Abstract
In temperate areas, insect larvae must decide between entering winter diapause or developing directly and reproducing in the same season. Long daylength and high temperature promote direct development, which is generally associated with a higher growth rate. In this work, we investigated whether the larval pathway decision precedes the adjustment of growth rate (state-independent), or whether the pathway decision is conditional on the individual’s growth rate (state-dependent), in the butterfly Pieris napi. This species typically makes the pathway decision in the penultimate instar. We measured growth rate throughout larval development under two daylengths: slightly shorter and slightly longer than the critical daylength. Results indicate that the pathway decision can be both state-independent and state-dependent; under the shorter daylength condition, most larvae entered diapause, and direct development was chosen exclusively by a small subset of larvae showing the highest growth rates already in the early instars; under the longer daylength condition, most larvae developed directly, and the diapause pathway was chosen exclusively by a small subset of slow-growing individuals. Among the remainder, the choice of pathway was independent of the early growth rate; larvae entering diapause under the short daylength grew as fast as or faster than the direct developers under the longer daylength in the early instars, whereas the direct developers grew faster than the diapausers only in the ultimate instar. Hence, the pathway decision was state-dependent in a subset with a very high or very low growth rate, whereas the decision was state-independent in the majority of the larvae, which made the growth rate adjustment downstream from the pathway decision.
Highlights
Generalist species often show diVerent phenotypes in diVerent environments due to either local genetic adaptations (Thompson 2005) or environmentally induced phenotypic plasticity (West-Eberhard 2003)
The major incentive for this study was to determine the causal factor for the higher larval growth rate under direct development compared to diapause development
We have investigated whether this growth rate adjustment reXects a state-independent pathway choice and is implemented downstream of the developmental pathway decision point, or whether the decision is in itself statedependent, so that larvae that grow slowly choose the diapause pathway while larvae that grow rapidly choose direct development
Summary
Generalist species often show diVerent phenotypes in diVerent environments due to either local genetic adaptations (Thompson 2005) or environmentally induced phenotypic plasticity (West-Eberhard 2003). The phenotypic response to diVerent environments can be either a gradual reaction norm (e.g., Nijhout 2003; West-Eberhard 2003; Oostra et al 2010) or the formation of two or several discrete phenotypes (e.g., Nijhout 2003; West-Eberhard 2003; Oostra et al 2010) induced in diVerent environments The formation of these discrete phenotypes is typically the result of a pathway switch that is developmentally upstream from the future phenotype. (b) which can be viewed as being equivalent to a decision point (Gotthard 2008), the diVerent discrete phenotypes follow diVerent developmental pathways This results in a nonoverlapping variety of phenotypes in diVerent environments (West-Eberhard 2003)
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