nonspecialized herbivores. By contrast, digestibility reducers or quantitative defenses
are large molecules (accounting for 5–40% of dry weight, [164]) that would occur in
plants with a predictable distribution (i.e., apparent plants like long-living trees, shrubs,
and perennial grasses), having a deterrent effect on specialized herbivores [59, 141]. A
classic example of deterrent compounds are tannins from oaks. Tannins are the most
abundant secondary metabolites made by plants, ranging from 5% to 10% dry weight
of leaves. In vertebrate herbivores, tannins can decrease protein digestion. In phytophagous insects, tannins are especially prone to oxidize with high pH guts, forming
semiquinone radicals and quinones [14]. Condensed tannins of Quercus robur have
shown to negative correlate with growth and survival of specialized butterfly
Operophtera brumata (Lepidoptera: Geometridae) larvae [58, 168]. Several experimental studies have shown the contrasting effect of plant defenses (especially toxins),
as a function of the degree of specialization of herbivores. For instance, in Arabidopsis,
the larvae of the generalist lepidopteran Helicoverpa armigera, “cotton worm,” avoids
feeding on rosette leaves with a high content of glucosinolates [153], while other
specialist lepidopteran, the butterfly Pieris rapae, successfully feed on plants
containing glucosinolates. After ingesting leaf tissue, P. rapae synthesizes a protein
in the intestine which prevents the formation of isothiocyanates by reorienting the
hydrolysis of glucosinolates toward the formation of nitriles which are excreted with
feces. Some insects are capable not only of disabling the glucosinolato-myrosinase
system but can even use glucosinolates as a cue to locate their host plants [177]. These
evidences show how a single chemical attribute can have multiple effects (e.g., toxic,
repellent, or attractant, [86]) on the diverse consumers that plants faced along their life.
On this regard, contrasting patterns of selection can be expected.
6.2
Natural Selection and Herbivores’ Community
Empirical evidence suggests that herbivores act as selecting agents for secondary
compounds of plants on natural and experimental populations [17, 32]. Using
quantitative genetics or measuring natural selection, studies have documented that
plant’s compounds that reduce the impacts of herbivores are favored by (positive)
directional selection (e.g., [3, 152]) or by balancing or disruptive selection (e.g.,
[117, 120]). Studies have demonstrated that several chemical compounds of plants,
such as glucosinolates have significant phenotypic and genetic variance [5, 125].
Detection of genetic variance for particular compounds indicates that those traits are
likely to evolve. However, given that once selection acts on a population, the
variance and heritability in the progeny is reduced [51], high genetic variation
may suggests that selection is weak or constrained by trade-offs among traits when
facing multiple selective forces simultaneously [6, 156, 174].
The effect of multiple phytophagous animals on fitness of a shared host plant may
differ between herbivores (e.g., [160]). Specialized and generalist herbivores can exert
opposite selective pressures on chemical defensive traits. In experimental populations of
Brassica nigra (Brassicaceae), Lankau [108] manipulated the presence of the generalist
slug (Agriliomax reticulate), and the specialist aphid (Brevicoryne brassicae), to test
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
67
are large molecules (accounting for 5–40% of dry weight, [164]) that would occur in
plants with a predictable distribution (i.e., apparent plants like long-living trees, shrubs,
and perennial grasses), having a deterrent effect on specialized herbivores [59, 141]. A
classic example of deterrent compounds are tannins from oaks. Tannins are the most
abundant secondary metabolites made by plants, ranging from 5% to 10% dry weight
of leaves. In vertebrate herbivores, tannins can decrease protein digestion. In phytophagous insects, tannins are especially prone to oxidize with high pH guts, forming
semiquinone radicals and quinones [14]. Condensed tannins of Quercus robur have
shown to negative correlate with growth and survival of specialized butterfly
Operophtera brumata (Lepidoptera: Geometridae) larvae [58, 168]. Several experimental studies have shown the contrasting effect of plant defenses (especially toxins),
as a function of the degree of specialization of herbivores. For instance, in Arabidopsis,
the larvae of the generalist lepidopteran Helicoverpa armigera, “cotton worm,” avoids
feeding on rosette leaves with a high content of glucosinolates [153], while other
specialist lepidopteran, the butterfly Pieris rapae, successfully feed on plants
containing glucosinolates. After ingesting leaf tissue, P. rapae synthesizes a protein
in the intestine which prevents the formation of isothiocyanates by reorienting the
hydrolysis of glucosinolates toward the formation of nitriles which are excreted with
feces. Some insects are capable not only of disabling the glucosinolato-myrosinase
system but can even use glucosinolates as a cue to locate their host plants [177]. These
evidences show how a single chemical attribute can have multiple effects (e.g., toxic,
repellent, or attractant, [86]) on the diverse consumers that plants faced along their life.
On this regard, contrasting patterns of selection can be expected.
6.2
Natural Selection and Herbivores’ Community
Empirical evidence suggests that herbivores act as selecting agents for secondary
compounds of plants on natural and experimental populations [17, 32]. Using
quantitative genetics or measuring natural selection, studies have documented that
plant’s compounds that reduce the impacts of herbivores are favored by (positive)
directional selection (e.g., [3, 152]) or by balancing or disruptive selection (e.g.,
[117, 120]). Studies have demonstrated that several chemical compounds of plants,
such as glucosinolates have significant phenotypic and genetic variance [5, 125].
Detection of genetic variance for particular compounds indicates that those traits are
likely to evolve. However, given that once selection acts on a population, the
variance and heritability in the progeny is reduced [51], high genetic variation
may suggests that selection is weak or constrained by trade-offs among traits when
facing multiple selective forces simultaneously [6, 156, 174].
The effect of multiple phytophagous animals on fitness of a shared host plant may
differ between herbivores (e.g., [160]). Specialized and generalist herbivores can exert
opposite selective pressures on chemical defensive traits. In experimental populations of
Brassica nigra (Brassicaceae), Lankau [108] manipulated the presence of the generalist
slug (Agriliomax reticulate), and the specialist aphid (Brevicoryne brassicae), to test
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
67