the dominant paradigm explaining floral diversification and is often invoked as an important plant speciation mechanism (reviewed in Kay and Sargent 2009). However, pollinator shifts, which are usually quantitative, not qualitative, often result only in weak reproductive isolation (Armbruster and Muchhala 2009). In addition, sympatric species that use different pollinators are rarely sister species, and most often also exhibit substantial postmating isolation. These observations question the validity of the link between pollinator shifts and reproductive isolation, which underlies the pollinator shift paradigm, and suggest that floral divergence associated with pollinator shifts is unlikely to be a product of selection for reproductive isolation in sympatry, except perhaps upon secondary contact. Instead, floral diversity might result largely from spatially variable influences on efficient gamete transfer (Johnson 2006), a perspective that opens the possibility of numerous mechanisms influencing the divergence of floral traits. A hallmark of this perspective should be substantial variation in floral traits between closely related allopatrically distributed species, but also between populations of species. Numerous studies document such floral diversity (Herrera et al. 2006; Ellis and Johnson 2009; Schlumpberger et al. 2009), suggesting that geographical variation in floral traits is ubiquitous. Approximately 20 % of British plant species (Warren and Mckenzie 2001), 40 % of the Polemoniaceae (Schemske and Bierzychudek 2007), 38 % of Cape Erica species (Rebelo and Siegfried 1985), and 40 % of Protea species (Carlson and Holsinger 2010) exhibit flower color polymorphisms. Although these numbers confound intra- and interpopulation variation, they do attest to the extent of variation in just a single floral trait.

Theory predicts that tighter correlation between floral traits and weaker relationship between floral and vegetative traits more likely occur in specialized flowers than generalized flowers, favoring by precise fit with pollinators. However, traits and trait correlations frequently vary under different environments. Through detecting spatiotemporal variation in phenotypic traits (floral organ size and vegetative size) and trait correlations in four Ranunculaceae species, we examined four predictions. Overall, our results supported these predictions to a certain degree. The mean coefficient of variation (CV) of floral traits in two specialized species (Delphinium kamaonense and Aconitum gymnandrum) was marginally significantly lower than that of another two generalized species (Trollius ranunculoides and Anemone obtusiloba). The two specialized species also showed marginally significantly smaller CV in floral traits than vegetative size across the two species. The absolute mean correlation between floral and vegetative traits, or that between floral traits in species with specialized flowers was not significantly lower, or higher than that in generalized plants, weakly supporting the predictions. Furthermore, we documented a large variation in trait correlations of four species among different seasons and populations. Study of covariance of floral and vegetative traits will benefit from the contrast of results obtained from generalized and specialized pollination systems.

Populations of the same plant species living in different locations but flowering at different times may vary in pollinator availability and floral traits. However, the spatial and temporal links between floral traits and pollination are rarely included in single studies. In this study, three populations of an alpine lousewort, Pedicularis rhinanthoides Schrenk subsp. tibetica (Bonati) Tsoong, were surveyed to detect the variations in floral traits and pollinator activity. We hypothesized that floral divergence was spatio-temporally correlated with pollen limitation (PL) in relation to pollinators. Sampled plants from each population were divided into three groups, according to flowering stage: early, peak, and late. Pollen-supplementation experiments and investigations into pollinators, reproductive success, and floral traits were conducted on the plants from the different flowering stages and across the populations. Our results showed that the extent of PL varies across populations and among flowering stages. Populations in which more pollinators were recorded displayed a lower extent of PL. Furthermore, the temporal differences in PL showed a similar pattern for the three populations; the plants from the peak flowering stage suffered slighter PL than those from the other two stages. Nevertheless, some of the floral traits displayed similar spatial and temporal patterns to the PL, while the others only varied among the populations spatially. The results indicated that the performance of floral traits in a particular spatial-temporal situation shows they are well adapted to the corresponding pollination environment, which might help plants to optimize their reproductive fitness under different abiotic factors.

Pollinators prefer flowers with traits that reliably indicate reward quality or quantity, a relationship defining 'honest signals'. Despite its prevalence in plant-pollinator interactions, genetic variation in floral honesty and its effects on plant fitness remain poorly understood. Using a clonal design, we propagated 41 genotypes of Turnera velutina from a natural population to estimate broad-sense heritability and genetic variation in floral morphological traits, nectar, and floral honesty (i.e. the signal-reward correlation). In a factorial experiment, we exposed combinations of 'less honest' and 'more honest' genotypes with above- or below-average nectar sugar content to natural pollinators and recorded pollinator visitation patterns and plant fitness. We found significant heritability and genetic variation in floral traits and the signal-reward correlation, indicating that floral honesty has the potential to evolve through pollinator-mediated selection. Pollinators preferred honest plants with larger flowers and higher nectar sugar content, spending more time on them. These plants also produced more seeds per fruit than other genotypes. Our study addresses key knowledge gaps in the evolution of floral honesty by revealing its genetic basis and demonstrating that a positive signal-reward relationship can be shaped by natural selection through plant-pollinator interactions.

PremiseWidespread associations between selfing rate and floral size within and among taxa suggest that these traits may evolve in concert. Does this association develop immediately because of shared genetic and/or developmental control, or stepwise with selection shaping the evolution of one trait following the other? If the former, then association ought to appear within and across selfing populations. We explore this fundamental question in three populations of the mixed‐mater Collinsia verna where autonomous selfing (AS) ability has been shown to be under selection by the pollination environment.MethodsWe grew clonal replicates of C. verna in a controlled environment to characterize broad‐sense genetic correlations among traits within populations and to assess whether divergence in mating system and floral traits among these populations is consistent with their previously observed selection pressures.ResultsAs predicted by their respective pollination environments, we demonstrate significant genetic divergence among populations in AS ability. However, patterns of divergence in floral traits (petal, stamen, and style size, stigmatic receptivity, and stigma‐anther distance) were not as expected. Within populations, genetic variation in AS appeared largely independent from floral traits, except for a single weak negative association in one population between flower size and AS rate.ConclusionsTogether, these results suggest that associations between selfing rate and floral traits across Collinsia species are not reflected at microevolutionary scales. If C. verna were to continue evolving toward the selfing syndrome, floral trait evolution would likely follow stepwise from mating system evolution.

When more than one closely related plant species share the same pollination niche, reproductive interference via interspecific pollen transfer should limit their coexistence. However, some studies have reported the sympatric coexistence of two native close relatives pollinated by the same pollinators under reproductive interference, even without niche partitioning. We examined the frequency dependency of reproductive interference between close relatives in natural conditions and the potential roles of autonomous selfing in mitigating the negative reproductive interference effects between congeneric species. We investigated sympatrically growing Commelina communis (Cc) and C. c. f. ciliata (Ccfc) populations. These species exhibit very large overlaps in habitat preference, flowering phenology and pollination niche, but seldom produce hybrids. First, we conducted a hand‐pollination experiment to examine the negative effects of heterospecific pollen deposition on seed production and the potential of self‐pollination to mitigate the effects in both species. Then, we examined the effects of reproductive interference on reproductive success and the potential for autonomous selfing in the field. We found significant negative effects of heterospecific pollen deposition on seed production and the mitigation effects of prior and competing self‐pollination, in both Cc and Ccfc. For both species in the field, intraspecific pollinator movements and reproductive success significantly decreased with an increase in the relative floral abundance of competing species, although the negative reproductive interference effect on reproductive success was lower in Cc than in Ccfc. We also found greater potential for prior autonomous selfing in Cc than in Ccfc. Our findings suggest that Cc flowers were less affected by reproductive interference from competing species, which was likely due to a higher prior selfing ability compared to Ccfc flowers. The asymmetry in susceptibility to reproductive interference may explain the Cc‐biased distribution in the study area. The study improves our understanding of how prior autonomous selfing can reduce the negative reproductive interference effect from competing species in mixed‐mating species with frequent pollinator visits. A plain language summary is available for this article.

Lack of strong selection pressures maintains wide variation in floral traits in a food-deceptive orchid.

Do floral and niche shifts favour the establishment and persistence of newly arisen polyploids? A case study in an Alpine primrose.