Plant size, latitude, and phylogeny explain within-population variability in herbivory

Interactions between plants and herbivores are central in most ecosystems, but their strength is highly variable. The amount of variability within a system is thought to influence most aspects of plant-herbivore biology, from ecological stability to plant defense evolution. Our understanding of what influences variability, however, is limited by sparse data. We collected standardized surveys of herbivory for 503 plant species at 790 sites across 116° of latitude. With these data, we show that within-population variability in herbivory increases with latitude, decreases with plant size, and is phylogenetically structured. Differences in the magnitude of variability are thus central to how plant-herbivore biology varies across macroscale gradients. We argue that increased focus on interaction variability will advance understanding of patterns of life on Earth.

Spatial habitat heterogeneity influences host-pathogen dynamics in a patchy population of Ranchman's tiger moth

Host-pathogen dynamics are influenced by many factors that vary locally, but models of disease rarely consider dynamics across spatially heterogeneous environments. In addition, theory predicts that dispersal will influence host-pathogen dynamics of populations that are linked, although this has not been examined empirically in natural systems. We examined the spatial dynamics of a patchy population of tiger moths and its baculovirus pathogen, in which habitat type and weather influence dynamics. Theoretical models of host-baculovirus dynamics predict that such variation in dynamics between habitat types could be driven by a range of factors, of which we predict two are likely to be operating in this system: (1) differences in the environmental persistence of pathogens or (2) differences in host intrinsic rates of increase. We used time series models and monitored infection rates of hosts to characterize population and disease dynamics and distinguish between these possibilities. We also examined the role of host dispersal (connectivity) and weather as important contributors to dynamics, using time series models and experiments. We found that the population growth rate was higher, delayed density dependence was weaker, and long-period oscillations had lower amplitudes in high-quality habitat patches. The infection rate was higher on average in high-quality habitat, and this was likely to have been driven by higher mean population densities and no differences in pathogen persistence in different habitats (delayed density dependence). Time series modeling and experiments also showed an interactive effect of temperature and precipitation on moth population growth rates (likely caused by variation in host plant quality and quantity), and an effect of connectivity. Our results showed that spatial heterogeneity, connectivity, climate, and their interactions were important in driving host-baculovirus dynamics. In particular, our study found that connected patches and spatial heterogeneity generated differences in dynamics that only partially aligned with theoretical predictions.

The Role of Repetitive Sequences in Repatterning of Major Ribosomal DNA Clusters in Lepidoptera

Genes for major ribosomal RNAs (rDNA) are present in multiple copies mainly organized in tandem arrays. The number and position of rDNA loci can change dynamically and their repatterning is presumably driven by other repetitive sequences. We explored a peculiar rDNA organization in several representatives of Lepidoptera with either extremely large or numerous rDNA clusters. We combined molecular cytogenetics with analyses of second- and third-generation sequencing data to show that rDNA spreads as a transcription unit and reveal association between rDNA and various repeats. Furthermore, we performed comparative long read analyses among the species with derived rDNA distribution and moths with a single rDNA locus, which is considered ancestral. Our results suggest that satellite arrays, rather than mobile elements, facilitate homology-mediated spread of rDNA via either integration of extrachromosomal rDNA circles or ectopic recombination. The latter arguably better explains preferential spread of rDNA into terminal regions of lepidopteran chromosomes as efficiency of ectopic recombination depends on the proximity of homologous sequences to telomeres.

Effect of ozone exposure on the foraging behaviour of Bombus terrestris

Tropospheric ozone (O₃) mixing ratios have increased substantially since preindustrial times and high O₃ peaks are increasingly common. Plant-pollinator interactions are central to natural ecosystem functioning and food production systems but could be negatively affected by unfavourable environmental conditions such as elevated O₃. Ecosystem functioning is threatened by O₃, which can degrade floral volatile organic compounds (VOCs) used by pollinators as olfactory cues during foraging. It can also exert oxidative stress on VOC-emitting plants and receiving organisms, potentially disturbing the sending and receiving of VOC signals. The aim of this study was to determine the effects of elevated ozone on the foraging behaviour of Bombus terrestris on three species of the Brassicaceae, with a particular focus on bumblebee choices and the mechanisms underpinning differences observed. Moreover, the study was designed to fill a gap between observations in small-scale laboratory experiments and large-scale modelling through empirical observations in polytunnels that represent a medium-large-scale artificial environment. Using 10 × 3 × 2 m polytunnels the effects of O₃ on pollinator foraging parameters on Sinapis alba, Sinapis arvensis and Raphanus raphanistrum were assessed. Significant effects of elevated O₃ (100 ± 10 ppb) on the time taken for the first bee to alight on a flower and the cumulative amount of time spent on flowers was observed. To further investigate the underlying mechanisms, a laboratory test was conducted to determine the effects of ozone on the VOC blend composition of S. alba flowers. Synthetic VOC blends representing O₃-altered and unaltered profiles were reconstituted and utilized in polytunnel and olfactometry experiments. The results indicated that a reduction of olfaction-mediated orientation, probably via VOC-degradation or direct effects of O₃ on bees, was responsible for the altered foraging parameters of B. terrestris, suggesting that the presence of elevated O₃ could have negative effects on the foraging efficiency of important pollinator species.

Volatile-Mediated Induced and Passively Acquired Resistance in Sagebrush (Artemisia tridentata)

Plants produce a diversity of secondary metabolites including volatile organic compounds. Some species show discrete variation in these volatile compounds such that individuals within a population can be grouped into distinct chemotypes. A few studies reported that volatile-mediated induced resistance is more effective between plants belonging to the same chemotype and that chemotypes are heritable. The authors concluded that the ability of plants to differentially respond to cues from related individuals that share the same chemotype is a form of kin recognition. These studies assumed plants were actively responding but did not test the mechanism of resistance. A similar result was possible through the passive adsorption and reemission of repellent or toxic VOCs by plants exposed to damage-induced plant volatiles (DIPVs). Here we conducted exposure experiments with five chemotypes of sagebrush in growth chambers; undamaged receiver plants were exposed to either filtered air or DIPVs from mechanically wounded branches. Receiver plants exposed to DIPVs experienced less herbivore damage, which was correlated with increased expression of genes involved in plant defense as well as increased emission of repellent VOCs. Plants belonging to two of the five chemotypes exhibited stronger resistance when exposed to DIPVs from plants of the same chemotypes compared to when DIPVs were from plants of a different chemotype. Moreover, some plants passively absorbed DIPVs and reemitted them, potentially conferring associational resistance. These findings support previous work demonstrating that sagebrush plants actively responded to alarm cues and that the strength of their response was dependent on the chemotypes of the plants involved. This study provides further support for kin recognition in plants but also identified volatile-mediated associational resistance as a passively acquired additional defense mechanism in sagebrush.

Hilltopping influences spatial dynamics in a patchy population of tiger moths

Dispersal is a key driver of spatial population dynamics. Dispersal behaviour may be shaped by many factors, such as mate-finding, the spatial distribution of resources, or wind and currents, yet most models of spatial dynamics assume random dispersal. We examined the spatial dynamics of a day-flying moth species (Arctia virginalis) that forms mating aggregations on hilltops (hilltopping) based on long-term adult and larval population censuses. Using time-series models, we compared spatial population dynamics resulting from empirically founded hilltop-based connectivity indices and modelled the interactive effects of temperature, precipitation and density dependence. Model comparisons supported hilltop-based connectivity metrics including hilltop elevation over random connectivity, suggesting an effect of hilltopping behaviour on dynamics. We also found strong interactive effects of temperature and precipitation on dynamics. Simulations based on fitted time-series models showed lower patch occupancy and regional synchrony, and higher colonization and extinction rates when hilltopping was included, with potential implications for the probability of persistence of the patch network. Overall, our results show the potential for dispersal behaviour to have important effects on spatial population dynamics and persistence, and we advocate the inclusion of such non-random dispersal in metapopulation models.

Risk of herbivory negatively correlates with the diversity of volatile emissions involved in plant communication

Plant-to-plant volatile-mediated communication and subsequent induced resistance to insect herbivores is common. Less clear is the adaptive significance of these interactions; what selective mechanisms favour plant communication and what conditions allow individuals to benefit by both emitting and responding to cues? We explored the predictions of two non-exclusive hypotheses to explain why plants might emit cues, the kin selection hypothesis (KSH) and the mutual benefit hypothesis (MBH). We examined 15 populations of sagebrush that experience a range of naturally occurring herbivory along a 300 km latitudinal transect. As predicted by the KSH, we found several uncommon chemotypes with some chemotypes occurring only within a single population. Consistent with the MBH, chemotypic diversity was negatively correlated with herbivore pressure; sites with higher levels of herbivory were associated with a few common cues broadly recognized by most individuals. These cues varied among different populations. Our results are similar to those reported for anti-predator signalling in vertebrates.

Induction of the sticky plant defense syndrome in wild tobacco

Many plants engage in protective mutualisms, offering resources such as extrafloral nectar and shelters to predatory arthropods in exchange for protection against herbivores. Recent work indicates that sticky plants catch small insects and provide this carrion to predators who defend the plants against herbivores. In this study, we investigated whether wild tobacco, Nicotiana attenuata, fits this sticky plant defense syndrome that has been described for other sticky plants. We developed a bioassay for stickiness involving the number of flies that adhered to flowers, the stickiest tissues. In surveys conducted over three field seasons at four sites, we found that the number of carrion that adhered to a plant was positively correlated with the number of predators that we observed foraging over its surfaces. The number of predators was positively correlated with the number of seed capsules that the plant produced, a measure of lifetime female reproductive success. Structural equation modeling indicated strong support for the causal path linking carrion numbers to predator numbers to capsule production. We investigated whether stickiness was an inducible trait and examined two potential cues. We found that experimental clipping of rosette leaves induced greater stickiness, although clipping of neighboring sagebrush leaves did not. Damage to leaf tissue is likely to be a more reliable predictor of risk than is damage to a neighboring plant. The sticky plant defense syndrome is a widespread protective mutualism; its strength and ecological relevance can adjust as risk of herbivory changes.

As temperature increases, predator attack rate is more important to survival than a smaller window of prey vulnerability

Climate change can have strong effects on species interactions and community structure. Temperature-dependent effects on predator-prey interactions are a major mechanism through which these effects occur. To understand the net effects of predator attack rates and dynamic windows of prey vulnerability, we examined the impacts of temperature on the interaction of a caterpillar (Arctia virginalis) and its ant predator (Formica lasioides). We conducted field experiments to examine attack rates on caterpillars relative to temperature, ant abundance, and body size, and laboratory experiments to determine the effects of temperature on caterpillar growth. We modeled temperature-dependent survival based on the integrated effects of temperature-dependent growth and temperature- and size-dependent predation. Attack rates on caterpillars increased with warming and ant recruitment, but decreased with caterpillar size. Caterpillar growth rates increased with temperature, narrowing the window of vulnerability. The model predicted that net caterpillar survival would decrease with temperature, suggesting that A. virginalis populations could be depressed with future climate warming. Theoretical work suggests that the net outcome of predator-prey interactions with increasing temperature depends on the respective responses of interacting species in terms of velocity across space, whereas the present study suggests the importance of effects of temperature on prey window of vulnerability, or "velocity" across time.

Wet years have more caterpillars: interacting roles of plant litter and predation by ants

Climate is widely recognized as an important factor that affects temporal and spatial patterns of occurrence and abundance of herbivorous insects, although the ecological mechanisms responsible are poorly understood. We found that precipitation and standing water were positively correlated with locations and years of high abundance of caterpillars of the ranchman's tiger moth, Platyprepia virginalis. We analyzed 30 years of survey data and found that the number of large rainfall events was a better predictor of caterpillar abundance than total annual accumulation. We considered three ecological mechanisms that could drive this relationship and conducted observations and manipulative experiments to evaluate these mechanisms. (1) Rainfall facilitates more plant growth, although we found no evidence that increased food quality or quantity was causing the positive association between precipitation and caterpillar abundance. (2) Large rainfall events cause predatory ground-nesting ants to be less abundant and we found that the number of ants that recruited to local sites was negatively associated with survival and abundance of caterpillars. (3) We found that litter from wet sites provided a refuge from ant predation; litter from wet sites was not beneficial to caterpillars in the absence of ants. Both abiotic factors (precipitation) and biotic factors (predatory ants) affected the temporal and spatial abundance of caterpillars directly and interactively. Climate models predict that rainfall will become more variable, suggesting that populations of this caterpillar may also become more variable in the future.

Plant structural complexity and mechanical defenses mediate predator-prey interactions in an odonate-bird system

Habitat-forming species provide refuges for a variety of associating species; these refuges may mediate interactions between species differently depending on the functional traits of the habitat-forming species. We investigated refuge provisioning by plants with different functional traits for dragonfly and damselfly (Odonata: Anisoptera and Zygoptera) nymphs emerging from water bodies to molt into their adult stage. During this period, nymphs experience high levels of predation by birds. On the shores of a small pond, plants with mechanical defenses (e.g., thorns and prickles) and high structural complexity had higher abundances of odonate exuviae than nearby plants which lacked mechanical defenses and exhibited low structural complexity. To disentangle the relative effects of these two potentially important functional traits on nymph emergence-site preference and survival, we conducted two fully crossed factorial field experiments using artificial plants. Nymphs showed a strong preference for artificial plants with high structural complexity and to a lesser extent, mechanical defenses. Both functional traits increased nymph survival but through different mechanisms. We suggest that future investigations attempt to experimentally separate the elements contributing to structural complexity to elucidate the mechanistic underpinnings of refuge provisioning.

Synovial fluid proteins differentiate between the subtypes of juvenile idiopathic arthritis

OBJECTIVE

Juvenile idiopathic arthritis (JIA) is a heterogeneous group of inflammatory diseases, and no clinically useful prognostic markers to predict disease outcome in children with JIA are currently available. Synovial fluid likely reflects the proteins present in the inflamed synovium. The purpose of this study was to delineate the synovial fluid proteome and determine whether protein expression differs in the different subtypes of JIA.

METHODS

Synovial fluid samples obtained from children with oligoarticular JIA, polyarticular JIA, or systemic JIA were compared. Two-dimensional gel electrophoresis for protein separation and matrix-assisted laser desorption ionization-time-of-flight mass spectrometry and quadripole time-of-flight mass spectrometry for protein identification were used for this study. Synovial fluid cells were analyzed by polymerase chain reaction (PCR) for the presence of haptoglobin messenger RNA (mRNA).

RESULTS

The synovial fluid proteome of the samples was delineated. The majority of proteins showed overexpression in JIA synovial fluid as compared with noninflammatory control samples. There were 24 statistically significantly differentially expressed spots (>2-fold change; P < 0.05) between the subtypes of JIA. PCR analysis revealed haptoglobin mRNA, suggesting that haptoglobin is locally produced in an inflamed joint in JIA.

CONCLUSION

Despite the similar histologic appearance of inflamed joints in patients with different subtypes of JIA, there are differences in protein expression according to the subtype of JIA. Haptoglobin is differentially expressed between the subtypes of JIA and is locally produced in an inflamed joint in JIA. Haptoglobin and other differentially expressed proteins may be potential biomarkers in JIA.