plants (Pteridophytes, Gymnosperms, Angiosperms) (Fig. 5a). A common theme for
the largest groups of invertebrates is that they either feed on plants and/or that they
are pollinators [19, 21, 67, 68]. Many insects are monophagous or oligophagous,
meaning that they have specialized on one or a few host plants [21]. There is
an estimate that a single plant species can be a host for more than 10 insect
species. (In the biodiversity crisis, which we see today, a dramatic loss of insects
has been recorded. This appears to be due to the increased application of insecticides
in agriculture. In addition, the loss of plant diversity due to herbicides and over
fertilization needs to considered. If we destroy the diversity of plants, we also
destroy a food source for all the adapted herbivores.)
Why are there so many mono- and oligophagous insects? As discussed before,
plants produce a wide diversity of PSM with deterrent and toxic properties, whose
composition differs between plant parts and developmental stages (Fig. 2). Many
insects nevertheless evolved strategies to feed on chemically defended plants or more
precise a particular organ, by acquiring tolerance against the respective PSMs. If a
PSM is directed to a specific target in insects, such as a receptor, the binding site
might be changed by mutations so that the PSM can no longer bind to it (examples are
Na
+
, K
+ ATPase, or amino acyl tRNA synthetase). Other strategies involve a sequestration of a toxic PSM into a compartment where it does not affect the metabolism. Or
the degradation of PSM with cytochrome p450 enzymes or their export via ABC
transporters can be additional strategies. These adaptive mechanisms are not general
but relevant for a single group of compounds. Thus, there are several beetles which
can cope with the defense chemistry of seeds of a particular plant species but not with
those of its leaves or roots [2, 8, 20, 21, 25, 26, 36, 37, 41].
On top of this, we have the specialization for nectar feeding in insects, several of
which are also specific for a restricted selection of plant species. Thus, the evolution
of a large number of angiosperm species probably offered many ecological niches
for speciation of specialized insects.
As discussed before, the insects visiting a flower should not feed on the carpels or
other parts of the flower, but only transfer the pollen, which they carry on their
integument, to the stigma of the pistil. We can speculate that plants should protect
their reproductive organ against herbivores. This is indeed the situation, and flowers
usually harbor substantial amounts of PSM, which could function as repellents or
deterrents. As mentioned before, many of the insect-pollinated angiosperms produce
neurotoxic alkaloids, which also accumulate in all parts of the flower (only rarely
in the nectar). Since neurotoxicity occurs immediately, such toxins should be
well suited to ward off herbivores and to prevent a pollinating insect to feed on
parts of the flower.
5
Conclusions
The abundance of alkaloid-producing plants among animal-pollinated angiosperms
(Fig. 4) suggests that alkaloid evolution was enhanced during the last 150 million
years when angiosperms started to radiate and to produce showy flowers that needed
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
171
the largest groups of invertebrates is that they either feed on plants and/or that they
are pollinators [19, 21, 67, 68]. Many insects are monophagous or oligophagous,
meaning that they have specialized on one or a few host plants [21]. There is
an estimate that a single plant species can be a host for more than 10 insect
species. (In the biodiversity crisis, which we see today, a dramatic loss of insects
has been recorded. This appears to be due to the increased application of insecticides
in agriculture. In addition, the loss of plant diversity due to herbicides and over
fertilization needs to considered. If we destroy the diversity of plants, we also
destroy a food source for all the adapted herbivores.)
Why are there so many mono- and oligophagous insects? As discussed before,
plants produce a wide diversity of PSM with deterrent and toxic properties, whose
composition differs between plant parts and developmental stages (Fig. 2). Many
insects nevertheless evolved strategies to feed on chemically defended plants or more
precise a particular organ, by acquiring tolerance against the respective PSMs. If a
PSM is directed to a specific target in insects, such as a receptor, the binding site
might be changed by mutations so that the PSM can no longer bind to it (examples are
Na
+
, K
+ ATPase, or amino acyl tRNA synthetase). Other strategies involve a sequestration of a toxic PSM into a compartment where it does not affect the metabolism. Or
the degradation of PSM with cytochrome p450 enzymes or their export via ABC
transporters can be additional strategies. These adaptive mechanisms are not general
but relevant for a single group of compounds. Thus, there are several beetles which
can cope with the defense chemistry of seeds of a particular plant species but not with
those of its leaves or roots [2, 8, 20, 21, 25, 26, 36, 37, 41].
On top of this, we have the specialization for nectar feeding in insects, several of
which are also specific for a restricted selection of plant species. Thus, the evolution
of a large number of angiosperm species probably offered many ecological niches
for speciation of specialized insects.
As discussed before, the insects visiting a flower should not feed on the carpels or
other parts of the flower, but only transfer the pollen, which they carry on their
integument, to the stigma of the pistil. We can speculate that plants should protect
their reproductive organ against herbivores. This is indeed the situation, and flowers
usually harbor substantial amounts of PSM, which could function as repellents or
deterrents. As mentioned before, many of the insect-pollinated angiosperms produce
neurotoxic alkaloids, which also accumulate in all parts of the flower (only rarely
in the nectar). Since neurotoxicity occurs immediately, such toxins should be
well suited to ward off herbivores and to prevent a pollinating insect to feed on
parts of the flower.
5
Conclusions
The abundance of alkaloid-producing plants among animal-pollinated angiosperms
(Fig. 4) suggests that alkaloid evolution was enhanced during the last 150 million
years when angiosperms started to radiate and to produce showy flowers that needed
8 Evolution of the Angiosperms and Co-evolution of Secondary Metabolites. . .
171