Plants And Herbivore Coevolved Over Millennia, Evaluate Evidence Herbivores Drive Evolution Of Plant Defences 13

Front 1

Erlich and Raven escape and radiate

Back 1

• Plant develops novel phytochemical that has some physiological function and is also noxious to herbivores
• Toxic or repellent properties of plant reduce herbivory
• Plant is released from herbivory, enters new adaptive zone, evolutionary raditation
• Herbivore evolves tolerance to novel plant chemical, released from competition, evolutionary radiation, may even be attracted to compound/ utilise it

Front 2

Berenbaum 1983- plants containing coumarins

Back 2

No. of plant families
Basic coumarins Many
Hydroxycoumarins 30
Linear furanocoumarins 8
Angular furanocoumarins 2 (legumes, umbellifers)
As you move down the table, increasing: molecular complexity, no of plant spp and no of herbivore spp

Front 3

problems with coevolution idea

Back 3

o Do insect herbivores exert enough selection pressure on plants to promote coevolution? Effect= plant abundance with/ without herbivore.
Plant Herbivore Effect
Prickly pear cactus Moth 0.002
St john’s wort Beetle <0.01
Ragwort Moth 0.003
Water hyacinth Weevil <10-5

Front 4

large impacts

Back 4

 Colarado potato beetle and Leptinotarsa decemlineata, serious pest of potatoes. Feed on foliage and skeletonise the crop. Most chemical unsuccessful when used against beetle, can rapidly develop resistance. High fecundity, withstand natural enemy pressure.

Front 5

o Hairston, smith and slobodkin

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 Most plants remain green- impact of herbivores relatively small
 Most insect herbivores remain scarce
 Insect herbivores usually kept in check by natural enemies
 Most spp too rare to exert strong selection pressure on their hosts

Front 6

o Features of insect herbivore communities

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 Many plants are attacked by several insect herbivores
 Some insect herbivores are oligophagous or polyphagous
 These features probably restrict or prevent the development of tight reciprocal co-evolutionary relationships.

Front 7

o Proving cause and effect

Back 7

 Some interactions are result of mutual congruence
 E.g. an introduced pollinator that is able to forage on the flowers of a native plant spp- e.g. bumblebees and tree lupin in Tasmania, no coevolurtionary history

Front 8

o Glucosinolate profiles in Arabidopsis thaliana

Back 8

 Annual in Brassiaceae
 Glucosinolates act as defences
 GS-ELONG gene determines length of side change on glucosinolate molecule
 Frequency across Europe varies with latitude and longitude
 2 aphid specialists:
• Cabbage aphid dominates where 4-C side chains produced
• Turnip aphid dominates where 3-C produced
 Experiment
• Created synthetic Arabidopsis pops with equal no of genotypes producing 4/3-C types.
• 5 gens of selection by each aphid spp
• Each aphid spp selected for glucosinolate profile identical to that in field pop where that aphid sp predominated
• No aphid treatment produced low level of glucosinolates
 Herbivorous insects can impose strong natural selection on plant genotypes for defensive chemical with specific structures, can drive plant evolution. Importance of ecological context: different aphid specialists produced different evolutionary outcomes

Front 9

Agrawal et al 2012

Back 9

Insect herbivores are hypothesized to be major factors affecting the ecology and evolution of plants. We tested this prediction by suppressing insects in replicated field populations of a native plant, Oenothera biennis, which reduced seed predation, altered interspecific competitive dynamics, and resulted in rapid evolutionary divergence. Comparative genotyping and phenotyping of nearly 12,000 O. biennis individuals revealed that in plots protected from insects, resistance to herbivores declined through time owing to changes in flowering time and lower defensive ellagitannins in fruits, whereas plant competitive ability increased. This independent real-time evolution of plant resistance and competitive ability in the field resulted from the relaxation of direct selective effects of insects on plant defense and through indirect effects due to reduced herbivory on plant competitors.