Guide 3

The growing appreciation for a parasite's true influence.

As indicated in the previous lesson, investigators have become interested in parasite effects upon hosts that go beyond those on survivorship. For a long time, a parasite's effects were all believed to be due to its nutritional demands, and the drain on potential energy it made on the host. We now recognize that their effects go well beyond “feeding” concerns, as ecologists began to chronicle all sorts of “mind games” played by parasites.

Look at the video below for an extreme example of mind control of a host yielded by a parasite.

View https://www.youtube.com/watch?v=ZgAEeisfHW8

Answer question 1.

We have already mentioned cases in which parasites induce behavior in their hosts that makes them more vulnerable to predation. Review the effect of flukes on frog development and the effect of toxoplasmosis on mice from previous lessons.

We have not fully evaluated such effects for any parasite on human species. You did read some proposed effects of toxoplasmosis on humans. It has for decades, been assumed a parasite only poses a treat to fetuses and immune compromised individuals. Now scientists feel there may be lasting effects on the way some otherwise healthy individuals behave. By affecting the way individuals interact with each other, parasies may even influence the resulting social structure.

Answer question two.

Evolution of parasitic relationships

In the last lesson we started also to look at a parasites effect on other species than its host. We at the end of lesson two, looked at some of the effects a parasite could have on on the ecosystem in which it resides. In this lesson, we will begin to look at what is know about the evolution of parasitic relationships. Both areas of study are new, but already have revealed that parasites can not only be extremely influential in structuring ecosystems, but may have greatly impacting the evolution of the biological world as we know it.

Parasitism and the evolution of sexual reproduction

It may surprise you that almost all evolutionary scientists have accepted that parasitism probably is playing a role in the maintenance of sexual reproduction (as opposed to asexual reproduction) in many species. Asexual reproduction is expected to always have the advantage given all other factors are equal. To see why this is so, go to the following website.

http://www.stanford.edu/group/Urchin/sex.htm ----------------------Answer question 3

(This website sometimes goes off line. Try a few times and yes, I know sexuals are misspelled in the simulation.)

Then how do parasites allow sexual subpopulations (demes) or populations to outdistance asexual demes or populations?

Go to this website. --------------http://www.bbc.co.uk/news/science-environment-14046316

Then explore this web activity produced by a student in my evolution classes a while back. The web activity again compares the advantages and disadvantages of asexual and sexual reproduction and examines a natural experiment with topminnow (fish) populations. Answer question 4.

Note that as you explore the modeling experiments in the activity, sex only wins some of the time. Study the figure below and answer question 5.

Sexual dimorphism

Most modern theories of sexual dimorphism focus on the flashy non combat displays and morphology of males, (such as the colorful and long tail feathers of male peacocks), as advertisements to females that they are parasite free, or can tolerate parasites so as to be able to grow these ornaments. In other words, males are advertising they carry “good” genes.

Skim through the paper by Hamilton and Zuk, reading carefully the two highlighted sections. Answer question 6.

Virulence

Parasitology has always been interested in understanding the factors regulating virulence, in hopes of understanding and so eventually controlling the factors that make one parasite species a deadly killer of man, the other barely noticed during the near normal lifetime of a human host. In few field studies, as you have learned, that among strains or different species of parasites infecting hosts, it is the most virulent strain or species that achieves the highest transmission or reproductive benefit. Given the advantage of a shorter life cycle among parasites (and so the opportunities for more selection among generations of parasite relative to one host generation), how do hosts evolve any effective resistance? Another major concern is what are the circumstances under which, (if any), selection works on the parasite itself to become less virulent.

Although we are far from understand all the factors controlling the evolution of virulence, a few principles have emerged that appear applicable to many different parasite species.

1. Parasites new to a host appear to be more virulent than parasite known to have long associated with a particular species of host. This is somewhat expected as the host has not as yet evolved any resistance to a new parasite.

How do we know how long a parasite has associated with a host?

Often we have fossil or DNA evidence by which we can draw a relationship (or phylogenetic) tree of hosts and parasites and compare them. Often the trees of hosts and parasites are mirror images of each other and give information on how parasites have changed after being inherited or jumping from one species of host to another.

Go to this website.

http://evolution.berkeley.edu/evosite/evo101/VC1hCospeciation.shtml

Answer question 7.

If you have trouble answering this question, go to this URL, also part of the Evo 101 site and review the three web pages on understanding phylogenies.

http://evolution.berkeley.edu/evosite/evo101/IIBPhylogenies.shtml

2. Vertical versus horizontal (lateral) transmission

Read the article linked here. Answer question 8.

Vertical transmission usually favors relative low virulence, since new hosts will more likely be found in the next generation. Also the most likely type of transmission is through eggs or larvae that inflect future generations of some indicated parental population. The idea is to keep the parental generation alive and in good enough condition to keep producing as many of those eggs and larvae as possible.

Lateral (Horizontal) transmission is associated with selection for relatively high virulence. The parasite is selected to produce young that will infect other hosts that are available at present. Therefore selection favors the parasite that reproduces quickly and whose propagules (young) are dispersed to new individuals despite its effect on the original hosts.

3. Other considerations

Although the general association of lateral transmission and more virulence and vertical transmission and less virulence is accepted, it is know that many other factors affect how virulence a parasite can be. For, example, anything that compromises host survival will lead to more virulence. So if predation pressures increase on a host, a parasite will also respond with increased virulence so as to reproduce more in a shorter period of time.

In parasites with complex life cycles, virulence can be different in different hosts. Here, in general, where such differences exist, the parasite is less virulent in its definitive host (host housing the adult stages of the parasite) then in the intermediate hosts (hosts housing larval stages) of the parasite.

Parasites are transferred sexually tend to be less virulent in general than parasites that are not transferred in this way. The reason for this appears twofold. These types of parasites generally are more host specific than their counterparts. Also they have essentially directed all the host’s energy into production of more parasites and little to gain from the host’s demise. Answer question 9.

You should begin to appreciate the many factors that can influence virulence and know these examples for the upcoming exam. We will be discussing specific cases as we go through the different groups of parasites.