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Subject: Re: (Fwd) Algal role in animal evolution?
From: "Ragan, Mark A." <[log in to unmask]>
Reply-To:Ragan, Mark A.
Date:Mon, 5 Apr 1999 16:23:10 -0400
Content-Type:text/plain
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Marc Washington asked for comments...   ; )

"...I'd like to ask for an opinion from you concerning
the possible role of a lateral gene during the formation of the
eukaryotic cell from algae. If a thing moves, actively seeks mates,
has "vision," has paired appendages for mobility, and in addition
"hair-like" bundles (in animals for mechanoreceptors and hair cells),
we might be inclined to say it is an animal..."

>>Equating characteristics willy-nilly from one group of organisms
(e.g., animals) to another (e.g, Chlamydomonas) -- for example,
considering both flagella and legs as "paired appendages" -- ignores
the most fundamental concept of comparative biology, i.e., homology.
No-one believes that flagella and legs derive from a common ancestral
structure, i.e, are homologs; they're (at best) analogs, and whatever
analogy might exist between flagella and legs certainly breaks down
long before we get to the point of supposing that animal locomotion
has anything whatsoever to do with genes transferred from algae.

>>Nor am I aware that there is any reason to assume that the eukaryotic
cell arose as, much less from, an alga (by which, I assume, is meant a
photosynthetic eukaryote -- if not this meaning, then what?).  "Algal
ancestry of eukaryotes" certainly goes against the main results of
molecular phylogenetics (that the deepest branches in the eukaryotic
tree are represented by colourless protozoa, a few of which, such as
euglenoids, were secondarily invaded by photosynthetic organisms).
An algal ancestry of eukaryotes would seem to require that all eukaryotes
are secondarily nonphotosynthetic.  Is there support for this idea in the
fossil record, or anywhere?

>>As a side-issue, I object to locomotion being used to characterise
animals.  This mistaken idea caused a great deal of trouble in the early
part of the Nineteenth Century (with algal zoospores).  Nathanael
Pringsheim showed that motility is not restricted to the animal kingdom,
and the issue has been pretty much moot since his work in the 1850s.

"...But, in this case, it would be
chlamydomonas. It is an algae which behaves and is formed like an
animal in, figuratively,  all things but name (and the obvious fact that
it photosynthesizes)."

>>The operative word here is "figuratively", which has its place in art
and poetry...

>>Lateral transfer of genes is probably real, at least in specific cases.
But hair bundles are NOT homologous with flagellae.  Do they share
any genes in common?  Are hair bundles based on the 9+2 ultrastructure
nearly ubiquitous among flagellae?  Do the two structures share a common
ontogeny?  Hair bundles and flagellae are, at best, analogous (and not very
exact or useful analogs either, in my opinion).

>>We also learn that Chlamydomonas has a "neuron" and "vision",
that some protista are "fish-like", and that Chlamy is a "prototype" for
animals (despite being on a different branch of the phylogenetic tree).
There is talk of "buds" in animals and plants -- let's not forget the "buds"
in yeast, and indeed in some bacteria!  Plants "bleed and heal" (but use a
completely different biochemistry and set of genes for wound-repair than is
used in coagulation).

>>Comparative biology must be based on homology, not on analogies,
similiarities, typology or prefigurations.  This imaginative email
unfortunately
provides a fine example of what happens when we lose sight of the
requirement
for homology as the basis of comparative biology.


Mark Ragan
CIAR Program in Evolutionary Biology, and
NRC Institute for Marine Biosciences
Halifax, Nova Scotia, Canada

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