Adult collembolans we:re fed with two different food types. Leaves of Taraxacum officinale and Dactylis glomerata were given as food sources in two physical forms: either as intact leaves or as powder. Foods were labelled with 15N. The 15N uptake curves were measured. Turnover rates and turnover times were calculated. Results show that Taraxacum leaves are preferred to Dactylis leaves. The physical condition of the leaves influenced the nitrogen turnover of the animals to a large extent. Fragmentation of Dactylis leaves enhanced nitrogen utilization by roughly two-fold and decreased turnover time by nearly half. Consumption of Taraxacum leaves has synergic effect on the nitrogen utilization of Dactylis leaves. Simultaneous consumption of Taraxacum and Dactylis enhanced the nitrogen uptake rate from Dactylis leaves compared to the treatment where Dactylis was the only food source.
Genetic variation for thermal plasticity plays an important role in the success or failure of a species with respect to the colonization of different thermal habitats and the ability to deal with climatic change. The aim of this paper is to study the relative contribution of the additive and non-additive components of genetic variation for the slope of the temperature reaction norm for juvenile growth rate in the springtail Orchesella cincta. We present the outcome of an artificial selection experiment for steep and flat temperature reaction norms and the results of a parent-offspring heritability experiment. There was a considerable phenotypic variation for the slope of the reaction norm. The selection experiment and the offspring to parent regression analysis, however, yielded no evidence for significant additive genetic variance. There were also no indications for maternal effects. The full-sib analysis, on the other hand, revealed a significant broad sense heritability of 0.76. An unforeseen result was that the slopes of females were steeper than those of males. This influenced the broad sense heritability of the full-sib analysis, since accidental female or male biased broods inflate the estimate of heritability. A randomization test showed that the probability level of the observed "between group" variance on the basis of the sexual differences alone was less than 10-5. From this we conclude that autosomal genetic variation played its own separate role. In conclusion, the thermal reaction norm for growth in juvenile O. cincta is not very much determined by the additive effects of a large number of independent genes, but more likely based on a still unknown but mainly non-additive, partially sex-related genetic mechanism, possibly including both dominance and epistatic effects. Hypotheses about the role of phenotypic plasticity in processes of local adaptation and speciation should thus be alert to such a complex genetic architecture.