The relationship between individual seed quality and maternal plant body size in crowded herbaceous vegetation

Aims In most natural plant populations, there is a strong right-skewed distribution of body sizes for reproductive plants—i.e. the vast majority are relatively small, suppressed weaklings that manage not just to survive effects of crowding/competition and other hazards but also to produce offspring. Recent research has shown that because of their relatively large numbers, these relatively small resident plants collectively contribute most of the seed offspring production available for the population in the next generation. However, the success of these offspring will depend in part on their quality, e.g. reflected by seed size and resource content. Accordingly, in the present study, we used material from natural populations of herbaceous species to test the null hypothesis that there is no significant relationship between body size variation in resident plants—resulting from between-site variation in the intensity of crowding/competition—and variation in the mass or N content of their individual seeds.Methods Using populations of 56 herbaceous species common in eastern Ontario, total above-ground dry plant mass, mean mass per seed and mean nitrogen (N) content per seed were recorded for a sample of the largest resident plants and also for the smallest reproductive plants growing in local neighbourhoods with the most severe crowding/competition from near neighbours.Important findings Mass per seed was numerically smaller from the smallest resident plants for most study species, but with few exceptions, this was not significantly different (P> 0.05) from mass per seed from the largest resident plants. The results therefore showed no general effect of maternal plant body size on individual seed mass, or N content. This suggests that the reproductive output of the smaller half of the resident plant size distribution within these populations is likely to contribute not just most of the seed production available for the next generation but also seed offspring that are just as likely—on a per individual basis—to achieve seedling/juvenile recruitment success as the seed offspring produced by the largest resident plants. This conflicts with the traditional 'size-advantage' hypothesis for predicting plant fitness under severe competition, and instead supports the recent 'reproductive-economy-advantage' hypothesis, where competitive fitness is promoted by capacity to produce offspring that—despite severe body size suppression imposed by neighbour effects—in turn have capacity to produce grand-offspring.     

Nurses experience reciprocal fitness benefits from their distantly related facilitated plants

It is well known that many plants benefit from growing beside a nurse plant of another species, but the possibility that the nurse also benefits has been rarely studied. We hypothesize that positive interactions are maintained not only because of the recruitment benefits for the facilitated plants but also because of fitness benefits for the nurse plant. We tested this hypothesis by comparing seed production, seed predation and seed viability of a dominant nurse plant species (Mimosa luisana) when growing alone and in patches surrounded by its facilitated species. We also tested whether fitness of the nurse species is dependent on the phylogenetic neighborhood formed by their facilitated species using an analysis that accounted for the abundance and pairwise phylogenetic similitude of all species in each patch. Nurses growing associated to their facilitated species produced more seeds (1.86 times) and these seeds were more viable (1.47 times) than those of nurses growing alone. Seed predation did not alter these fitness differences. Seed number and viability increased in phylogenetically diverse neighborhoods. We conclude that distantly related partners are more likely to cause reciprocal increases in fitness, and that such effects contribute to species coexistence.     

Seed weight and seed number affect subsequent fitness in outcrossing and selfing Primula species

Using the outcrossing Primula farinosa and its autogamous selfing relatives P. scotica , P. scandinavica and P. stricta , we compared the fitness of light and heavy seeds. Heavy seeds germinated in greater numbers and more quickly. In competition with seedlings grown from lighter seeds, heavy seeds produced larger rosettes. In P. farinosa such seedlings went on to produce more seeds, and in two populations heavier seeds, than plants from lighter seeds. After transplantation to natural populations, seedlings of P. farinosa derived from heavy seeds produced larger rosettes, more flowers and seeds than those from lighter seeds in certain populations so that seedlings born of heavy seeds were much fitter than seedlings from lighter seeds. Average seed weight varied in inverse proportion to seed number per capsule. The autogamous species produced on average about twice as many seeds per capsule as P. farinosa . In P. scotica and P. stricta this difference appears to be due in part to assured fertilization, but this high fecundity did not cause disadvantageously light seeds. As these species produced fewer capsules per scape, their overall seed production was on average no greater than for P. farinosa . P. farinosa traded-off fitness between capsules with large seed numbers, which donated more offspring to the next generation, and those with small seed numbers, whose heavy seeds would be more likely to reproduce themselves in the next generation. We conclude that low fecundity in outcrossing species might at times be advantageous.     

EVOLUTION OF BRASSICA RAPA L. (CRUCIFERAE) POPULATIONS IN INTRA- AND INTERSPECIFIC COMPETITION

Populations of Brassica rapa were grown for three generations in each of two environments: intraspecific competition, with four surrounding Brassica rapa neighbors per pot, and interspecific competition, with two Raphanus sativus neighbors per pot. In each environment, the largest (by flower number) 10% of the plants were outcrossed and provided seeds for the next generation. As a control, a randomly chosen 10% of the plants in each environment were outcrossed to produce seed for the next generation. Each of these four treatments, the selected lines in intra- and interspecific competition and the corresponding control lines, was maintained for three generations. After a single generation of growth in a common, no-competition environment, replicate plants from each treatment were grown with no competition and with intra- and interspecific competition for determination of growth responses. After two generations of selection, flower number in the intraspecific-selection line had increased by more than 50% over that in the control line and by more than 19% over that under interspecific selection. After a common-environment generation, plants from the intraspecific-selection line were shown to have significantly faster growth in height and flower number as seedlings. Plants in the interspecific-selection line showed similar but nonsignificant trends. No differences in seed mass, emergence time, or photosynthetic rate were found between control and selected lines in either intra- or interspecific competition. Some differences between control and selected lines were noted in biomass allocation related to differences in phenology. The results demonstrate that performance in competitive environments can evolve through changes in plant development but that rates of evolution will differ in intra- and interspecific competition.     

Effects of foliar herbivory by insects on the fitness of Raphanus raphanistrum: damage can increase male fitness

Generally, effects of herbivory on plant fitness have been measured in terms of female reproductive success (seed production). However, male plant fitness, defined as the number of seeds sired by pollen, contributes half of the genes to the next generation and is therefore crucial to the evolution of natural plant populations. This is the first study to examine effects of insect herbivory on both male and female plant reproductive success. Through controlled field and greenhouse experiments and genetic paternity analysis, we found that foliar damage by insects caused a range of responses by plants. In one environment, damaged plants had greater success as male parents than undamaged plants. Neither effects on pollen competitive ability nor pollinator visitation patterns could explain the greater siring success of these damaged plants. Success of damaged plants as male parents appeared to be due primarily to changes in allocation to flowers versus seeds after damage. Damaged plants produced more flowers early in the season, but not more seeds, than undamaged plants. Based on total seed production, male fitness measures from the first third of the season, and flower production, we estimated that damaged and undamaged plants had equal total reproductive success at the end of the season in this environment. In a second, richer environment, damaged and undamaged plants had equal male and female plant fitness, and no traits differed significantly between the treatments. Equal total reproductive success may not be ecologically or evolutionarily equivalent if it is achieved differentially through male versus female fitness. Genes from damaged plants dispersed through pollen may escape attack from herbivores, if such attack is correlated spatially from year to year.     

Consequences of the overproduction of methyl jasmonate on seed production, tolerance to defoliation and competitive effect and response of Arabidopsis thaliana

* Accumulation of methyl jasmonate (MeJA) after herbivore attack in plants is associated with the induction of defenses that can benefit fitness, but are costly to express; effects often explored using exogenous application of jasmonates. * Here I explored the consequences of the overexpression of MeJA on seed production, tolerance to defoliation and competitive effect and response, using a genotype of Arabidopsis thaliana that overexpresses jasmonic acid carboxyl methyltransferase (JMT) and contains threefold higher levels of MeJA than wild-type plants. * Without competition, JMT plants produced 37-40% less total seed mass than vector controls or wild-type plants, and had reduced seed germination. Defoliation reduced height more strongly in wild-type than in JMT plants, but reduced total seed production equally. In a competition experiment, the presence of a neighbor reduced fitness more strongly in wild-type than in JMT plants, but JMT plants exhibited dampened opportunity costs and benefits of induction with jasmonic acid of itself or its neighbor. This may have related to the higher constitutive expression but reduced inducibility of jasmonate-mediated defenses, including trypsin inhibitors, exhibited by JMT plants. * In natural plant populations, overexpression of MeJA-mediated responses should be beneficial to resistance to herbivores, pathogens and competitors, but is directly costly to fitness and probably constrains plasticity in response to changing environmental conditions.     

The effects of pollen load size and donor diversity on pollen performance, selective abortion, and progeny vigor in Mirabilis jalapa

The influence of pollen competitive environment on pollen performance (pollen germination, stigmatic penetration, and pollen tube growth rate), the maturation or abortion of initiated fruit, seed size, and seedling vigor was explored by manipulating the size and diversity of stigmatic pollen loads on Mirabilis jalapa. All aspects of pollen performance significantly increased with the number of pollen grains on a stigma or pollen tubes in a style, but was not influenced by the diversity of pollen donors. Plants tended to mature single-ovulate fruits that came from flowers where pollen load size and diversity were greatest and aborted those where these were lowest. No plants from seeds resulting from pollinations with a single pollen grain survived, but other fitness measures were mostly determined by maternal plant. The data suggest that pollen performance is influenced by pollen competitive environment, and both the genetic diversity of the pollen load and number of competing pollen tubes are important determinants of seed/fruit abortion.     

Offspring performance in three cleistogamous Viola species

Seed germination and seedling/juvenile fitness in the cleistogamous perennials Viola hirta, V. mirabilis, and V. riviniana were investigated during three growing seasons, to compare the performance of chasmogamously (CH) and cleistogamously (CL) derived progeny. For V. hirta and V. mirabilis the effects of sibling competition were examined, for V. riviniana the effects of interspecific (grass) competition. Seed abortion and seed weight were also taken into account as fitness measures.In none of the species, seed abortion rate differed between CH and CL capsules. In V. mirabilis and V. riviniana, CL seeds had a lower germination rate than CH seeds. In V. hirta the two seed types did not differ in germinability. Mortality did not differ between the two seedling types in any of the species. In V. hirta and V. riviniana, CL progeny had shorter mean length of largest leaf than CH progeny. In V. mirabilis plant size did not differ between progeny types. Sibling competition had little effect on offspring performance, but grass competition increased mortality and reduced plant size of V. riviniana progeny. The two progeny types did not differ in their response to sibling and grass competition.The differences in performance between progeny types could be attributable to inbreeding depression in the CL phase, but the slightly lower fitness of CL offspring is probably balanced by their lower production costs. It is suggested that a dimorphic reproductive system is maintained in perennial Viola species to maximize total seed output in the face of environmental variation.     

Winning and losing with microbes: how microbially mediated fitness differences influence plant diversity

Interactions between plants and soil microbes can strongly influence plant diversity and community dynamics. Soil microbes may promote plant diversity by driving negative frequency‐dependent plant population dynamics, or may favor species exclusion by providing one species an average fitness advantage over others. However, past empirical research has focused overwhelmingly on the consequences of frequency‐dependent feedbacks for plant species coexistence and has generally neglected the consequences of microbially mediated average fitness differences. Here we use theory to develop metrics that quantify microbially mediated plant fitness differences, and show that accounting for these effects can profoundly change our understanding of how microbes influence plant diversity. We show that soil microbes can generate fitness differences that favour plant species exclusion when they disproportionately harm (or favour) one plant species over another, but these fitness differences may also favor coexistence if they trade off with competition for other resources or generate intransitive dominance hierarchies among plants. We also show how the metrics we present can quantify microbially mediated fitness differences in empirical studies, and explore how microbial control over coexistence varies along productivity gradients. In all, our analysis provides a more complete theoretical foundation for understanding how plant–microbe interactions influence plant diversity.     

Sexually antagonistic evolution caused by male–male competition in the pistil

Although sexual selection and sexual conflict are important evolutionary forces in animals, their significance in plants is uncertain. In hermaphroditic organisms, such as many plants, sexual conflict may occur both between mating partners (interlocus conflict) and between male and female sex roles within an individual (intralocus conflict). We performed experimental evolution, involving lines that were crossed with either one or two pollen donors (monogamous or polyandrous lines), in the hermaphroditic plant (Collinsia heterophylla) where early fertilizations are associated with female fitness costs (reduced seed set). Artificial polyandry for four generations resulted in enhanced pollen performance and increased female fitness costs compared to the monogamous and source (starting material) lines. Female fitness was also reduced in the monogamous line, indicating a possible trade‐off between sex roles, resulting from early pollination. We performed a second experiment to investigate a potential harming effect of pollen performance on seed set. We found that high siring success of early arriving pollen competing with later‐arriving pollen was associated with high female fitness costs, consistent with an interlocus sexual conflict. Our study provides evidence for the importance of sexual selection in shaping evolution of plant reproductive strategies, but also pinpoints the complexity of sexual conflict in hermaphroditic species.     

Fitness costs associated with acetyl‐coenzyme A carboxylase mutations endowing herbicide resistance in American sloughgrass (Beckmannia syzigachne Steud.)

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Size-dependent ESS sex allocation in wind-pollinated cosexual plants: fecundity vs. stature effects

To theoretically investigate the single and compound effects of relative fecundity and relative stature of plants on size-dependent sex allocation (SDS) in wind-pollinated cosexual species, we developed a game model and analysed ESS sex allocation of large and small plants having totally or partially different reproductive resources and different pollen and seed dispersal areas in a population. We found that e.g. when both sized plants have large pollen dispersal areas relative to their seed dispersal areas, which plants are male-biased is largely determined by relative fecundity (t) and relative size of seed dispersal area (k) of the large plants to the small plants: If t >k, large plants tend to be more male-biased even if relative size of pollen dispersal area of large to small plants (l) is smaller than k. If t相似文献   

Inclusive fitness,asymmetric competition and kin selection in plants

The findings that some plants alter their competitive phenotype in response to genetic relatedness of its conspecific neighbour (and presumed competitor) has spurred an increasing interest in plant kin‐interactions. This phenotypic response suggests the ability to assess the genetic relatedness of conspecific competitors, proposing kin selection as a process that can influence plant competitive interactions. Kin selection can favour restrained competitive growth towards kin, if the fitness loss from reducing own growth is compensated by increased fitness in the related neighbour. This may lead to positive frequency dependency among related conspecifics with important ecological consequences for species assemblage and coexistence. However, kin selection in plants is still controversial. First, many studies documenting a plastic response to neighbour relatedness do not estimate fitness consequences of the individual that responds, and when estimated, fitness of individuals grown in competition with kin did not necessarily exceed that of individuals grown in non‐kin groups. Although higher fitness in kin groups could be consistent with kin selection, this could also arise from mechanisms like asymmetric competition in the non‐kin groups. Here we outline the main challenges for studying kin selection in plants taking genetic variation for competitive ability into account. We emphasize the need to measure inclusive fitness in order to assess whether kin selection occurs, and show under which circumstances kin selected responses can be expected. We also illustrate why direct fitness estimates of a focal plant, and group fitness estimates are not suitable for documenting kin selection. Importantly, natural selection occurs at the individual level and it is the inclusive fitness of an individual plant – not the mean fitness of the group – that can capture if a differential response to neighbour relatedness is favoured by kin selection.     

Seed sex ratio in dioecious plants depends on relative dispersal of pollen and seeds: an example using a chessboard simulation model

Two principles are important for the optimal sex ratio strategy of plants. (1) Sib mating. Because seed dispersal is restricted, sib mating may occur which selects for a female bias in the seed sex ratio. (2) Local resource competition (LRC). If a plant produces pollen its nuclear genes are dispersed in two steps: first through the pollen and then, if the pollen is successful in fertilizing an ovule on another plant, through the seed. If the plant produces an ovule, its genes are dispersed only through the seed. By making pollen instead of ovules the offspring of a single plant is then spread out over a wider area. This reduces the chance that genetically related individuals are close together and need to compete for the same resource. The effect is the strongest if pollen is dispersed over a much wider area than seeds. Less LRC for paternally vs. maternally derived offspring selects for a male bias in sex allocation. We study the above‐mentioned opposite effects in dioecious plants (with separate male and female individuals), with maternal control over the sex ratio (fraction males) in the seeds. In a two‐dimensional spatial model female‐biased sex ratios are found when both pollen and seed dispersal are severely restricted. If pollen disperses over a wider area than seeds, which is probably the common situation in plants, the seed sex ratio becomes male‐biased. If pollen and seeds are both dispersed over a wide area, the sex ratio approaches 0.5. Our results do not change if the offspring of brother–sister matings are less fit because of inbreeding depression.     

Does experience with competition matter? Effects of source competitive environment on mean and plastic trait expression in Erodium cicutarium

Interspecific competition is likely to act as an agent for selection on local scales, although evidence in plants is sparse so far. We hypothesize that in annual shade-avoiding grassland species, heterogeneity in the intensity of aboveground competition for light may shape patterns of genetic variation and induce phenotypic plasticity in traits affecting competitive ability. We collected maternal seed families of Erodium cicutarium from replicated high and low competition environments and exposed them to different levels of aboveground competition in a glasshouse. We examined effects of seed source and competition treatment on expression of plant traits related to competitive ability and fitness. Source environments with high levels of competition were significantly more heterogeneous in competition intensity at both intermediate (approx. 10 m) and small (approx. 0.1 m) spatial scales. Seed source and competition treatment both had highly significant effects on trait expression. Greater intensity of competition experienced by maternal plants was coupled with lower vegetative biomass production and slower growth rates, and at the same time lower abortion rates in the offspring. We interpret these findings as an indication of greater reproductive efficiency in the next generation, in response to competition experienced by parents. There was higher total phenotypic variability in the plants from high competition source sites, but equivalent levels of phenotypic plasticity across source-site competition levels; no costs of phenotypic plasticity were detected. We concluded that differences in competition intensity can lead to trait differentiation in the next generation. For E. cicutarium, experience with competition matters: it leads to substantial phenotypic differences and more total variability in the offspring generation.     

Asymmetrical intraspecific competition in <Emphasis Type="Italic">Echinochloa crus</Emphasis>-<Emphasis Type="Italic">galli</Emphasis> is related to differences in the timing of seedling emergence and seedling vigour

Individual size is an important factor that determines fitness in annual plants. Variation in size originates at the seedling stage based on differences in seedling vigour and time of emergence, ensuing asymmetrical intraspecific competition between early- and late-established plants. The effects of the order of seedling emergence on characteristics of barnyard grass (Echinochloa crus-galli) were studied. In the first experiment, competition between pairs of plants, one of which was sown and emerged 0, 3, 6 or 12 days earlier than the other, was determined. In the second experiment, competition between two plants that were sown at the same time, but emerged at different times because of variation in seed quality, was determined. Competition decreased plant mass, tillering, height and the number of caryopses per panicle, but not time to earing or root/shoot ratio. Plants that were sown 3 days after the first plants in competing pairs grew to 32% of the size of solitary control plants, with the effect of competition accounting for 40% and asymmetrical competition 28% of the total reduction in size. When a 3-day lag in germination was determined intrinsically, plant size was reduced to 19% of the control, with the effect of competition accounting for 34%, asymmetrical competition 15% and poor intrinsic quality of the plant 32% of the total reduction in size. Small environmentally induced delays and more importantly, intrinsically induced delays in the time of emergence result in a considerable decrease in final plant size and fitness.     

Inbreeding depression in Lychnis flos-cuculi (Caryophyllaceae): effects of different levels of inbreeding

In a controlled crossing experiment on Lychnis flos-cuculi plants in the greenhouse, outbred and selfed maternal plants were each treated with pollen from unrelated plants, siblings and selves. The seeds thus obtained had expected inbreeding coefficients of 0, 0.25 and 0.5 for the outbred maternal plants, and 0, 0.5 and 0.75 for the selfed maternal plants. Seed abortion rate, seed weight and germination rate were estimated. Seedlings were transplanted to an outdoor garden, and monitored for survival, probability of flowering, number of capsules and area of capsules next spring. Inbred seeds germinated slower and in lower proportions than those less inbred, and seedlings had lower survival, flowering, fruit set and area of capsules if inbred. Combined fitness values were estimated from the survival and fecundity components, and severe inbreeding depression was detected for these estimates (0.51 and 0.56 for one generation of selfing). The fitness function decreased linearly with the increase in inbreeding coefficient, which is as expected if the inbreeding depression is additive among loci.     

Parasitoid load affects plant fitness in a tritrophic system

Plants attacked by herbivorous insects emit volatile compounds that attract predators or parasitoids of the herbivores. Plant fitness increases when these herbivorous insects are parasitized by solitary parasitoids, but whether gregarious koinobiont parasitoids also confer a benefit to plant fitness has been disputed. We investigated the relationship between parasitoid load of the gregarious Cotesia glomerata (L.) (Hymenoptera: Braconidae), food consumption by larvae of their host Pieris brassicae L. (Lepidoptera: Pieridae), and seed production in a host plant, Brassica nigra L. (Brassicaceae), in a greenhouse experiment. Plants damaged by caterpillars containing single parasitoid broods produced a similar amount of seeds as undamaged control plants and produced significantly more seeds than plants with unparasitized caterpillars feeding on them. Increasing the parasitoid load to levels likely resulting from superparasitization, feeding by parasitized caterpillars was significantly negatively correlated with plant seed production. Higher parasitoid brood sizes were negatively correlated with pupal weight of Cotesia glomerata , revealing scramble competition leading to a fitness trade-off for the parasitoid. Our results suggest that in this tritrophic system plant fitness is higher when the gregarious parasitoid deposits a single brood into its herbivorous host. A prediction following from these results is that plants benefit from recruiting parasitoids when superparasitization is prevented. This is supported by our previous results on down-regulation of synomone production when Brassica oleracea was fed on by parasitized caterpillars of P. brassicae . We conclude that variable parasitoid loads in gregarious koinobiont parasitoids largely explain existing controversies about the putative benefit of recruiting these parasitoids for plant reproduction.     

Friend or foe? A parasitic wasp shifts the cost/benefit ratio in a nursery pollination system impacting plant fitness

Nursery pollination systems are species interactions where pollinators also act as fruit/seed herbivores of the plant partner. While the plants depend on associated insects for pollination, the insects depend on the plants’ reproductive structures for larval development. The outcome of these interactions is thus placed on a gradient between mutualism and antagonism. Less specialized interactions may fluctuate along this gradient with the ecological context, where natural enemies can play an important role. We studied whether a natural enemy may impact the level of seed consumption of a nursery pollinator and how this in turn may influence individual plant fitness. We used the plant Silene latifolia, its herbivore Hadena bicruris, and its ectoparasitoid Bracon variator as a model plant–herbivore–natural enemy system. We investigated seed output, germination, survival, and flower production as proxies for individual plant fitness. We show that B. variator decreases the level of seed consumption by H. bicruris larvae which in turn increased seed output in S. latifolia plants, suggesting that parasitism by B. variator may act as a regulator in the system. However, our results also show that plant survival and flower production decrease with higher seed densities, and therefore, an increase in seed output may be less beneficial for plant fitness than estimated from seed output alone. Our study should add another layer to the complex discussion of whether parasitoids contribute to plant fitness, as we show that taking simple proxies such as seed output is insufficient to determine the net effect of multitrophic interactions.     

Effects of two types of herbivores on the population dynamics of a perennial herb

The joint effects of multiple herbivores on their shared host plant have received increasing interest recently. The influence of herbivores on population dynamics of their host plants, especially the relative roles of different types of damage, is, however, still poorly understood. Here, we present a modelling approach, including both deterministic and stochastic matrix modelling, to be used in estimating fitness effects of multiple herbivores on perennial plants. We examined the effects and relative roles of two specialist herbivores, a pre-dispersal seed predator, Euphranta connexa, and a leaf-feeding moth, Abrostola asclepiadis, on the population dynamics and long-term fitness of their shared host plant, a long-lived perennial herb Vincetoxicum hirundinaria (Asclepiadaceae). We collected demographic data during 3 years and combined these data with the effects of natural levels of herbivory measured from the same individuals. We found that both seed predation and leaf herbivory reduced population growth of V. hirundinaria, but only very high damage levels changed the growth trend of the vigorously growing study populations from positive to negative. Demographic modelling indicated that seed predation had a greater impact on plant population growth than leaf herbivory. The effect of leaf herbivory was weaker and diminished with increasing level of seed predation. Evaluation of individual fitness components, however, suggested that leaf herbivory contributed more strongly to host plant fitness than seed predation. Our results emphasize that understanding the effects of a particular herbivore on plant population dynamics requires also knowledge on other herbivores present in the system, because the effect of a particular type of herbivory on plant population dynamics is likely to vary according to the intensity of other types of herbivory. Furthermore, evaluating herbivore impact from using individual fitness components does not necessarily reflect the long-term effects on total plant fitness.