Bottlenecks and spatiotemporal variation in the sexual reproduction pathway of perennial meadow plants

Sexual reproduction is important for the growth of populations and the maintenance of genetic diversity. Several steps are involved in the sexual reproduction pathway of plants: the production of flowers, the production of seeds and the establishment of seedlings from seeds. In this paper we quantify the relative importance and spatiotemporal variability of these different steps for four grassland perennials: Centaurea jacea, Cirsium dissectum, Hypochaeris radicata and Succisa pratensis. We compared undisturbed meadows with meadows where the top soil layer had been removed as a restoration measure. Data on the number of flower heads per flowering rosette, the numbers of flowers and seeds per flower head, and the seedling establishment probabilities per seed were collected by field observations and experiments in several sites and years. Combination of these data shows that H. radicata and S. pratensis have higher recruitment rates (1.9 and 3.3 seedlings per year per flowering rosette, respectively) than the more clonal C. dissectum and C. jacea (0.027 and 0.23, respectively). Seedling establishment is the major bottleneck for successful sexual reproduction in all species. Large losses also occurred due to failing seed set in C. dissectum. Comparison of the coefficients of variation per step in space and time revealed that spatiotemporal variability was largest in seedling establishment, followed closely by flower head production and seed set.     

Optimum reproduction and dispersal strategies of a clonal plant in a metapopulation: a simulation study with Hieracium pilosella

Clonal spread is favoured in many plants at the expense of seed production in order to expand rapidly into open habitats or to occupy space by forming dense patches. However, for the dynamics of a population in a patchy landscape seed dispersal remains important even for clonal plants. We used a spatially explicit individual-based metapopulation model to examine the consequences of two trade-offs in Hieracium pilosella L: first, between vegetative and sexual reproduction, and second, between short and far-distance dispersal of seeds. Our main question was, what are the environmental conditions that cause a mixed strategy of vegetative and sexual reproduction to be optimal. The model was parameterised with field data on local population dynamics of H. pilosella. Patch dynamics were given firstly by disturbance events that opened patches in a matrix of a clonal grass that were colonisable for H. pilosella, and secondly by the gradual disappearance of H. pilosella patches due to the expanding grass. Simulations revealed opposing selection pressures on traits determined by the two trade-offs. Vegetative reproduction is favoured by local dynamics, i.e. the need for maintenance and expansion of established populations, whereas seed production is favoured by the necessity to colonise empty habitats. Similar pressures act on the proportion of seeds dispersed over short and far distances. Optimum reproductive and dispersal strategies depended on habitat quality (determined by seedling establishment probability), the fraction of dispersed seeds, and the fraction of seeds lost on unsuitable ground. Under habitat conditions supporting moderate to low seedling establishment, between 20% and 40% of reproductive effort in H. pilosella should be devoted to sexual reproduction with at least 10% of the seeds dispersed over distances suitable to attain empty patches. We conclude that in a spatially heterogeneous landscape sexual seed production in a clonal plant is advantageous even at the expense of local vegetative growth.     

Causes and consequences of extreme variation in reproductive strategy and vegetative growth among invasive populations of a clonal aquatic plant, <Emphasis Type="Italic">Butomus umbellatus</Emphasis> L. (Butomaceae)

The mode of reproduction may influence the spread of invasive species by affecting evolutionary potential and dispersal ability. We sampled 51 introduced North American populations of the clonal aquatic plant Butomus umbellatus L. (flowering rush) and found extreme variation in sexual fertility caused by polyploidy. Populations consisted of diploids that produced thousands of viable seeds or of sexually sterile triploids. Although a trade-off between sexual and clonal reproduction predicts that the sexual sterility of triploids would be compensated for by greater clonal reproduction, a greenhouse experiment involving eight diploid and 10 triploid populations showed that diploid plants not only invest substantially in sexual structures but also make hundreds of tiny clonal bulbils on both rhizomes and inflorescences. In contrast, triploids do not make bulbils and have very limited scope for clonal multiplication and dispersal. Diploid populations were more frequent than triploid populations, especially in the Great Lakes region. This is probably because of the difference between cytotypes in clonal rather than sexual reproduction, as genetic analyses indicate a general lack of sexual recruitment in North America. Although triploids were less common, they have a wider geographical distribution. This could be due to a greater ecological tolerance resulting from polyploidy. However, genetic evidence suggests that triploids have become widespread via their use in and escape from horticulture. North America is being colonized by two distinct forms of B. umbellatus that differ strongly in reproductive strategy as well as the vectors and pathways of invasion.     

Floral distribution,clonal structure,and their effects on pollination success in a self-incompatible <Emphasis Type="Italic">Convallaria keiskei</Emphasis> population in northern Japan

In plant species, when clonal growth produces a patchy structure and flowering ramets are clustered, the amount of pollen contributing to reproductive success is often regulated by pollinator efficiency and geitonogamy. The spatial population structure may influence reproductive success. We examined the clonal structure, the spatial ramet distribution, and their combined effects on fruit set in a natural population of the insect-pollinated, self-incompatible clonal herb, Convallaria keiskei, in northern Japan. The number of shoots, flowers, and fruits in 1-m² quadrats were counted at every 5 m grid point in an established 100 × 90-m study plot. From all the quadrats where shoots existed, leaf samples were collected for allozyme analysis. Using the two spatial parameters of flowering ramet densities and genotypes, we then constructed individual-based fruit-set models. A total of 236 quadrats contained shoots, and 135 contained flowering ramets, which indicated expanded distribution of this plant throughout the study plot, while shoots, flowers and fruits all showed clustering distributions. Allozyme analysis of 282 samples revealed 94 multilocus genotypes. The largest clone extended to more than 40 m, whereas 56 genotypes were detected in only one sample. Several large clones and many small clones were distributed close to each other. Fine-scale spatial modelling revealed that the neighbouring flower numbers of different genotypes, compared with local genet or flower diversity, more influenced fruit set, in which the range of the neighbour was 14.5 m. These findings indicate that the compatible pollen dispersed by insect pollinators has a significant effect on sexual reproduction, in this C. keiskei population. Consequently, the spatial structure, which includes both genet distribution and clonal expansion by ramets, had a significant effect on pollination success.     

Structure and Dynamics of Populations of Japanese Barberry (Berberis Thunbergii DC.) in Deciduous Forests of New Jersey

Japanese barberry, Berberis thunbergii DC., has become a prominent exotic species in deciduous forests throughout the eastern and midwestern US. Populations range from small plants occurring at low densities to dense, impenetrable thickets of plants with up to 40 stems/individual. A study was undertaken at Morristown National Historical Park in New Jersey to document plant densities, plant size, recruitment through vegetative processes of new shoot initiation and clonal spread and recruitment from seedling establishment, and mortality of stems and plants. Nearly 2000 shoots on 370 plants were individually marked and followed for two growing seasons, and over 1000 seedlings were also individually marked and followed. Populations vary much more in total shoots/area than they do in plant individuals/area, or in mean plant size (shoots/plant), as even the sparse populations have a few large individuals. Shoot mortality is less than new shoot initiation, but most plants do not change in size or change by small numbers of stems. However, the number of new shoots per plant increases as plant size increases. Once plants have three stems, they suffer little or no mortality. Seedling establishment is proportional to the density of shoots, so that as plants grow in size, local recruitment from seed increases. Large numbers of seedlings, and a survival rate of 10%, combine to make seedling recruitment a major component of population increase. The combination of multiple forms of vegetative and seed-based population growth, and the very low rates of plant mortality due to the multi-stemmed growth form explains the ability of this invasive species to rapidly produce dense, persistent populations.     

Demographic consequences of greater clonal than sexual reproduction in Dicentra canadensis

Clonality is a widespread life history trait in flowering plants that may be essential for population persistence, especially in environments where sexual reproduction is unpredictable. Frequent clonal reproduction, however, could hinder sexual reproduction by spatially aggregating ramets that compete with seedlings and reduce inter‐genet pollination. Nevertheless, the role of clonality in relation to variable sexual reproduction in population dynamics is often overlooked. We combined population matrix models and pollination experiments to compare the demographic contributions of clonal and sexual reproduction in three Dicentra canadensis populations, one in a well‐forested landscape and two in isolated forest remnants. We constructed stage‐based transition matrices from 3 years of census data to evaluate annual population growth rates, λ. We used loop analysis to evaluate the relative contribution of different reproductive pathways to λ. Despite strong temporal and spatial variation in seed set, populations generally showed stable growth rates. Although we detected some pollen limitation of seed set, manipulative pollination treatments did not affect population growth rates. Clonal reproduction contributed significantly more than sexual reproduction to population growth in the forest remnants. Only at the well‐forested site did sexual reproduction contribute as much as clonal reproduction to population growth. Flowering plants were more likely to transition to a smaller size class with reduced reproductive potential in the following year than similarly sized nonflowering plants, suggesting energy trade‐offs between sexual and clonal reproduction at the individual level. Seed production had negligible effects on growth and tuber production of individual plants. Our results demonstrate that clonal reproduction is vital for population persistence in a system where sexual reproduction is unpredictable. The bias toward clonality may be driven by low fitness returns for resource investment in sexual reproduction at the individual level. However, chronic failure in sexual reproduction may exacerbate the imbalance between sexual and clonal reproduction and eventually lead to irreversible loss of sex in the population.     

Population demographics and trade-offs to reproduction of an invasive and noninvasive species of <Emphasis Type="Italic">Rubus</Emphasis>

Do trade-offs between growth and reproduction differ between an invasive and noninvasive plant species and how do such trade-offs relate to population demographics? To help address these questions, we compared demographics for an invasive plant species, Rubus discolor, with a noninvasive congener, R. ursinus, in several populations of varying density. Removal of floral buds from reproductive canes increased the size of juvenile canes that arose from clonal sprouting in R. ursinus, suggesting a trade-off between current reproduction and growth. Removal of floral buds had no effect on growth of R. discolor. R. ursinus displayed trade-offs between reproduction (sexual and vegetative) and future growth based on negative correlations between leaf area production and both clonal sprouting and seedling production during the previous year. R. discolor did not exhibit these trade-offs. Both species had high population growth rates in low-density populations, but exhibited little or no growth in high-density populations. A life table response experiment was used to determine the underlying cause for the effect of density on population growth. For R. ursinus, lack of population growth in high-density populations was due primarily to increased mortality of clonally sprouting canes, while for R. discolor, it was due to decreased clonal cane production. Elasticity analysis revealed that clonal growth was more important than sexual reproduction for population growth of both species. However, elasticity values for sexual reproduction in R. discolor were greater in high- than low-density populations. This suggests an increased reliance on sexual reproduction in populations that had reached stable sizes, which could increase the capacity of R. discolor to disperse to new sites. Elasticity analyses were also used to simulate the efficacy of various control strategies for R. discolor. Control of this species could be attained by reducing clonal production within existing populations while reducing seed production to limit establishment of new populations.     

Vegetative reproduction and clonal diversity in Rhytidium rugosum (Rhytidiaceae, Bryopsida) inferred by morpho-anatomical and molecular analyses

Many bryophytes exhibit specific life forms that result in the formation of distinct patches. This is primarily achieved by consequent vegetative multiplication through indeterminate growth and vegetative reproduction. Little, however, is known about the underlying mechanisms and genetic patterns. In this study on vegetative multiplication in bryophytes, we apply morpho-anatomical and molecular (AFLP fingerprinting) techniques to analyze the vegetative reproduction system, clonal diversity, and habitat colonization of a pleurocarpous moss, Rhytidium rugosum (Rhytidiaceae). Morpho-anatomically, three types of vegetative diaspores are identified and illustrated: ramets, separated after decay and disintegration of older shoot parts (clonal reproduction); brood branches/branchlets; and caducous shoot apices (both vegetative reproduction s.str.). The AFLP fingerprinting of 35 samples (from two plots in Thuringia, further German populations, and from France, Russia, and Canada) resulted in the identification of three clones from the two plots, each comprising two to 15 samples with identical fingerprints. Samples from one clone occurred in both plots, thus proving dispersal of vegetative diaspores. The AFLP analysis further revealed close relationships of the plot samples, which suggest a clonal rather than generative (sexual) origin of the population.     

Stochastic LTRE analysis of the effects of herbivory on the population dynamics of a perennial grassland herb

Herbivores can have strong deleterious effects on vital rates (growth, reproduction, and survival) and thus negatively impact the population dynamics of plant species. In practice, however, these effects might be strongly correlated, for example as a result of tradeoffs between vital rates. To get better insights into the effects of herbivory on the population dynamics of the long‐lived grassland plant Primula veris population projection matrices were constructed from demographic data collected between 1999 and 2008 (nine annual transitions). Data were collected in two large grassland populations, each of which was subjected to two treatments (grazing by cattle versus a mowing treatment), yielding a total of 36 matrices. We applied a lower‐level vital rate life table response experiment (LTRE) using the small noise approximation (SNA) of the stochastic population growth rate to disentangle the contributions of changes in mean vital rates, variability in vital rates, correlations between vital rates and vital rate elasticities to the difference in the stochastic growth rate. Stochastic growth rates (a= log λ_S) were significantly lower in grazed than in mown plots (a= 0.0185 and 0.1019, respectively). SNA LTRE analysis showed that contributions of mean vital rates by far made the largest contribution to the observed difference in a between grazed and control plots. In particular, changes in sexual reproduction rates made the largest contributions to lower the stochastic growth rate in grazed plots: both adult flowering probabilities and flower and seed production were importantly lower in grazed populations, but these negative effects were largely buffered by increased establishment and seedling survival rates. Among the stochastic terms of the SNA decomposition, contributions of covariance and correlations between vital rates had the largest impact, whereas contributions of elasticities were smaller. The strongest correlation driver was the association between adult survival and seedling establishment, suggesting that environmental conditions favouring adult survival also are beneficial for seedling establishment. Overall, our results show that herbivory had a strong negative effect on the long‐term population growth rate of P. veris that was primarily mediated by differences in fecundity (flower and seed production) and germination.     

When sex is not enough: ecological correlates of resprouting capacity in congeneric tropical forest shrubs

In moist tropical forests resprouting may be an important component of life history, contributing to asexual reproduction through the clonal spread of individuals derived from shoot fragments. However, in contrast to other ecosystems where resprouting is common, the ecological correlates of resprouting capacity in tropical forests remain largely unexplored. In this study we characterized shade tolerance, resprouting capacity and sexual reproductive success of eight co-occurring Piper species from lowland forests of Panama. In field experiments we found that shade-tolerant Piper species had a higher capacity to regenerate from excised or pinned stem fragments than light-demanding species in both gap and understory light conditions. In contrast, shade-tolerant species had lower recruitment probabilities from seeds, as a consequence of lower initial seed viability, and lower seedling emergence rates. All Piper species needed gap conditions for successful seedling establishment. Of 8,000 seeds sown in the understory only 0.2% emerged. In gaps, seed germination of light-demanding species was between 10 and 50%, whereas for shade-tolerant species it was 0.5–9.8%. We propose that the capacity to reproduce asexually from resprouts could be adaptive for shade-tolerant species that are constantly exposed to damage from falling litter in the understory. Resprouting may allow Piper populations to persist and spread despite the high rate of pre-dispersal seed predation and low seed emergence rates. Across Piper species, we detected a trade-off between resprouting capacity and the annual viable seed production per plant but not with annual seed mass produced per plant. This suggests that species differences in sexual reproductive success may not necessarily result from differential resource allocation. Instead we suggest that low sexual reproductive success in the understory may in part reflect reduced genetic diversity in populations undergoing clonal growth, resulting in self-fertilization and in-breeding depression.     

Conditions for seedling establishment and probable function of seed in the clonal sedge Carex rugulosa Kük. in riverside marshes

Carex rugulosa Kük. forms large meadows in moist zones along estuarine riversides. These meadows are usually maintained by rhizomatous ramet production (clonal growth), and the appearance of seedlings is uncommon. We studied the conditions required for seedling establishment. In areas in which clonal ramets were dense (906–1050/m²), and the relative light intensity at ground level was low (0.8–3.8%), seed germination was entirely suppressed. In contrast, many seedlings (288–513/m²) were observed in areas in which clonal ramets had been considerably reduced (13–269/m²) by continuous inundation in the previous year. In these areas, the relative light intensity had increased (20.5–79.3%). It was inferred that seeds resisted the prolonged inundation that killed the ramets, and that germination was induced under these open conditions. These results suggest that the main functions of sexual reproduction in C. rugulosa are recovery after severe damage to vegetation using seeds from the seed bank and the dispersal of seeds to areas without dense vegetation. Most seedlings died when the marshy meadow was temporarily inundated by heavy rain. However, transplanted seedlings survived better at artificially elevated sites that were not submerged. This shows that inundation during the seedling stage impedes seedling survival. Therefore, seeds can contribute to recovery after ramet decline or colonization to open area when (i) the rainfall is not heavy to cause seedling submergence or (ii) seeds are dispersed to higher area which is not inundated.     

Sexual and vegetative reproduction in the aboveground part of a dioecious clonal plant, <Emphasis Type="Italic">Dioscorea japonica</Emphasis> (Dioscoreaceae)

In dioecious clonal plants, the reproductive effort required to set seeds will be responsible for the larger investment in sexual reproduction by females. If there will be a trade-off in resource allocation between sexual and clonal reproduction, this differential sexual reproduction will lead to sexual differentiation in the relative amount of clonal reproduction. To test this prediction, we studied differences between the sexes in their phenologies and investments in sexual and vegetative reproduction (clonal reproduction by means of bulbils) with respect to ramet size in a dioecious clonal plant, Dioscorea japonica Thunb. The period of bulbil production overlapped the period during which infructescences developed. Females flowered later, produced heavier inflorescences, and fewer flowers per inflorescence than did males. Regression analysis using the size of the individual plants demonstrated that large females made smaller investments in inflorescences and larger investments in sexual reproduction than did large males. In contrast, females invested fewer resources in vegetative reproduction than did males. However, the total investments in sexual and vegetative reproduction did not differ between the sexes. These results supported our hypothesis on the sexual differentiation in sexual and clonal reproduction.     

Effects of clonality on the genetic variability of rare,insular species: the case of Ruta microcarpa from the Canary Islands

Many plant species combine sexual and clonal reproduction. Clonal propagation has ecological costs mainly related to inbreeding depression and pollen discounting; at the same time, species able to reproduce clonally have ecological and evolutionary advantages being able to persist when conditions are not favorable for sexual reproduction. The presence of clonality has profound consequences on the genetic structure of populations, especially when it represents the predominant reproductive strategy in a population. Theoretical studies suggest that high rate of clonal propagation should increase the effective number of alleles and heterozygosity in a population, while an opposite effect is expected on genetic differentiation among populations and on genotypic diversity. In this study, we ask how clonal propagation affects the genetic diversity of rare insular species, which are often characterized by low levels of genetic diversity, hence at risk of extinction. We used eight polymorphic microsatellite markers to study the genetic structure of the critically endangered insular endemic Ruta microcarpa. We found that clonality appears to positively affect the genetic diversity of R. microcarpa by increasing allelic diversity, polymorphism, and heterozygosity. Moreover, clonal propagation seems to be a more successful reproductive strategy in small, isolated population subjected to environmental stress. Our results suggest that clonal propagation may benefit rare species. However, the advantage of clonal growth may be only short‐lived for prolonged clonal growth could ultimately lead to monoclonal populations. Some degree of sexual reproduction may be needed in a predominantly clonal species to ensure long‐term viability.     

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.     

Heavy metals modify costs of reproduction and clonal growth in the stoloniferous herb Potentilla anserina

We examined costs of sexual reproduction and clonal propagation, and their consequences for resource allocation in the clonal stoloniferous herb, Potentilla anserina, a typical pioneer species in disturbed areas. We used heavy-metal treatment in soil to create unfavourable growing conditions, because costs of reproduction are more likely to be expressed when resources are limited. We also studied whether heavy metals affect the plasticity of clonal growth form that enables the plants to avoid poor growing conditions. Ramets collected from field were grown in a greenhouse under the heavy-metal treatment consisting of a control and two levels of heavy-metals added in soil. Clonal propagation was costly in terms of total biomass of flowering ramets. Also the costs of sexual reproduction were detected in flowering ramets. Contrary to our predictions, the costs of flower production were visible in the control but not in the heavy-metal contaminated plants. Only the flowering ramets were able to produce longer stolons under heavy-metal stress, but the stolon biomass was not affected by heavy metals. Results indicate that clonal propagation and sexual reproduction may be costly for P. anserina. However, the costs are modified by heavy-metal contamination.Co-ordinating editor: J. Tuomi     

Sexual dimorphism in nuclear DNA content and floral morphology in populations of Silene latifolia (Caryophyllaceae)

The evolution of flower size has become a major focus of plant population biology. In order to gain insight into the causal basis for flower-size variation, we have explored the relationship between nuclear DNA content, flower size, and cell size within and among populations of the dioecious plant Silene latifolia. We found significant variation among populations for both DNA content and flower size, with a consistent sexual dimorphism within all populations (males have a bigger genome, but smaller flowers). The overall correlation between DNA content and flower size was negative, especially within males. The cell dimensions of calyx and petal cells were not significantly different between the sexes, indicating that females have bigger flowers because they contain more cells. These findings are discussed in the context of nucleotype theory, which predicts a slower growth rate (division rate) for cells with greater DNA content. This leads to the suggestion that males have smaller flowers because of the relatively slow rate of cell division due to their larger genome. It would be of great interest to know whether associated effects on flower size of changes in genome size of the type investigated in the present study can be generalized to other species.     

The use of periodic matrices to model the population dynamics of the long-lived semelparous <Emphasis Type="Italic">Furcraea parmentieri</Emphasis> (Asparagaceae) in a temperate forest in central Mexico

Furcraea parmentieri (Roezl ex Ortgies) García-Mend. is a long-lived, semelparous threatened species with a restricted geographic distribution. We studied the demography of this species in southwest of Mexico City using periodic matrices. We followed the fate of 404 individuals (in 2006–2007) and performed seed germination and seedling establishment experiments in the field (in 2007–2008) to estimate survival, growth and reproduction probabilities of individuals in eight size categories, according to plant height. Population structure was characterized by a high number of saplings, as well as medium and large plants. We built three annual matrices that described the demography in different phases of the population cycle: regular years (RG) characterized by low fecundity and low adult mortality; mast-seeding years (MS) with low seedling emergence and high adult mortality (related to semelparity); and post mast-seeding years (PS) with a high emergence of seedlings and bulbils. Periodic matrices were built simulating three scenarios, which consisted of mast-seeding events occurring every 5, 10, or 15 years. Finite population growth rates (λ) were 1.03 (RG year), 0.99 (MS year), and 1.25 (PS year). The long term population growth rate values projected over a 90 year period (estimated on an annual basis) were higher as the frequency of mast-seeding events increased. We estimated that individuals reproduce at an age of 60–90 years and that most new recruits originate via sexual reproduction. Although the population has a potential for growth, its persistence may be threatened due to land use change.     

Reproductive Consequences of Clonal Growth in Stenocereus eruca, a Rare Clonal Cactus of the Sonoran Desert

Stenocereus eruca is a prostrated, self-incompatible cactus endemic to the Sonoran Desert that regenerates primarily through clonal propagation. Clonal growth is expected to affect mate availability by influencing the number and spatial distribution of mating types. In this paper we examine the role of clonal growth on female fecundity through a series of pollination experiments in a population of S. eruca. We set up a pollen supplementation experiment using five distance treatments with pollen collected at 1, 10, 100, 1000 and 25000 m from receptor flowers during the years 2001 and 2002 and evaluated genetic sifmilarities between pairs of receptor-donor ramets through RAPD markers. Our data on fruit set, number of seeds/fruit, germination and overall fecundity revealed that S. eruca show a significant reduction in female fecundity when pollination occurs between ramets located at short distances (1 and 10 m), while genetic data showed high levels of similarity at those distances. The reduction in female fecundity is apparently a consequence of geitonogamy and inbreeding depression. Our data suggest that clonal growth and geitonogamy are likely to be partially responsible for the low levels of sexual reproduction and seedling recruitment observed in populations of S. eruca. Co-ordinating editor: H. Kudoh     

Clonal growth is enhanced in the absence of a mating morph: a comparative study of fertile stylar polymorphic and sterile monomorphic populations of Nymphoides montana (Menyanthaceae)

Background and Aims

Many aquatic species with stylar polymorphisms have the capacity for clonal and sexual reproduction and are sensitive to the balance of the two reproductive modes when there are a limited number of mating morphs within a population. This study asked how the clonal and sexual reproductive modes perform in populations that contain only a single morph and where fitness gain through sexual reproduction is rare. In clonal aquatic Nymphoides montana, polymorphic populations normally contain two mating morphs in equal frequencies. Populations are sexually fertile and appear to be maintained by pollen transfer between the two partners. However, in a monomorphic population of N. montana where mating opportunities are unavailable, female and male function is impaired and clonality maintains the population. Here, the consequences of intraspecific variation in sexuality were explored between monomorphic and polymorphic N. montana populations in eastern Australia.

Methods

Comparative measurements of male and female fertility, total dry mass and genotypic diversity using ISSR markers were made between populations with variable sexuality.

Key Results and Conclusions

Very few seeds were produced in the monomorphic population under natural and glasshouse conditions due to dysfunctional pollen and ovules. Stigma–anther separation was minimal in the monomorphic population, which may be a consequence of the relaxed selective pressures that regulate the maintenance of sexual function. However, clonal reproduction was favoured at the expense of sexual reproduction in the monomorphic population; this may facilitate the establishment of sterility throughout the population via resource reallocation or pleiotropic effects. The ISSR results showed that the monomorphic population was one large, single genotype, unlike the multi-genotypic fertile polymorphic populations. Evolutionary loss of sex in a clonal population in which a mating morph is absent was evident; under these conditions clonal growth may assure reproduction and expand the population via spreading stolons.     

A comparison of native and invasive populations of three clonal plant species in Germany and New Zealand

Aim Our aim was to test for changes in growth patterns of three clonally growing plant species (Achillea millefolium, Hieracium pilosella and Hypericum perforatum) between native and invaded regions. We addressed the hypotheses that with differing important life‐history traits, invasive populations perform better than native populations, and that this expected better performance is linked to weakened trade‐offs between individual growth and sexual and clonal reproduction. Location Germany and New Zealand. Methods We conducted field surveys for the three above‐mentioned species in both native German and invasive New Zealand populations, and collected data at both population and individual levels. Results At the population level, the proportion of flowering plants, population size and population density were all higher in invasive populations. Similarly, at the individual level, the number of stolons per plant, stolon–biomass ratio and population crowdedness (local plant density in a specified area around a target plant) were significantly higher in New Zealand. Plant height did not differ between countries, and plant biomass was lower in New Zealand than in Germany for Achillea millefolium and Hypericum perforatum. These two species showed significant trade‐offs between individual growth and sexual and clonal reproduction. Achillea millefolium exhibited a weakened trade‐off in its invaded range, where the same proportion of flowering plants was sustained at much higher levels of population crowdedness than in its native range. Main conclusions The apparent invasion success of the three study species is generally due to better overall performance in their respective invaded ranges. In respect of both Achillea millefolium and Hypericum perforatum, this is driven primarily by increased vegetative reproduction. In contrast, Hieracium pilosella seems to benefit more from increased sexual reproduction in its invaded range. Shifts in trade‐offs as a general trend seem to be of minor importance.