High Temperature Triggers Latent Variation among Individuals: Oviposition Rate and Probability for Outbreaks

It is anticipated that extreme population events, such as extinctions and outbreaks, will become more frequent as a consequence of climate change. To evaluate the increased probability of such events, it is crucial to understand the mechanisms involved. Variation between individuals in their response to climatic factors is an important consideration, especially if microevolution is expected to change the composition of populations.Here we present data of a willow leaf beetle species, showing high variation among individuals in oviposition rate at a high temperature (20 °C). It is particularly noteworthy that not all individuals responded to changes in temperature; individuals laying few eggs at 20 °C continued to do so when transferred to 12 °C, whereas individuals that laid many eggs at 20 °C reduced their oviposition and laid the same number of eggs as the others when transferred to 12 °C. When transferred back to 20 °C most individuals reverted to their original oviposition rate. Thus, high variation among individuals was only observed at the higher temperature. Using a simple population model and based on regional climate change scenarios we show that the probability of outbreaks increases if there is a realistic increase in the number of warm summers. The probability of outbreaks also increased with increasing heritability of the ability to respond to increased temperature.If climate becomes warmer and there is latent variation among individuals in their temperature response, the probability for outbreaks may increase. However, the likelihood for microevolution to play a role may be low. This conclusion is based on the fact that it has been difficult to show that microevolution affect the probability for extinctions. Our results highlight the urge for cautiousness when predicting the future concerning probabilities for extreme population events

Induced responses in willow determine feeding success of a gall-forming insect

In the willow Salix viminalis L. (Salicaceae), growth and defence are key responses determining feeding success of the gall midge Dasineura marginemtorquens Bremi (Diptera: Cecidomyiidae). The interaction includes induced growth (gall formation), and induced defence (hypersensitive response, HR). Salix viminalis exhibits large genotypic variation in resistance (larval mortality) against D. marginemtorquens. This thesis focuses on testing hypotheses developed from field observations of interactions between the willow and the gall midge. A specific aim was to gain insight into the mechanism(s) of resistance. Combined greenhouse and field experiments were used to test whether abiotic factors induce susceptibility in otherwise resistant genotypes at the time of gall initiation. The results suggest that plasticity in plant resistance is linked to ambient conditions, probably light. Resistance was associated with HR, but to different degrees among willow genotypes. Hydrogen peroxide, a marker of HR, was induced in genotypes expressing HR but not in resistant genotypes not showing symptoms. The genetic architecture of plant traits that determine larval mortality and HR was investigated by means of a QTL analysis. The presence of one particular allele at one single locus was enough to determine life or death of the gall midge. The defence hypothesis was not supported when examining the homologous region in the poplar genome. However, an auxin gene was located within the QTL area, lending some support for the starvation and anti-manipulation hypothesis. The place of birth (leaf position) was found to be important for insect fitness, and the quality of a gall seemed to be associated with the duration of the site as a resource sink. In conclusion, data suggest that the mechanism of resistance involves anti-manipulation of growth rather than defence. Such, nonreactivity genes may have pleiotropic effects, and the frequency of genes insensitive to manipulation might be restricted in nature due to selection compared with the gall stimulus phenotype

Induced plant responses in willow to a gall-forming insect

The interaction between the basket willow Salix viminalis, and the gall midge Dasineura marginemtorquens, includes responses known to enhance as well as reduce herbivore performance i.e., gall formation, and hypersensitive response (HR), respectively. There is large genotypic variation in resistance in S.viminalis against D. marginemtorquens. In the first of two studies in this thesis we report, variation in larval survival and induced plant responses within a full-sib S. viminalis family. Resistance (larval mortality) was associated with HR, but to different degrees among willow genotypes. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR but not in resistant genotypes without symptoms. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, yet unknown, mechanism of resistance may therefore be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. Resistant willows usually have a very few galled leaves, whereas on the great majority of leaves all larvae die before any galls are formed. This observation, of high but not complete resistance, formed the basis for the second study. We found support for rapid induced susceptibility i.e., factors acting on the target leaf immediately before and during gall formation, but no support for delayed induced susceptibility, i.e., responses induced in the shoot the previous year and expressed when leaves develop the next season. In a combined greenhouse and field experiment the possibilities that susceptibility was induced by abiotic factors at the time of gall initiation were tested. Although resistance remained strong, significantly more leaves with surviving larvae, and galls were found on potted plants moved to the field, compared with identical plants that remained in the greenhouse. These findings suggest that plasticity in plant resistance may be linked to ambient light conditions. This system is a semi-natural one, where no intentional breeding for insect resistance has taken place. The interaction between S. viminalis and D.marginemtorquens includes responses and phenomena important for understanding plant-herbivore interactions in general. Thus, results from this research contribute to general models of plant resistance as well as to breeding programs in connection with short-rotation forestry based on willow

Summary of the regional climate change scenarios.

<p><i>T_RCM</i> is the threshold temperature in the regional climate scenarios that corresponds to <i>T_obs</i> = 16°C during the control period 1961–2005. This is the 98th percentile, which also means that the threshold is exceeded in 2% of the cases during the reference period. Columns <i>P_ex</i> is the probability of exceeding this threshold in the future scenario periods. The ERA40 control simulation only covers the control period and is not included in the ensemble summary statistics. Because the CCSM3 driven scenario exhibits a rather different time evolution towards the end of the century compared to the other models, we present ensemble statistics including this scenario included, and in within parentheses also ensemble statistics excluding the CCSM3 driven scenario. Column “Exp.” refer to different experiments with the same GCM; this is explained in section Data and Methods.</p

Probability for outbreaks increase with level of heritability and frequency of warm summers.

<p>Results from a model describing the relationship between heritability of the trait to be able to lay more eggs at high temperatures and the probability for outbreaks among leaf beetles feeding on willows. How this relationship is affected by the frequency of warm summers, corresponding roughly to different global warming scenarios, is presented; 0 =  no change in climate, 0.1 = 10% of the summers are so warm that individuals with the ability to substantially increase the number of eggs they lay per day can express their maximum potential, 0.2 = 20% of the summers are that warm, and 0.5 = 50% of the summers are that warm. It is assumed in the model that the ability to lay many eggs is determined by one allele in a single locus, and that the allele frequency in the population at start is low.</p

The oviposition rate of individual beetles varies more at high temperature than at low temperature.

<p>Average number of eggs laid per day by individual <i>Phratora vulgatissima</i> females kept either at constant temperature (20–20–20°C) during all three experimental periods (upper graph) or transferred from high to low and then back to high temperature (20–12–20°C).</p