Floral and ecological isolation between Aquilegia formosa and Aquilegia pubescens

A few essential genetic loci distinguish Penstemon species with flowers adapted to pollination by bees or hummingbirds

In the formation of species, adaptation by natural selection generates distinct combinations of traits that function well together. The maintenance of adaptive trait combinations in the face of gene flow depends on the strength and nature of selection acting on the underlying genetic loci. Floral pollination syndromes exemplify the evolution of trait combinations adaptive for particular pollinators. The North American wildflower genus Penstemon displays remarkable floral syndrome convergence, with at least 20 separate lineages that have evolved from ancestral bee pollination syndrome (wide blue-purple flowers that present a landing platform for bees and small amounts of nectar) to hummingbird pollination syndrome (bright red narrowly tubular flowers offering copious nectar). Related taxa that differ in floral syndrome offer an attractive opportunity to examine the genomic basis of complex trait divergence. In this study, we characterized genomic divergence among 229 individuals from a Penstemon species complex that includes both bee and hummingbird floral syndromes. Field plants are easily classified into species based on phenotypic differences and hybrids displaying intermediate floral syndromes are rare. Despite unambiguous phenotypic differences, genome-wide differentiation between species is minimal. Hummingbird-adapted populations are more genetically similar to nearby bee-adapted populations than to geographically distant hummingbird-adapted populations, in terms of genome-wide dXY. However, a small number of genetic loci are strongly differentiated between species. These approximately 20 "species-diagnostic loci," which appear to have nearly fixed differences between pollination syndromes, are sprinkled throughout the genome in high recombination regions. Several map closely to previously established floral trait quantitative trait loci (QTLs). The striking difference between the diagnostic loci and the genome as whole suggests strong selection to maintain distinct combinations of traits, but with sufficient gene flow to homogenize the genomic background. A surprisingly small number of alleles confer phenotypic differences that form the basis of species identity in this species complex.

Microhabitat and Pollinator Differentiation Drive Reproductive Isolation between Two Sympatric Salvia Species (Lamiaceae)

Evaluation of multiple barriers contributing to reproductive isolation between sympatric plant species is key to understanding the mechanism of their coexistence; however, such investigations in biodiversity hotspots are still rare. In this study, we investigated and compared geography, microhabitat, phenology, flora, and pollinators, in addition to pollen–pistil interactions, seed production, and seed germination of the closely related sympatric Salvia digitaloides and S. flava on Yulong Snow Mountain, Southwestern Yunnan, China. The geographic distribution of these species overlapped, but their adaptation to physical and chemical properties of soil microhabitats differed. They shared the same flowering time but differed in flower size, style length, nectar volume, sugar concentration, and flower longevity. Both species shared bumblebees as effective pollinators, but flower constancy for the two species was relatively strong. Pollen tube growth, seed production, and seed germination were lower in interspecific than in intraspecific crosses. Our study suggested that microhabitat and pollinator isolation acted as the most important isolating barriers in maintaining the coexistence of the two Salvia species. Our study also highlighted that post-pollination barriers play an important role in preventing the gene flow between these two Salvia species.

Adaptation to lower latitudes and lower elevations precedes the evolution of hummingbird pollination in western North American Penstemon

PREMISE

A switch in pollinator can occur when a plant lineage enters a new habitat where the ancestral pollinator is less common, and a novel pollinator is more common. Because pollinator communities vary according to environmental tolerances and availability of resources, there may be consistent associations between pollination mode and specific regions and habitats. Such associations can be studied in lineages that have experienced multiple pollinator transitions, representing evolutionary replicates.

METHODS

Our study focused on a large clade of Penstemon wildflower species in western North America, which has repeatedly evolved hummingbird-adapted flowers from ancestral bee-adapted flowers. For each species, we estimated geographic ranges from occurrence data and inferred environmental niches from climate, topographical, and soil data. Using a phylogenetic comparative approach, we investigated whether hummingbird-adapted species occupy distinct geographic regions or habitats relative to bee-adapted species.

RESULTS

Hummingbird-adapted species occur at lower latitudes and lower elevations than bee-adapted species, resulting in a difference in their environmental niche. Bee-adapted species sister to hummingbird-adapted species are also found in relatively low elevations and latitudes, similar to their hummingbird-adapted sister species, suggesting ecogeographic shifts precede pollinator divergence. Sister species pairs-regardless of whether they differ in pollinator-show relatively little geographic range overlap.

CONCLUSIONS

Adaptation to a novel pollinator may often occur in geographic and ecological isolation from ancestral populations. The ability of a given lineage to adapt to novel pollinators may critically depend on its ability to colonize regions and habitats associated with novel pollinator communities.

Natural hybridization among three Rhododendron species (Ericaceae) revealed by morphological and genomic evidence

BACKGROUND

Natural hybridization can influence the adaptive response to selection and accelerate species diversification. Understanding the composition and structure of hybrid zones may elucidate patterns of hybridization processes that are important to the formation and maintenance of species, especially for taxa that have experienced rapidly adaptive radiation. Here, we used morphological traits, ddRAD-seq and plastid DNA sequence data to investigate the structure of a Rhododendron hybrid zone and uncover the hybridization patterns among three sympatric and closely related species.

RESULTS

Our results show that the hybrid zone is complex, where bi-directional hybridization takes place among the three sympatric parental species: R. spinuliferum, R. scabrifolium, and R. spiciferum. Hybrids between R. spinuliferum and R. spiciferum (R. ×duclouxii) comprise multiple hybrid classes and a high proportion of F₁ generation hybrids, while a novel hybrid taxon between R. spinuliferum and R. scabrifolium dominated the F₂ generation, but no backcross individuals were detected. The hybrid zone showed basically coincident patterns of population structure between genomic and morphological data.

CONCLUSIONS

Natural hybridization exists among the three Rhododendron species in the hybrid zone, although patterns of hybrid formation vary between hybrid taxa, which may result in different evolutionary outcomes. This study represents a unique opportunity to dissect the ecological and evolutionary mechanisms associated with adaptive radiation of Rhododendron species in a biodiversity hotspot.

Divergence in the Aquilegia ecalcarata complex is correlated with geography and climate oscillations: Evidence from plastid genome data

Quaternary climate oscillations and geographical heterogeneity play important roles in determining species and genetic diversity distribution patterns, but how these factors affect the migration and differentiation of East Asian plants species at the population level remains poorly understood. The Aquilegia ecalcarata complex, a group that originated in the Late Tertiary and is widely distributed throughout East Asia, displays high genetic variation that is suitable for studying elaborate phylogeographic patterns and demographic history related to the impact of Quaternary climate and geography. We used plastid genome data from 322 individuals in 60 populations of the A. ecalcarata complex to thoroughly explore the impact of Quaternary climate oscillations and geography on the phylogeographic patterns and demographic history of the A. ecalcarata complex through a series of phylogenetic, divergence time estimation, and demographic history analyses. The dry, cold climate and frequent climate oscillations that occurred during the early Pleistocene and the Mid-Pleistocene transition led to the differentiation of the A. ecalcarata complex, which was isolated in various areas. Geographically, the A. ecalcarata complex can be divided into Eastern and Western Clades and five subclades, which conform to the divergence of the East Asian flora. Our results clearly show the impact of Quaternary climate and geography on evolutionary history at the population level. These findings promote the understanding of the relationship between plant genetic differentiation and climate and geographical factors of East Asia at the population level.

Speciation along the elevation gradient: Divergence of Roscoea species within the south slope of the Himalayas

The Himalayas with dramatic elevation gradient is one of the global biodiversity hotspots. Although origin of biodiversity of the Himalayas is of great concern, the speciation process within the Himalayas is poorly known. Roscoea within the Himalayas serve as a good model system to test the speciation process along an elevation gradient. 32,375 unlinked SNPs were used to reconstruct phylogenetic relationships and introgression analyses in D-statistics and Fastsimicoal2. Species distribution modeling (SDM) was used to simulate habitat shift of Roscoea species during climate changes. Phylogeny suggested that the speciation order, except R. capitata, was from highland to lowland. D-statistics analyses suggested significant bidirectional ancient introgression between elevation-neighboring clades but no introgression between R. capitata and othern clades and no introgression among extant species. Fastsimicoal2 suggested interspecific introgressions were asymmetric. SDM predicted that habitats of Roscoea shifted to low elevation during cooling age. These results suggested that the sudden uplift of the Himalayas likely promoted speciation by vicariance, and climate cooling drove species divergence towards lower elevation. This study provides explanations for the origin of biodiversity within the Himalayas, and an insight to understand speciation along elevation in the mountainous regions.

Evolutionary history of Hemerocallis in Japan inferred from chloroplast and nuclear phylogenies and levels of interspecific gene flow

The perennial herb genus Hemerocallis (Asphodelaceae) shows four flowering types: diurnal half-day, diurnal one-day, nocturnal half-day, and nocturnal one-day flowering. These flowering types are corresponding to their main pollinators, and probably act as a primary mechanism of reproductive isolation. To examine how the four flowering types diverged, we reconstructed the phylogeny of the Japanese species of Hemerocallis using 1615 loci of nuclear genome-wide SNPs and 2078 bp sequences of four cpDNA regions. We also examined interspecific gene flows among taxa by an Isolation-with-Migration model and a population structure analysis. Our study revealed an inconsistency between chloroplast and nuclear genome phylogenies, which may have resulted from chloroplast capture. Each of the following five clusters is monophyletic and clearly separated on the nuclear genome-wide phylogenetic tree: (I) two nocturnal flowering species with lemon-yellow flowers, H. citrina (half-day flowering) and H. lilioasphodelus (one-day flowering); (II) a diurnal one-day flowering species with yellow-orange flowers, H. middendorffii; (III) a variety of a diurnal half-day flowering species with reddish orange flowers, H. fulva var. disticha; (IV) another variety of a diurnal half-day flowering species with reddish orange flowers, H. fulva var. aurantiaca, and a diurnal one-day flowering species with yellow-orange flowers, H. major; (V) a diurnal half-day flowering species with yellow-orange flowers, H. hakuunensis. The five clusters are consistent with traditional phenotype-based taxonomy (cluster I, cluster II, and clusters III-V correspond to Hemerocallis sect. Hemerocallis, Capitatae, and Fulvae, respectively). These findings could indicate that three flowering types (nocturnal flowering, diurnal one-day flowering, and diurnal half-day flowering) diverged in early evolutionary stages of Hemerocallis and subsequently a change from diurnal half-day flowering to diurnal one-day flowering occurred in a lineage of H. major. While genetic differentiation among the five clusters was well maintained, significant gene flow was detected between most pairs of taxa, suggesting that repeated hybridization played a role in the evolution of those taxa.

Within-day temporal isolation of two species of Iris (Iridaceae) sharing the same pollinator

Knowledge of factors driving reproductive isolation is essential to understand the process of speciation. To study the reproductive isolation of two closely related species with overlapping flowering seasons, Iris domestica and Iris dichotoma, we compared their reproductive system, floral biology and pollination biology. The results indicated that I. domestica was facultatively xenogamous, whereas I. dichotoma was facultatively autogamous. Although the two species differed significantly in floral colour, floral diameter, floral structure, nectar volume, flower opening and closing times, they shared the same diurnal pollinator, the honeybee Apis cerana. The frequency of pollination by A. cerana did not differ significantly between the two species, but honeybee pollination of I. domestica was more efficient compared with that of I. dichotoma. Despite the difference in floral structure between the two species, both species deposited pollen on the same parts of the body of honeybees. The temporal partitioning of within-day flowering times between I. domestica (from 07.15 to 08.15 h to 18.00 to 19.00 h) and I. dichotoma (from 15.45 to 16.15 h to 22.00 to 23.00 h), together with the time memory of honeybees, meant that the two species did not overlap in the time of their pollination, thus leading to temporal isolation as a major driver of reproductive isolation between the two species.

Reproductive isolation in alpine gingers: How do coexisting Roscoea (R. purpurea and R. tumjensis) conserve species integrity?

Multiple barriers may contribute to reproductive isolation between closely related species. Understanding the relative strength of these barriers can illuminate the ecological factors that currently maintain species integrity and how these factors originally promoted speciation. Two Himalayan alpine gingers, Roscoea purpurea and R. tumjensis, occur sympatrically in central Nepal and have such similar morphology that it is not clear whether or how they maintain a distinct identity. Our quantitative measurements of the components of reproductive isolation show that they are, in fact, completely isolated by a combination of phenological displacement of flowering, earlier for R. tumjensis and later for R. purpurea, and complete fidelity of visitation by different pollinator species, bumblebees for R. tumjensis and a long-tongued fly for R. purpurea. Furthermore, the nectar of R. tumjensis flowers is available to the shorter tongued bumblebees while R. purpurea nectar is less accessible, requiring deep probing from long-tongued flies. Although flowering phenology is a strong current barrier that seemingly obviates any need for pollinator discrimination, this current pattern need not reflect selective forces occurring at the initial divergence of R. tumjensis. There has been considerable pollinator switching during the radiation of the Himalayan Roscoea, and the association of flowering time with type of pollinator in these sympatric species may have originated among the earliest or latest flowering individuals or populations of an ancestor to exploit either bumblebee activity early in the breeding season or long-tongued fly abundance later in the season. These two sympatric Roscoea species add to accumulating evidence of the primacy of prezygotic pollination traits in speciation among angiosperms even in the absence of postzygotic incompatibility.

Evolution of Darwin's Peloric Gloxinia (Sinningia speciosa) Is Caused by a Null Mutation in a Pleiotropic TCP Gene

Unlike most crops, which were domesticated through long periods of selection by ancient humans, horticultural plants were primarily domesticated through intentional selection over short time periods. The molecular mechanisms underlying the origin and spread of novel traits in the domestication process have remained largely unexplored in horticultural plants. Gloxinia (Sinningia speciosa), whose attractive peloric flowers influenced the thoughts of Darwin, have been cultivated since the early 19th century, but its origin and genetic basis are currently unknown. By employing multiple experimental approaches including genetic analysis, genotype-phenotype associations, gene expression analysis, and functional interrogations, we showed that a single gene encoding a TCP protein, SsCYC, controls both floral orientation and zygomorphy in gloxinia. We revealed that a causal mutation responsible for the development of peloric gloxinia lies in a 10-bp deletion in the coding sequence of SsCYC. By combining genetic inference and literature searches, we have traced the putative ancestor and reconstructed the domestication path of the peloric gloxinia, in which a 10-bp deletion in SsCYC under selection triggered its evolution from the wild progenitor. The results presented here suggest that a simple genetic change in a pleiotropic gene can promote the elaboration of floral organs under intensive selection pressure.

Breakdown of species boundaries in Mandevilla: floral morphological intermediacy, novel fragrances and asymmetric pollen flow

Phenotypic intermediacy is an indicator of putative hybrid origin and has provided the main clues to discovering hybrid plants in nature. Mandevilla pentlandiana and M. laxa (Apocynaceae) are sister species with clear differences in floral phenotype and associated pollinator guilds: diurnal Hymenoptera and nocturnal hawkmoths, respectively. The presence of individuals with intermediate phenotypes in a wild population raises questions about the roles of visual and olfactory signals (i.e. corolla morphology and floral fragrances) as barriers to interbreeding, and how the breakdown of floral isolation occurs. We examined phenotypic variation in a mixed Mandevilla population, analysing the chemical composition of floral fragrances, characterising floral shape through geometric morphometrics and assessing individual grouping through taxonomically relevant traits and an unsupervised learning algorithm. We quantified the visitation frequencies of floral visitors and tracked their foraging movements using pollen analogues. The presence of morphologically intermediate individuals and pollen analogue movement suggested extensive hybridisation between M. laxa and M. pentlandiana, along with asymmetrical rates of backcrossing between these putative hybrids and M. laxa. Floral volatiles from putative hybrid individuals showed a transgressive phenotype, with additional compounds not emitted by either parental species. Our results suggest the presence of a hybrid swarm between sympatric M. pentlandiana and M. laxa and indicate that initial hybridisation events between these parental species are rare, but once they occur, visits between putative hybrids and M. laxa are common and facilitate continued introgression.

On the origin of orphan hybrids between Aquilegia formosa and Aquilegia flavescens

We report the investigation of an Aquilegia flavescens × A. formosa population in British Columbia that is disjunct from its parents-the latter species is present locally but ecologically separated, while the former is entirely absent. To confirm hybridity, we used multivariate analysis of floral characters of field-sampled populations to ordinate phenotypes of putative hybrids in relation to those of the parental species. Microsatellite genotypes at 11 loci from 72 parental-type and putative hybrid individuals were analysed to assess evidence for admixture. Maternally inherited plastid sequences were analysed to infer the direction of hybridization and test hypotheses on the origin of the orphan hybrid population. Plants from the orphan hybrid population are on average intermediate between typical A. formosa and A. flavescens for most phenotypes examined and show evidence of genetic admixture. This population lies beyond the range of A. flavescens, but within the range of A. formosa. No pure A. flavescens individuals were observed in the vicinity, nor is this species known to occur within 200 km of the site. The hybrids share a plastid haplotype with local A. formosa populations. Alternative explanations for this pattern are evaluated. While we cannot rule out long-distance pollen dispersal followed by proliferation of hybrid genotypes, we consider the spread of an A. formosa plastid during genetic swamping of a historical A. flavescens population to be more parsimonious.

Fitness benefits and costs of floral advertising traits: insights from rayed and rayless phenotypes of Anacyclus (Asteraceae)

PREMISE OF THE STUDY

Ray flowers commonly observed in daisies' flowering heads are a well-known example of advertising structures for enhancing pollinator attraction. Despite this, ray loss has occurred in multiple lineages, which still rely on pollinators, suggesting that rayless phenotypes could also be adaptive for animal-pollination. Here, we investigate the benefits and costs of these specialized floral advertising structures by comparing rayed and rayless phenotypes in two hybridizing closely related species.

METHODS

We assessed the advantages and costs of ray production in terms of floral visitor's attraction, pollen limitation, and female reproductive success using the broad natural variation on ray size and number at the contact zone of A. clavatus (rayed) and A. valentinus (rayless). In addition, we experimentally explored the effect of rays under controlled neighborhoods and the effect of ray removal on fruit production.

KEY RESULTS

In sympatry, rayed phenotypes attracted significantly more visitors than rayless plants, in which seed production was pollen limited. However, rayed phenotypes did not show higher fruit set or seed production than rayless phenotypes. Fruit set and seed production benefited from denser neighborhood displays and larger individual floral displays, respectively. The removal of ray florets did not appear to enable resource reallocation to fruit production.

CONCLUSIONS

Rayless heads compensated their lower visitation rate by means of a higher number of flowers per head achieving similar fecundity levels to rayed plants. The larger size of rayless heads might thus indicate an inflorescence-level trade-off between attraction and fertility.

Reproductive isolation between Salvia elegans and S. fulgens, two hummingbird-pollinated sympatric sages

The integrity of species in sympatric contact sites is dependent on the existence of reproductive isolating mechanisms, which restrict gene flow between them. However, we know little about the mechanisms that enable the coexistence of species with similar floral morphologies. Here, we evaluated several reproductive isolation barriers between Salvia elegans and S. fulgens, two sympatric sages with a similar ornithophilous floral syndrome, offering nectar as the main reward. Over 3 years, we evaluated broad-scale geographic isolation, floral phenologies and floral visitors as pre-pollination barriers, and fruit set, seed number and seed germination as post-pollination barriers. We found considerable geographic isolation and significant altitudinal differences between the two sages. The flowering period of both sages always overlapped extensively during the 3 years of this study, but hummingbirds were highly specific, visiting one or the other Salvia species and showing aggressive territorial behaviour. Interspecific experimental crosses revealed that hybrid seeds might be formed although strong asymmetric barriers were found depending on the species acting as the maternal donor. Despite the low level of flowering asynchrony, reproductive isolation was remarkably high in the two sages. Geographic isolation and pollinator fidelity were the main factors responsible for maintaining species integrity. Despite an extensive review, we found very few studies quantifying the efficiency of isolation barriers in Neotropical plants or even the importance of hummingbirds as pollinators.

The Aquilegia genome provides insight into adaptive radiation and reveals an extraordinarily polymorphic chromosome with a unique history

The columbine genus Aquilegia is a classic example of an adaptive radiation, involving a wide variety of pollinators and habitats. Here we present the genome assembly of A. coerulea 'Goldsmith', complemented by high-coverage sequencing data from 10 wild species covering the world-wide distribution. Our analyses reveal extensive allele sharing among species and demonstrate that introgression and selection played a role in the Aquilegia radiation. We also present the remarkable discovery that the evolutionary history of an entire chromosome differs from that of the rest of the genome - a phenomenon that we do not fully understand, but which highlights the need to consider chromosomes in an evolutionary context.

Shifts to earlier selfing in sympatry may reduce costs of pollinator sharing

Coexisting plant congeners often experience strong competition for resources. Competition for pollinators can result in direct fitness costs via reduced seed set or indirect costs via heterospecific pollen transfer (HPT), causing subsequent gamete loss and unfit hybrid offspring production. Autonomous selfing may alleviate these costs, but to preempt HPT, selfing should occur early, before opportunities for HPT occur (i.e., "preemptive selfing hypothesis"). We evaluated conditions for this hypothesis in Collinsia sister species, C. linearis and C. rattanii. In field studies, we found virtually identical flowering times and pollinator sharing between congeners in sympatric populations. Compared to allopatric populations, sympatric C. linearis populations enjoyed higher pollinator visitation rates, whereas visitation to C. rattanii did not differ in sympatry. Importantly, the risk of HPT to each species in sympatry was strongly asymmetrical; interspecies visits comprised 40% of all flower-to-flower visits involving C. rattanii compared to just 4% involving C. linearis. Additionally, our greenhouse experiment demonstrated a strong cost of hybridization when C. rattanii was the pollen donor. Together, these results suggest that C. rattanii pays the greatest cost of pollinator sharing. Matching predictions of the preemptive selfing hypothesis, C. rattanii exhibit significantly earlier selfing in sympatric relative to allopatric populations.

Male mate choice via cuticular hydrocarbon pheromones drives reproductive isolation between Drosophila species

Mate discrimination is a key mechanism restricting gene flow between species. While studied extensively with respect to female mate choice, mechanisms of male mate choice between species are far less studied. Thus, we have little knowledge of the relative frequency, importance, or overall contribution of male mate discrimination to reproductive isolation. In the present study, we estimated the relative contributions of male and female choice to reproductive isolation between Drosophila simulans and D. sechellia, and show that male mate discrimination accounts for the majority of the current isolation between these species. We further demonstrate that males discriminate based on female cuticular hydrocarbon pheromones, and collect evidence supporting the hypothesis that male mate discrimination may alleviate the costs associated with heterospecific courtship and mating. Our findings highlight the potentially significant contribution of male mate choice to the formation of reproductive isolating barriers, and thus the speciation process.

Factors contributing to the accumulation of reproductive isolation: A mixed model approach

The analysis of large datasets describing reproductive isolation between species has been extremely influential in the study of speciation. However, the statistical methods currently used for these data limit the ability to make direct inferences about the factors predicting the evolution of reproductive isolation. As a result, our understanding of iconic patterns and rules of speciation rely on indirect analyses that have clear statistical limitations. Phylogenetic mixed models are commonly used in ecology and evolution, but have not been applied to studies of reproductive isolation. Here I describe a flexible framework using phylogenetic mixed models to analyze data collected at different evolutionary scales, to test both categorical and continuous predictor variables, and to test the effect of multiple predictors on rates and patterns of reproductive isolation simultaneously. I demonstrate the utility of this framework by re-analyzing four classic datasets, from both animals and plants, and evaluating several hypotheses that could not be tested in the original studies: In the Drosophila and Bufonidae datasets, I found support for more rapid accumulation of reproductive isolation in sympatric species pairs compared to allopatric species pairs. Using Silene and Nolana, I found no evidence supporting the hypothesis that floral differentiation elevates postzygotic reproductive isolation. The faster accumulation of postzygotic isolation in sympatry is likely the result of species coexistence determined by the level of postzygotic isolation between species. In addition, floral trait divergence does not appear to translate into pleiotropic effects on postzygotic reproductive isolation. Overall, these methods can allow researchers to test new hypotheses using a single statistical method, while remedying the statistical limitations of several previous methods.

The Sexual Advantage of Looking, Smelling, and Tasting Good: The Metabolic Network that Produces Signals for Pollinators

A striking feature of the angiosperms that use animals as pollen carriers to sexually reproduce is the great diversity of their flowers with regard to morphology and traits such as color, odor, and nectar. These traits are underpinned by the synthesis of secondary metabolites such as pigments and volatiles, as well as carbohydrates and amino acids, which are used by plants to lure and reward animal pollinators. We review here the knowledge of the metabolic network that supports the biosynthesis of these compounds and the behavioral responses that these molecules elicit in the animal pollinators. Such knowledge provides us with a deeper insight into the ecology and evolution of plant-pollinator interactions, and should help us to better manage these ecologically essential interactions in agricultural ecosystems.

Maintenance of species boundaries in a Neotropical radiation of Begonia

A major goal of evolutionary biology is to determine the mechanisms generating biodiversity. In Begonia, one of the largest plant genera (1900+ species), it has been postulated that the high number of endemic species is a by-product of low gene flow among populations, which predisposes the group to speciation. However, this model of divergence requires that reproductive barriers accumulate rapidly among diverging species that overlap in their geographic ranges, otherwise speciation will be opposed by homogenizing gene flow in zones of secondary contact. Here, we test the outcomes of secondary contact in Begonia by genotyping multiple sympatric sites with 12 nuclear and seven plastid loci. We show that three sites of secondary contact between B. heracleifolia and B. nelumbiifolia are highly structured, mostly containing parental genotypes, with few F1 hybrids. A sympatric site between B. heracleifolia and B. sericoneura contains a higher proportion of F1s, but little evidence of introgression. The lack of later-generation hybrids contrasts with that documented in many other plant taxa, where introgression is extensive. Our results, in conjunction with previous genetic work, show that Begonia demonstrate properties making them exceptionally prone to speciation, at multiple stages along the divergence continuum. Not only are populations weakly connected by gene flow, promoting allopatric speciation, but species often show strong reproductive barriers in secondary contact. Whether similar mechanisms contribute to diversification in other large genera remains to be tested.

The genetics of reproductive organ morphology in two Petunia species with contrasting pollination syndromes

Switches between pollination syndromes have happened frequently during angiosperm evolution. Using QTL mapping and reciprocal introgressions, we show that changes in reproductive organ morphology have a simple genetic basis. In animal-pollinated plants, flowers have evolved to optimize pollination efficiency by different pollinator guilds and hence reproductive success. The two Petunia species, P. axillaris and P. exserta, display pollination syndromes adapted to moth or hummingbird pollination. For the floral traits color and scent, genetic loci of large phenotypic effect have been well documented. However, such large-effect loci may be typical for shifts in simple biochemical traits, whereas the evolution of morphological traits may involve multiple mutations of small phenotypic effect. Here, we performed a quantitative trait locus (QTL) analysis of floral morphology, followed by an in-depth study of pistil and stamen morphology and the introgression of individual QTL into reciprocal parental backgrounds. Two QTLs, on chromosomes II and V, are sufficient to explain the interspecific difference in pistil and stamen length. Since most of the difference in organ length is caused by differences in cell number, genes underlying these QTLs are likely to be involved in cell cycle regulation. Interestingly, conservation of the locus on chromosome II in a different P. axillaris subspecies suggests that the evolution of organ elongation was initiated on chromosome II in adaptation to different pollinators. We recently showed that QTLs for pistil and stamen length on chromosome II are tightly linked to QTLs for petal color and volatile emission. Linkage of multiple traits will enable major phenotypic change within a few generations in hybridizing populations. Thus, the genomic architecture of pollination syndromes in Petunia allows for rapid responses to changing pollinator availability.

Strong premating reproductive isolation drives incipient speciation in Mimulus aurantiacus

Determining which forms of reproductive isolation have the biggest impact on the process of divergence is a major goal of speciation research. These barriers are often divided into those that affect the potential for hybridization (premating isolation), and those that occur after mating (postmating isolation), and much debate has surrounded the relative importance of these categories. Within the species Mimulus aurantiacus, red- and yellow-flowered ecotypes occur in the southwest corner of California, and a hybrid zone occurs where their ranges overlap. We show that premating barriers are exclusively responsible for isolation in this system, with both ecogeographic and pollinator isolation contributing significantly to total isolation. Postmating forms of reproductive isolation have little or no impact on gene flow, indicating that hybrids likely contribute to introgression at neutral loci. Analysis of molecular variation across thousands of restriction-site associated DNA sequencing (RAD-seq) markers reveals that the genomes of these taxa are largely undifferentiated. However, structure analysis shows that these taxa are distinguishable genetically, likely due to the impact of loci underlying differentiated adaptive phenotypes. These data exhibit the power of divergent natural selection to maintain highly differentiated phenotypes in the face of gene flow during the early stages of speciation.

Floral isolation is the major reproductive barrier between a pair of rewarding orchid sister species

The crucial role of reproductive isolation in speciation has long been recognized; however, a limited number of studies quantify different isolation barriers and embed reproductive isolation in a phylogenetic context. In this study, we investigate reproductive isolation between the often sympatrically occurring orchid species, Gymnadenia conopsea and G. odoratissima. We examine the phylogenetic relationship between the two species and analyse floral isolation, fruit set and seed viability from interspecies crosses, as well as the ploidy level. Additionally, we quantify interspecies differences in floral signals and morphology. The results suggest that the two species have a sister-species relationship. In terms of reproductive isolation, we found complete floral isolation between the two species, but little to no post-pollination isolation; the species also mostly had the same ploidy level in the studied populations. We also show clear distinctions in floral signals, as well as in floral size and spur length. We propose that respective adaptation to short- vs. long-tongued pollinators was the driver of speciation in the here studied Gymnadenia species. Our study supports the key role of floral isolation in orchid speciation and shows that floral isolation is not restricted to highly specialized pollination systems, but can also occur between species with less specialized pollination.

Coevolution and the diversification of life

Coevolution, reciprocal adaptation between two or more taxa, is commonly invoked as a primary mechanism responsible for generating much of Earth's biodiversity. This conceptually appealing hypothesis is incredibly broad in evolutionary scope, encompassing diverse patterns and processes operating over timescales ranging from microbial generations to geological eras. However, we have surprisingly little evidence that large-scale associations between coevolution and diversity reflect a causal relationship at smaller timescales, in which coevolutionary selection is directly responsible for the formation of new species. In this synthesis, we critically evaluate evidence for the often-invoked hypothesis that coevolution is an important process promoting biological diversification. We conclude that the lack of widespread evidence for coevolutionary diversification may be best explained by the fact that coevolution's importance in diversification varies depending on the type of interaction and the scale of the diversification under consideration.

Altitudinal gradients, plant hybrid zones and evolutionary novelty

Altitudinal gradients are characterized by steep changes of the physical and biotic environment that present challenges to plant adaptation throughout large parts of the world. Hybrid zones may form where related species inhabit different neighbouring altitudes and can facilitate interspecific gene flow and potentially the breakdown of species barriers. Studies of such hybrid zones can reveal much about the genetic basis of adaptation to environmental differences stemming from changes in altitude and the maintenance of species divergence in the face of gene flow. Furthermore, owing to recombination and transgressive effects, such hybrid zones can be sources of evolutionary novelty. We document plant hybrid zones associated with altitudinal gradients and emphasize similarities and differences in their structure. We then focus on recent studies of a hybrid zone between two Senecio species that occur at high and low altitude on Mount Etna, Sicily, showing how adaptation to local environments and intrinsic selection against hybrids act to maintain it. Finally, we consider the potential of altitudinal hybrid zones for generating evolutionary novelty through adaptive introgression and hybrid speciation. Examples of homoploid hybrid species of Senecio and Pinus that originated from altitudinal hybrid zones are discussed.

Genetic Drift Linked to Heterogeneous Landscape and Ecological Specialization Drives Diversification in the Alpine Endemic Columbine Aquilegia thalictrifolia

The European Alpine system is an extensive mountain range, whose heterogeneous landscape together with Quaternary climatic oscillations significantly affected organismal diversity and distribution in Europe. The model genus Aquilegia represents a textbook example of a rapid and recent radiation through the Northern hemisphere, with the majority of the European taxa occuring in the Alpine system. However, the processes governing genetic differentiation of the genus in this complex geographic area are still widely unexplored. In this work, we used 9 microsatellite loci to study the genetic structure and diversity of 11 populations of Aquilegia thalictrifolia Schott & Kotschy, an alpine taxon characterized by a marked ecological specificity. We found that, despite the endemic and fragmented distribution, A. thalictrifolia has overall high levels of heterozygosity, which is consistent to the substantial inbreeding depression that characterizes the genus. Strong spatial genetic structuring of populations suggests a historical prevalence of genetic drift over gene flow, with natural barriers and ecological niche hindering migration. An analytical comparison of fixation and population differentiation indexes allowed us to infer hypotheses of the postglacial history and more recent demographic events that have influenced the genetics of the species. Overall, our results indicate allopatry as a major force of differentiation in the European scenario, likely to underlie the development of taxonomic boundaries in a broader geographic context. This adds to previous notions on the primary evolutionary forces shaping the Aquilegia radiation in Europe.

Adaptation and exogenous selection in a Picea glauca × Picea engelmannii hybrid zone: implications for forest management under climate change

The nature of selection responsible for the maintenance of the economically and ecologically important Picea glauca × Picea engelmannii hybrid zone was investigated. Genomic, phenotypic and climatic data were used to test assumptions of hybrid zone maintenance and to model future scenarios under climate change. Genome-wide estimates of admixture based on a panel of 86 candidate gene single nucleotide polymorphisms were combined with long-term quantitative data on growth and survival (over 20 yr), as well as one-time assessments of bud burst and bud set phenology, and cold hardiness traits. A total of 15,498 individuals were phenotyped for growth and survival. Our results suggest that the P. glauca × P. engelmannii hybrid zone is maintained by local adaptation to growing season length and snowpack (exogenous selection). Hybrids appeared to be fitter than pure species in intermediate environments, which fits expectations of the bounded hybrid superiority model of hybrid zone maintenance. Adaptive introgression from parental species has probably contributed to increased hybrid fitness in intermediate habitats. While P. engelmannii ancestry is higher than P. glauca ancestry in hybrid populations, on average, selective breeding in managed hybrid populations is shifting genomic composition towards P. glauca, potentially pre-adapting managed populations to warmer climates.

Evolutionary history of the Afro-Madagascan Ixora species (Rubiaceae): species diversification and distribution of key morphological traits inferred from dated molecular phylogenetic trees

BACKGROUND AND AIMS

Previous work on the pantropical genus Ixora has revealed an Afro-Madagascan clade, but as yet no study has focused in detail on the evolutionary history and morphological trends in this group. Here the evolutionary history of Afro-Madagascan Ixora spp. (a clade of approx. 80 taxa) is investigated and the phylogenetic trees compared with several key morphological traits in taxa occurring in Madagascar.

METHODS

Phylogenetic relationships of Afro-Madagascan Ixora are assessed using sequence data from four plastid regions (petD, rps16, rpoB-trnC and trnL-trnF) and nuclear ribosomal external transcribed spacer (ETS) and internal transcribed spacer (ITS) regions. The phylogenetic distribution of key morphological characters is assessed. Bayesian inference (implemented in BEAST) is used to estimate the temporal origin of Ixora based on fossil evidence.

KEY RESULTS

Two separate lineages of Madagascan taxa are recovered, one of which is nested in a group of East African taxa. Divergence in Ixora is estimated to have commenced during the mid Miocene, with extensive cladogenesis occurring in the Afro-Madagascan clade during the Pliocene onwards.

CONCLUSIONS

Both lineages of Madagascan Ixora exhibit morphological innovations that are rare throughout the rest of the genus, including a trend towards pauciflorous inflorescences and a trend towards extreme corolla tube length, suggesting that the same ecological and selective pressures are acting upon taxa from both Madagascan lineages. Novel ecological opportunities resulting from climate-induced habitat fragmentation and corolla tube length diversification are likely to have facilitated species radiation on Madagascar.

Long prereproductive selection and divergence by depth in a Caribbean candelabrum coral

Long-lived corals, the foundation of modern reefs, often follow ecological gradients, so that populations or sister species segregate by habitat. Adaptive divergence maintains sympatric congeners after secondary contact or may even generate species by natural selection in the face of gene flow. Such ecological divergence, initially between alternative phenotypes within populations, may be aided by immigrant inviability, especially when a long period separates larval dispersal and the onset of reproduction, during which selection can sort lineages to match different habitats. Here, we evaluate the strength of one ecological factor (depth) to isolate populations by comparing the genes and morphologies of pairs of depth-segregated populations of the candelabrum coral Eunicea flexuosa across the Caribbean. Eunicea is endemic to the Caribbean and all sister species co-occur. Eunicea flexuosa is widespread both geographically and across reef habitats. Our genetic analysis revealed two depth-segregated lineages. Field survivorship data, combined with estimates of selection coefficients based on transplant experiments, suggest that selection is strong enough to segregate these two lineages. Genetic exchange between the Shallow and Deep lineages occurred either immediately after divergence or the two have diverged with gene flow. Migration occurs asymmetrically from the Shallow to Deep lineage. Limited recruitment to reproductive age, even under weak annual selection advantage, is sufficient to generate habitat segregation because of the cumulative prolonged prereproductive selection. Ecological factors associated with depth can act as filters generating strong barriers to gene flow, altering morphologies, and contributing to the potential for speciation in the sea.

Characterization of Linaria KNOX genes suggests a role in petal-spur development

Spurs are tubular outgrowths of perianth organs that have evolved iteratively among angiosperms. They typically contain nectar and often strongly influence pollinator specificity, potentially mediating reproductive isolation. The identification of Antirrhinum majus mutants with ectopic petal spurs suggested that petal-spur development is dependent on the expression of KNOTTED 1-like homeobox (KNOX) genes, which are better known for their role in maintaining the shoot apical meristem. Here, we tested the role of KNOX genes in petal-spur development by isolating orthologs of the A. majus KNOX genes Hirzina (AmHirz) and Invaginata (AmIna) from Linaria vulgaris, a related species that differs from A. majus in possessing long, narrow petal spurs. We name these genes LvHirz and LvIna, respectively. Using quantitative reverse-transcription PCR, we show that LvHirz is expressed at high levels in the developing petals and demonstrate that the expression of petal-associated KNOX genes is sufficient to induce sac-like outgrowths on petals in a heterologous host. We propose a model in which KNOX gene expression during early petal-spur development promotes and maintains further morphogenetic potential of the petal, as previously described for KNOX gene function in compound leaf development. These data indicate that petal spurs could have evolved by changes in regulatory gene expression that cause rapid and potentially saltational phenotypic modifications. Given the morphological similarity of spur ontogeny in distantly related taxa, changes in KNOX gene expression patterns could be a shared feature of spur development in angiosperms.

Genetic diversity of Aquilegia (Ranunculaceae) species and cultivars assessed by AFLPs

Species of the genus Aquilegia are exceptionally diverse in their floral morphology and color, commonly known as columbine. They are widely planted ornamentals and are highly attractive for hummingbirds. However, little is known about their genetic diversity. We examined the genetic diversity of the species and cultivars using amplified fragment length polymorphism (AFLP) markers. Sixteen EcoRI/MseI AFLP primer combinations produced 327 informative polymorphic bands, with a mean of 20.4 bands scored per primer. Jaccard's coefficient of similarity varied from 0.61 to 0.93, indicative of high levels of genetic variation. Cluster analysis using the unweighted pair group method with arithmetic mean algorithm placed the 64 accessions into two main clusters, each divided into two sub-clusters. The AFLP variability was significantly associated with the geographic origins, as the Asian species and the North American species grouped into two distinct clusters. The genetic diversity found among Aquilegia demonstrated the potential value of Chinese germplasm for cultivar improvement and for widening the genetic basis of breeding programs and breeding material selection. We concluded that AFLPs are informative and can provide significant insights for genetic diversity research in columbine species.

Intra-population comparison of vegetative and floral trait heritabilities estimated from molecular markers in wild Aquilegia populations

Measuring heritable genetic variation is important for understanding patterns of trait evolution in wild populations, and yet studies of quantitative genetic parameters estimated directly in the field are limited by logistic constraints, such as the difficulties of inferring relatedness among individuals in the wild. Marker-based approaches have received attention because they can potentially be applied directly to wild populations. For long-lived, self-compatible plant species where pedigrees are inadequate, the regression-based method proposed by Ritland has the appeal of estimating heritabilities from marker-based estimates of relatedness. The method has been difficult to implement in some plant populations, however, because it requires significant variance in relatedness across the population. Here, we show that the method can be readily applied to compare the ability of different traits to respond to selection, within populations. For several taxa of the perennial herb genus Aquilegia, we estimated heritabilities of floral and vegetative traits and, combined with estimates of natural selection, compared the ability to respond to selection of both types of traits under current conditions. The intra-population comparisons showed that vegetative traits have a higher potential for evolution, because although they are as heritable as floral traits, selection on them is stronger. These patterns of potential evolution are consistent with macroevolutionary trends in the European lineage of the genus.

Reproductive isolation and pollination success of rewarding Galearis diantha and non-rewarding Ponerorchis chusua (Orchidaceae)

BACKGROUND AND AIMS

Increasing evidence challenges the conventional perception that orchids are the most distinct example of floral diversification due to floral or prezygotic isolation. Regarding the relationship between co-flowering plants, rewarding and non-rewarding orchids in particular, few studies have investigated whether non-rewarding plants affect the pollination success of rewarding plants. Here, floral isolation and mutual effects between the rewarding orchid Galearis diantha and the non-rewarding orchid Ponerorchis chusua were investigated.

METHODS

Flowering phenological traits were monitored by noting the opening and wilting dates of the chosen individual plants. The pollinator pool and pollinator behaviour were assessed from field observations. Key morphological traits of the flowers and pollinators were measured directly in the field. Pollinator limitation and interspecific compatibility were evaluated by hand-pollination experiments. Fruit set was surveyed in monospecific and heterospecific plots.

KEY RESULTS

The species had overlapping peak flowering periods. Pollinators of both species displayed a certain degree of constancy in visiting each species, but they also visited other flowers before landing on the focal orchids. A substantial difference in spur size between the species resulted in the deposition of pollen on different regions of the body of the shared pollinator. Hand-pollination experiments revealed that fruit set was strongly pollinator-limited in both species. No significant difference in fruit set was found between monospecific plots and heterospecific plots.

CONCLUSIONS

A combination of mechanical isolation and incomplete ethological isolation eliminates the possibility of pollen transfer between the species. These results do not support either the facilitation or competition hypothesis regarding the effect of nearby rewarding flowers on non-rewarding plants. The absence of a significant effect of non-rewarding P. chusua on rewarding G. diantha can be ascribed to low levels of overlap between the pollinator pools of two species.

Genomic tools development for Aquilegia: construction of a BAC-based physical map

BACKGROUND

The genus Aquilegia, consisting of approximately 70 taxa, is a member of the basal eudicot lineage, Ranuculales, which is evolutionarily intermediate between monocots and core eudicots, and represents a relatively unstudied clade in the angiosperm phylogenetic tree that bridges the gap between these two major plant groups. Aquilegia species are closely related and their distribution covers highly diverse habitats. These provide rich resources to better understand the genetic basis of adaptation to different pollinators and habitats that in turn leads to rapid speciation. To gain insights into the genome structure and facilitate gene identification, comparative genomics and whole-genome shotgun sequencing assembly, BAC-based genomics resources are of crucial importance.

RESULTS

BAC-based genomic resources, including two BAC libraries, a physical map with anchored markers and BAC end sequences, were established from A. formosa. The physical map was composed of a total of 50,155 BAC clones in 832 contigs and 3939 singletons, covering 21X genome equivalents. These contigs spanned a physical length of 689.8 Mb (~2.3X of the genome) suggesting the complex heterozygosity of the genome. A set of 197 markers was developed from ESTs induced by drought-stress, or involved in anthocyanin biosynthesis or floral development, and was integrated into the physical map. Among these were 87 genetically mapped markers that anchored 54 contigs, spanning 76.4 Mb (25.5%) across the genome. Analysis of a selection of 12,086 BAC end sequences (BESs) from the minimal tiling path (MTP) allowed a preview of the Aquilegia genome organization, including identification of transposable elements, simple sequence repeats and gene content. Common repetitive elements previously reported in both monocots and core eudicots were identified in Aquilegia suggesting the value of this genome in connecting the two major plant clades. Comparison with sequenced plant genomes indicated a higher similarity to grapevine (Vitis vinifera) than to rice and Arabidopsis in the transcriptomes.

CONCLUSIONS

The A. formosa BAC-based genomic resources provide valuable tools to study Aquilegia genome. Further integration of other existing genomics resources, such as ESTs, into the physical map should enable better understanding of the molecular mechanisms underlying adaptive radiation and elaboration of floral morphology.

Aquilegia as a model system for the evolution and ecology of petals

The ranunculid genus Aquilegia holds extraordinary promise as a model system for investigating a wide range of questions relating to the evolution and ecology of petals. New genetic and genomic resources, including an extensive EST database, BAC libraries and physical maps, as well as virus-induced gene silencing are facilitating this research on multiple fronts. At the developmental genetic level, Aquilegia has been important for elucidating the developmental programme for specifying petals and petaloid characteristics. Data suggest that duplication events among the petal and stamen identity genes have resulted in sub- and neofunctionalization. This expansion of gene function does not include the petaloidy of Aquilegia sepals, however, which does not depend on the same loci that control identity of the second whorl petals. Of special interest is the elaboration of the petal into a nectar spur, a major innovation for the genus. Intra- and interspecific variation in the shape and colour of petals, especially the spurs, has been shown to be adaptative for different pollinators. Thus, understanding the genetic basis of these traits will help us connect the ecological interactions driving speciation with the genetic changes responsible for remodelling morphology. Progress in this area has focused on the multiple, parallel transitions in flower colour and nectar spur length across the genus. For flower colour, upstream transcription factors appear to be primarily targets of natural selection. Thus research in Aquilegia spans the initial evolution of petals and petaloidy to the diversification of petal morphology to the ecological basis of petal form, thereby providing a comprehensive picture of the evolutionary biology of this critical angiosperm feature.

Does human-induced habitat transformation modify pollinator-mediated selection? A case study in Viola portalesia (Violaceae)

Pollinator-mediated selection is one of the most important factors driving adaptation in flowering plants. However, as ecological conditions change through habitat loss and fragmentation, the interactions among species may evolve in new and unexpected directions. Human-induced environmental variation is likely to affect selection regimes, but as yet no empirical examples have been reported. In the study reported here, we examined the influence of human-induced habitat transformation on the composition of pollinator assemblages and, hence, pollinator-mediated selection on the flower phenotype of Viola portalesia (Violaceae). Our results indicate that pollinator assemblages differed substantially in terms of species composition and visitation rate between nearby native and transformed habitats. Similarly, the insect species that contributed most to visitation rates differed between plant populations. While the magnitude and sign of pollinator-mediated selection on flower length and width did not differ between sites, selection for flower number lost significance in the transformed habitat, and a significant pattern of disruptive selection for flower shape, undetected in the native habitat, was present in the transformed one. Overall, the results of this study suggest that human-induced habitat change may not only modify the species composition of pollinator assemblages, relaxing the selection process on some flower characters, but they may also create new opportunities for fitness-trait covariation not present in pristine conditions.

Genetic variation at nuclear loci fails to distinguish two morphologically distinct species of Aquilegia

Aquilegia formosa and pubescens are two closely related species belonging to the columbine genus. Despite their morphological and ecological differences, previous studies have revealed a large degree of intercompatibility, as well as little sequence divergence between these two taxa. We compared the inter- and intraspecific patterns of variation for 9 nuclear loci, and found that the two species were practically indistinguishable at the level of DNA sequence polymorphism, indicating either very recent speciation or continued gene flow. As a comparison, we also analyzed variation at two loci across 30 other Aquilegia taxa; this revealed slightly more differentiation among taxa, which seemed best explained by geographic distance. By contrast, we found no evidence for isolation by distance on a more local geographic scale. We conclude that the extremely low levels of genetic differentiation between A. formosa and A. pubescens at neutral loci will facilitate future genome-wide scans for speciation genes.

Evolution and coexistence of pollination ecotypes in an African Gladiolus (Iridaceae)

Pollinator-mediated selection has been suggested as a key driver of speciation in plants. We examined the potential role of hawkmoth pollinators in driving allopatric divergence and maintaining sympatric coexistence of morphotypes in the African iris Gladiolus longicollis. Floral tube length in this species varies from 35 mm to 130 mm across its geographic range and reflects the prevailing tongue lengths of local hawkmoth assemblages. The distribution of floral tube lengths is bimodal with two relatively discrete categories--long (about 90 mm) or short (about 50 mm)--that match the bimodal distribution of hawkmoth tongue lengths in eastern South Africa. At a contact site between these two floral morphs, we found few individuals of intermediate length, suggesting limited gene flow between morphs despite their interfertility. A difference in flowering phenology appears to be the main isolating barrier between morphs at this site. Long- and short-tubed morphs differed markedly in the chemical composition of their floral fragrance, a trait that could be used as a cue for morph-specific foraging by hawkmoths. Positive directional selection on tube length was found to occur in both morphs.

Identification of conserved Aquilegia coerulea microRNAs and their targets

Aquilegia is an emerging model organism that is phylogenetically intermediate between the core eudicot and monocot models, Arabidopsis and Oryza. In this study, we have used a comparative genomics approach to identify 45 Aquilegia microRNAs that comprise 20 separate plant microRNA families. We have predicted 84 targets of these newly identified Aquilegia microRNAs including transcription factors and loci involved in metabolism, stress responses, transport, and auxin signaling. microRNA families from 16 plant species and the newly identified microRNAs from Aquilegia were analyzed in a phylogenetic context revealing 40 distantly conserved microRNA families. In addition to these highly conserved plant microRNA families, several families with disjointed phylogenetic distribution were identified. This study provides a phylogenetically important dataset for plant microRNA evolution studies. The current study is the first to identify miRNAs in a lower eudicot in which comprehensive genomic resources are becoming available.

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

BACKGROUND AND AIMS

A South American cactus species, Echinopsis ancistrophora (Cactaceae), with dramatic among-population variation in floral traits is presented.

METHODS

Eleven populations of E. ancistrophora were studied in their habitats in northern Argentina, and comparisons were made of relevant floral traits such as depth, stigma position, nectar volume and sugar concentration, and anthesis time. Diurnal and nocturnal pollinator assemblages were evaluated for populations with different floral trait combinations.

KEY RESULTS

Remarkable geographical variations in floral traits were recorded among the 11 populations throughout the distribution range of E. ancistrophora, with flower lengths ranging from 4.5 to 24.1 cm. Other floral traits associated with pollinator attraction also varied in a population-specific manner, in concert with floral depth. Populations with the shortest flowers showed morning anthesis and those with the longest flowers opened at dusk, whereas those with flowers of intermediate length opened at unusual times (2300-0600 h). Nectar production varied non-linearly with floral length; it was absent to low (population means up to 15 microL) in short- to intermediate-length flowers, but was high (population means up to 170 microL) in the longest tubed flowers. Evidence from light-trapping of moths, pollen carriage on their bodies and moth scale deposition on stigmas suggests that sphingid pollination is prevalent only in the four populations with the longest flowers, in which floral morphological traits and nectar volumes match the classic expectations for the hawkmoth pollination syndrome. All other populations, with flowers 4.5-15 cm long, were pollinated exclusively by solitary bees.

CONCLUSIONS

The results suggest incipient differentiation at the population level and local adaptation to either bee or hawkmoth (potentially plus bee) pollination.

Aquilegia: a new model for plant development, ecology, and evolution

The lower eudicot genus Aquilegia holds enormous potential for investigating aspects of development, ecology, and evolution that are otherwise unrepresented among existing model systems. Its evolutionary history is of particular interest because it represents a phylogenetic midpoint between models such as Arabidopsis and Oryza but, at the same time, has experienced a recent adaptive radiation within the genus. To take advantage of these features, a collaborative group has developed a number of genetic and genomic resources for Aquilegia that have facilitated the study of its distinct morphology. This work has demonstrated that although the petaloid sepals of Aquilegia do not depend on B-class genes for their identity, these loci do control development of the petals, stamens, and novel staminodium. Overall, Aquilegia stands as a key example of the potential utility and speed of developing new genetic model systems.

Genetic and morphological patterns show variation in frequency of hybrids between Ipomopsis (Polemoniaceae) zones of sympatry

Variation in rates of hybridization among zones of sympatry between a pair of species provides a useful window into the effect of local conditions on the evolution of reproductive isolation. We employed floral morphological traits and neutral genetic markers to quantify the frequency of individuals intermediate to the two parental species in two zones of sympatry between Ipomopsis aggregata and I. tenuituba, using clustering methods that make no a priori assumptions about population structure. The sites differed not only in the frequency of intermediate individuals, but also in climate, pollinator abundance and behavior and spatial structure of plant populations. Both floral traits, which are likely to be under natural selection and molecular markers, which are quasi-neutral, indicated more population structure at one site than the other, the pattern being more pronounced for floral morphology. One likely explanation for this difference between sites is that local ecological conditions, particularly pollinator choice of flowers, have promoted different rates of hybridization between these species. Hence, the evolution of reproductive isolation might depend in part on local conditions, and thus differ among populations of the same pair of species.

Is floral diversification associated with pollinator divergence? Flower shape, flower colour and pollinator preference in Chilean Mimulus

BACKGROUND AND AIMS

Adaptation to different pollinators is thought to drive divergence in flower colour and morphology, and may lead to interspecific reproductive isolation. Floral diversity was tested for association with divergent pollinator preferences in a group of four closely related wildflower species: the yellow-flowered Mimulus luteus var. luteus and the red-pigmented M. l. variegatus, M. naiandinus and M. cupreus.

METHODS

Patterns of pollinator visitation were evaluated in natural plant populations in central Chile, including both single-species and mixed-species sites. Floral anthocyanin pigments were identified, and floral morphology and nectar variation were quantified in a common garden experiment using seeds collected from the study sites.

KEY RESULTS

Mimulus l. luteus, M. l. variegatus and M. naiandinus are morphologically similar and share a single generalist bumblebee pollinator, Bombus dahlbomii. Mimulus cupreus differs significantly from the first three taxa in corolla shape as well as nectar characteristics, and had far fewer pollinator visits.

CONCLUSIONS

This system shows limited potential for pollinator-mediated restriction of gene flow as a function of flower colour, and no evidence of transition to a novel pollinator. Mimulus cupreus may experience reduced interspecific gene flow due to a lack of bumblebee visitation, but not because of its red pigmentation: rare yellow morphs are equally undervisited by pollinators. Overall, the results suggest that factors other than pollinator shifts may contribute to the maintenance of floral diversity in these Chilean Mimulus species.