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Lecture 10

BIOL 3170 Lecture 10: Lecture 10 - Inducible and fixed defences

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York University
BIOL 3170
Mark Vicari

Inducible and fixed defences Inducible (aka plastic) defenses are produced/altered in reaction to damage or stress • A form of phenotypic plasticity, where different phenotypes can be produced by environmental effects (reaction norm) that influence the expression of a genotype Constitutive (aka fixed) defences are always present In animals: • Body form is altered during development if it is growing in an area where predators are common o E.g. animals grow thicker shells or longer spines in the presence of predators • E.g. mayfly nymphs o Have two defensive structures: tail filaments and abdominal spines o In presence of predators, tail filaments grew longer ▪ Therefore, potential inducible defence ▪ Next, tested whether longer filaments actually reduced predation: • Results showed they did o However, abdominal spines were similar length ▪ Therefore, constitutive defence So, if inducible defences reduce predation, why are the defences not constitutive i.e. why are they not always on? • Trade-off: Investment in the defence requires energy o E.g. Nymphs with longer filament are smaller which lowers reproductive output In plants: • Can be activated as needed, not only in development as in animals • Plants often form chemicals that are toxic to predators o E.g. nicotine and other alkaloids • Manufacturing of these chemicals is drastically increased when the plant is damaged o It is not always high because manufacturing of the toxic compounds is costly (trade-off) • For example, when tobacco plants are slightly induced to produce these compounds, they tend to be smaller or produce less seeds than those which are not (=cost-savings function) o However, when exposed to predators or extreme
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