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model organisms

Richard M Kliman rkliman at runet.edu
Fri May 2 10:01:12 EST 1997

Reminder: A researcher had asked for suggestions for an organism that 
shares an ancestor with vertebrates that predates the divergence of 
Drosophila and postdates the divergence of C. elegans.  This individual 
used language that irks some evolutionary biologists, but it seemed clear 
to me that he was not ignorant of the biology - only of the jargon.  I 
had suggested that a mollusc, Aplysia, might be suitable.

Larry Moran wrote:
     Molluscs are definitely out. They are proterostomes, just like 
     insects. In fact, there may not be an organism that meets your

This may be true.  To the best of my knowledge, not too many 
people are studying the phylum Chaetognatha.

However, molluscs are not necessarily a bad candidate.  They are, of 
course, protostomes.  That does not mean that all protostomes are
equally diverged (in time) from all deuterostomes - i.e., the protostomes 
may not be monophyletic with respect to the deuterostomes.  Most trees that 
I've seen have the annelids/arthropods sharing an ancestor that postdates 
the divergence of the molluscs.  I was, however, under (the possibly 
mistaken) impression that the molluscs might have diverged before the time 
of the most recent common ancestor of annelids/arthropods and deuterostomes.
In Futuyma's text, four possible phylogenies of animals are shown, two of 
which have molluscs diverging early, and two of which make the protostomes 
monophyletic.  In a recent news article in Science, reference was made to a
finding by Gehring et al that the squid homologue to Drosophila eyeless was
able to induce ectopic eye expression in the same way that mouse small-eye
did.  In that article, it was suggested (perhaps erroneously) that the squid
gene was more distantly related to the Drosophila gene than was the mouse
gene.  On the other hand, a phylogeny based on 18S rRNA sequence 
(Winnepenninckx et al., MBE 13:1306) is consistent with monophyly of the
protostomes  - though the extent to which you accept this depends on your 
personal feelings about boostrapping and decay index.  I should point out 
that their NJ tree placed the urochordates as an outgroup to vertebrates, 
cephalochordates *and* hemichordates.  Brachiopods, annelids and "lesser" 
protostomes were found within the mollusc clade.  So, I think we 
(including Larry) should be careful about assuming monophyly of the 

I'll grant Larry that molluscs are risky, but there's a lot more known 
about the neurobiology of Aplysia than of Chaetognatha.

I had previously written:
     And unless there's been a change that I don't know about, it's 
     reasonable to assume a priori that the Aplysia gene will be more 
     similar to the human gene than either is to the C. elegans gene.  
     The experimental approach, at least on the surface, strikes me as

To which Larry responded:
     I'm afraid I don't agree. Could you please explain how you could 
     clone the C. elegans gene using Aplysia as an intermediate? There 
     seem to have been some changes to evolution that you don't know 

Why would I want to do something as stupid as that?  Of course we would not
assume that the Aplysia gene is any more similar to the C. elegans gene.
And that's not what I inferred to be the researcher's intended approach.  
The experimental approach being taken by the originator of the thread is
as rational as that taken by Gehring et al.

Rich Kliman
Dept. of Biology
Radford University

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