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Herbivore adaptations to plant defense

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Herbivores are dependent on plants for food, and have evolved mechanisms to obtain this food despite plants’ diverse arsenal of defenses. Herbivores’ feeding adaptations have been likened to “offensive traits” and consist of those traits that currently allow increased feeding and use of a host (Karban and Agrawal 2002). Plants, on the other hand, are adapted with an arsenal of defenses to protect their resources for their own requirements, specifically growth and reproduction. Attack by herbivores selects for a defensive response by the plant, whether it be incorporated biochemically or physically, or induced as a counterattack. Relationships between herbivores and their host plants often result in reciprocal evolutionary change. In cases where this relationship demonstrates “specificity” (the evolution of each trait is due to the other), and “reciprocity” (both traits must evolve), the species are thought to have coevolved (Futuyma and Slatkin 1983). The escape and radiation mechanism for coevolution, described in Ehrlich and Raven (1964), presents the idea that adaptations in herbivores and their host plants, many of which are discussed below, result in speciation (Thompson 1999).

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Herbivore Adaptations

Physical Adaptations

Herbivores have developed a diverse range of physical structures to facilitate the consumption of plant material. To break up intact plant tissues, mammals develop teeth structures that reflect their feeding preferences. For instance, frugivores (animals that feed primarily on fruit) and herbivores that feed on soft foliage have low-crowned teeth specialized for grinding foliage and seeds. Grazers that eat hard silica-rich grasses have high-crowned teeth, which grind tough plant tissues and do not wear down as quickly as low-crowned teeth (Romer 1959). Birds grind plant material or crush seeds using their beaks.

Insect herbivores have a wide range of tools to facilitate feeding. Often these tools reflect an individual’s feeding strategy and its preferred food type (Bernays 1991). Bernays and Janzen (1988) observed that within the family Sphingidae (e.g. moths), the species that eat relatively soft leaves are equipped with incisors for tearing and chewing, while the species that feed on touch matures leaves and grasses cut them with toothless snipping mandibles (uppermost pair of jaws in insects, used for feeding).

An herbivore’s diet can shape its feeding phenotype. Thompson (1992) demonstrated that grasshopper head size, and thus chewing power, was greater for individuals raised on rye grass (a relatively hard grass) when compared to individuals raised on red clover (a soft diet). Larval lepidoptera that feed on plants with high levels of condensed tannins (as in trees) have more alkaline midguts when compared to lepidoptera that feed on herbs and forbs (pH of 8.67 vs. 8.29 respectively; Berenbaum 1980). This morphological difference can be explained by the fact that insoluble tannin-protein complexes disassociate at alkaline pH levels.

Microbial Symbionts

Herbivores are unable to digest complex cellulose and rely on mutualistic, internal symbiotic bacteria, fungi, or protozoa to exploit this resource. Microbial symbionts also allow herbivores to eat plants that would otherwise be inedible by detoxifying plant secondary compounds. For example, fungal symbionts of cigarette beetles use certain plant allelochemicals as their source of carbon in addition to producing detoxification enzymes (esterases) for other toxins (Dowd 1991). Microbial symbionts assist in the acquisition of plant material by weakening a host plant’s defenses. Some herbivores are more successful at feeding on damaged hosts (Karban and Agrawal 2002). As an example, many species of bark beetle introduce blue stain fungi of the genera Ceratocystis and Ophiostoma to trees before feeding. The blue stain fungi cause lesions that reduce the trees’ defensive mechanisms and allow the bark beetles to feed (Whitney 1982, Nebeker et al. 1993).

Biochemical Adaptations

Herbivores have enzymes that counter the numerous toxic secondary metabolic products produced by plants and that reduce their effectiveness. One such enzymes group, mixed function oxidases (MFOs), detoxify harmful plant compounds by catalyzing oxidative reactions (Feyereisen 1999). P-450 enzymes, a specific class of MFOs, have been specifically connected to detoxification of plant secondary metabolic products. Snyder and Glendinning (1996) linked herbivore feeding on plant material protected by chemical defenses with P-450 detoxification in larval tobacco hornworms. Snyder and Glendinning found that the induction of P-450 after initial nicotine ingestion allowed the larval tobacco hornworms to increase feeding on the toxic plant tissues (Snyder and Glendinning 1996).

Herbivores may also produce salivary enzymes that reduce the degree of defense generated by a host plant. Musser et al. (2002) demonstrated that the enzyme glucose oxidase, a component of saliva for the caterpillar Helicoverpa zea, counteracts the production of induced defenses in tobacco. Similarly, aphid saliva reduces its host’s induced response by forming a barrier between the aphid’s stylet and the plant’s cells (Felton and Eichenseer 1999).

Behavioral Adaptations

Herbivores can avoid plant defenses by eating plants selectively in space and time. Feeny (1970) determined that for the winter moth, feeding on oak leaves early in the season maximized the amount of protein and nutrients available while minimizing the amount of tannins. Herbivores can also spatially avoid plant defenses. The piercing mouthparts of species in Hemiptera allow them to feed around areas of high toxic concentration. Hagan and Chabot (1986) documented several species of caterpillar “window feeding” on maple leaves by feeding on pieces of leaf and avoiding tough areas of high lignin concentration. Similarly, the cotton leaf perforator selectively avoids eating the epidermis and pigment glands of their hosts, which contain defensive terpenoid aldehydes (Karban and Agrawal 2002).

Plant defense may explain, in part, why herbivores employ different life history strategies. Monophagous species (animals that eat plants in a single genus) must produce specialized enzymes to detoxify their food, or they must develop specialized structures to deal with sequestered chemicals. Polyphagous species (animals that eat plants from many different families), on the other hand, produce more MFOs (Krieger et al. 1971) to deal with a diversity of plant chemical defenses. Polyphagy often develops when an herbivore’s host plants are rare as a necessity to gain enough food. Monophagy is favored when there is interspecific competition for food, where specialization often increases a species’ competitive ability to utilize a resource (see Jaenike 1990).

Herbivore Use of Plant Chemicals

Plant chemical defenses can be eaten by herbivores, stored, and used in defense against predators. To be effective defensive agents, the sequestered chemicals cannot be metabolized into inactive products. Utilizing plant chemicals can be costly to herbivores because it often requires specialized handling, storage, and modification (Bowers 1992). This cost can be seen when plants that utilize plant chemicals are compared to those plants that do not in a situation where herbivores are excluded. Caterpillar and adult monarch butterflies store cardiac glycosides from milkweed, making these organisms distasteful. After eating a monarch caterpillar or butterfly, its bird predator will vomit and will avoid eating similar individuals in the future (Huheey 1984). Species that feed on milkweeds are usually aposematically colored. Aposomatic species are those that “advertise” their distastefulness by being brightly colored (see Guilford 1990).

Secondary metabolic products can also be useful to herbivores because the antibiotic properties of the toxins can protect the herbivore against pathogens (see Frings et al. 1948). Additionally, secondary metabolic products can act as cues to identify a plant for feeding or oviposition by herbivores.

Host Manipulation

Herbivores often manipulate their hosts in order to better utilize them as resources. Herbivorous insects favorably alter the microhabitat in which the herbivore feeds to counter existing plant defenses. For example, caterpillars from the families Pyralidae and Ctenuchidae roll mature leaves of the neotropical shrub Psychotria horizontalis around an expanding bud that they consume. This rolling reduces the entering light by 95% and this shading decreases the leaf toughness and leaf tannin concentration of the expanding bud while maintaining the amount of nutritional gain of nitrogen (Sagers 1992). Sanberg and Berenbaum (1989) demonstrated that the tying of leaves together and feeding inside by lepidoptera larvae could decrease the effectiveness of the phototoxin hypericin in St. John’s-wort.

Herbivores also manipulate their microhabitat by forming galls, plant structures comprised of plant tissue but controlled by the herbivore. Galls act as both domatia (housing), and food sources for the gall maker. The interior of a gall is composed of edible nutritious tissue. Larson and Whitham (1991) demonstrated that aphid galls in narrow leaf cottonwood acted as “physiologic sinks,” concentrating resources in the gall from the surrounding plant parts. Galls may also provide protection from predators for these herbivores (Weis and Kapelinski 1994).

Some herbivores exhibit feeding behaviors that disarm the defenses of their host plants. One such plant defensive strategy is the use of latex and resin canals that contain sticky toxins and digestibility reducers. These canal systems store fluids under pressure, and when ruptured (i.e. from herbivory) secondary metabolic products flow to the release point (Dussourd and Denno 1994). Herbivores can evade this defense, however, if they are able to damage leaf veins. This minimizes the outflow of latex or resin beyond the cut and allows herbivores to freely feed above the damage. There are several strategies employed by herbivores to relieve canal pressure, including vein cutting and trenching, and the technique used corresponds to the architecture of the canal system (Dussourd and Denno 1991). Dussourd and Denno (1991) examined the behavior of 33 species of insect herbivores on 10 families of plants with canals and found that herbivores on plants with branching canal systems used vein cutting, while herbivores found on plants with net-like canal systems employed trenching to evade plant defenses.

See also

Work Cited

 

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