Inducible Defenses

Induced defenses are phenotypically plastic traits which are only expressed in the presence of potential biotic threats. They are widespread in nature and include responses to predators, herbivores, pathogens, parasites, and competitors. Chemical deterrents produced by grazed plants, reduced foraging behaviour or life-history shifts in animals under risk of predation, and the vertebrate immune system are among the many examples.

Current theory specifies four criteria for the maintenance of inducible defenses. (Harvell and Tollrian, 1998). Most importantly, proximity of the inducing agent must be unpredictably variable but occasionally exert strong selection. There must be a reliable cue which indicates potential threat and triggers the response. The defense must be effective, and it must incur costs which offset any advantage of the defense in the absence of threat.

If there is no such trade-off, the defensive trait should be fixed in the genome. This is supported by the general observation that induced responses are graded, which allows organisms to adjust defense to a level that is protective while minimizing associated costs.

Predator-induced defenses have recently been described in a number of ciliate protists. In particular, several freshwater species of the hypotrich genus Euplotes are known to express defensive morphologies (Kuhlmann and Heckman, 1985) and behaviour (Kusch, 1993b, Kuhlmann, 1994) in response to their predators.

Normal and induced morphs of Euplotes octocarinatus.

Scale bar is 40 microns.

Morphogenetic transformation in Euplotes involves a cytoskeletal reorganization in which lateral "wings" and dorsal and ventral projections develop (Jerka-Dziadosz et al, 1987). This results in an enlarged circular cell relative to the typical ovoid form. The dimensions of the defended morphotype exceed those of the mouth parts of inducing predators, preventing engulfment and ingestion (Kuhlmann and Heckmann, 1994).

Transformation occurs within hours of exposure to predators, continues for 24-36 hours and is reversed upon removal of predators or subculture of prey into fresh predator-free medium. The degree of defense depends on the density of predators and prey (Kusch, 1993a, Wiackowski and Staronska, 1999) and the availability of food (Wiackowski and Szkarlat, 1996).

Defensive transformation in Euplotes octocarinatus decreases population mortality rates by as much as 90-100% (Kuhlmann and Heckmann, 1994). Costs of defense are observed as a delay in division during transformation and a reduction of the population growth rate in subsequent reproductive cycles of defended cells (Kusch and Kuhlmann, 1994).

To date, predator-induced defenses have been shown to provide a long-term advantage to Euplotes while decreasing the fitness of predators in single-species prey populations (Kusch, 1998).

Stenostomum virginianum, a gape limited predator that induces morphological changes in Euplotes. Scale bar is 100 microns

References

Harvell, C.D. and R. Tollrian. 1998. Why Inducible Defenses? Pages 3-9 in Tollrian, R. and C.D. Harvell, (eds.), The Ecology and Evolution of Inducible Defenses. Princeton University Press, Princeton New Jersy.

Jerka-Dziadosz, M., Dosche, C., Kuhlmann, H-W. and Heckmann, K. 1987. Signal-induced reorganization of the microtubular cytoskeleton in the ciliated protozoon Euplotes octocarinatus. Journal of Cell Science 87:555-564.

Kuhlmann, H-W. and Heckmann, K. 1985. Interspecific morphogens regulating prey-predator relationships in protozoa. Science 227:1347-1349

Kuhlmann, H-W. and Heckmann, K. 1994. Predation risk of typical ovoid and ïwingedÍ morphs of Euplotes (Protozoa, Ciliophora). Hydrobiologia 284:219-227.

Kusch, J. 1993a. Induction of morphological changes in ciliates. Oecologia 94:571-575.

Kusch, J. 1993b. Behavioural and morphological changes in ciliates induced by the predator Amoeba proteus. Oecologia 96:354-359..

Kusch, J. 1998. Long-term effects of inducible defense. Ecoscience 5:1-7.

Kusch, J. and Kuhlmann, H-W. 1994. Cost of Stenostomum-induced morphological defense in the ciliate Euplotes octocarinatus. Archiv fuer Hydrobiologie 130:257-267.

Wiackowski, K. and A. Staronska. 1999. The effect of predator and prey density on the induced defense of a ciliate. Functional Ecology 13:59-65.

Wiackowski, K. and Szkarlat, M. 1996. Effects of food availability on predator-induced morphological defence in the ciliate Euplotes octocarinatus (Protista)

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Last modified 5.July.2005.