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Amateur Question

Mark Siddall mes at zoo.toronto.edu
Sun Mar 5 22:45:18 EST 1995


In article <dat-0103952013340001 at dat.port.net> dat at interport.net (Dave Terry) writes:
>Leucochloridium paradoxum, which spends part of its life cycle in a snail
>and part in a bird. To move from the snail to the bird, large populations
                                                          ^^^^^^^^^^^^^^^^^
>of these worms migrate to the snail's eyestalks, where they cause the
 ^^^^^^^^^^^^^
>eyestalks to swell and change in coloration. Such changes result in the
>millions of worms to effect the change in the snail's eyestalks. But can
 ^^^^^^^^^^^^^^^^^^^^
>entire populations of individuals undergo the same genetic mutation at the
>same time and thus secure an advantage? If not, why would such a
>behavioral mechanism evolve, since the behavior could not confer any
>advantages on a single individual?

The answer to your question, as undoubtedly many will post, surrounds
a misunderstanding of the life-cycle as you have understood it (and
perhaps as described in Sci-Am).
Inasmuch as there are "millions of worms" they are descended from one
individual.
It is a single sporocyst that protrudes into the tentacle.
This sporocyst contains many (I doubt millions) daughter organisms.
But they are just-that, progeny.
The sporocyst was once (ontogenetically, not phylogenetically) a single worm   that, through differentiation
of internal germinal tissues, produced many daughter organisms.
This is not unusual among parasitic trematodes. 
Though unproven, one can state with reasonable confidence that the
production of many progeny internally, predates the adaptation seen
in the pulsating sporocyst of Leucochloridium.
(It may not be unproven, Brooks' work on the phylogeny of trematodes
may well already corroborate this).
Either way, the sporocyst is an individual organism in its own right.
The progeny form by asexual internal clonal reproduction.
It is not until the snail is eaten and the sporocyst ruptures that there
is any realization of the progeny's individuality.
>
-- 
Mark E. Siddall                "I don't mind a parasite...
mes at vims.edu                    I object to a cut-rate one" 
Virginia Inst. Marine Sci.                     - Rick
Gloucester Point, VA, 23062



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