Abstract
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.
Highlights
Growth and developmental rates of plants and ectothermal animals are plastic traits that are strongly affected by environmental temperature
We found a considerable phenotypic variation for the slope of the thermal reaction norm for growth rate in juvenile Orchesella cincta
We have carefully tried to keep these factors constant and we found no indications in our computerized temperature and humidity registrations that any of these factors varied in such a way that it could have explained our results
Summary
Growth and developmental rates of plants and ectothermal animals are plastic traits that are strongly affected by environmental temperature. A particular lower threshold temperature the metabolism of ectotherms is practically idle and no growth or development occurs. This threshold growth and developmental rates increase more or less linearly with temperature up to an upper limit after which further temperature increments rapidly become detrimental, Gilbert & Raworth, 1996; Trudgill et al, 2005). The functional relationship between temperature and performance is called a thermal reaction norm. A roughly linear relationship in the increasing part of the temperature reaction norm is rather common for many behavioural and physiological traits (Angilletta et al, 2002)
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