Costs, benefits and the evolution of inducible defences: a case study with Daphnia pulex

Abstract

Phenotypic plasticity is one major source of variation in natural populations. Inducible defences, which can be considered threshold traits, are a form of plasticity that generates ecological and evolutionary consequences. A simple cost–benefit model underpins the maintenance and evolution of these threshold, inducible traits. In this model, a rank‐order switch in expected fitness, defined by costs and benefits of induction between defended and undefended morphs, predicts the risk level at which individuals should induce defences. Here, taking predator‐induced morphological defences in Daphnia pulex as a threshold trait, we provide the first comprehensive investigation into the costs and benefits of a threshold trait, and how they combine to reflect fitness and predict the switchpoint at which induction should occur. We develop reaction norms that show genetic variation in switchpoints. Further experiments show that induction can confer a survival benefit and a cost in terms of lifetime reproductive success. Together, these two traits combine to estimate expected fitness and can predict the switchpoint between an undefended and a defended strategy. The predictions match the reaction norm data for clones that experience these costs and benefits, and correspond well to independent field data on induction. However, predictions do not, and cannot, match for clones that do not gain a benefit from induction. This study confirms that a simple theory, based on life history costs and benefits, is a sufficient framework for understanding the ecology and evolution of inducible, threshold traits.

Phenotypic convergence along a gradient of predation risk

“…[16,18]). The neckteeth, similar to many defences induced in rotifer predation systems (for review, see [19]), confers a survival benefit to juveniles, which are the preferred prey of Chaoborus [20,21].…”

“…Experiments were conducted at 218C in controlled-temperature rooms on a 16 : 8 light cycle. For experiments involving predator cue, we extracted kairomone from frozen C. flavicans (Honka, Germany), following the method developed by Tollrian [23] (see also [21]). …”

“…Five neonates of the subsequent brood from each mother (a total of 15 neonates per genotype) were distributed individually into glass jars containing 50 ml of hard artificial pond water [22], food (2 Â 10 5 cells ml 21 algae) and the appropriate concentration of chemical cue. Daily, each animal was photographed, neckteeth induction scored (sensu [21,23]) and animals transferred to a new jar containing fresh media and cue. The experiment continued until all animals had reached maturity (the appearance of eggs in the brood pouch).…”

“…(d) Data analysis Univariate reaction norms: Reaction norms of neckteeth induction across the predator treatments were analysed with a three-parameter logistic model, using a nonlinear mixed-effects model (see [21,25]), with predator cue concentration as the fixed effect and genotype as a random effect. The three-parameter logistic model is a sigmoid curve with an asymptote (maximum induction), an inflection point (threshold concentration) and a scale parameter (steepness of the inflection).…”

Investigating the genetic architecture of conditional strategies using the environmental threshold model

“…Owing to the predominance of environmental influence over genetic control, the genetic component is often overlooked when studying conditional strategies. However, genetic variation still plays a central role [12], because a 'pure polyphenism' or 'pure conditional strategy', with no genetic variance underlying the response to the environment, can only be manifested in clonal populations that lack genetic variation [13]. In fact, the role of genetic variation in polyphenisms [14][15][16] should be expected, because it allows adaptive evolution on ecological timescales in changing environments, keeping phenotypes close to their optimum [17].…”