plant secondary metabolites can be ascribed to the dominance of monarchs on
milkweeds [129]. These events highlight the influence of insects on the development
of plant secondary metabolites. Variable cardenolides production in milkweed has
been espied with different insect species. The sap-sucking insects (aphids) induce
lesser cardenolides production compared to the leaf-chewing insects. Thus, there is
disorder in the coevolution of monarchs and milkweeds [130]. Along with herbivores, geographical location has also determined the regulation of cardenolide
synthesis in milkweeds. Therefore, resistance profile (Na+/K+ ATPase) of monarchs
against milkweeds must deviate according to varied locations. Nevertheless,
researchers did not find any discrepancy in an extensive study on six monarch
populations [131].
Another example of coevolution is that of parsnip webworm (Depressaria
pastinacella) and the wild parsnip (Pastinaca sativa). In defense to the parsnip
webworm, the wild parsnip produces a large amount of furanocoumarins. Parsnip
webworms resist the furanocoumarins by detoxifying it by enzyme, P450 (as seen in
Sect. 4.2.1). They also pose behavioral defiance by feeding exclusively on parthenocarpic fruits of parsnip, which have lower levels of furanocoumarins [132]. In
spite of this phenomenon, in a study conducted, parsnip webworms were unable to
distinguish between high and low levels of furanocoumarins in artificial diet. It was
further speculated that octyl butyrate (deterrent), present in parallel with
furanocoumarins, is responsible for the behavioral alteration of webworms [132].
Here, it can be noted that only one secondary metabolite may not be the cause of a
certain modification in insect, rather a medley of chemicals are involved in the interrelation. Also, in this study, unlike the previous monarch-milkweed example, the
resistance offered by webworms is comparable to the change in the furanocoumarin
levels in parsnips.
From the above instances, it is apparent that the herbivores drive the evolution of
plant defenses. Nevertheless, not much is established about the impact of plant
defenses on herbivore diversification. A recent phylogenetic analysis of Inga and
lepidopteran coevolution suggests that the plant defense drive host selection of
herbivore and not plant evolution. Thus, the closely related Lepidopteran insects
fed on plants with similar defenses and not evolutionarily closely related Inga
species [133]. They further indicate an asymmetry in the coevolution of Inga and
Lepidoptera. As seen earlier, many coevolution theories propose the diversification
of plants due to the pressure exerted by herbivores. Nonetheless, this might not be
true for the insects. They adapt to a new host plant having defenses similar to its own
traits. If a host plant changes its defense, the insect might change the host to which it
is now resistant.
6
Conclusion
Numerous studies have revealed the molecular complexity in Plant-Insect interactions. It comprises an enumeration of molecules involved in insect perception, plant
defense system, and resistance in herbivore insects. Thus, any transition at the
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
37
milkweeds [129]. These events highlight the influence of insects on the development
of plant secondary metabolites. Variable cardenolides production in milkweed has
been espied with different insect species. The sap-sucking insects (aphids) induce
lesser cardenolides production compared to the leaf-chewing insects. Thus, there is
disorder in the coevolution of monarchs and milkweeds [130]. Along with herbivores, geographical location has also determined the regulation of cardenolide
synthesis in milkweeds. Therefore, resistance profile (Na+/K+ ATPase) of monarchs
against milkweeds must deviate according to varied locations. Nevertheless,
researchers did not find any discrepancy in an extensive study on six monarch
populations [131].
Another example of coevolution is that of parsnip webworm (Depressaria
pastinacella) and the wild parsnip (Pastinaca sativa). In defense to the parsnip
webworm, the wild parsnip produces a large amount of furanocoumarins. Parsnip
webworms resist the furanocoumarins by detoxifying it by enzyme, P450 (as seen in
Sect. 4.2.1). They also pose behavioral defiance by feeding exclusively on parthenocarpic fruits of parsnip, which have lower levels of furanocoumarins [132]. In
spite of this phenomenon, in a study conducted, parsnip webworms were unable to
distinguish between high and low levels of furanocoumarins in artificial diet. It was
further speculated that octyl butyrate (deterrent), present in parallel with
furanocoumarins, is responsible for the behavioral alteration of webworms [132].
Here, it can be noted that only one secondary metabolite may not be the cause of a
certain modification in insect, rather a medley of chemicals are involved in the interrelation. Also, in this study, unlike the previous monarch-milkweed example, the
resistance offered by webworms is comparable to the change in the furanocoumarin
levels in parsnips.
From the above instances, it is apparent that the herbivores drive the evolution of
plant defenses. Nevertheless, not much is established about the impact of plant
defenses on herbivore diversification. A recent phylogenetic analysis of Inga and
lepidopteran coevolution suggests that the plant defense drive host selection of
herbivore and not plant evolution. Thus, the closely related Lepidopteran insects
fed on plants with similar defenses and not evolutionarily closely related Inga
species [133]. They further indicate an asymmetry in the coevolution of Inga and
Lepidoptera. As seen earlier, many coevolution theories propose the diversification
of plants due to the pressure exerted by herbivores. Nonetheless, this might not be
true for the insects. They adapt to a new host plant having defenses similar to its own
traits. If a host plant changes its defense, the insect might change the host to which it
is now resistant.
6
Conclusion
Numerous studies have revealed the molecular complexity in Plant-Insect interactions. It comprises an enumeration of molecules involved in insect perception, plant
defense system, and resistance in herbivore insects. Thus, any transition at the
2 Plant-Insect Interaction: The Saga of Molecular Coevolution
37