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Evolution of Chemical Defenses in Plants? lecture by Ranessa Cooper

Biology 606 Lecture Summary by Yazdan Keivany

Several morphological and chemical defenses are used by plants against herbivorous insects. The morphological traits are: epidermis, color, shape, thorns, spines, prickles, surface waxes, silica, cellulose, lignin, cell proliferation, and trichomes. Trichomes vary in density, erection, length, shape, and chemical contents. The chemical contents of trichomes are toxic or sticky. Plants produce two kinds of chemicals; primary and secondary. The primary chemicals are the metabolites. The secondary chemicals are phenolics (e.g., Tannins and phenols), Nitrogen components (e.g., nonprotein amino acids, Coniine), and Terpenoids (e.g., cardenolides, cucurbitacins). How these defenses evolved, is a key question.

There are two competing theories which address this problem; coevolution and sequential theories. Coevolution is the interdependent evolution of two or more species that occurs as a result of their interactions over a long period of time. In this context, plants evolve new chemicals for defense and insects evolve detoxification mechanisms. In the sequential evolution theory, each of these characters is achieved independently to serve other ecological or physiological needs, so natural enemies do not affect plant evolution. One of the main premises of the coevolutionary theory is that phytophagous insects reduce plant fitness and therefore are important selection factors in plant evolution. The other premise is that insect attack selects for resistance in plants and may result in radiation of plants. Based on the sequential theory, evolution of biochemical diversity in angiosperms serve as the major factor in their radiation.

To test the coevolution versus sequential theory of defenses in plants, Mauricio and Rausher (1997) chose Arabidopsis thaliana, which is found through North America, as the host plant and its natural enemies (e.g., two species of beetles) as the predators. They planted 1728 individuals of A. thaliana in North Carolina and exposed half of them to the natural enemies at natural densities and sprayed the other half regularly with pesticides in order to remove herbivores and fungi. They measured the trichome density and glucosinolates concentration as the indicators of selection, the total number of leaf damage holes and their diameter as indicators of herbivory, and the total number of their fruit as indicator of fitness.

Based on table 1, in the sprayed treatment directional selection favors a decrease in trichome density and glucosinolate concentration, and stabilizing selection occurred on glucosinolate. Based on table 2A there is no significant difference between treatments in the directional selection gradient, in other words, natural enemies did not modify the pattern of directional selection for these characters. Based on table 2B, the stabilizing selection gradients were significantly different between treatments for trichome density and glucosinolate concentration. This means that natural enemies modify the pattern of stabilizing selection on these two characters.

The three requirements for coevolution are genetic variation for a trait, fitness variation with the trait, and contribution of the putative selection agents. This paper is interesting and is a good example of coevolution. Authors looked at several characters to contrast coevolution and sequential theories in explaining the defense evolution in plants. They concluded that this study satisfies the three aforementioned requirements and supports the coevolution of defensive characters in plants.

Focal Paper:

Mauricio, R. and M. D. Rausher. 1997. Experimental manipulation of putative selective agents provides evidence for the natural enemies in the evolution of plant defense. Evolution, 51(5): 1435-1444.

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Summary of the discussion based on the focal paper by Mauricio & Rausher (1997)

Discussants: Keith Jackson and Greg Wilson

The originality of the idea presented in the focal paper was questioned. However, it was asserted that this paper is crucial to defend the theory of coevolution and to show the pattern that insects really influence the defensive characters in plants.

There was a concern about the difficulty of interpreting diagrams and statistical analyses in the focal paper. Three dimensional graphs and counter plots were difficult to interpret fully without any types of error bars. It was suggested that the authors should have presented the data itself, so that we can assess the accuracy of the figures.

Someone suggested that since predation has real influence, if the amount of predation were used as a continuous character instead of binary (present/absent), it would better show the correlation between, trichome density, glucosinolate concentration, and fitness, as measured by seed production.

It was asked whether hairs act against predation. There was an agreement that they do protect plants. However, it was indicated that the point in the paper is to show the coevolution of the organisms not the effectiveness of the hairs in predation.

Since the fitness was accounted for by total seed production, there was a concern that germination may affect fitness. Selection may be affected by germination and can also be in different directions in different stages of life and may be no selection in one stage or the other at all. On the other hand, selection can act in opposite directions in different stages, so that the net effect is zero selection.

In discussing the coevolution versus sequential theory it was suggested that there might not be enough time for sequential effects in this study. Evolution can not be shown in only one generation. What they showed was that the fitness of a given phenotype was dependent on the presence or absence of herbivores, so, they demonstrated the potentials for evolution, not the evolution itself. While coevolution implies reciprocal influence of organisms, in this paper, only the effects of insects on plants was studied, and there was nothing about the effects of plants on insects. In fact, they showed the coevolution in one direction.

A question was raised about the validity of the paper in showing evolution. What if the response triggered by insects was just a physiological adaptation or phenotypic plasticity with no evolutionary consequences? They looked at defensive evolution of trichome density and fitness. In control plots there was advantage to produce more trichome and glucosinolate. This selective pressure lead to produce more seeds in the fittest plant. If this measure is correct, it is showing evolution because they show differential fitness which is heritable and results in divergence of two populations. Although different plants respond differently under pressure, as long as genotypes vary, selection pressures can act.

Genetic variability in the plants was high, so does this mean that there is no selection? Because beetle and plants have lived together for a long time, if they selected why there is so much genetic variation? Why did the variation persist? Some of the reasons for this problem might be due to their distribution in different habitats with different density, spatial and temporal heterogeneity in the levels of herbivory, and migration. It was also suggested that the populations may not have reached a state of genetic equilibrium.

 

Several suggestions were made for improving this study:

  • - Use seeds of the same plant instead of taking lots of plants.
  • - Design better experiments in many plots or natural gradients.
  • - Study this plant in their natural environment in Europe or work on native plants to North America.
  • - Study the effects of pesticides since they are not just pesticides, but chemicals which may affect plant physiology in different ways (e.g., pesticides may change seed production).
  • - Do a laboratory experiment on plants with pesticides and without pesticides.
  • - Study the glucosinolate fluctuation in this plant and the effects of environment on the quality of seeds which is not heritable.

 

There was an attempted to explain selection surfaces, directional selection, disruptive selection and stabilizing selection in terms of linear and polynomial fit, based on table 1.

 

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