Evolution of inducible defenses

Predation is a major selective force structuring biological communities and causing the evolution of defenses in many prey organisms. While permanent defenses evolve under constant predation pressure, inducible defenses are adaptations to heterogeneity in predation risk [1] and likely evolved under divergent selection regimes. Costs for the production or maintenance of defenses are saved during times when these defenses are not required. A fascinating aspect of the study of inducible defenses is that organisms with the same genotype can display drama ti cally different phenotypes in response to par­ ticular environmental factors that are required for activat ing genes that control the formation of these defenses. The interplay of genes and environment is far from being fully understood, above all since the genes and regulatory pathways have not been identified in most systems. The question whether ‘plasticity genes’ or inter­ actions between multiple loci regulate plasticity, is an unresolved question (for example, [2]). Inducible defenses evolved in a range of evolutionarily diverse organisms from bacteria to vertebrates [1], but waterfleas (Daphnia), small planktonic crustaceans, are particularly famous for their large variety of spectacular inducible defenses. These inducible defenses include prominent morpho­ logical changes preventing capture [3,4], adaptive shifts in body size contrasting predator’s prey­size selectivity [5], depth­selection behavior [6] to spectacular pheno­ mena such as vertical migration where tons of zoo­ plankton avoid predator encounters by changing their position in a diurnal rhythm.