which are attractive to fruit eating animals (frugivores), such as certain birds,
primates, or bats. In this case, the seeds can withstand the digestive process and
are discarded in the feces, usually at a different site. To be attractive, mature fruits are
often rich in nutritive sugars and have an aromatic smell. As only mature fruits
should be consumed, unripe fruits are often rich in bitter or acidic PSMs and do not
advertise themselves by conspicuous colors, i.e., they are mostly green [11, 15].
Plants use some of their PSM in addition as mobile nitrogen storage compounds,
as antioxidants or for UV defense (Fig. 2) [11, 15]. Thus, PSMs can have multiple
purposes for the plants producing and storing them.
1.4
Adaptation of Herbivores
Whereas chemical defense works in most instances against most polyphagous
herbivores, some monophagous herbivores have evolved a tolerance towards a
particular toxin of its host plants. Some insects can quickly detoxify and eliminate
dietary PSMs, a few other sequester these PSM and use them for their own defense
against predators. These specialists can thus exploit a specific host plant without
being poisoned. But they cannot feed on other unrelated plants [2, 20, 21, 25–30].
Special adaptations are required for this process, often involving several mutational steps. For example, larvae of the Monarch butterfly can feed on Asclepias
plants, which produce cardiac glycosides which are poisonous for most animals. In
the Monarch, a mutation of Na
+
, K
+ -ATPase in the binding site for cardiac
glycosides confers resistance. Larvae and adult Monarch butterflies become toxic
themselves and are avoided by insectivorous birds [31–35]. Similar adaptations have
been observed in insects, mostly in moths, butterflies, beetles, and bugs, and
comprise several neurotoxic PSM, such as cardiac glycosides, pyrrolizidine alkaloids, quinolizidine alkaloids, cyanogenic glucosides, but also iridoid glucosides,
glucosinolates, and polyphenols [2, 13, 21, 36–43]. In most instances, the exact
molecular adaptations have not been discovered.
As a consequence of these adaptations, many insects are mono- and
oligophagous.
2
Evolution and Function of Alkaloids
As mentioned above, plants produce a wide diversity of PSMs, which can
be classified by their ring structures or biochemical pathways from which
they derived (Fig. 1). Formally, we can distinguish between nitrogen-containing
PSM and nitrogen-free PSM, such as terpenoids, phenolics and tannins, anthraquinones and polyenes. The class of PSM with nitrogen in their molecule is dominated
by alkaloids, of which more than 27,000 structures have been described. In this
class, we also find nonprotein amino acids (NPAA; more than 700 structures),
glucosinolates (ca. 100 structures), cyanogenic glucosides (80 structures), and
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M. Wink
primates, or bats. In this case, the seeds can withstand the digestive process and
are discarded in the feces, usually at a different site. To be attractive, mature fruits are
often rich in nutritive sugars and have an aromatic smell. As only mature fruits
should be consumed, unripe fruits are often rich in bitter or acidic PSMs and do not
advertise themselves by conspicuous colors, i.e., they are mostly green [11, 15].
Plants use some of their PSM in addition as mobile nitrogen storage compounds,
as antioxidants or for UV defense (Fig. 2) [11, 15]. Thus, PSMs can have multiple
purposes for the plants producing and storing them.
1.4
Adaptation of Herbivores
Whereas chemical defense works in most instances against most polyphagous
herbivores, some monophagous herbivores have evolved a tolerance towards a
particular toxin of its host plants. Some insects can quickly detoxify and eliminate
dietary PSMs, a few other sequester these PSM and use them for their own defense
against predators. These specialists can thus exploit a specific host plant without
being poisoned. But they cannot feed on other unrelated plants [2, 20, 21, 25–30].
Special adaptations are required for this process, often involving several mutational steps. For example, larvae of the Monarch butterfly can feed on Asclepias
plants, which produce cardiac glycosides which are poisonous for most animals. In
the Monarch, a mutation of Na
+
, K
+ -ATPase in the binding site for cardiac
glycosides confers resistance. Larvae and adult Monarch butterflies become toxic
themselves and are avoided by insectivorous birds [31–35]. Similar adaptations have
been observed in insects, mostly in moths, butterflies, beetles, and bugs, and
comprise several neurotoxic PSM, such as cardiac glycosides, pyrrolizidine alkaloids, quinolizidine alkaloids, cyanogenic glucosides, but also iridoid glucosides,
glucosinolates, and polyphenols [2, 13, 21, 36–43]. In most instances, the exact
molecular adaptations have not been discovered.
As a consequence of these adaptations, many insects are mono- and
oligophagous.
2
Evolution and Function of Alkaloids
As mentioned above, plants produce a wide diversity of PSMs, which can
be classified by their ring structures or biochemical pathways from which
they derived (Fig. 1). Formally, we can distinguish between nitrogen-containing
PSM and nitrogen-free PSM, such as terpenoids, phenolics and tannins, anthraquinones and polyenes. The class of PSM with nitrogen in their molecule is dominated
by alkaloids, of which more than 27,000 structures have been described. In this
class, we also find nonprotein amino acids (NPAA; more than 700 structures),
glucosinolates (ca. 100 structures), cyanogenic glucosides (80 structures), and
156
M. Wink