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Definition of Parasite and embryos

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Definition of Parasite and embryos

Postby kokigami » Wed Jul 26, 2006 4:50 am

I am new here, and on another forum I was in an arguement which revolved around the definition of parasite. More specifically, if a fetus met the definition. I content it does. But I am not a biologist. Nor, I think, were any of those who were arguing against the idea.

I am not trying to start a flame war, despite the controversial topic.

As I can tell, a parasite is defined only in its relationship to its host. That relationship being parasitic...

By that definition a parasite need not even be a living thing. An air conditioner unit is parasitic to an automobile engine (where as an alternator is symbiotic).

But I am curious if the Biology community definition has any reason to oppose this usage of the term. For example, does anyone know of a parasite that uses its on species as host? I found reference to parasitic plants that live on other parasitic plants, and I found articles on the competitive relationship between mother and fetus. But I can't seem to find any place that makes a sound arguement for or against the idea that an embryo (through gestation) is in a parasitic phase of life..
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Postby SU_reptile » Wed Jul 26, 2006 10:49 am

Hmmm. Alghough, it's very often difficult to distinguish a parasite from symbiont, there are some traits that help in recognizing them.

We have a definition of parasite in our dictionary, however it may be confusing and may make misunderstanding of that problem.

As fa as I understand what the parasite is, is an organic object, mostly living organism, that lives by means of antagonistic symbiosis called parasitism. That organism gets nurishment from the host's body (tissues, body fluids etc), however it doesn't give anything in return, but instead it affects host's health, although rarely cause it's death (it would have meant parasite's death too). Some organisms may only need other organism for a shelter for some stage in it's life cycle but may also use host's tissues as food - they are also parasites and it's believed that obligatory parasites arised from organisms having that way of surviving inside host's body.

An air conditioner unit is parasitic to an automobile engine (where as an alternator is symbiotic)

I personally think that we can't use word parasite in that context. I think that parasite has to be a organic matter (I'd say organism but viruses are obligatory parasites, however not organisms).

As far as fetus or transplants are concerned, it's quite difficult topic, however worth serious discusion. I hope other users will join to it. As far as I am concerned, I will try to find some explanations for that fetus problem and some more parasitic interactions . :wink:
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Postby MrMistery » Wed Jul 26, 2006 6:53 pm

First of all, a parasite is a type of symbiont. Symbiosis, by a generally accepted definition can be of 4 types: mutualism, parasitism, amensalism and comensalism.
A fetus is not a parasite. Why? simple. because a parasite, by definition, brings absolutely no advantage to the host. Whereas a fetus brings the highest advantage possible to the host: it directly increases it's fitness, by allowing the host to pass it's genes forward.
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Postby LilKim » Wed Jul 26, 2006 8:41 pm

MrMistery wrote: . . . a fetus brings the highest advantage possible to the host: it directly increases it's fitness, by allowing the host to pass it's genes forward.


I absolutely agree. Survival of the fittest ... thus, those that are most fit are best suited to pass their genes to their offspring... and create more 'creatures' that are fit (like their parents)
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Postby kokigami » Wed Jul 26, 2006 11:59 pm

MrMistery wrote:First of all, a parasite is a type of symbiont. Symbiosis, by a generally accepted definition can be of 4 types: mutualism, parasitism, amensalism and comensalism.
A fetus is not a parasite. Why? simple. because a parasite, by definition, brings absolutely no advantage to the host. Whereas a fetus brings the highest advantage possible to the host: it directly increases it's fitness, by allowing the host to pass it's genes forward.
An interesting arguement. I am not convinced, however. The embryos presence puts the host at risk, and manipulates the host to its own advantage. The advantage to the host, if any, is incidental to the embryos goals. The condition of the host plays a critical role in determining if the embyros presence is beneficial.

edited to add a question.
I understood symbiotics to be separate from parasitics in that symbiotics offered a mutual benefit, and parasitics were not mutual. If parasitic is a sub division of symbiotics, which sub division would apply more.

(PS, I do recognize that there are sound arguements that in many cases host and embryo do have a symbiotic relationship.)
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Postby SU_reptile » Thu Jul 27, 2006 10:31 am

I just think that the definition of parasite should be modified, so that it should include information that stages of sexual reproduction cycle musn't be regarded as parasite. For example eggs, fetus etc.
What I am worried about is the issue of sporophyte in Bryophyta - it's more less parasitic, isn't it?
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Postby MrMistery » Thu Jul 27, 2006 6:41 pm

ok i had this argument with an ecology professor a month ago- and won! Symbiosis doesn't mean mutualism. Symbiosis means "living together". It doesn't mean you have to like it. In current ecology books, symbiosis is divided into the 4 categories i stated earlier. If you want to clasify it as something, clasify it as mutualism, because it is benefical for both, as i said earlier.

About the sporogon in bryophyta, all the textbooks i have ever read define it as being a parasite on the plant...
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Postby kokigami » Thu Jul 27, 2006 7:41 pm

MrMistery wrote:ok i had this argument with an ecology professor a month ago- and won! Symbiosis doesn't mean mutualism. Symbiosis means "living together". It doesn't mean you have to like it. In current ecology books, symbiosis is divided into the 4 categories i stated earlier. If you want to clasify it as something, clasify it as mutualism, because it is benefical for both, as i said earlier.

About the sporogon in bryophyta, all the textbooks i have ever read define it as being a parasite on the plant...


That arguement creates a paradox under current technology. (well, paradox may be overstating it, but this is the internet, and hyperbole is worth extra points.). It only takes into account embryo that secure to the parent host. With suragate technologies, the embryo would not be living off the host and providing the advantage of fitness. Which would mean that, in at least some cases, an embryos actions would be parasitic, though its nature has not changed. Wouldn't the fact that the embryo does not need to live off the parental host, but can, in fact, live of any appropriate host from the species challenge that arguement? Its existence has already met the test of fitness for the mother, but its parasitic relationship to the mother is mostly a matter of convenience and location. The embryo implants on the first available implantable surface..


Of course, taxonomy is not an exact science.
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Postby surya » Fri Jul 28, 2006 1:38 pm

I'm really confused.what i learnt(from my bio teachers and text books) is that symbiosis is a relationship btw 2 organisms in which both r mutually benefitted.but reading all the posts above i'm totally confused.upto now,for me,symbiosis was a relationship seen btw,for example ,leguminous plants n rhizobium.
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Postby MrMistery » Sat Jul 29, 2006 7:15 pm

That is what is called mutualistic symbiosys. The definition you have is the old definition of symbiosys that was abandoned because it was too restrictive. But most teachers are not up to date with this, because they don't move their lazy ass to the library or the internet to read a book once in a while...
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Postby kokigami » Sat Jul 29, 2006 10:38 pm

MrMistery wrote:That is what is called mutualistic symbiosys. The definition you have is the old definition of symbiosys that was abandoned because it was too restrictive. But most teachers are not up to date with this, because they don't move their lazy ass to the library or the internet to read a book once in a while...


See, now that's kinda the whole problem. FLoating definitions. I don't know if there was really a good reason for a change from sybiosis to Mutualism. What are the working defs for the other four categories of symbiosis then?

the dictonary here defines parasite as

An organism which obtains food and shelter from another organism (for example giardia).


here http://www.biology-online.org/dictionary/Parasite

by that simple definition, an embryo is a parasite. But I suspect the taxonomy system has a more detailed definition of parasite.
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Postby MrMistery » Sun Jul 30, 2006 5:15 pm

The definition on the dictionary is not exactly accurate. By that definition, a fungus in a lichen is a parasite. The definition should say something like "An organism which obtains food and shelter from another organism without giving anything in return".
But you know this is really an unimportant argument. Because it is just how you define parasite.
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