Evolution of a predator-induced, nonlinear reaction norm

Abstract

Inducible, anti-predator traits are a classic example of phenotypic plasticity. Their evolutionary dynamics depend on their genetic basis, the historical pattern of predation risk that populations have experienced and current selection gradients. When populations experience predators with contrasting hunting strategies and size preferences, theory suggests contrasting micro-evolutionary responses to selection. Daphnia pulex is an ideal species to explore the micro-evolutionary response of anti-predator traits because they face heterogeneous predation regimes, sometimes experiencing only invertebrate midge predators and other times experiencing vertebrate fish and invertebrate midge predators. We explored plausible patterns of adaptive evolution of a predator-induced morphological reaction norm. We combined estimates of selection gradients that characterize the various habitats that D. pulex experiences with detail on the quantitative genetic architecture of inducible morphological defences. Our data reveal a fine scale description of daphnid defensive reaction norms, and a strong covariance between the sensitivity to cues and the maximum response to cues. By analysing the response of the reaction norm to plausible, predator-specific selection gradients, we show how in the context of this covariance, micro-evolution may be more uniform than predicted from size-selective predation theory. Our results show how covariance between the sensitivity to cues and the maximum response to cues for morphological defence can shape the evolutionary trajectory of predator-induced defences in D. pulex.

Ecology of predator-induced morphological defense traits in Daphnia longispina (Cladocera, Arthropoda)

“…Neckteeth expression was also dependent on Chaoborus kairomone concentration for one of our investigated D. longispina clones. Such concentration-dependent responses have been previously observed in numerous D. pulex clones (Havel 1985;Parejko and Dodson 1991;Tollrian 1993;Hammill et al 2008;Dennis et al 2011;Carter et al 2017). The sensitivity and magnitude of these responses appear to depend on the historical predation regime of the habitat.…”

“…Similar to the D. pulex clones from the Chaoborus-ponds (cf. Hammill et al 2008;Dennis et al 2011;Carter et al 2017) our clone showed close to maximum induction (70-80%) at 0.25 µL extract mL −1 , which corresponds to a Chaoborus density of ~ 13 larvae L −1 (Hammill et al 2008). This density may seem a little high compared to natural densities, but note that neckteeth can be induced already at densities < 13 larvae L −1 (Fig.…”

“…For instance, D. pulex clones from Chaoborus-free ponds generally showed weaker responses to imposed predation threat than clones from Chaoborus containing ponds (Parejko and Dodson 1991). Similarly, clones from Chaoborus-harboring but fishless ponds showed higher neckteeth induction at lower kairomone concentrations than clones from ponds with Chaoborus and fish (Hammill et al 2008;Dennis et al 2011;Carter et al 2017). Thus, the occurrence of neckteeth along the gradient of predation threat may depend on the duration a particular clone has spent in a predator-scarce environment in combination with the strength of selection against neckteeth formation (assuming fish can control Chaoborus densities to low levels).…”

Phenological mismatch drives selection on elevation, but not on slope, of breeding time plasticity in a wild songbird

“…Most of what we know about the evolutionary dynamics of the reaction norms stems from theoretical work or laboratory experiments (see Scheiner 1993;De Jong 1999;Van Asch et al 2007;Lande 2009) or from phylogenetic and population comparisons of reaction norms (Murren et al 2014), with very few empirical examples of how selection as well as (additive) genetic variation led to an evolutionary change in the reaction norm in wild populations (cf. Carter et al 2017;Van Asch et al 2013).…”

Quantifying evolution in wild populations

“…This linear function is often assumed to adequately describe the thermal reaction norms of phenological traits, but may be over-simplistic in other contexts (e.g. Brommer et al 2012;Carter et al 2017), in which case some higher polynomial (e.g. a quadratic term) may be more suitable.…”