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Selected Abstracts

Pollination ecology of Isoglossa woodii, a long-lived, synchronously monocarpic herb from coastal forests in South Africa

PLANT BIOLOGY, Issue 3 2010
M. E. Griffiths
Abstract Synchronous monocarpy in long-lived plants is often associated with pollination by wind, in part because infrequent mass flowering may satiate pollinators. Selfing in synchronous monocarps may provide reproductive assurance but conflict with the benefits of outcrossing, a key evolutionary driver of synchrony. We predicted that animal-pollinated species with synchronous flowering would have unspecialised flowers and attract abundant generalised pollinators, but predictions for selfing and outcrossing frequencies were not obvious. We examined the pollination biology of Isoglossa woodii (Acanthaceae), an insect-pollinated, monocarpic herb that flowers synchronously at 4,7-year intervals. The most frequent visitor to I. woodii flowers was the African honeybee, Apis mellifera adansonii. Hand-pollination failed to enhance seed production, indicating that the pollinators were not saturated. No seed was set in the absence of pollinators. Seed set was similar among selfed and outcrossed flowers, demonstrating a geitonogamous mixed-mating strategy with no direct evidence of preferential outcrossing. Flowers contained four ovules, but most fruits only developed one seed, raising the possibility that preferential outcrossing occurs by post-pollination processes. We argue that a number of the theoretical concerns about geitonogamous selfing as a form of reproductive assurance do not apply to a long-lived synchronous monocarp such as I. woodii. [source]

The Reproductive Biology of Two Species of Diplusodon Pohl (Lythraceae) from Serra do Cipó, Southeastern Brazil

PLANT BIOLOGY, Issue 6 2000
C. M. Jacobi
Abstract: The reproductive biology of two shrub species of Diplusodon (Lythraceae), D. hirsutus and D. orbicularis, was studied. Their distribution is restricted, mainly associated with "campos rupestres", a montane biome in SE Brazil. Both species proved to be self-compatible but because of herkogamy they are greatly dependent on visitors to effect pollination. The main pollinators are medium-sized bees. In experiments where pollinators were excluded or discouraged the percentage of abscised fruits was total or extremely high; most of these fruits dropped within the first two weeks after pollination in D. orbicularis and three weeks in D. hirsutus. The percentage of surviving fruits from control or manual pollination was higher in D. orbicularis (ca. 70 %) than in D. hirsutus (ca. 50 %). Seed set was very variable, with slightly lower means in D. hirsutus. Within each species the mean seed set was similar in self-, cross-pollinated fruits, and controls. In both species, the similarity between control and manual pollination fruit survival curves suggests that factors other than pollen limitation are the cause of abortion in the study area. The high rate of abscission, which was also observed in unmanipulated fruits, was attributed to resource limitation. In D. hirsutus this was caused by drought during seed development. In D. orbicularis it was enhanced by nutrient drain by shoot-galling flies. The lack of selective abortion of either selfed fruits or fruits with a small number of seeds may be a useful mechanism to overcome this detrimental effect which is felt later during fruit development. [source]

Reproduction in Wild Populations of the Threatened Tree Macadamia tetraphylla: Interpopulation Pollen Enriches Fecundity in a Declining Species

BIOTROPICA, Issue 3 2009
Philip C. Pisanu
ABSTRACT Macadamia tetraphylla is a subtropical rain forest tree from fragmented lowlands in eastern Australia. Owing to habitat loss and fragmentation, this commercially important species is vulnerable to extinction. Breeding system and fecundity were investigated in nine populations incorporating three habitat types (moderately disturbed, highly disturbed, and intact) to determine if seed set, seed weight, and genetic diversity are compromised by disturbance. Breeding success was also tested using pollen donors from distant (30,100 km), local (2,3 km), neighbor (10,20 m), and near-neighbor (< 10 m) sources. Macadamia tetraphylla is weakly self-compatible but incapable of automatic self-pollination. Across populations, seed to flower ratios were always < 0.1 percent in open-pollinated trees and trees from moderately disturbed habitats had the highest fruit production. Outcross pollen produced more seed per raceme than open-pollinated or self-pollination treatments. Seed set and seed weights were positively influenced by pollen source with local pollen and distant pollen effecting more or heavier seeds. Germination rates and genetic diversity did not vary significantly in seedlings from different pollen sources. Results suggest a pollen source from at least a 2 km distance is an optimal outbreeding distance; however, many wild populations do not have conspecifics at optimal distances owing to habitat fragmentation. Highly disturbed populations are producing seed but the longevity of these sites is threatened by weed invasions. We conclude that small populations in degraded habitats that are at risk of being overlooked should not be ignored but should be a focus for restoration efforts as they are a valuable asset for the conservation of M. tetraphylla. [source]

Population Size, Genetic Variation, and Reproductive Success in a Rapidly Declining, Self-Incompatible Perennial (Arnica montana) in The Netherlands

CONSERVATION BIOLOGY, Issue 6 2000
Sheila H. Luijten
In 26 populations in The Netherlands we investigated the relationship between population size and genetic variation using allozyme markers. Genetic variation was low in A. montana ( He = 0.088). There were positive correlations between population size and the proportion of polymorphic loci, the number of effective alleles, and expected heterozygosity, but not with observed heterozygosity. There was a significantly positive correlation between population size and the inbreeding coefficient. Generally, small populations showed heterozygote excess, which decreased with increasing population size. Possibly, the heterozygous individuals in small populations are survivors from the formerly larger populations with relatively high fitness. The F statistics showed a moderately high level of differentiation among populations ( FST = 0.140 ± 0.02), implying a low level of gene flow. For three out of four allozyme loci, we found significant inbreeding ( FIS = 0.104 ± 0.03). Only 14 of 26 populations were in Hardy-Weinberg equilibrium at all four polymorphic loci. In a subset of 14 populations of various size, we investigated natural seed production and offspring fitness. Population size was positively correlated with seed set, seedling size, number of flowering stems and flowerheads, adult survival, and total relative fitness, but not with the number of florets per flowerhead, germination rate, or the proportion of germination. Offspring performance in the greenhouse was not associated with genetic diversity measured on their mothers in the field. We conclude that the fitness of small populations is significantly reduced, but that there is as yet no evidence that this was caused by inbreeding. Possibly, the self-incompatibility system of A. montana has been effective in reducing selfing rates and inbreeding depression. Resumen:Arnica montana es una especie de planta rara, en declinación rápida y autoincompatible. En 26 poblaciones de los Países Bajos investigamos la relación entre el tamaño poblacional y la variación genética mediante el uso de alozimas marcadoras. La variación genética fue baja en A. montana ( He = 0.088). Existió una correlación positiva entre el tamaño poblacional y la proporción de emplazamientos polimórficos, el número de alelos efectivos y la heterocigocidad esperada, pero no con la heterocigocidad observada. Existió una correlación positiva significativa entre el tamaño poblacional y el coeficiente de endogamia. Generalmente, las poblaciones pequeñas mostraron una heterocigocidad excesiva con disminuciones en el tamaño poblacional. Posiblemente, los individuos heterocigóticos de poblaciones pequeñas son sobrevivientes de poblaciones anteriormente grandes con una adaptabilidad relativamente alta. Las pruebas de F mostraron un nivel de diferenciación moderadamente alto entre poblaciones ( FST = 0.140 ± 0.02) lo que implica un nivel bajo de flujo de genes. Para tres de cuatro de los emplazamientos de alozimas encontramos una endogamia significativa ( FIS = 0.104 ± 0.03). Solamente 14 de las 26 poblaciones estuvieron en equilibrio Hardy-Weinberg para los cuatro emplazamientos polimórficos. En un subconjunto de 14 poblaciones de varios tamaños, investigamos la producción natural de semillas y la adaptabilidad de la descendencia. El tamaño poblacional estuvo positivamente correlacionado con el juego de semillas, el tamaño del almácigo, el número de tallos en flor y de inflorescencias, la supervivencia de adultos y la adaptabilidad total relativa, pero no con el número de flores por inflorescencia, la tasa de germinación ni la proporción de la germinación. El rendimiento de la descendencia en invernaderos no estuvo asociado con la diversidad genética medida en sus madres en el campo. Concluimos que la adaptabilidad de poblaciones pequeñas está significativamente reducida, pero no existe aún evidencia de que esto sea ocasionado por endogamia. Es posible que el sistema de autoincompatibilidad de A. montana haya sido efectivo en la reducción de tasas de autofecundación y depresión de la endogamia. [source]

Pollinator genetics and pollination: do honey bee colonies selected for pollen-hoarding field better pollinators of cranberry Vaccinium macrocarpon?

ECOLOGICAL ENTOMOLOGY, Issue 2 2001
James H. Cane
Summary 1. Genetic polymorphisms of flowering plants can influence pollinator foraging but it is not known whether heritable foraging polymorphisms of pollinators influence their pollination efficacies. Honey bees Apis mellifera L. visit cranberry flowers for nectar but rarely for pollen when alternative preferred flowers grow nearby. 2. Cranberry flowers visited once by pollen-foraging honey bees received four-fold more stigmatic pollen than flowers visited by mere nectar-foragers (excluding nectar thieves). Manual greenhouse pollinations with fixed numbers of pollen tetrads (0, 2, 4, 8, 16, 32) achieved maximal fruit set with just eight pollen tetrads. Pollen-foraging honey bees yielded a calculated 63% more berries than equal numbers of non-thieving nectar-foragers, even though both classes of forager made stigmatic contact. 3. Colonies headed by queens of a pollen-hoarding genotype fielded significantly more pollen-foraging trips than standard commercial genotypes, as did hives fitted with permanently engaged pollen traps or colonies containing more larvae. Pollen-hoarding colonies together brought back twice as many cranberry pollen loads as control colonies, which was marginally significant despite marked daily variation in the proportion of collected pollen that was cranberry. 4. Caloric supplementation of matched, paired colonies failed to enhance pollen foraging despite the meagre nectar yields of individual cranberry flowers. 5. Heritable behavioural polymorphisms of the honey bee, such as pollen-hoarding, can enhance fruit and seed set by a floral host (e.g. cranberry), but only if more preferred pollen hosts are absent or rare. Otherwise, honey bees' broad polylecty, flight range, and daily idiosyncrasies in floral fidelity will obscure specific pollen-foraging differences at a given floral host, even among paired colonies in a seemingly uniform agricultural setting. [source]

How do plants know when other plants are flowering?

ECOLOGY LETTERS, Issue 11 2009
Resource depletion, mast-seeding in a perennial wildflower, pollen limitation
Abstract Mast-seeding is the synchronous and periodic reproduction by plant populations. This phenomenon has been widely studied from a community-level perspective, but we know extremely little about how plants are able to synchronize reproduction. Here, we present the first experimental test of proximate mechanisms of mast-seeding, by preventing reproduction in an iteroparous, mast-seeding wildflower. Through a series of experiments, we show that mobile carbohydrate stores (NSC) control alternate-year flowering by individual plants; seed set depletes NSC which prevents flowering the following year. Plants are synchronized by density-dependent pollen limitation; when plants flower asynchronously, they set fewer seeds, which prevents NSC depletion. Therefore, these individual plants flower in subsequent years and become synchronized. Because mast-seeding is a consequence of physiological controls of reproduction, differences in plant resource acquisition and allocation could dramatically change patterns of seed production, and changes in plant consumers and pollinators could change selection on physiological and developmental pathways. [source]

ALLEE EFFECT AND SELF-FERTILIZATION IN HERMAPHRODITES: REPRODUCTIVE ASSURANCE IN DEMOGRAPHICALLY STABLE POPULATIONS

EVOLUTION, Issue 12 2004
Pierre-Olivier Cheptou
Abstract The fact that selfing increases seed set (reproductive assurance) has often been put forward as an important selective force for the evolution of selfing. However, the role of reproductive assurance in hermaphroditic populations is far from being clear because of a lack of theoretical work. Here, I propose a theoretical model that analyzes selffertilization in the presence of reproductive assurance. Because reproductive assurance directly influences the per capita growth rate, I developed an explicit demographic model for partial selfers in the presence of reproductive assurance, specifically when outcrossing is limited by the possibility of pollen transfer (Allee effect). Mating system parameters are derived as a function of the underlying demographical parameters. The functional link between population demography and mating system parameters (reproductive assurance, selfing rate) can be characterized. The demographic model permits the analysis of the evolution of self-fertilization in stable populations when reproductive assurance occurs. The model reveals some counterintuitive results such as the fact that increasing the fraction of selfed ovules can, in certain circumstances, increase the fraction of outcrossed ovules. Moreover, I demonstrate that reproductive assurance per se cannot account for the evolution of stable mixed selfing rates. Also, the model reveals that the extinction of outcrossing populations depends on small changes in population density (ecological perturbations), while the transition from outcrossing to selfing can, in certain cases, lead the population to extinction (evolutionary suicide). More generally, this paper highlights the fact that self-fertilization affects both the dynamics of individuals and the dynamics of selfing genes in hermaphroditic populations. [source]

CROSSING RELATIONSHIPS AMONG ANCIENT AND EXPERIMENTAL SUNFLOWER HYBRID LINEAGES

EVOLUTION, Issue 3 2000
Loren H. Rieseberg
Abstract., Reproductive barrier formation between newly derived hybrid taxa and their parental species represents a major evolutionary hurdle. Here, I examine the development of a sterility barrier during hybrid speciation by examining the fertility of progeny from all combinations of crosses involving three experimentally synthesized sunflower hybrid lineages, their natural hybrid counterpart, Helianthus anomalus, and their parents, H. annuus and H. petiolaris. Crosses between the parental species and H. anomalus generated almost completely sterile offspring (pollen viability < 5%; seed set < 1%). A fairly strong sterility barrier also has developed between three hybrid lineages and both parental species (pollen viability 11.1,41.6%; seed set 0.84,20.1%). In contrast, the three hybrid lineages are almost fully interfertile (pollen viabilities 83.1,88.6%; seed set 72.1,75.3%), as predicted by molecular mapping studies that indicate they have converged on a similar set of gene combinations and chromosomal rearrangements. A modest decline in compability is observed in crosses between the three hybrid lineages and H. anomalus (pollen viabilities 64.1,70.7%; seed set 37,43%), a result that agrees well with prior data demonstrating significant congruence between the genomes of the natural and experimental hybrid lineages. These observations not only indicate that reproductive isolation can arise as a by-product of fertility selection in hybrid populations, but also testify to the repeatability of this mode of speciation. [source]

The importance of scent and nectar filters in a specialized wasp-pollination system

FUNCTIONAL ECOLOGY, Issue 5 2009
Adam Shuttleworth
Summary 1.,Plants with open flowers and exposed nectar should attract a wide diversity of flower visitors, yet, for reasons that are not yet well understood, some plants with these ,generalist' floral traits have highly specialized pollination systems. 2.,We investigated this problem in the African milkweed Pachycarpus grandiflorus which has open flowers that produce copious amounts of exposed and concentrated nectar, yet is visited almost exclusively by spider-hunting wasps in the genus Hemipepsis. 3.,These wasps were the only visitors found to consistently carry pollinaria and a cage experiment showed that they are capable of successfully pollinating this plant. Furthermore, experimental hand-pollinations showed that P. grandiflorus is genetically self-incompatible and thus reliant on pollinators for seed set. 4.,We investigated the roles of chemical (nectar and floral scent) and spectral properties in the selective attraction of wasps and the filtering out of other potential flower visitors. Nectar palatability experiments showed that the nectar is unpalatable to honeybees but palatable to the wasps. Choice experiments conducted in the field and using a Y-maze in the laboratory showed that wasps are attracted primarily by scent rather than visual cues. Analysis of scent using Gas Chromatography-Mass Spectrometry showed that these inflorescences produce 36 different compounds, mostly monoterpenes and aliphatics. Analysis of spectral reflectance showed that flowers have similar colouring to the background vegetation. 5.,We conclude that P. grandiflorus is specialized for pollination by Hemipepsis wasps, and in the absence of morphological filters, achieves specialization through unpalatable nectar, cryptic colouring and scent as a selective pollinator attractant. 6.,This study demonstrates that plants whose flowers are not morphologically adapted to exclude particular floral visitors can achieve specialization through non-morphological filters. [source]

Nectar ,theft' by hummingbird flower mites and its consequences for seed set in Moussonia deppeana

FUNCTIONAL ECOLOGY, Issue 1 2001
Lara C.
Summary 1,Mites (Acari: Mesostigmata: Ascidae) that live and feed in the flowers of about 100 plant species are transported in the nares of hummingbirds (Trochilidae). Mites may compete with hummingbirds for nectar secreted by the host plants, and this could affect the dynamics and reproductive outcomes of the mutualism between plants and their pollinating hummingbirds. 2,Here we combined field observations and experimental manipulations to assess the role of hummingbird flower mites (Tropicoseius sp. nov.) on nectar secretion and reproductive output of protandrous Moussonia deppeana (Schlecht. & Cham.) Hanst. (Gesneriaceae) during their flowering period in a cloud forest remnant. 3,During the 4 days that the flowers of M. deppeana last, flowers were visited exclusively by hummingbirds (Lampornis amethystinus). Bud production per inflorescence peaked in December. There were few open flowers per inflorescence in November, but numbers increased as the flowering season progressed (December and January). 4,The availability of each flower phase differed over the flowering season. Staminate-phase flowers were more abundant over the flowering season than pistillate-phase flowers. These differences were statistically significant over time. 5,Nectar availability was reduced by up to 50% in the presence of hummingbird flower mites. Over the 4 days of observation, significantly more nectar was secreted to flowers from which mites were excluded than to flowers with no mite exclusion. The same effect was observed during flowering, but mites consumed a greater percentage of the total nectar secreted in December. 6,Significantly more nectar was secreted during the staminate phase than in the pistillate phase, independent of time and treatment. 7,A manual pollination experiment suggested that mites act like secondary pollinators in this self-compatible, non-autogamous plant, at least in flowers that were not pollinated manually and had no access to pollinating hummingbirds. 8,Although seed production was not reduced significantly by flower mites, our results suggest that the presence of floral mites can affect pollen transmission, as the amount of nectar available to hummingbirds was reduced drastically. This can directly affect hummingbird foraging patterns and reduce the fitness of the host plants. [source]

Effects of Water Shortage and Air Temperature on Seed Yield and Seed Performance of Lucerne (Medicago sativa L.) in a Mediterranean Environment

JOURNAL OF AGRONOMY AND CROP SCIENCE, Issue 6 2009
A. J. Karamanos
Abstract Seed production and performance of lucerne is characterized by fluctuating yields with often poor seed quality, and is dependent on environmental conditions, genetic characteristics and agronomic techniques applied during seed set, development, maturation and storage. A field experiment was carried out in two successive growing seasons at Kopais (southern Greece) to evaluate the effects of drought stress imposed by three irrigation treatments, and temperature during flowering and seed filling on lucerne seed yield and quality. Plant water status, expressed in terms of the water potential index (WPI), growth in leaf area and dry weight, seed yield and yield components, flowering and seed quality parameters were measured throughout the growing seasons. The adopted irrigation schemes produced a clear differentiation among treatments concerning their plant water status. Seed yield and leaf growth showed close positive correlations with WPI. An irrigation effect was also detected for the number of pods/plant, but not for the average weight of seeds/pod. Less negative values of WPI, and, especially, higher temperatures during flowering were also positively associated with a longer duration of flowering, as well as with higher total numbers of inflorescences. A very good description of the time course of seed germination was performed by fitting the Richards' function to the real data. By examining the germination parameters derived from this function it was found that final germination and germination rate were improved, while germination duration was shortened with more negative values of WPI. The effects of growing season and seeding period were occasionally equally or more important than irrigation effects. These results are also discussed in terms of their practical implications for seed producing lucerne crops. [source]

Disruption of an exotic mutualism can improve management of an invasive plant: varroa mite, honeybees and biological control of Scotch broom Cytisus scoparius in New Zealand

JOURNAL OF APPLIED ECOLOGY, Issue 2 2010
Quentin Paynter
Summary 1.,A seed-feeding biocontrol agent Bruchidius villosus was released in New Zealand (NZ) to control the invasive European shrub, broom Cytisus scoparius, in 1988 but it was subsequently considered unable to destroy sufficient seed to suppress broom populations. We hypothesized that an invasive mite Varroa destructor, which has caused honeybee decline in NZ, may cause pollinator limitation, so that the additional impact of B. villosus might now reach thresholds for population suppression. 2.,We performed manipulative pollination treatments and broad-scale surveys of pollination, seed rain and seed destruction by B. villosus to investigate how pollinator limitation and biocontrol interact throughout the NZ range of broom. 3.,The effect of reduced pollination in combination with seed-destruction was explored using a population model parameterized for NZ populations. 4.,Broom seed rain ranged from 59 to 21 416 seeds m,2 from 2004 to 2008, and was closely correlated with visitation frequency of honeybees and bumblebees. Infestation of broom seeds by B. villosus is expected to eventually reach 73% (the average rate observed at the localities adjacent to early release sites). 5.,The model demonstrated that 73% seed destruction, combined with an absence of honeybee pollination, could cause broom extinction at many sites and, where broom persists, reduce the intensity of treatment required to control broom by conventional means. 6.,Nevertheless, seed rain was predicted to be sufficient to maintain broom invasions over many sites in NZ, even in the presence of the varroa mite and B. villosus, largely due to the continued presence of commercial beehives that are treated for varroa mite infestation. 7.,Synthesis and applications. Reduced pollination through absence of honeybees can reduce broom seed set to levels at which biocontrol can be more effective. To capitalize on the impact of the varroa mite on feral honeybees, improved management of commercial beehives (for example, withdrawal of licences for beekeepers to locate hives on Department of Conservation land) could be used as part of a successful integrated broom management programme at many sites in NZ. [source]

Oilseed rape crops distort plant,pollinator interactions

JOURNAL OF APPLIED ECOLOGY, Issue 1 2010
Tim Diekötter
Summary 1. New incentives at the national and international level frequently lead to substantial structural changes in agricultural landscapes. Subsidizing energy crops, for example, recently fostered a strong increase in the area cultivated with oilseed rape Brassica napus across the EU. These changes in landscape structure affect biodiversity and associated ecosystem services. 2. Mass-flowering oilseed rape has been shown to positively affect colony growth and densities of bumblebees, which may enhance pollination services in agroecosystems. Not considered, however, have been species-specific traits of pollinators resulting in disproportionate benefits from these recurrent resource pulses. A subsequent community shift towards the subsidized species potentially distorts plant,pollinator interactions in the surrounding landscape. 3. We analysed the effects of mass-flowering crops on the abundance of legitimate long-tongued bumblebee pollinators, nectar robbing by illegitimate short-tongued bumblebees and seed set in the long-tubed flowers of red clover Trifolium pratense in 12 landscape sectors with differing amounts of oilseed rape. 4. Densities of long-tongued bumblebees visiting long-tubed plants decreased with increasing amounts of oilseed rape. The simultaneous increase of nectar robbing suggests that resource depletion is a likely explanation for this decline which may lead to a distortion in plant,pollinator interactions. The decline in long-tongued bumblebees, however, did not result in an immediate effect on seed set. In contrast, seed set increased with increasing amounts of semi-natural habitats, indicating the positive effects of these habitats on the legitimate long-tongued pollinators. 5.Synthesis and applications. Accounting for species-specific traits is essential in evaluating the ecological impacts of land-use change. The disproportional trait-specific benefits of increasing oilseed rape to short-tongued bumblebees may abet an increasingly pollinator-dependent agriculture but simultaneously threaten the more specialized and rare long-tongued species and their functions. Semi-natural habitats were found to positively affect seed set in long-tubed plants indicating that they can counteract the potentially distorting effects of transient mass-flowering crops on plant,pollinator interactions in agroecosystems. Future agri-environmental schemes should aim to provide diverse and continuous resources matching trait-specific requirements of various pollinators in order to avoid resource competition. Thereby they harmonize the economic interest in abundant pollinators and the conservation interest in protecting rare species. [source]

Changes in population biology of two succulent shrubs along a grazing gradient

JOURNAL OF APPLIED ECOLOGY, Issue 4 2003
Corinna Riginos
Summary 1Heavy livestock grazing in Namaqualand, South Africa, is threatening the region's unique diversity of succulent shrubs. This is especially true in the communally managed lands, where grazing is centred around fixed enclosures (stockposts) in which animals stay overnight. In this study we set out to determine the effects of a semi-permanent stockpost on the composition of the surrounding vegetation and the mechanisms by which grazing limits the persistence of leaf-succulent shrub populations. 2We used the grazing gradient created by a stockpost to examine the impacts of grazing on vegetation composition and changes in mortality, reproductive output and seedling establishment for the leaf-succulent species Ruschia robusta and Cheiridopsis denticulata. 3Vegetation composition was found to change from a community dominated by the unpalatable shrub Galenia africana at high grazing intensities to a community dominated by the palatable leaf-succulent shrub R. robusta at lower grazing intensities. 4Mortality of the leaf-succulents R. robusta and C. denticulata was high at the sites closest to the stockpost, while fruit production and seedling germination were substantially reduced over distances of 800 m and 2 km for the two species, respectively. Seedling establishment was not limited by either grazing or microsite availability. Thus reduction in reproductive output is the greatest impact of heavy grazing on these two species. 5Synthesis and applications. This study demonstrates that marked zonation in vegetation composition and population biology can develop around a fixed stockpost and that the greatest impact of grazing on the two leaf-succulent species studied is the suppression of flower and fruit production. Consistent suppression of reproductive output could have long-term consequences for the persistence of succulent shrub populations in the heavily grazed communal lands of Namaqualand. We recommend that (i) herders should be encouraged to relocate their stockposts regularly to prevent the development of centres of degradation, and (ii) areas should be relieved periodically of all grazing pressure to allow for successful seed set of native shrubs. [source]

Effects of patch size and density on flower visitation and seed set of wild plants: a pan-European approach

JOURNAL OF ECOLOGY, Issue 1 2010
Jens Dauber
Summary 1.,Habitat fragmentation can affect pollinator and plant population structure in terms of species composition, abundance, area covered and density of flowering plants. This, in turn, may affect pollinator visitation frequency, pollen deposition, seed set and plant fitness. 2.,A reduction in the quantity of flower visits can be coupled with a reduction in the quality of pollination service and hence the plants' overall reproductive success and long-term survival. Understanding the relationship between plant population size and/or isolation and pollination limitation is of fundamental importance for plant conservation. 3.,We examined flower visitation and seed set of 10 different plant species from five European countries to investigate the general effects of plant populations size and density, both within (patch level) and between populations (population level), on seed set and pollination limitation. 4.,We found evidence that the effects of area and density of flowering plant assemblages were generally more pronounced at the patch level than at the population level. We also found that patch and population level together influenced flower visitation and seed set, and the latter increased with increasing patch area and density, but this effect was only apparent in small populations. 5.,Synthesis. By using an extensive pan-European data set on flower visitation and seed set we have identified a general pattern in the interplay between the attractiveness of flowering plant patches for pollinators and density dependence of flower visitation, and also a strong plant species-specific response to habitat fragmentation effects. This can guide efforts to conserve plant,pollinator interactions, ecosystem functioning and plant fitness in fragmented habitats. [source]

Differential performance of reciprocal hybrids in multiple environments

JOURNAL OF ECOLOGY, Issue 6 2008
Sarah Kimball
Summary 1Closely related taxa may be maintained as distinct species by a variety of reproductive isolating mechanisms. These include: inability to produce hybrid offspring, endogenous selection against hybrids in the form of genomic incompatibilities, and exogenous selection observable in genotype-by-environment interactions. To understand the relative importance of these three isolating mechanisms, we performed hand-pollination and reciprocal transplant experiments in a natural plant hybrid zone. 2We measured reproductive isolation by making crosses between two parent species of Penstemon and naturally occurring hybrids. Inclusion of reciprocal hybrid crosses allowed us to determine whether fitness components differed depending on the identity of the mother. 3Hybrid performance was evaluated in the greenhouse and in a reciprocal transplant experiment in the field. We measured fruit set, seed set, seed weight, time required for fruits to mature and seedling growth for potted plants. To test for exogenous isolation, we planted pure parents, reciprocal F1 hybrids and later generation hybrids in a reciprocal transplant experiment, and measured survival. 4On average, interspecific crosses produced as many seeds as conspecific crosses. Hybrid performance was also equal to or greater than parents in all environments, including the greenhouse and all field gardens, indicating a lack of endogenous isolation. Parent species and reciprocal F1 hybrids differed in many traits measured. In each field garden, the hybrid with the native cytoplasm had a higher survival rate, suggesting local adaptation to different elevations. 5Synthesis. Exogenous factors that differ along elevational gradients can be more important than intrinsic genetic incompatibilities in determining the fitness of plant hybrids. Our results illustrate the importance of studying hybrid performance in multiple environments and in generating reciprocal hybrids to test for isolating mechanisms in natural hybrid zones. [source]

Effects of plant abundance on reproductive success in the biennial Sabatia angularis (Gentianaceae): spatial scale matters

JOURNAL OF ECOLOGY, Issue 2 2008
Rachel B. Spigler
Summary 1Small and low-density populations often suffer significant reductions in reproduction, as exemplified by studies on rare and threatened species. While this phenomenon is less studied in common species, if general, it should not be restricted to those in peril. We addressed this assertion by examining the effects of plant abundance, measured at population-level (population size, density) and local-level (local neighbourhood size, LNS) spatial scales, on fruit set, seed set and subsequent reproductive success (RS) across 19 natural populations of the widespread biennial Sabatia angularis (Gentianaceae). We also examined stigmatic pollen loads across a subset of populations to address whether changes in reproduction were related to reduced pollination. 2Effects of plant abundance on reproduction were evident at both spatial scales. At the population level, population size , but not density , affected fruit set significantly. This effect was non-linear. Fruit set in populations larger than 15 individuals was relatively constant, but dropped abruptly in populations below that size. This relationship did not prevail between population size and RS. Instead, RS declined significantly with population density. 3At the local level, increases in LNS between 1-m and 4-m radii from study plants increased fruit set significantly. In contrast, increases within 1 m significantly reduced seed set. Both of these effects prevailed at different spatial scales for RS; increases in 1-m LNS reduced RS, but increases beyond that distance and within 4 m increased RS. 4Although non-significant, trends in the data are consistent with facilitative interactions for pollination influencing local-level effects on fruit set. However, negative interactions, presumably because of competition for resources, are more important for determining seed set at local scales. Both interactions influence RS, but the effect of competition appears to dominate at the population level. 5Synthesis. This study demonstrates that although significant reductions in fruit set in small S. angularis populations imply that common species are not impervious to reproductive disadvantages, small populations need not experience reduced RS. Furthermore, low-density populations actually enjoy increased RS. As such, this study highlights the complexity of ecological interactions affecting reproduction and the importance of incorporating multiple spatial scales when examining population dynamics. [source]

Morph-ratio variation, population size and female reproductive success in distylous Pulmonaria officinalis (Boraginaceae)

JOURNAL OF EVOLUTIONARY BIOLOGY, Issue 5 2008
R. BRYS
Abstract Theory predicts that morph ratios in heterostylous populations are governed by negative frequency-dependent selection typically resulting in equal morph ratios at equilibrium. Previous work on the distylous perennial herb Pulmonaria officinalis, however, showed asymmetric mating between floral morphs and a weak self-incompatibility system, with the long-styled morph (L-morph) producing significantly higher seed set following intramorph crosses and even selfing than the short-styled morph (S-morph), two aspects thought to affect female fecundity and morph-ratio variation. Here, we evaluated morph ratios and population size of all known P. officinalis populations in the northern part of Belgium. Morph ratios deviated significantly from 1 : 1 (range 0.09,1 L-morph frequency, mean = 0.58). Relative fecundity of the S-morph (i.e. mean seed set of the S-morph/mean seed set of the L-morph) was on average 0.73, was positively related to the frequency of the L-morph, and reached 1 (similar levels of female fecundity) at an average L-morph frequency of 0.66 in the population. As some small populations had the S-morph in majority, our results suggest that local morph ratios are influenced both by the relative fecundity of L- and S-morph individuals and by stochastic processes in small populations. [source]

Selection on apomictic lineages of Taraxacum at establishment in a mixed sexual,apomictic population

JOURNAL OF EVOLUTIONARY BIOLOGY, Issue 4 2000
De Kovel
A species' mode of reproduction, sexual or asexual, will affect its ecology and evolution. In many species, asexuality is related to polyploidy. In Taraxacum, apomicts are triploid, and sexuals are diploid. To disentangle the effects of ploidy level and reproductive mode on life-history traits, we compared established apomictic Taraxacum genotypes with newly synthesized apomictic genotypes, obtained from diploid,triploid crosses. Diploid,triploid crossing is probably the way that most apomictic lineages originate. New genotypes had on average a much lower seed set than established genotypes. Established genotypes differed on average from new genotypes, in particular under shaded conditions: the established genotypes had longer leaves and flowered later. The differences between new and established triploids resembled the differences that have been found between sexual diploids and established apomictic triploids. We conclude that ploidy differences alone are not directly responsible for observed differences between sexual diploid and apomictic triploid dandelions. [source]

Identifying genetic components controlling fertility in the outcrossing grass species perennial ryegrass (Lolium perenne) by quantitative trait loci analysis and comparative genetics

NEW PHYTOLOGIST, Issue 3 2008
I. P. Armstead
Summary ,,Mutational load and resource allocation factors and their effects on limiting seed set were investigated in ryegrass by comparative mapping genomics and quantitative trait loci (QTL) analysis in two perennial ryegrass (Lolium perenne) mapping families sharing common genetic markers. ,,Quantitative trait loci for seed-set were identified on chromosome (LG) 7 in both families and on LG4 of the F2/WSC family. On LG7, seed-set and heading date QTLs colocalized in both families and cannot be unequivocally resolved. Comparative genomics suggests that the LG7 region is syntenous to a region of rice LG6 which contains both fertility (S5n) and heading date (Hd1, Hd3a) candidate genes. The LG4 region is syntenous to a region of rice LG3 which contains a fertility (S33) candidate gene. QTL maxima for seed-set and heading date on LG4 in the F2/WSC family are separated by c. 8 cm, indicating distinct genetic control. ,,Low seed set is under the control of recessive genes at both LG4 and LG7 locations. ,,The identification of QTLs associated with seed set, a major component of seed yield in perennial ryegrass, indicates that mutational load associated with these genomic regions can be mitigated through marker-assisted selection. [source]

Interspecific seed discounting and the fertility cost of hybridization in an endangered species

NEW PHYTOLOGIST, Issue 1 2008
Kevin S. Burgess
Summary ,,Hybrid fertilizations can have negative demographic effects on taxa by usurping ovules that would otherwise give rise to nonhybrid offspring. The consequent reduction in conspecific matings may be exaggerated in rare taxa and constitutes a fertility cost that has rarely been quantified. ,,Here, the effect of interspecific mating was estimated on the fecundity of locally rare red mulberry (Morus rubra), which hybridizes with introduced white mulberry (Morus alba) and red ¥ white hybrids. First, the asymmetry in pollen production among red, white and hybrid mulberry in two sympatric populations was quantified. The fertility cost of hybridization was then assessed experimentally by estimating seed production and rates of interspecific mating in red mulberry trees from plots where white and hybrid mulberry trees were selectively removed. ,,On average, the percentage of mulberry pollen per plot produced by red mulberry (8%) was significantly lower than the mean for white and hybrid mulberry combined (92%). Experimentally removing white and hybrid mulberry increased the siring fertility of red mulberry by 14% but produced no change in seed set. ,,Results indicate that seeds of red mulberry, ordinarily sired by conspecific pollen, are being discounted through fertilization of ovules by heterospecific pollen, which may contribute to local decline of red mulberry. [source]

Pollination processes and the Allee effect in highly fragmented populations: consequences for the mating system in urban environments

NEW PHYTOLOGIST, Issue 4 2006
Pierre-Olivier Cheptou
Summary ,,The urban environment was used to study the plant reproductive system in small fragmented populations as well as the potential adaptations of plants to urban conditions. We examined the effect of density on the pollination process and on reproduction in urban populations of the allogamous species Crepis sancta. The habitat is composed of small uncultivated square patches (c. 2 m2) regularly spaced along the pavement in streets of the city of Montpellier, France. ,,Pollinator behaviour (the presence of pollinators, the number of flowers visited and the duration of each visit) and seed set as a function of the number of plants in patches and selfing rates, determined using progeny array analysis, were studied. The propensity for the urban populations to produce seeds by self-fertilization in insect-proof glasshouse was also analysed. ,,We found strong evidence of reduced pollinator activities at low densities, resulting in reduced pollination and a reduction in seed set from 80 to 20% of ovules fertilized (the Allee effect). ,,Progeny array analysis revealed a slight increase (marginally significant) in selfing rates in urban populations compared with large populations. In spite of lower pollinator activity, urban populations did not show a greater ability to self-fertilize compared with rural populations from the nearby countryside. [source]

Pollen source and resource limitation to fruit production in the rare species Eremosparton songoricum (Fabaceae)

NORDIC JOURNAL OF BOTANY, Issue 4 2010
Xiang Shi
Eremosparton songoricum (Litv.) Vass. is a rare, central Asian desert species which shows lower fruit set and seed set (<16%) than most hermaphroditic species. We hypothesized that fruit production was limited by pollen and resources. To evaluate potential fruit abortion due to pollen limitation, supplemental hand-pollination was undertaken, the mating system was investigated and the foraging behavior of pollinators was recorded. To investigate possibile resource limitation, flowers and young pods were artificially removed and fertilization were manipulated. The results showed that under natural pollination, the number of pollen deposited on the stigma greatly exceed the number of ovules per ovary. Mating system experiments showed that the species is self-compatible, but depended on pollinators to set seeds. Supplemental outcross pollination increased fruit set significantly. The most frequent effective pollinator Megachile terminate Morawitz, was observed pollinating many flowers of the same individual plant (74.5±1.3%). These results suggested that fruit production is affected by insufficient outcross pollen rather than by pollen quantity. Removal of 2/3 of the flowers and young pods led to significantly higher fruit set, as did addition of fertilizers (N,P,K: 0.025,0.05,0.013,g, N,P,K: 0.05,0.1,0.025,g) showing that reducing resource acceptors and increasing inorganic resources both helps to improve fruit set. We therefore conclude that reproductive success of E. songoricum is limited by both outcross pollen and available nutrients. [source]

Do pollen carryover and pollinator constancy mitigate effects of competition for pollination?

OIKOS, Issue 7 2009
Benjamin R. Montgomery
Pollinator constancy and pollen carryover are both thought to mitigate competitive effects that result when shared pollinators cause loss of pollen to heterospecific flowers. I present analytical and simulation models to investigate how pollinator constancy and pollen carryover interact with each other and with the relationship between pollen receipt and seed set to determine pollination success in competitive environments. With inconstant pollinators, increased pollen carryover reduces variance in pollen receipt without affecting average pollen receipt. Consequently, for flowers requiring at least a threshold quantity of pollen for success, rare flowers with inconstant pollinators benefit from reduced carryover, especially for high pollen receipt thresholds, whereas common flowers benefit from increased carryover, especially for low receipt thresholds. Pollinator constancy is predicted to increase pollen receipt, especially if pollen carryover rates are low. As a result, increased pollinator constancy reduces the range of pollen receipt thresholds for which carryover is beneficial. Similarly, for flowers whose pollination success is a convex function of pollen receipt, carryover is expected to increase fecundity if pollinators are inconstant, but with even a low degree of pollinator constancy, carryover reduces fecundity. These results predict that rare plants with many ovules per flower benefit from dispersing aggregations of pollen, especially if their pollinators exhibit constancy, whereas plants with inconstant pollinators and low thresholds of pollen receipt benefit from pollen grains dispersing individually to increase the number of flowers reached by the pollen. [source]

Spatially contingent interactions between an exotic and native plant mediated through flower visitors

OIKOS, Issue 1 2009
Daniel P. Cariveau
Exotic plants can negatively impact the fitness of native plants by changing the behavior of flower visitors and thus affecting pollen transfer. The presence of an exotic plant may decrease the visitation rate to native plants and thus increase pollen limitation. Flower visitors may also switch between exotic and native plants and if pollen from an exotic plant is transferred to native plant stigmas this may impede siring by conspecific pollen. As flower visitors forage within a spatial context, the distribution of plants may affect the type and magnitude of pollinator-mediated competition. In this study we examined two questions: 1) does the exotic plant, Carduus nutans (Asteraceae) interact with the native Monarda fistulosa (Lamiaceae) through flower visitors by changing visitation rate and/or through heterospecific pollen transfer, and does this affect seed set of the native plant? 2) Does spatial context affect how the native and exotic plants interact through flower visitors? We created plots containing potted M. fistulosa with and without the presence of potted C. nutans. In the presence of C. nutans, M. fistulosa stigmas had significantly fewer conspecific and more C. nutans pollen grains. Visitation rate and seed set tended to be lower in these invaded plots, however they were not significant. In a second experiment, we examined whether changes in visitation rate to M. fistulosa due to the presence of C. nutans was a function of M. fistulosa distance from C. nutans. We found that visitation rate did not decrease in the presence of C. nutans when M. fistulosa were adjacent to C. nutans or 15 meters from C. nutans. However, floral visitation rate to M. fistulosa decreased at 1 and 5 meters from C. nutans. Our results suggest interactions between plant species through flower visitors may depend on spatial scale. [source]

Reproductive assurance through self-fertilization does not vary with population size in the alien invasive plant Datura stramonium

OIKOS, Issue 8 2007
Mark van Kleunen
Autonomous self-fertilization is suggested to be associated with invasiveness in plants because it offers reproductive assurance when there is a shortage of suitable mates or pollinators. Given that shortages of mates and pollinators are a common cause of Allee effects in small plant populations, we predict that the benefits of self-fertilization in terms of reproductive assurance should be greatest in small populations. We tested this idea for the invasive herb Datura stramonium, a self-fertilizing species which is also cross-pollinated to some extent by insects (mainly hawkmoths and honeybees). During two consecutive years, we studied 20 and 55 populations, respectively, of different sizes. Untreated flowers of D. stramonium showed high levels of fruit and seed set in all populations studied. Although, fruit and seed set were generally reduced by about 90% in flowers in which self-fertilization was prevented through emasculation, this effect did not vary according to population size. By using a natural color (anthocyanin) dimorphism in 12 populations, we showed that the average outcrossing rate was low (1.3%) and that there was no relationship between outcrossing rate and population size. Pollen removal from flowers also did not vary according to population size, suggesting that the pollinator visitation rate is not lower in small populations. However, decreasing deviations of observed from expected fruit set with population size imply that small populations may have an increased chance of extinction due to demographic stochasticity. Overall, our results suggest that reproductive assurance through self-fertilization in invasive plants may be important for all stages of population establishment, and not just in the founder population. [source]

Presence of Lythrum salicaria enhances the bodyguard effects of the parasitoid Asecodes mento for Filipendula ulmaria

OIKOS, Issue 3 2007
Johan A. Stenberg
This paper reports significant effects of a co-occurring plant species (Lythrum salicaria, Lythraceae) on the reproductive success of the perennial herb Filipendula ulmaria (Rosaceae). We studied 15 Filipendula populations in the Skeppsvik Archipelago; seven of which were monospecific and eight mixed with Lythrum. All the Filipendula populations studied harbored the chrysomelid beetle Galerucella tenella, and in 2005 seed set was strongly negatively correlated with the percentage leaf area consumed. Moreover, data from 2004 showed that 25,100% of the G. tenella larvae were parasitized by the hymenopteran parasitoid Asecodes mento, and we found a strong cascading top-down effect of parasitism in 2004 on Filipendula seed set in 2005. In 2004, parasitism (at the population level) was negatively correlated with percentage leaf area consumed and positively correlated with seed set in 2005. The parasitoid Asecodes also parasitized G. calmariensis, which is monophagous on Lythrum. Mixed populations of Filipendula and Lythrum supported higher densities of their shared ,bodyguard'Asecodes. Further, Y-tube bioassays showed that floriferous Filipendula attracted more than twice as many gravid Asecodes females as floriferous Lythrum. Taken together, these findings suggest that coexistence of the two plants results in ,associational resistance' for Filipendula and ,associational susceptibility' for Lythrum. This scenario was supported for Filipendula since, for this species, we found lower leaf consumption followed by higher seed production in mixed than in monospecific populations. Considered together, our results show that bodyguards may increase the reproductive fitness of a perennial herb, and that the strength of the cascading ,bodyguard' effect can be strongly influenced by co-occurring plants through ,apparent competition'. This is the first paper to demonstrate that, in the wild, plant species may use odors to compete for ,bodyguards', thereby causing asymmetrical ,apparent competition' between the herbivores involved. Our data emphasize the need to consider community factors in studies of trophic interactions. [source]

Pollinators and Reproductive Success of the Wild Cucurbit Cucurbita maxima ssp. andreana (Cucurbitaceae)

PLANT BIOLOGY, Issue 4 2001
L. Ashworth
Abstract: We studied the reproductive success and pollinators of Cucurbita maxima ssp. andreana in different disturbed habitats where it grows naturally. Data were obtained from three populations. One grew within a soybean crop, the other within a corn crop, and the third in an abandoned crop field. Cucurbita maxima ssp. andreana is an annual vine with a flowering period from December to April. Male flowers appear first, thereafter female and male flowers appear together. Flower lifetime (9 h) was similar in male and female flowers. The pollinator guild was comparable for the three populations but some differences in the frequency of the insect species were observed. Native bees were the main pollinators in the population in the abandoned field, while beetles pollinated the populations in crop fields. These differences were not linked with the pre-emergent reproductive success, fruit and seed set, or fruit quality. This is a self-compatible plant. Fruit and seed set and fruit traits (total mass, width and length of fruits, number of seeds per fruit, and seed mass) did not show significant differences between hand-cross and hand-self pollinated flowers. This wild cucurbit is a generalist with respect to pollinator guild, and flower visitors seem to be highly efficient in pollen transference. Cucurbita maxima ssp. andreana is well adapted to disturbed habitats because plants ripened fruits successfully, regardless of the group of insects visiting flowers. [source]

The Reproductive Biology of Two Species of Diplusodon Pohl (Lythraceae) from Serra do Cipó, Southeastern Brazil

Yield assessment of integument-led seed growth following targeted repair of auxin response factor 2

PLANT BIOTECHNOLOGY JOURNAL, Issue 8 2008
Rhiannon Hughes
Summary It is becoming increasingly vital to improve the yield of seed crops to feed an expanding population and, more recently, for biofuel production. One strategy to increase the yield is to increase the seed size, provided that there is not a concomitant decrease in seed number. In a previous study, we described a mutant in the AUXIN RESPONSE FACTOR 2 (ARF2) gene which produced extra cells in the seed coat and, subsequently, enlarged seeds. However, arf2 mutant plants also show severely reduced self-fertility caused, in part, by over-elongated sepals that prevent flower opening. As a low seed set increases individual seed size, a meaningful comparison of the yield in arf2 and wild-type plants could not be conducted. In this study, we show that targeted expression of wild-type ARF2 in the sepals and petals of arf2-9 mutant flowers restores flower opening and dramatically increases seed set. The restored plants retain both enlarged integuments and increased seed size, reinforcing previous evidence that arf2 mutations increase seed weight through their effect on integuments and not only via reduced fertility. We also show that the measurement of the harvest index in Arabidopsis is useful in assessing the impact of introduced traits on the yield. [source]