Seed Reproduction Is Associated with a Transient Escape from Parasite Damage in American Beech (original) (raw)
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Evolutionary Ecology, 2005
Genetic variation in plant populations for resistance to pathogens and herbivores might be maintained by parasite-mediated negative frequency-dependent selection (FDS). But it is difficult to observe the time-lagged oscillations between host and parasite genotypes that should result from FDS. To evaluate the potential for FDS, we tested for local adaptation of parasites to common clones, the role of host genetic diversity in resistance to parasites, and genetic correlations among fitness, parasitism, and the frequency of host clones. We studied three populations of Arabis holboellii, a short-lived apomictic (asexual by seed) plant attacked by rust fungi and insect herbivores. To estimate clone frequency, we used polymorphic allozyme markers on 200 individuals in each population in 1990 and in 2000. We also recorded levels of parasitism and host fitness (fruit production). Only the rust fungi showed evidence for local host adaptation; they usually increased in incidence as a function of clone frequency, and they tracked temporal change in clone frequency. In further support of FDS, parasitism was lower in populations with higher genetic diversity. However, total parasitism (herbivory and disease combined) decreased as host clone frequency and fitness increased. Thus, although the highly virulent rust pathogen showed potential for driving the cycles that result from FDS, this apparently does not occur in the populations studied because the host clones were also attacked by herbivores.
Indirect positive effects of a parasitic plant on host pollination and seed dispersal
Oikos, 2014
Parasitic plants often have a strong fitness-impact on their plant hosts through increased host mortality and reduced or complete suppression of reproduction. Tristerix corymbosus (Loranthaceae) is a hemiparasitic mistletoe that infects a wide range of host species along its distribution range. Among such species, Rhaphithamnus spinosus (Verbenaceae) is a frequent host with a flowering and fruiting season partially synchronized with mistletoe reproductive phenology. As parasitized hosts have, in principle, a larger flower display and fruit crop size than non-parasitized hosts, we examined whether host and parasite reproductive synchrony make infected hosts more attractive for pollinators and seed dispersers than uninfected hosts. Our results showed that pollinator visit rates did not differ between parasitized and non-parasitized hosts. Conversely, seed rain was higher in parasitized than non-parasitized individuals. The number of seeds fallen under non-parasitized plants was spatially associated with crop size, while parasitized plants did not show such association. Finally, the number of seedlings of R. spinosus was significantly larger near parasitized than non-parasitized hosts. Our results suggest that the presence of the mistletoe might be responsible of the higher reproductive success showed by the parasitized fraction of R. spinosus. This effect, however, seems to be related to seed dispersal processes rather than pollination effects.
Inbreeding alters a plant-predispersal seed predator interaction
Evolutionary Ecology, 2011
The effect of inbreeding on genetic diversity is expected to decrease plant defences or vigour-related traits that, in turn, can modify the pattern of attack by herbivores. The selective damage caused by herbivores can produce variable fitness costs between inbred and outcrossed progenies influencing the evolution of a species’ plant mating system. By exposing inbred and outcrossed plants to natural conditions of seed predation, we assessed whether inbreeding increases weevil incidence and infestation, and how weevil seed predation affects the fitness of inbred and outcrossed progeny. To test if inbreeding affected the host’s plant quality, we weighed the biomass of weevils developed in inbred and outcrossed progenies. An additional experiment was carried out to examine whether weevils preferentially attack vigorous plants regardless from the level of inbreeding. The average value of leaf size was 21% larger in outcrossed plants than in inbred plants. Likewise, weevil incidence and infestation were 13 and 40%, respectively, higher on outcrossed plants relative to their inbred counterparts. However, the relative impact of seed predation was significantly lower in outcrossed progeny than in inbred progeny. In contrast, inbreeding did not alter host plant quality and weevils developed in inbred and outcrossed plants had a similar biomass. Variations in fruit number were consistently associated with the infestation level in both experiments, whereas leaf size only predicted the number of weevils in one experiment, suggesting that fruit number is the most influential vigour-related characteristic of a weevil attack. These findings indicate that the costs of inbreeding of the interaction D. stramonium-T. soror were higher for inbred plants than for outcrossed plants. The interaction between seed predation and inbreeding depression could prevent the fixation of selfing as a unique reproductive strategy in D. stramonium.
Austral Ecology, 2011
Interactions between species pairs are almost always mediated by interactions with other species. The outcomes of these multispecies interactions are often difficult to predict and are rarely studied. In addition to their theoretical importance, multispecies interactions are also important for management situations. Where multiple agents are used to control invasive species, interactions between agents may either enhance or reduce the impacts on the target species, or may simply have additive effects. In this study, conducted in a Mediterranean-type woodland in Australia, we examined how the interaction between an invasive legume, Cytisus scoparius (Leguminosae), its pollinator, Apis mellifera and a seed predator, Bruchidius villosus (Bruchidae), are modified by a native hemiparasitic vine, Cassytha pubescens (Loranthaceae). The parasite had a direct negative effect on C. scoparius, reducing flowering by 50% and consequently fruit and seed production. Despite having fewer flowers, infected plants had the same proportion of 'tripped' flowers, an indirect measure of pollinator visitation, as uninfected plants; although fruit formed on infected plants it was more likely to abort prematurely. Seed predation by B. villosus was lower on parasite-infected C. scoparius plants than in uninfected plants. Although Ca. pubescens had an antagonistic effect on B. villosus, in consort, the two agents reduced overall seed production by an average of 62%. The acquired parasite Ca. pubescens was more effective in reducing reproductive output than the introduced seed predator, B. villosus, and shows potential as a biocontrol agent for C. scoparius. We documented a subadditive effect of two biological enemies on the invasive species, where the acquired parasite had a stronger effect than the introduced seed predator.
Differences in life history strategies between an invasive and a competing resident seed predator
Biological Invasions, 2008
Competitive interactions may arise from biological invasion if a successful invasive species requires the ecological niche of a resident one. Lifehistory traits that make a species a successful invader are of particular interest in elucidating both invasion success and how interspecific competition may emerge. In southeastern France, the invasion of cedar forests by the seed chalcid Megastigmus schimitscheki generated competitive relationships with the resident M. pinsapinis for the exploitation of the seed resource. Among the numerous ecological traits allowing these seed predators to exploit their niche successfully, the timing of adult emergence, initial egg load and age-specific realized fecundities of females were investigated to help understanding the issue of such interspecific relationships. Spring adult emergence of M. schimitscheki under natural conditions was significantly earlier than that of M. pinsapinis, suggesting that an advantage for the access to the seed resource for the invasive species may be associated with this trait. Initial egg load was significantly higher in M. schimitscheki than in M. pinsapinis and the analysis of age-specific realized fecundity in semi-natural conditions indicated that both M. schimitscheki and M. pinsapinis females lay a large proportion of their eggs during the early days of their lives. In the light of these findings, both earlier phenology and higher reproduction abilities of M. schimitscheki may have the potential to confer an advantage within a competitive context with M. pinsapinis through enhanced seed resource preemption. This may potentially explain the invasiveness of M. schimitscheki in southern France despite the presence of its closely related competitor M. pinsapinis.
The predator satiation hypothesis states that synchronous periodic production of seeds is an adaptive strategy evolved to reduce the pressure of seed predators. The seed production pattern is crucial to the predator satiation hypothesis, but there are few studies documenting the success of individuals that are in synchrony and out of synchrony with the whole population. This study is based on long-term data on seed production of Sorbus aucuparia and specialised pre-dispersal seed predation by Argyresthia conjugella, in a subalpine spruce forest in the Western Carpathians (Poland). At the population level, we tested whether functional and numerical responses of predators to the variation of fruit production operate. At the individual level, we tested whether individuals with higher interannual variability in their own seed crops and higher synchrony with the population have higher percentages of uninfested fruits. The intensity of pre-dispersal seed predation was high (average 70 %; range 19-100 %). There were both functional and numerical responses of predators to the variation of fruit production at the population level. We found that individuals that were expected to be preferred under seed predator pressure had higher reproductive success. With increasing synchrony of fruit production between individual trees and the population, the percentage of infested fruits decreased. There was also a negative relationship between the interannual variation in individual fruit production and the percentage of infested fruits. These results confirm selection for individuals with a masting strategy. However, the population does not seem well adapted to strong seed predation pressure and we suggest that this may be a result of prolonged diapause of A. conjugella.
Assessment of parasite-mediated selection in a host-parasite system in plants
Ecology, 2000
A two-year field study was conducted to evaluate the potential of two cactus species, Echinopsis chilensis and Eulychnia acida, to evolve defensive traits against the parasitic mistletoe Tristerix aphyllus (Loranthaceae). The adaptive value of host traits against parasitism was inferred through: (1) identification of the relevant characters of cacti to prevent infection, (2) evaluation of the fitness impact of parasitism on cacti, and (3) estimation of the linear and nonlinear selection coefficients on the relevant characters. Different lines of experimental and correlative evidence indicated that spine length was important in preventing individuals of the two cactus species from becoming parasitized. However, the impact of the mistletoe on cactus fecundity was contingent on the species involved. Even though parasitism decreased fruit production, seed number per fruit, and the total seed output in E. chilensis, low and nonsignificant linear and nonlinear gradients of selection were prevalent in this species, indicating absence of directional and stabilizing/disruptive selection for spine length. Additional analysis based on logistic regression, however, revealed that long-spined E. chilensis had a higher probability of reproduction than did short-spined individuals. Unlike its effect on E. chilensis, the mistletoe had no fitness impact on E. acida, and the maintenance and evolution of spines in this species could not be attributed to parasite-mediated selection. Even though spines act as a first line of defense against parasitism in the two cactus species, selection was detected only on E. chilensis. These results indicate that inferences on the adaptive value of host traits based only upon their role in preventing infection run the risk of overestimating parasite-mediated selection, and thus the potential for host–parasite coevolution.