On Fri, 31 Mar 1995, Charles T. Faulkner wrote:
> So if we develope a test for anti-E.coli IgA, this would imply
> that it is a parasite, rather than a commensal because we have
> demonstrated an immune response ?
>> I guess I still see metabolic dependency as the important criteria
> because I think we can get an immunologic response from living and nonliving
> pertubations to the host. I believe the questions still goes back to
> measurement scale for describing net effect on host. Pathogenicity is a crude
> meassure.... nutrient robbing (metabolic dependence) no matter how small
> seems more appropriate.
I would not disagree that metabolic dependence is an important criteria,
but in order to differentiate parasitism from commensalism or mutualism,
some aspect of the metabolic dependance (physical damage, "nutrient
robbing") must cause a net negative effect on the host. I also agree that
the net effect may be difficult to measure, which is why I stated that if
there is no benefit to having an endosymbiont then there is nothing to
offset the potentially negative effect its presence has on the host, and
it is therefore a parasite. I think that true commensalism would be hard
to show without examining energy budgets, as a symbiont cannot have *no*
effect on a host. I would even go as far to say that, given natural
selection (at least temporarily) favours "cheaters" that mutualistic
relationships oscillate around the stable balanced energy budgets.
I guess it all comes down to how restrictive we wish our boundaries to
be; for the most part I think that flexibility is practical. I also,
however, enjoy arguing semanitics ... if only for the sake of discussion.
Derek A. Zelmer