place of the worm
David H. A. Fitch
fitch at ACF2.NYU.EDU
Mon Jul 29 16:59:15 EST 1996
David Bailey writes:
>"And people, who are genetically much closer to flies than to worms, would
>probably die in the womb without programmed cell death."
>What do other C. elegans workers think of this novel proposal? I was not
>aware of any phylogeny that places insects on the route to man.
> Even using molecular data, I have seen no evidence that C. elegans
>is either closer or further from humans than are fruit flies. Can anyone
>shed real light on this issue? I assume that the distance between C. elegans
>and man is determined mainly by the time to the invertebrate:vertebrate
>divergence. The same applies to flies.
First, I'm not sure I understand what you mean when you say "the distance
between C. elegans and man is determined mainly by the
invertebrate:vertebrate divergence. Vertebrates may be monophyletic, but
invertebrates most certainly are not. Different invertebrate lineages
diverged at different times from the lineage that eventually produced
vertebrates (e.g., sea squirts diverged from us much more recently than did
arthropods). The point at which vertebrates diverged from the
invertebrates was undoubtedly much more recent than the point at which
nematodes diverged from the rest of the invertebrates.
Second, the proposal that we are more closely related to (i.e., diverged
more recently from) arthropods than to nematodes is not a novel one.
However, there are so few phylogenetically informative (morphological)
characters that the position of nematodes relative to other metazoa is
highly contentious. Conway Morris (1993, Nature 361:219-225) placed
nematodes as an early branch of the Protostoma, but considered this fairly
shaky. If that were the case, flies and worms would have diverged from us
together because they would have shared a protostom ancestor which diverged
from our deuterostom ancestor.
Third, the molecular data so far seem in pretty good agreement that worms
diverged early from a lineage more recently shared between flies and
humans. The evidence is presented especially well by Sidow and Thomas
(1994, Current Biology, 4:596-603). Molecular sequences that favor this
"outgroup" position for nematodes include RNA Pol II and III, mt rDNA,
cytochrome c and 18S rRNA. Now, it could be that changes just accumulate
more rapidly in nematode lineages (maybe because of their rapid generation
times?) and that this "outgroup" position is an artefact of that faster
rate. Hopefully, as data accumulate to represent genomes more fully, these
competing hypotheses can be tested more rigorously.
Finally, if you just want an estimate of "difference" between taxa,
regardless of when they diverged from one another, then molecularly at
least, worms are definitely more different from us than flies. In their
"trees of life" using 18S rRNA sequences, several investigators have simply
disregarded the C. elegans 18S sequence because the sequence and secondary
structure is more different from other animals than are fungal or plant
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