Variation In Flower Size Research Articles

‘Junk’ DNA and phenotypic evolution inSilenesectionSiphonomorpha

One of the long-standing mysteries in genomic evolution is the observation that much of the genome is composed of repetitive DNA, resulting in inter- and intraspecific variation in nuclear DNA content. Our discovery of a negative correlation between nuclear DNA content and flower size in Silene latifolia has been supported by our subsequent investigation of changes in DNA content as a correlated response to selection on flower size. Moreover, we have observed a similar trend across a range of related dioecious species in Silene sect. Elisanthe. Given the presence of sex chromosomes in dioecious Silene species, and the tendency of sex chromosomes to accumulate repetitive DNA, it seems plausible that dioecious species undergo genomic evolution in ways that differ from what one might expect in hermaphroditic species. Specifically, we query whether the observed relationship between nuclear DNA content and flower size observed in dioecious Silene is a peculiarity of sex chromosome evolution. In the present study we investigated nuclear DNA content and flower size variation in hermaphroditic species of Silene sect. Siphonomorpha, as close relatives of the dioecious species studied previously. Although the nuclear DNA contents of these species were lower than those for species in sect. Elisanthe, there was still significant intra- as well as interspecific variation in nuclear DNA content. Flower size variation was found among species of sect. Siphonomorpha for petal claw and petal limb lengths, but not for calyx diameter. This last trait varies extensively in sect. Elisanthe, in part due to sex-specific selection. A negative correlation with nuclear DNA content was found across populations for petal limb length, but not for other floral dimensions. We conclude that impacts of nuclear DNA content on phenotypic evolution do manifest themselves in hermaphroditic species, so that the effects observed in sect. Elisanthe, and particularly in S. latifolia, while perhaps amplified by the genomic impacts of sex chromosomes, are not limited to dioecious taxa.

The reproductive assurance benefit of selfing: importance of flower size and population size

Autonomous selfing can provide reproductive assurance (RA) for flowering plants that are unattractive to pollinators or in environments that are pollen limited. Pollen limitation may result from the breakdown of once-continuous habitat into smaller, more isolated patches (habitat fragmentation) if fragmentation negatively impacts pollinator populations. Here we quantify the levels of pollen limitation and RA among large and small populations of Collinsia parviflora, a wildflower with inter-population variation in flower size. We found that none of the populations were pollen limited, as pollen-supplemented and intact flowers did not differ in seed production. There was a significant effect of flower size on RA; intact flowers (can self) produced significantly more seeds than emasculated flowers (require pollen delivery) in small-flowered plants but not large-flowered plants. Population size nested within flower size did not significantly affect RA, but there was a large difference between our two replicate populations for large-flowered, small populations and small-flowered, large populations that appears related to a more variable pollination environment under these conditions. In fact, levels of RA were strongly negatively correlated with rates of pollinator visitation, whereby infrequent visitation by pollinators yielded high levels of RA via autonomous selfing, but there was no benefit of autonomous selfing when visitation rates were high. These results suggest that autonomous selfing may be adaptive in fragmented habitats or other ecological circumstances that affect pollinator visitation rates.

Testing the rare-alleles model of quantitative variation by artificial selection.

The rare-alleles model of quantitative variation posits that a common allele (the 'wild-type') and one or more rare alleles segregate at each locus affecting a quantitative trait; a scenario predicted by several distinct evolutionary hypotheses. Single locus arguments suggest that artificial selection should substantially increase the genetic variance (Vg) if the rare-alleles model is accurate. This paper tests the 'DeltaVg prediction' using a large artificial selection experiment on flower size of Mimulus guttatus. Vg for flower size does evolve, increasing with selection for larger flower while decreasing in the other direction. These data are consistent with a model in which flower size variation is caused by rare, partially dominant alleles. However, this explanation becomes increasingly tenuous when considered with other data (correlated responses to selection and the effects of inbreeding). A combination of modern (marker-based mapping) and classical (biometric) techniques will likely to be required to determine the distribution of allele frequencies at loci influencing quantitative traits.

DELETERIOUS MUTATIONS AND GENETIC VARIATION FOR FLOWER SIZE IN MIMULUS GUTTATUS

Abstract.— Mimulus guttatus is a wildflower that exhibits substantial genetic variation in flower size. Here, we test the hypothesis that this variation is caused by deleterious mutations maintained through mutation-selection balance. The deleterious-mutation model predicts that rare, partially recessive alleles will be the primary source of variation. We test this prediction by measuring the change in the mean flower size (ΔM) and the directional dominance of flower size (ΔB) within a selection experiment. If variation is due to rare (partially) recessive alleles, ΔB/ΔM is expected to be positive and exceed one. However, we obtain negative values for ΔB/ΔM from three independent selection lines. This result is statistically inconsistent with the deleterious-mutation model.

Correlated variation of floral and leaf traits along a moisture availability gradient

Variation in flower size is an important aspect of a plant's life history, yet few studies have shown how flower size varies with environmental conditions and to what extent foliar responses to the environment are correlated with flower size. The objectives of this study were to (1) develop a theoretical framework for linking flower size and leaf size to their costs and benefits, as assessed using foliar stable carbon isotope ratio (delta(13)C) under varying degrees of water limitation, and then (2) examine how variation in flower size within and among species growing along a naturally occurring moisture availability gradient correlates with variation in delta(13)C and leaf size. Five plant species were examined at three sites in Oregon. Intra- and inter-specific patterns of flower size in relation to moisture availability were the same: the ratios of the area of flower display to total leaf area and of individual flower area to leaf area were greater at sites with more soil moisture compared to those sites with less soil moisture. The increase in flower area per unit increase in leaf area was greater at sites with more soil moisture than at sites where water deficit can occur. Values of delta(13)C, an index of water-use efficiency, were greater for plants with larger floral size. The patterns we observed generalize across species, irrespective of overall plant morphology or pollination system. These correlations between flower size, moisture availability, and delta(13)C suggest that water loss from flowers can influence leaf responses to the environment, which in turn may indirectly mediate an effect on flower size.

Cost and benefits of attractive floral traits in the annual species Madia sativa (Asteraceae)

The maintenance of flower size variation within populations might be explained by conflicting selection pressures on floral traits that may involve bio- logical agents, such as mutualists and antagonists, and allocation costs associated with floral display. The annual species Madia sativa (Asteraceae) exhibits ample variation in the number of ray florets in natural populations. This field study aimed at evaluating the costs and benefits associated with floral traits in M. sativa .I n particular, we addressed two main questions: (1) Is the number of ray florets posi- tively associated with pollinator visitation rate? (2) Is there a fitness cost of ray floret maintenance when pollinators are absent? We detected one benefit of conspicuous ray capitula: a strong preference by insect pollinators. We also confirmed the occurrence of costs: when pollinators were excluded conspicuous ray capitula had a reduced reproductive assurance via autogamous selfing, and there were trade-offs between the number of ray florets and seed mass and seed germination. Results suggest that the maintenance of within-population variation in the number of ray florets in M. sativa is explained, at least in part, by the balance between costs and benefits associated with this floral trait.

The influence of floral symmetry and pollination systems on flower size variation

We compared the amount of variation in flower size between autogamous and insect‐pollinated species to examine the hypothesis that pollinator‐mediated selection stabilizes flower size in plant populations. One would expect the flower size variation to be larger in selfing species that are less affected by pollinator‐mediated stabilizing selection than in insect‐pollinated species. The results of phylogenetic comparisons between autogamous and insect‐pollinated flowers supported the pollinator‐mediated stabilizing selection hypothesis, although the non‐phylogenetic comparison did not. According to our results, we discuss the factors influencing the flower size variation.

Breeding system of Macromeria viridiflora (Boraginaceae) and geographic variation in pollinator assemblages

This study explores the association between variation in pollinator type and flower size in Macromeria viridiflora (Boraginaceae) by studying the breeding system of the plant and the pollinator effectiveness of floral visitors. Studies were conducted at two sites where plants differ in flower size and floral visitors. Breeding system studies showed that while plants are self-compatible and occasionally produce seed autogamously, pollinators are important for reproductive success in the plants. However, plants are not pollinator-limited at these sites. Combining visitation rate and pollen deposition as measures of pollinator effectiveness, I found hummingbirds to be the most effective pollinators at both sites. Although hawkmoths also pollinate the flowers, they visit the flowers less frequently and, at one of the two sites, deposit less pollen. These results are consistent with the hypothesis that geographic variation in corolla size is the result of selection by different hummingbird species.

The relationship between flower size, inbreeding coefficient and inferred selfing rate in British Euphrasia species.

The genus Euphrasia in Britain comprises a taxonomically complex group of self-compatible, morphologically similar, hemi-parasitic, annual plant species of high conservation importance. The 19 diploid and tetraploid taxa currently recognised show striking variation in flower size. The objective of this paper is to determine whether a relationship exists between flower size and breeding system within Euphrasia. Following a survey of flower size variation among the 19 taxa, seven diploid populations, encompassing a broad range of flower sizes, were selected for detailed study. Four nuclear microsatellite loci were used to estimate the inbreeding coefficient Fis within each population. Fis values varied from to 0.17-0.77 and showed a significant, negative correlation with flower size. These results are best explained as the consequence of variation in selfing rate among the Euphrasia populations, with selfing rate increasing as flower size decreases. The potential factors influencing breeding system evolution in Euphrasia are discussed, together with the role of autogamy in generating taxonomic complexity and facilitating lineage differentiation within the genus.

Flower size dimorphism in diclinous plants native to La R�union Island

Although many diclinous plants have a flower size dimorphism, the causes and ecological correlates of dimorphism in flower size remain poorly understood. In this paper we quantify the frequency and distribution of flower size dimorphism in the native flora of La Reunion Island. The frequency of flower size dimorphism is lower than in a previous global study suggesting that the dimorphism may limit colonisation success. Flower size dimorphism is significantly more frequent in endemic taxa compared to indigenous taxa, a trend that is particularly evident at the species level, due to the large number of species with male flowers bigger than female flowers and is more common in species-rich lineages. These results suggest that flower size dimorphism may be associated with species divergence. The direction of dimorphism varies between dioecious and monoecious species, suggesting that the mechanisms acting on flower size variation differ in species with these different sexual systems.

Reproductive assurance varies with flower size in Collinsia parviflora (Scrophulariaceae)

A central question in plant evolutionary ecology is how mixed mating systems are maintained in the face of selection against self-pollination. Recently, attention has focused on the potential reproductive assurance (RA) benefit of selfing: the ability to produce seeds via autonomous selfing when the potential for outcrossing is reduced or absent. To date, there is little experimental support for this benefit under natural pollination conditions. In addition, the RA hypothesis has not been tested experimentally in a species displaying morphological variation for traits expected to influence the mating system, such as flower size, which affects both attractiveness to pollinators and ability to self autonomously. Here, we document significant among-population variation in flower size in Collinsia parviflora and show that pollinators preferred large flowers over small flowers in experimental arrays. The pollinator community varied among three study sites, and two small-flowered populations had lower pollinator visitation rates than one large-flowered population. We compared seed production between intact flowers (can self) and experimentally emasculated flowers (require a pollinator) on large- and small-flowered plants. As predicted by the RA hypothesis, small-flowered plants show a greater RA benefit of selfing than large-flowered plants; emasculated, small flowers produced very few seeds, relative to intact, small flowers or either emasculated or intact, large flowers. We also show that the RA benefit is pollination-context dependent, differing between small- and large-flowered test sites, likely due to a combination of pollinator discrimination against small flowers and differences between test sites in the pollinator community. This paper is the first experimental evidence showing a trait-dependent RA benefit of selfing under natural pollination conditions.

Large variation in flower size of the myco‐heterotrophic plant, Monotropastrum globosum: effect of floral display on female reproductive success

Large variation in flower size of the myco‐heterotrophic plant, <i>Monotropastrum globosum</i>: effect of floral display on female reproductive success

Are there predictable clines in plant–pollinator interactions along altitudinal gradients? The example of Cytisus scoparius (L.) Link in the Sierra de Guadarrama (Central Spain)

Abstract. We analyse the hypothesis that predictable gradients occur in plant–pollinator interactions along altitudinal gradients due to thermal constraints on insect body size and floral traits being selected for by pollinators. Pollinators’ size should follow Bergmann's rule in mountains (larger body size at higher altitudes) and selection should, in turn, lead to larger flower size at higher altitudes. The study focuses on the flower and pollinator size variation in 11 Cytisus scoparius populations located between 680 and 1300 m a.s.l., and on the relationship between flower size and pollination success of plants in each population. Significant differences among populations were found in flower size, pollinator size and fruiting success (anova, P &lt; 0.001 in all cases). Regression models show that (i) pollinator size increases significantly with altitude, (ii) a parallel quadratic increase in flower size and (iii) a quadratic decrease in pollination success with altitude. Moreover, there is a tendency of plants with smaller flowers in each population to have a higher percentage of flowers initiating development into fruits (ancova, P &lt; 0.05). All these observations support the presence of the predicted gradient in the studied species and they suggest the possibility of it to be somewhat common among plants that occupy large altitudinal ranges due to the simple basis on which it could be rooted.

The importance of phylogeny and ecology in microgeographical variation in the morphology of four Canarian species of Aeonium (Crassulaceae)

The relative importance of natural selection in the diversification of organisms can be assessed indirectly using matrix correspondence. The present study determines the environmental and genetic correlates of microgeographical variation in the growth form, leaf form and flower morphology in populations of four Aeonium species from section Leuconium using partial regression methods. The phylogeny of the four species and the other 12 species in the section was deduced from amplified fragment length polymorphism (AFLP). Pubescence of floral organs and flower size correlate with the phylogeny while traits related to growth form, leaf form, flower construction and inflorescence size correlate with ecological factors. The variation in the latter four traits may therefore reflect selection by current ecological conditions while variation in pubescence and flower size may reflect historical events like neutral mutations, founder events and drift. Additionally, the morphological analyses revealed a large amount of variation in all traits within populations. This suggests a possible influence of microhabitat on the variation in morphology of Aeonium in the Canary Islands.

Genetic variation for sexual dimorphism in flower size within and between populations of gynodioecious Thymus vulgaris

Abstract There has been very little empirical study of quantitative genetic variation in flower size in sexually dimorphic plant species, despite the frequent occurrence of flower size differences between sexual phenotypes. In this study we quantify the nature of quantitative flower size variation in females and hermaphrodites of gynodioecious Thymus vulgaris. In a field study, females had significantly smaller flowers than hermaphrodites, and the degree of flower size dimorphism varied significantly among populations. To quantify the genetic basis of flower size variation we sampled maternal progeny from 10 F0 females in three populations (across the range of variation in flower size in the field), performed controlled crosses on F1 offspring in the glasshouse and grew F2 progeny to flowering in uniform field conditions. A significant population * sex interaction was again observed, hence the degree of sexual dimorphism shows genetic variation among populations. A significant family * sex interaction was also observed, indicating that the degree of sexual dimorphism shows genetic variation among families. Females showed significantly greater variation among populations and among families than hermaphrodites. Female flower size varied significantly depending on the degree of stamen abortion, with morphologically intermediate females having flowers more similar to hermaphrodites than to other females. The frequency of female types that differ in the degree of stamen abortion varied among populations and families and mean family female flower size increased as the proportion of intermediate female types increased across families. Variation in the degree of flower size dimorphism thus appears to be a result of variation in the degree of stamen abortion in females, the potential causes of which are discussed.

Natural enemies and sex: how seed predators and pathogens contribute to sex-differential reproductive success in a gynodioecious plant.

In insect-pollinated plants flowers must balance the benefits of attracting pollinators with the cost of attracting natural enemies, when these respond to floral traits. This dilemma can have important evolutionary consequences for mating-system evolution and polymorphisms for floral traits. We investigate the benefits and risks associated with flower size and sex morph variation in Dianthus sylvestris, a gynodioecious species with pistillate flowers that are much smaller than perfect flowers. We found that this species is mainly pollinated by nocturnal pollinators, probably moths of the genus Hadena, that also oviposit in flowers and whose caterpillars feed on developing fruits and seeds. Hadena preferred larger flowers as oviposition sites, and flowers in which Hadena had deposited eggs bore more pollen on their stigmas, suggesting that Hadena is indeed the principle pollinator, or that pollinators and these seed predators employ the same choice criteria for flowers. Globally, perfect flowers suffered more predation by seed predators than did pistillate flowers, suggesting that seed predators play an important role in reproductive system dynamics in this species. On the other hand, female flowers were more likely to be contaminated with spores of another natural enemy, the pathogenic fungus Microbotryum violaceum, that are transmitted by pollinating insects. This complex interplay between two natural enemies, one of which, Hadena, is also a pollinator and vector of fungal spores, may contribute to the maintenance of the polymorphic reproductive system of the host plant.

The Variability of Organs Differentially Involved in Pollination, and Correlations of Traits in Genisteae (Leguminosae: Papilionoideae)

Size differences in organs involved to varying degrees in pollen exchange (standard and keel petals, calyx, pedicel and inflorescence stem diameter), together with correlations among these traits, were investigated at both the intraspecific and interspecific levels in populations of 15 southern Spanish species of the papilionoid legume tribe Genisteae. Characteristics less related to pollination (e.g. pedicel length and stem diameter) were roughly twice as variable between individuals as petal or calyx dimensions. The coefficients of variation were in the range 16–19%vs. 7–10%, respectively. Standard and keel petal size varied in parallel, not only in conspecific individuals but also across species and genera, suggesting a tight developmental relationship between these organs and a low potential for evolutionary modification. The remaining characteristics covaried in different ways within and between species and thus appeared much less mutually constrained (particularly so for calyx size). Multivariate analyses of variance revealed significant population divergence in five out of six species. Overall, 28% of all variation in flower size occurred among populations and 72% within populations, but the proportion of variance accounted for by population membership varied extensively across species (from 1 to 64%) and was highest in two selfing annuals (Lupinus). Outcrossing perennials may show either high (Stauracanthus, Genista) or low (Cytisus) population differentiation. In addition to contributing to the increasing amount of data on quantitative floral variation in plants, results of this study may help understand how pleiotropic effects limit the evolutionary modification of some flower traits more than others in diverse plant groups.

The allometry of flower size dimorphism in dioecious Dombeya species on La Reunion

In this paper we examine whether variation in male and female flower size follows an allometric relation for 10 dioecious species in the genus Dombeya endemic to La Réunion. Male flowers are significantly larger than female flowers and the degree of dimorphism varies significantly across species. There is a significant allometric relationship between male and female flower size: as mean flower size decreases the degree of flower size dimorphism increases. This is the first study to document an allometric relationship between male and female flower size in unisexual plants. We discuss this allometric relationship in relation to two characteristics frequently observed in dioecious flowers: flower size dimorphism is common in unisexual plants and dioecious species often have smaller flowers than hermaphrodites.

DELETERIOUS MUTATIONS AND GENETIC VARIATION FOR FLOWER SIZE IN MIMULUS GUTTATUS

Mimulus guttatus is a wildflower that exhibits substantial genetic variation in flower size. Here, we test the hypothesis that this variation is caused by deleterious mutations maintained through mutation-selection balance. The deleterious-mutation model predicts that rare, partially recessive alleles will be the primary source of variation. We test this prediction by measuring the change in the mean flower size (deltaM) and the directional dominance of flower size (deltaB) within a selection experiment. If variation is due to rare (partially) recessive alleles, deltaB/deltaM is expected to be positive and exceed one. However, we obtain negative values for deltaB/deltaM from three independent selection lines. This result is statistically inconsistent with the deleterious-mutation model.

Floral Display in Narcissus: Variation in Flower Size and Number at the Species, Population, and Individual Levels.

Floral display (the size, number, and arrangement of open flowers) influences pollinator visitation to animal-pollinated plants and should be an important determinant of reproductive success. We examined variation in the size and number of open flowers in wild daffodils (Narcissus). Our analysis of published data on 45 taxa showed that flower number varied negatively with flower diameter among Narcissus species, which supports the widespread assumption that there is a trade-off between these traits. In contrast, field measurements indicated a positive relation between flower number and diameter within two populations of Narcissus dubius, and no relation was evident after we controlled for variation in bulb size. The discrepancy between inter- and intraspecific patterns may have occurred because variable resource levels obscure trade-offs when variation in flower size is low (e.g., within species). Size-related increases in floral tube length were half as great as corresponding increases in flower diameter, a result that is consistent with stronger stabilizing selection on tube length. Staggered flowering within N. dubius inflorescences limited the mean number of open flowers to <66% of total flower number, and slow expansion by later opening flowers resulted in significant differences in flower size throughout flowering. Although pollinators preferred large flowers, experimental reductions in flower diameter did not affect seed production. Our results illustrate how the relative importance of the factors influencing floral display can vary among levels of biological organization. Interspecific variation in flower size and number appeared to be constrained by allocation trade-offs, but intraspecific variation in both traits was more greatly influenced by plant resource status. Within plants, the size and number of open flowers reflected the relative age of individual flowers and floral longevity.