"Astonishing" Parasite
 

Just a fascinating note from the 6-April-2007 Random Samples in the journal [I]Science[/I]. I never realized a two-host parasite could pull off something this nifty:

[QUOTE]"Toxoplasma gondii is a parasite that requires two hosts. It’s born in a cat’s intestines, develops in another animal- such as a rat- and must return to a cat to reproduce. To boost its chances of making that return trip, researchers at Oxford University have shown, Toxoplasma makes rodents less afraid of cats. Now, a Stanford University team led by Ajai Vyas has found that rats carrying the parasite don’t mellow out across the board; they just lose their fear of the smell of cats.

In the lab, infected rats showed much less aversion than normal ones to bobcat urine. But they reacted normally when the researchers probed other types of fear responses. That means Toxoplasma has a “remarkably specific behavioral effect, says co-autor Robert Sapolsky. He says most parasites control behavior in much cruder ways- for example, by destroying muscle metabolism so an organism can’t evade a predator. In the 2 April online [I]Proceedings of the National Academy of Sciences[/I], the scientists report that Toxoplasma cysts form preferentially on the rat amygdale, which Sapolsky calls “ground zero” for fear in the brain.

I always found it incredible that the parasite would be able to alter a response, cat aversion, that is so ingrained in the rat’s psyche,” says Oxford veterinary scientist Manuel Berdoy, an author of the earlier work. He says the new research shows that the parasite may have the “astonishing” ability to zero in on the neural pathways for processing cat odors."[/QUOTE]


Norm

Ain't evolution cool? It may seem "astonishing" to us, but the parasite certainly doesn't "know" it's doing anything special. You have this parasite that over the course of evolutionary history has become specialized to cats and some of their common prey. Like all species, this 'single" parasite is actually a huge dispersed population of bugs which are nearly, but not quite, identical in their genetic makeup, due to random mutations, genome-duplication errors, lateral gene transfer within the species and perhaps with other microbes inhabiting the same kinds of hosts; in other words, the usual suspects. The various mutant forms perhaps inhabit different parts of the host(s) with varying efficacy. Over time, the ones who best manage to reproduce - by whatever mechanism that happens to be - become prevalent. Soemwhere along the way, one mutant variety happened to hit the lottery, in that it wound up with a taste for the rodent amygdala, and happened to have a chemical effect that made the rodent easier prey, but without drastically affecting the rodent host's reproductive success. (It would be interesting to see if the no-fear effect is more pronounced in older rodents, i.e. those whose best reproductive days are behind them -- that would seem to be the optimal kind of compromise for the parasite here.) Nothing clever, nothing planned or designed, just sheer blind chance, inexorably amplified by thousands or millions of successive generations.

The specificity of the action in question actually makes perfect sense, but not in the "ooh, look how clever this bug was in developing such a remarkably specific action" manner. Rather, the specificity allows for the effect to occur as a result of a much smaller set of genetic mutations in the parasite (perhaps just a single point mutation, in fact), rather than a much-less-likely-to-occur complex set of mutations which might end up having a similar effect. In fact, the simpler the required mutation and/or the more beneficial its effect on its owner's reproductive success, the more chance its owner's descendants have of achieving dominance in what is quite possibly a very short evolutionary time span.

Nifty stuff.

ewmayer,

I have a question. What factors make you so sure that it was "random mutatations, genome-duplication errors...amplified by thousands or millions of generations" which resulted in the formation of this parasite's abilities?

Disclaimer: I am not arguing that these were not the factors which led to this parasites abilities. I am simply interested in what evidences ewmayer has which give him his certainty that these were the mechanisms.

Best,
Zeta-Flux

[QUOTE=Zeta-Flux;107343]What factors make you so sure that it was "random mutatations, genome-duplication errors...amplified by thousands or millions of generations" which resulted in the formation of this parasite's abilities?[/QUOTE]

That is the same menu of evolutionary mechanisms which has been shown to be at work in the evolution of innumerable other species, with no known exceptions of which I am aware. The only other known evolutionary mechanism of note is sexual reproduction, but I left that out here since it seems we're talking about an asexually reproducing organism.

Also, as I pointed out above, one has to very careful in using anthrpomorphic or other implication-laden terms like "abilties," "design", "exquisitely specific" and even "astonishing" in such discussions. It may strike *us* as remarkable, but that is an entirely subjective impression.

There are species of Hummingbird which only feed on a single type of flower - "exquisitely specific" to us, "greedy optimization" from an evolutionary perspective. (And again, "greedy" has unwanted anthropomorphic baggage, and even "optimization" is loaded with unwanted "design" implications here - I used these terms for lack of a better phrase, strictly in the "whatever works as far as increasing reporductive fitness in a particular environmental niche" sense).

The really exciting/scary thing is that we humans are just now discovering how to alter genetics and cause our own evolutionary effects. Just recently, though I forget the details, scientists took a protein, and through intelligent evolution(stressing the organism with a certain chemical agent) they doubled it's ability to bind a certain protein.

[QUOTE=jasong;107349]The really exciting/scary thing is that we humans are just now discovering how to alter genetics and cause our own evolutionary effects. Just recently, though I forget the details, scientists took a protein, and through intelligent evolution(stressing the organism with a certain chemical agent) they doubled it's ability to bind a certain protein.[/QUOTE]

Not really -- humans have been doing this sort of "guided" selection for millennia, e.g.

wolves --> domestic dogs
wild crops --> domestic crops
wild yeasts --> specialized brewer's/baker's/viniculture/cheesemaking yeasts

In fact, it was precisely this kind of human breeding experimentation which Darwin used (with great success, although it was not universally accepted in the early days of the field) as an accelerated model for natural "unguided" evolution. Same mchanisms, just humans providing the selection pressure.

Admittedly, our technical toolkit for doing this kind of stuff has expanded dramatically in recent times, sometimes in scary directions (e.g. bioweapons).

Ewmayer, I think Jasong was just remarking on the fact that now we can, maybe soon, change the human species itself on purpose in specific ways.

ewmayer,

Thanks. It just occured to me that I don't understand the term "random mutation." Or maybe it is just the word "random." What is random about such mutations? If I understand correctly, there are almost always 'reasons' for the mutations; miscopying DNA, etc...

Thanks,
Zeta-Flux

The site where the mutation occurs is random. (Though certain segments of the genome are more prone to mutations, because they form tertiary structures and get in the way of DNA polymerase from replicating.)

The reason why the mutation occurs can also be random. (eg. what chemical the DNA was exposed too)

I can understand that (more-or-less) every site is (approximately) as likely to be a place for mutation, under certain pressures (but not under others). I can also understand that some mutations occur due to the presence of chemicals whose presence at any given moment is dependent on so many factors as to approximate randomness. Is it this small subset of mutations that are called "random mutations"? Are not almost all of these harmful to the organism?

[QUOTE]Are not almost all of these harmful to the organism?[/QUOTE]

That's my understanding, yes. The vast majority of mutations are neutral or detrimental to the organism. I assume also that a mutation that is detrimental at one time in one enviroment could be helpful at some different time in a different environment.

Norm