The relationship between floral display size, pollen carryover and geitonogamy in Myosotis colensoi (Kirk) Macbride (Boraginaceae)

The relative amounts of self- and cross-pollen deposited on stigmas depends on both the number of pollinator visits that occur within plants and the amount of pollen carryover. Data collected for Myosotis colensoi (Kirk) Macbride and compiled from a survey of the literature, reveal that pollen carryover is frequently very high (upwards of 80%) and this at least partially relieves some of the effects of geitonogamous pollinator movements. It is suggested that in some cases, selection for traits that confer a high rate of pollen carryover may occur. Aspects of the plant–pollinator interaction that are likely to influence pollen carryover are discussed.     

Dinamica della vegetazione forestale su tempi plurimillenari: problemi e prospettive

Abstract

The pollen records, combining long duration with high resolution and so providing information on different types of processes occurring on different time-scales, stress the importance of the temporal dimension in the study of biological processes. The history of the vegetation of the last tens of thousands of years indicates that the presentday widespread diffusion of forests at the middle latitudes is exceptional rather than normal. An example of processes seemingly similar, but with presumably different causes, is shown: the exponential growth of past plant populations, ascribed in some cases to the reproductive potential of the species concerned, and in other cases to changes of climatic and/or environmental conditions.     

Modelling pollen‐mediated gene flow in rice: risk assessment and management of transgene escape

Fast development and commercialization of genetically modified plants have aroused concerns of transgene escape and its environmental consequences. A model that can effectively predict pollen‐mediated gene flow (PMGF) is essential for assessing and managing risks from transgene escape. A pollen‐trap method was used to measure the wind‐borne pollen dispersal in cultivated rice and common wild rice, and effects of relative humidity, temperature and wind speed on pollen dispersal were estimated. A PMGF model was constructed based on the pollen dispersal pattern in rice, taking outcrossing rates of recipients and cross‐compatibility between rice and its wild relatives into consideration. Published rice gene flow data were used to validate the model. Pollen density decreased in a simple exponential pattern with distances to the rice field. High relative humidity reduced pollen dispersal distances. Model simulation showed an increased PMGF frequency with the increase of pollen source size (the area of a rice field), but this effect levelled off with a large pollen‐source size. Cross‐compatibility is essential when modelling PMGF from rice to its wild relatives. The model fits the data well, including PMGF from rice to its wild relatives. Therefore, it can be used to predict PMGF in rice under diverse conditions (e.g. different outcrossing rates and cross‐compatibilities), facilitating the determination of isolation distances to minimize transgene escape. The PMGF model may be extended to other wind‐pollinated plant species such as wheat and barley.     

Mating system as a barrier to gene flow

Understanding mating system as one of reproductive isolating barriers remains important although this barrier is classified in a different sense from behavioral, ecological, and mechanical isolating barriers. Selfing enhances incipient speciation while outcrossing facilitates species integrity. Here, I study how mating system affects gene exchanges between genetically diverging species in a hybrid zone. Results show that a predominant selfing species has a greater barrier to selective gene flow than does a predominant outcrossing species. Barrier to neutral gene flow convexly changes with the selfing rate due to linkage disequilibrium, with a maximum at around intermediate selfing rate. Asymmetric transient or steady‐state barriers to neutral gene flow occur between two sides of a hybrid zone when the neutral gene is affected by its linked selective gene whose alternative alleles are adaptive to heterogeneous habitats. Selfing interacts with both a physical barrier and a density‐dependent ecological regulation (a logarithmic model) to strengthen the barriers to neutral and selective gene flow. This theory helps to interpret incipient speciation driven by selfing or to explain the asymmetric gene flow or unequal genomic mixtures between closely related species caused by their asymmetric mating systems in natural hybrid zones.     

Florally rich habitats reduce insect pollination and the reproductive success of isolated plants

Landscape heterogeneity in floral communities has the potential to modify pollinator behavior. Pollinator foraging varies with the diversity, abundance, and spatial configuration of floral resources. However, the implications of this variation for pollen transfer and ultimately the reproductive success of insect pollinated plants remains unclear, especially for species which are rare or isolated in the landscape. We used a landscape‐scale experiment, coupled with microsatellite genotyping, to explore how the floral richness of habitats affected pollinator behavior and pollination effectiveness. Small arrays of the partially self‐compatible plant Californian poppy (Eschscholzia californica) were introduced across a landscape gradient to simulate rare, spatially isolated populations. The effects on pollinator activity, outcrossing, and plant reproduction were measured. In florally rich habitats, we found reduced pollen movement between plants, leading to fewer long‐distance pollination events, lower plant outcrossing, and a higher incidence of pollen limitation. This pattern indicates a potential reduction in per capita pollinator visitation, as suggested by the lower activity densities and richness of pollinators observed within florally rich habitats. In addition, seed production reduced by a factor of 1.8 in plants within florally rich habitats and progeny germination reduced by a factor of 1.2. We show this to be a consequence of self‐fertilization within the partially self‐compatible plant, E. californica. These findings indicate that locally rare plants are at a competitive disadvantage within florally rich habitats because neighboring plant species disrupt conspecific mating by co‐opting pollinators. Ultimately, this Allee effect may play an important role in determining the long‐term persistence of rarer plants in the landscape, both in terms of seed production and viability. Community context therefore requires consideration when designing and implementing conservation management for plants which are comparatively rare in the landscape.     

Long-distance pollen flow and tolerance to selfing in a neotropical tree species

Outcrossing rates, pollen dispersal and male mating success were assessed in Dicorynia guianensis Amshoff, a neotropical tree endemic to the Guiana shield. All adult trees within a continuous area of 40 ha (n = 157) were mapped, and were genotyped with six microsatellite loci. In addition, progenies were genotyped from 22 mature trees. At the population level, the species was mostly outcrossing (tm = 0.89) but there was marked variation among individuals. One tree exhibited mixed mating, confirming earlier results obtained with isozymes that D. guianensis can tolerate selfing. A Bayesian extension of the fractional paternity method was used for paternity analysis, and was compared with the neighbourhood method used widely for forest trees. Both methods indicated that pollen dispersal was only weakly related to distance between trees within the study area, and that the majority (62%) of pollen came from outside the study stand. Using maximum likelihood, male potential population size was estimated to be 1119, corresponding to a neighbourhood size of 560 hectares. Male mating success was, however, related to the diameter of the stem and to flowering intensity assessed visually. The mating behaviour of D. guianensis is a combination of long-distance pollen flow and occasional selfing. The species can still reproduce when it is extremely rare, either by selfing or by dispersing pollen at long distances. These results, together with the observation that male mating success was correlated with the size of the trees, could be implemented in management procedures aiming at regenerating the species.     

The relative importance of reproductive assurance and automatic selection as hypotheses for the evolution of self-fertilization

Background

The field of plant mating-system evolution has long been interested in understanding why selfing evolves from outcrossing. Many possible mechanisms drive this evolutionary trend, but most research has focused upon the transmission advantage of selfing and its ability to provide reproductive assurance when cross-pollination is uncertain. We discuss the shared conceptual framework of these ideas and their empirical support that is emerging from tests of their predictions over the last 25 years.

Scope

These two hypotheses are derived from the same strategic framework. The transmission advantage hypothesis involves purely gene-level selection, with reproductive assurance involving an added component of individual-level selection. Support for both of these ideas has been garnered from population-genetic tests of their predictions. Studies in natural populations often show that selfing increases seed production, but it is not clear if this benefit is sufficient to favour the evolution of selfing, and the ecological agents limiting outcross pollen are often not identified. Pollen discounting appears to be highly variable and important in systems where selfing involves multiple floral adaptations, yet seed discounting has rarely been investigated. Although reproductive assurance appears likely as a leading factor facilitating the evolution of selfing, studies must account for both seed and pollen discounting to adequately test this hypothesis.

Conclusions

The transmission advantage and reproductive assurance ideas describe components of gene transmission that favour selfing. Future work should move beyond their dichotomous presentation and focus upon understanding whether selection through pollen, seed or both explains the spread of selfing-rate modifiers in plant populations.     

Inbreeding and outbreeding effects on pollen fitness and zygote survival in Silene nutans (Caryophyllaceae)

In plants, selfing and outcrossing may be affected by maternal mate choice and competition among pollen and zygotes. To evaluate this in Silene nutans, we pollinated plants with mixtures of (1) self‐ and outcross pollen and (2) pollen from within a population and from another population. Pollen fitness and zygote survival was estimated from the zygote survival and paternity of seeds. Self pollen had a lower fitness than outcross pollen, and selfed zygotes were less likely, or as likely, to develop into seeds. Hybrid zygotes survived as frequently or more than local zygotes, and pollen from one of the populations fertilized most ovules in both populations. Our results thus indicate strong maternal discrimination against selfing, whereas the success of outbreeding seems mostly affected by divergent pollen performance. The implications for the evolution of maternal mate choice are discussed.     

Effects of distinct pollinators on the mating system and reproductive success in Incarvillea sinensis,an annual with large floral displays

Aims Most flowering plants engage in mutualisms with animals to move pollen between individuals, and it is expected that pollinators play an essential role in the evolution of selfing, yet few studies have determined how distinct pollinator types affect a plant’s mating system and reproductive success differentially. We investigated the effect of two different pollinators on the reproductive success ofIncarvillea sinensis, an annual with showy, insect-pollinated, one-day flowers.     

DIFFERENTIAL SUCCESS OF POLLEN DONORS IN A SELF-COMPATIBLE LILY

If pollen donors are equally effective at siring seeds, the presence of equal proportions of pollen from two pollen donors on a stigma will lead to equal proportions of seeds sired by each pollen donor. Variation in germination rates, pollen-tube growth, and embryo viability may cause one donor to sire more seed than another. We looked for differential donor success in the field by simultaneously applying equal amounts of pollen from two pollen donors. We simultaneously applied equal amounts of self and outcross pollen to receptive stigmas and simultaneously applied pollen from two donors at different physical distances from the recipient. Following simultaneous application of self and outcross pollen, significantly more of the seeds were sired by outcross pollen donors. Seed set following simultaneous application of two outcross donors was also nonrandom. Pollen donors from 100 m were more likely to sire seeds when competing with pollen from plants nearby (1 m). To determine whether pollen-tube growth rates were responsible for these patterns of paternity, we varied the timing of deposition of outcross pollen allowing self pollen tubes a head start on the stigma. Outcross pollen was applied 3 or 24 h after self pollen. In spite of this time delay, the majority of the seeds were again sired by outcross pollen. There was no significant difference in the amount of seeds sired by self pollen between the two delay treatments. This result suggests that mechanisms operating after ovule fertilization may contribute to the discordance between the proportions of the pollen present and the proportions of seeds sired.     

ECOLOGICAL AND GENETIC CONSEQUENCES OF POLLINATION BY SEXUAL DECEPTION IN THE ORCHID CALADENIA TENTACTULATA

Only orchids affect pollination by the deceptive sexual attraction of male insects, a syndrome particularly well developed in Australia. We examined the ecological and genetic consequences of exclusive pollination by sexually attracted male thynnine wasps in the orchid Caladenia tentaculata. Male wasps respond rapidly to flowers artificially presented in 1 × 1 m² experimental patches. Sixty of 287 wasps approached within centimeters of the flower, but did not land. Of the remaining 79% who made floral contact, only 7.5% attempted copulation, the step critical for pollination. Wasps only rarely moved among patches (19% of flights) and none attempted copulation a second time, resembling observations in natural populations. We confirmed outcrossing and long distance pollen flow by monitoring how colored pollen moved in natural populations. Pollen movements approximated a linear rather than a leptokurtic distribution (mean distance: 17 m; maximum: 58 m). Pollinator visits varied independently of flower density in three of four populations with most solitary flowers being visited. Allozyme analysis revealed within-population fixation indices (F) close to zero and low levels of differentiation (FST) among populations. Despite behavioral evidence for long distance pollen flow, significant local genetic structure exists, perhaps reflecting restricted seed dispersal. Long distance pollen flow in C. tentaculata may therefore promote outbreeding by minimizing pollen transfers among related neighbors. Although this species is self-compatible, outcrossed progeny develop significantly faster than selfed progeny. Effective pollination at low flower densities could accentuate this advantage. The data are consistent with the predictions that deceptive pollination will result in long distance pollen flow, which may be of selective advantage at low density. Comparative studies of how food reward, food deceptive, and sexual deceptive pollination systems vary within a phylogenetic framework could further illuminate the evolution of sexual deception.     

Does anthocyanin affect outcrossing rates in Datura Stramonium (Solanaceae)?

In this paper, I investigate whether the presence or absence of anthocyanin is neutral with respect to reproduction in Datura stramonium. The observations concern the portion of the life cycle spanning pollination to germination. Pollinators do not appear to distinguish between floral morphs, as revealed by nonbiased distribution of fluorescent powder used as a pollen analogue. Pollen-tube growth is also equal for the two morphs. Seed germination is affected by the presence of anthocyanin, but apparently only by the genotype of the mother at the anthocyanin locus, and not by the genotype of the embryo itself. In addition, there was an interaction between maternal morph and the maternal source population, with seeds from high-elevation anthocyanin-producing mothers germinating most rapidly and seeds from low-elevation anthocyanin-producing mothers germinating most slowly in a common garden at low elevation. However, because germination of anthocyanin-producing and anthocyaninless progeny proceed equally quickly, the anthocyanin marker provides unbiased estimates of outcrossing rates. The results overall support the use of anthocyanin as a neutral marker, with the alternate phenotypes unlikely to be differentially impacted by the processes of pollination, pollen-tube growth, and germination. Fluctuating selection on conditions for dormancy release may be partially responsible for the maintenance of the anthocyanin polymorphism in Datura stramonium.     

A Lichenological Excursion to the West of Scotland

ABSTRACT

Background: Hybridisation can be a threat for the survival of a rare species because, in the case of insufficient numbers of appropriate mates, a rare form is much more likely to cross with a widespread taxon.

Aims: In the present study, we tested hypotheses concerning the level of hybridisation between endangered Betula humilis and its widespread congeners: B. pendula and B. pubescens as a function of habitat conditions.

Methods: We genotyped 312 individuals of three species using AFLP markers. B. humilis specimens were sampled in populations with low and high groundwater levels. Morphological identification of B. pubescens and B. pendula was verified using the Atkinson discriminant function.

Results: Altogether, 15 individuals (4.8%) were indicated as putative hybrids. The B. humilis hybrids were found in dry habitats and they could be classified as F1 or F2 generation. Tree hybrids could represent backcrosses to either B. pendula or B. pubescens.

Conclusions: Genetic analyses contradicted the idea that hybridisation between B. humilis and its close relatives was extensive. On the other hand, the presence of introgressed individuals in the populations in areas with low groundwater levels implied that pollen swamping might be a threat for declining B. humilis stands.     

The long and the short of it: long-styled florets are associated with higher outcrossing rate in Senecio vulgaris and result from delayed self-pollen germination

Background: It has been reported that some plants of the self-compatible species Senecio vulgaris produce capitula containing long-styled florets which fail to set seed when left to self-pollinate, although readily set seed when self-pollinated by hand.

Aims: To determine if production of long-styled florets is associated with higher outcrossing rate in S. vulgaris, and whether long-styles occur in non-pollinated florets, whereas short-styles are present in self-pollinated florets.

Methods: The frequency of long-styled florets was compared in the radiate and non-radiate variants of S. vulgaris, known to exhibit higher and lower outcrossing rates, respectively. In addition, style length was compared in emasculated florets that were either self-pollinated or left non-pollinated.

Results: Long-styled florets were more frequent in the higher outcrossing radiate variant. Following emasculation, long styles occurred in non-pollinated florets, while short styles were present in self-pollinated florets. The two variants did not differ in style length within the non-pollinated or within the self-pollinated floret categories.

Conclusions: A high frequency of long-styled florets is associated with higher outcrossing rate in S. vulgaris and results from delayed self-pollination and pollen germination on stigmas.     

Seasonal changes in pollinator activity influence pollen dispersal and seed production of the alpine shrub Rhododendron aureum (Ericaceae)

In alpine ecosystems, microscale variation in snowmelt timing often causes different flowering phenology of the same plant species and seasonal changes in pollinator activity. We compared the variations in insect visitation, pollen dispersal, mating patterns, and sexual reproduction of Rhododendron aureum early and late in the flowering season using five microsatellites. Insects visiting the flowers were rare early in the flowering season (mid-June), when major pollinators were bumblebee queens and flies. In contrast, frequent visitations by bumblebee workers were observed late in the season (late July). Two-generation analysis of pollen pool structure demonstrated that quality of pollen-mediated gene flow was more diverse late in the season in parallel with the high pollinator activity. The effective number of pollen donors per fruit (N(ep)) increased late in the season (N(ep) = 2.2-2.7 early, 3.4-4.4 late). However, both the outcrossing rate (t(m)) and seed-set ratio per fruit were smaller late in the season (t(m) = 0.89 and 0.71, seed-set ratio = 0.52 and 0.18, early and late in the season, respectively). In addition, biparental inbreeding occurred only late in the season. We conclude that R. aureum shows contrasting patterns of pollen movement and seed production between early and late season: in early season, seed production can be high but genetically less diverse and, during late season, be reduced, possibly due to higher inbreeding and inbreeding depression, but have greater genetic diversity. Thus, more pollinator activity does not always mean more pollen movement.     

The absence of cryptic self-incompatibility in Clarkia unguiculata (Onagraceae)

Many species exhibit reduced siring success of self-relative to outcross-pollen donors. This can be attributed either to postfertilization abortion of selfed ovules or to cryptic self-incompatibility (CSI). CSI is a form of self-incompatibility whereby the advantage to outcross pollen is expressed only following pollinations where there is gametophytic competition between self and outcross pollen. Under the definition of CSI, this differential success is due to the superior prefertilization performance (pollen germination rate and pollen tube growth rate) of outcross pollen relative to self pollen. Although CSI has been demonstrated in several plant species, no studies have assessed among-population variation in the expression of CSI. We conducted a greenhouse study on Clarkia unguiculata (an annual species with a mixed-mating system) to detect CSI, and we compare our observations to previous reports of CSI in C. gracilis and another population of C. unguiculata. In contrast to these previous studies of CSI in Clarkia, we used genetic rather than phenotypic markers to measure the relative performance of selfed vs. outcross pollen. In this study, we measured the intensity of CSI in C. unguiculata from a large population in southern California and we determined whether the magnitude of pollen competition (manipulated by controlling the number of pollen grains deposited on a stigma) influenced the outcome of competition between self and outcross pollen. In contrast to previous investigations of Clarkia, we found no evidence for CSI. The mean number of seeds sired per fruit did not differ between self and outcross pollen following either single-donor or mixed pollinations. In addition, the relative success of selfed vs. outcross pollen was independent of the magnitude of pollen competition. These results suggest that: (1) one of the few nonheterostylous species previously thought to be cryptically self-incompatible is completely self-compatible (at least in the population studied here) or (2) phenotypic markers may be problematic for the detection of CSI.     

Pollen limitation and fruit abortion in a declining rare tree,the Eurasian yew (Taxus baccata L.): A reproductive cost of ecological marginality

Populations of rare tree species such as the dioecious and anemophilous yew (Taxus baccata) are especially prone to extinction if they occupy marginal habitats. This is the case of yew populations growing in Mediterranean mountains under dry climate and a severe anthropogenic disturbance regime, which show insufficient regeneration. We examined two of the likely mechanisms driving this population decline. The study was conducted in a central Spanish yew population in 2005 and 2006. On one hand, we tested for the effect of the amount of pollen received by female flowers by means of a pollen addition experiment. On the other hand, we tested for the effect of initial fruit set on abortion and mature fruit production. Finally, we evaluated the effect of the spatial arrangement of male and female yew trees on realized fecundity. The success of fertilization and fruiting in the experimental population was pollen-limited. Female trees aborted ca. 70% of the fruit set in both years and treatments. Fruit set was significantly influenced by the females' neighbourhood, with fecundity decreasing exponentially with distance to the nearest male in both years. Overall, our results indicate that factors associated with ecological marginality have a strong influence on yew performance.