From owner-celegans@net.bio.net Sun Jul 28 23:00:00 1996 Path: biosci!ACF2.NYU.EDU!fitch From: fitch@ACF2.NYU.EDU (David H. A. Fitch) Newsgroups: bionet.celegans Subject: Re: place of the worm Date: 29 Jul 1996 14:59:15 -0700 Organization: BIOSCI International Newsgroups for Molecular Biology Lines: 68 Sender: daemon@net.bio.net Distribution: world Message-ID: <9607292156.AA23439@acf2.NYU.EDU> NNTP-Posting-Host: net.bio.net 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. > >Dave Baillie 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 rRNAs. Best wishes, ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~ ~ - - ~ David Fitch ~ \ / / / ~ Department of Biology ~ \ / / ~ New York University ~ [] / / ~ Main Building, Room 1009 ~ \ / / ~ New York, NY 10003 ~ \ / ~ 212-998-8254 ~ \ / ~ fax 212-995-4015 ~ / ~ fitch@acf2.nyu.edu ~ / ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .