species. Along with cardenolides, alteration of Na
+
/K
+ ATPase receptor also confers
resistance to ouabain in D. melanogaster and Oncopeltus fasciatus [123, 125].
Until now, very few insect species have been found to have developed target site
mutations against plant secondary metabolites. Although a very high resistance is
determined by the target site mutations, it results in high specificity and restricts the
ligand binding classes. Contrarily, other strategies like detoxification act against a
wide range of chemicals. This might be the leading cause of the metabolic resistance
strategies of detoxification and sequestration being evolutionarily more favourable
than target site mutations [126].
5
Coevolution in Plant Secondary Metabolites and Insects
The field of plant-insect interactions was revolutionized by the study done in 1964
by Ehrlich and Raven “Butterflies and Plants: A Study in Coevolution”. Though the
significance of plant-insect interaction was put forth by Darwin, Ehrlich and Raven
highlighted the impact of plant-insect arm race in the evolution of plant defense.
Thus, the concept of “coevolution” was introduced. Ever since then many studies are
being carried out to find the evolutionary cause of plant secondary metabolites and
their diversification. Ehrlich and Raven also proposed the “Escape and radiate”
theory according to which, insects render new resistance strategies and radiate into
a new species [127, 128]. Thus, the enormous speciation and dominance of plants
and insects observed today is the consequence of their intercommunication over
millions of years.
The arms race between plants and herbivores is long being debated to be the
primary root for secondary metabolites escalation. This has not only led to chemical
changes in these phytochemicals but also has expanded their complexity in plants. In
2009, Becerra et al. carried out a phylogenetic study of volatile chemicals among 70
Bursera species. This study perceived an evolutionary hike in the volatile compounds’ complexity. Thus, we can propose that over time, plants tend to adapt to the
virulent herbivores by increasing the intricacies of allele chemicals, rather than
introducing a new compound. Hence, this makes it difficult for herbivores to develop
new resistance strategies [87].
One of the most successful plant-insect interaction examples is that of monarchs
and milkweeds. There is a negative relationship between monarchs and milkweed.
Monarch caterpillars eat only milkweed plants and butterflies use milkweed to lay
eggs. Though the monarchs benefit from the milkweed, they are only pests for the
host plant and are ineffective in milkweed pollination. Thus, the coevolution of
monarchs and milkweed is not synergistic. The dominance of monarchs on milkweeds has led to the expansion and modification in the plant’s defense. One of the
coevolution theories predicts that the secondary metabolites might have evolved in
response to diversifying herbivores. Therefore, according to this, the newer plant
species must exhibit higher production of plant secondary metabolites. However,
Agrawal et al. in 2008 discovered chronological decrement in cardenolides and an
exclusive rise in phenolic levels in Asclepias species. This phenolytical reduction in
36
S. S. Zunjarrao et al.
+
/K
+ ATPase receptor also confers
resistance to ouabain in D. melanogaster and Oncopeltus fasciatus [123, 125].
Until now, very few insect species have been found to have developed target site
mutations against plant secondary metabolites. Although a very high resistance is
determined by the target site mutations, it results in high specificity and restricts the
ligand binding classes. Contrarily, other strategies like detoxification act against a
wide range of chemicals. This might be the leading cause of the metabolic resistance
strategies of detoxification and sequestration being evolutionarily more favourable
than target site mutations [126].
5
Coevolution in Plant Secondary Metabolites and Insects
The field of plant-insect interactions was revolutionized by the study done in 1964
by Ehrlich and Raven “Butterflies and Plants: A Study in Coevolution”. Though the
significance of plant-insect interaction was put forth by Darwin, Ehrlich and Raven
highlighted the impact of plant-insect arm race in the evolution of plant defense.
Thus, the concept of “coevolution” was introduced. Ever since then many studies are
being carried out to find the evolutionary cause of plant secondary metabolites and
their diversification. Ehrlich and Raven also proposed the “Escape and radiate”
theory according to which, insects render new resistance strategies and radiate into
a new species [127, 128]. Thus, the enormous speciation and dominance of plants
and insects observed today is the consequence of their intercommunication over
millions of years.
The arms race between plants and herbivores is long being debated to be the
primary root for secondary metabolites escalation. This has not only led to chemical
changes in these phytochemicals but also has expanded their complexity in plants. In
2009, Becerra et al. carried out a phylogenetic study of volatile chemicals among 70
Bursera species. This study perceived an evolutionary hike in the volatile compounds’ complexity. Thus, we can propose that over time, plants tend to adapt to the
virulent herbivores by increasing the intricacies of allele chemicals, rather than
introducing a new compound. Hence, this makes it difficult for herbivores to develop
new resistance strategies [87].
One of the most successful plant-insect interaction examples is that of monarchs
and milkweeds. There is a negative relationship between monarchs and milkweed.
Monarch caterpillars eat only milkweed plants and butterflies use milkweed to lay
eggs. Though the monarchs benefit from the milkweed, they are only pests for the
host plant and are ineffective in milkweed pollination. Thus, the coevolution of
monarchs and milkweed is not synergistic. The dominance of monarchs on milkweeds has led to the expansion and modification in the plant’s defense. One of the
coevolution theories predicts that the secondary metabolites might have evolved in
response to diversifying herbivores. Therefore, according to this, the newer plant
species must exhibit higher production of plant secondary metabolites. However,
Agrawal et al. in 2008 discovered chronological decrement in cardenolides and an
exclusive rise in phenolic levels in Asclepias species. This phenolytical reduction in
36
S. S. Zunjarrao et al.