route of the aromatic amino acids phenylalanine, tyrosine, and tryptophan. This
route is restricted to microorganisms and plants and is responsible for the synthesis
of alkaloids and glycosides, among others [81]. (2) The mevalonate or the isoprenoid
pathway is the route by which terpenes and steroids are synthetized. This pathway is
present in eukaryotes, archaea, and some bacteria. Through this route, cholesterol,
vitamin K, and all steroid hormones are synthetized. (3) The acetate-malonate
pathway is the route of fatty acids and polyketides. This pathway is found in bacteria,
fungi and plants, and is responsible for the synthesis of aromatic and aliphatic
compounds, prostaglandins and some flavonoids, among many others [41]. Each
of these classes of phytochemicals that often characterize botanical families and even
a single genus have specific effects on animals (Table 3). Related compounds tend to
share a similar range of effects on plants’ consumers, affecting specialized cells and
tissues. For instance, alkaloids such as cocaine (found in Erythroxylum spp.:
Erythroxylaceae), atropine (found in Datura spp.: Solanaceae), and nicotine
(found in Nicotiana spp.: Solanaecea and other members of the nightshade family
of plants) affect nervous cells by inhibiting the reuptake of neurotransmitters and by
competing for the muscarinic receptors of acetylcholine [137, 143]. Also alkaloids,
in particular those derived from tropane, may affect the enzymatic activity of plants’
consumers inhibiting phosphodiesterase (e.g., [126]) and α-mannosidase enzymes
(e.g., [34]). The pharmacological and phytochemical study of secondary metabolites
have helped to understand their nature and the extent in to which they can affect
plants’ consumers either causing sickness or death by poisoning. For instance,
quantification of the lethal median dose (LD50) or median lethal concentration
(LC50) allows to determine the required dose to kill half the members of a tested
population after a specified test duration. For nicotine, the LC50 of humans ranges
from 6.5 to 13 mg/kg [118], whereas the LD50 of neonicotinoids (a class of
neuroactive insecticide chemically similar to nicotine) for bees typical ranges from
0.03 to 3.6 μg/bee [148]. Nicotine and neonicotinoids cause hyperactivity and death
in insects. Their extreme toxicity to insects (but not for specialized ones, see [176])
contrasts with their relative low toxicity to vertebrate taxa [116]. This selectivity is
due to a different kind of nicotinic acetylcholine receptors (nACh-R) found in
vertebrates [148, 169]. Work derived from bioassays and study cases of herbivores’
sensitivity to plants’ phytochemicals at cellular and molecular level exhibits the finetuning mechanisms of plants when faced consumers. These evidences support the
coevolutionary assumption of the reciprocal adaptive adjustments between plants
and consumers, mainly driven by plant chemistry.
6
Ecological Patterns: The Defensive Role of Secondary
Metabolites
In accordance with macroevolutionary patterns, many studies within species have
shown that current associations of plants and herbivores are often related to the
presence of a particular kind of phytochemical (e.g., [36, 164]). Unlike the phylogenetic approach, the ecological approach allows to directly measure natural selection,
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
65
route is restricted to microorganisms and plants and is responsible for the synthesis
of alkaloids and glycosides, among others [81]. (2) The mevalonate or the isoprenoid
pathway is the route by which terpenes and steroids are synthetized. This pathway is
present in eukaryotes, archaea, and some bacteria. Through this route, cholesterol,
vitamin K, and all steroid hormones are synthetized. (3) The acetate-malonate
pathway is the route of fatty acids and polyketides. This pathway is found in bacteria,
fungi and plants, and is responsible for the synthesis of aromatic and aliphatic
compounds, prostaglandins and some flavonoids, among many others [41]. Each
of these classes of phytochemicals that often characterize botanical families and even
a single genus have specific effects on animals (Table 3). Related compounds tend to
share a similar range of effects on plants’ consumers, affecting specialized cells and
tissues. For instance, alkaloids such as cocaine (found in Erythroxylum spp.:
Erythroxylaceae), atropine (found in Datura spp.: Solanaceae), and nicotine
(found in Nicotiana spp.: Solanaecea and other members of the nightshade family
of plants) affect nervous cells by inhibiting the reuptake of neurotransmitters and by
competing for the muscarinic receptors of acetylcholine [137, 143]. Also alkaloids,
in particular those derived from tropane, may affect the enzymatic activity of plants’
consumers inhibiting phosphodiesterase (e.g., [126]) and α-mannosidase enzymes
(e.g., [34]). The pharmacological and phytochemical study of secondary metabolites
have helped to understand their nature and the extent in to which they can affect
plants’ consumers either causing sickness or death by poisoning. For instance,
quantification of the lethal median dose (LD50) or median lethal concentration
(LC50) allows to determine the required dose to kill half the members of a tested
population after a specified test duration. For nicotine, the LC50 of humans ranges
from 6.5 to 13 mg/kg [118], whereas the LD50 of neonicotinoids (a class of
neuroactive insecticide chemically similar to nicotine) for bees typical ranges from
0.03 to 3.6 μg/bee [148]. Nicotine and neonicotinoids cause hyperactivity and death
in insects. Their extreme toxicity to insects (but not for specialized ones, see [176])
contrasts with their relative low toxicity to vertebrate taxa [116]. This selectivity is
due to a different kind of nicotinic acetylcholine receptors (nACh-R) found in
vertebrates [148, 169]. Work derived from bioassays and study cases of herbivores’
sensitivity to plants’ phytochemicals at cellular and molecular level exhibits the finetuning mechanisms of plants when faced consumers. These evidences support the
coevolutionary assumption of the reciprocal adaptive adjustments between plants
and consumers, mainly driven by plant chemistry.
6
Ecological Patterns: The Defensive Role of Secondary
Metabolites
In accordance with macroevolutionary patterns, many studies within species have
shown that current associations of plants and herbivores are often related to the
presence of a particular kind of phytochemical (e.g., [36, 164]). Unlike the phylogenetic approach, the ecological approach allows to directly measure natural selection,
3 Coevolution: Plant-Herbivore Interactions and Secondary Metabolites of Plants
65