Lecture 29

Myth of the benign parasite: parasites (parasites and diseases) are purported to evolve to become less damaging to their hosts and thereby preserve their means of reproduction.

In nature, parasites evolve like all other predators, they are molded by natural selection to maximize the number of viable descendants produced. Natural selection may yield deadly (highly virulent) or benign (poorly virulent) parasites depending on the requirements for parasite reproduction and transmission. The condition of the host is important to a parasite only as it relates to the parasite’s reproduction and transmission (see Ewald, 1994 for a more detailed discussion).

If transmission rates are slow or if an active host is necessary for transmission, then selection may yield a more benign parasites. This may occur with vector borne parasites.

If transmission rates are rapid or if host lifespan is unimportant in transmission, then selection may yield a more virulent, destructive parasite. This may occur with air or water borne parasites.

European rabbits were introduced to Australia in 1859 and rapidly became an uncontrollable invading exotic species. Myxoma virus from South America was introduced to control rabbits in Australia in 1950. This virus is transmitted by mosquitos and causes a fibroma cancer in rabbits called myxomatosis. The following mortality rates were observed:

	Rabbit Mortality Rate
1st Epidemic	99.8%
2nd Epidemic	90%
3rd Epidemic	40% - 60%

This pattern of decreasing mortality among rabbits is the consequence of evolution in both the rabbits and the myxoma virus. The rabbit hosts evolved resistance to the virus, resulting from very strong selection against non-resistant individuals. The myxoma virus evolved toward decreased virulence which maximizes rates of mosquito transmission. Myxoma strains that kill their host very fast do not get transmitted to a new host because host death occurs before a mosquito takes a blood meal from the infected rabbit.

The term symbiosis, which means living close together, is more general and includes several functional categories of between species interactions: mutualism, commensalism, and parasitism.

Mutualisms occur between almost all combinations of living things: plant-plant, animal-animal, plant-animal, plant-bacteria, fungus-plant, fungus-bacteria, protista-bacteria, and animal-bacteria.

Mutualisms are important phenomena to study because they typically involve an extensive degree of specialization in one species that may appear to be for the exclusive good of another species. If organisms are indeed molded by natural selection, then phenotypes are ultimately selfish. It is a challenge to reveal the true nature of these often complex co-evolved interactions. Mutualisms are examples of cooperation between species for biologically selfish gain.

Rhizobium is a free living soil bacterium that enters the root cells of plants in the family Fabaceae (peas, beans, peanuts, clover, soybean) and the plant root forms nodules around the Rhizobium containing cells.

Rhizobium fixes N₂ gas to form NH₃ (ammonia) and NH₄⁺ (ammonium ion) which is used by the bacterium to form organic nitrogen compounds. Nitrogen fixation is not performed by any plants, and nitrogen is a limiting nutrient for terrestrial plant growth.

The plant provides the bacteria with shelter, provide anaerobic conditions necessary for nitrogen fixation to occur (hemoglobin-like compounds in the outer cells of the root nodule trap oxygen), and provide carbohydrates to the bacteria.

Lichens are a mutualistic organism consisting of fungi, and either green algal cell or cyanobacterial cells. Fungal mycelia surround the algae or cyanobacteria.

The fungus is heterotrophic and requires an external source of carbohydrate for energy. The internally located algae or cyanobacteria are photosynthetic autotrophs that require sunlight exposure, water, and mineral nutrients for their metabolism.

The cyanobacteria or algae receive shelter, water and nutrient. Fungal mycelium has a very high surface to volume ratio, and they are very efficient at absorbing water and nutrients. The combined organism, the lichen, can live in places where neither fungus nor algae (nor cyanobacteria) could live alone. Typically in very exposed locations with full sunlight exposure (tree bark and rock surfaces).

Mycorrhizae are fungi associated with plant roots. In many cases, this association is essential for seed germination and normal plant growth. This is very common among tree species.

The plant receives a greatly increased and more efficient absorptive surface for collecting water and mineral nutrients from the soil. This is shown in an experiment evaluating growth of tomato plants, Lycopersicon esculentum, with and without its mycorrhizal fungus, Endogone macrocarpa. The fungus has the most dramatic positive effect on tomato plant growth when phosphate is least abundant (Ricklefs, 1996, p 172, Fig. 8.5).

The heterotrophic fungus receives an abundant and reliable supply of carbohydrates and amino acids from the plant .

Although this interaction may have evolved from a parasite (fungus)-host (plant) interaction to a mutualistic state. Some plants (for example, all members of the family Orchidaceae) attract and parasitize the fungi that form mycorrhizal relationships with their roots. Whether these were once mutualistic interactions is not clear. In these cases, the fungus gets no resource from the plant, but the plant gets resources from the fungus, including carbohydrates and amino acids. The most extreme of these parasitic interactions are cases in which the plant species has lost the ability to conduct photosynthesis and depends entirely on fungi for all energy resource needs (for example, some species in the family Monotropaceae, indian-pipes).

Many coral polyps contain algal cells, called zooxanthellae, inside the body cavity (within the coelom). The coral provides the algae with shelter, nutrients (nitrogen and phosphorus) and exposure to sunlight. The algae provide the coral with carbohydrates.

Some marine fishes seek anemones for shelter, protection from predators, and for nest sites. The development of this interaction may vary for different fish species and anemone species. These fishes are either immune to the stinging cells of the anemone and/or the anemone does not sting these fishes. Other fishes are readily stung, captured, and consumed by the anemone. The anemone fish brings food to the anemone.

Cleaner fishes are small marine fishes that remove the ectoparasites (crustacea) from the body and mouth cavity of larger fishes (cleanee). Fishes seeking to be cleaned visit established cleaning stations on a reef where the cleaner fishes are found. The cleanees get parasites removed from their body and they do not attempt to consume the cleaner fishes. The cleaner fishes get food brought to them.