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Model Organisms

L.A. Moran lamoran at gpu.utcc.utoronto.ca
Thu May 8 14:55:54 EST 1997

Richard M. Kliman wrote,
>>Richard M. Kliman wrote,
>>>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.

Chaetognatha (arrow worms) may indeed be a valid candidate although
most biologists seem to classify them as deuterostomes. If they are
correct then arrow worms don't fulfil your criterion either.

>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.

Most trees that I have seen suggest that the protostomes form a monophyletic
group. This would suggest that the evolutionary distance between humans and
insects is the same as that between humans and molluscs. This interpretation
is largely supported by the work of Wray et al. (1996, Science 274, 568).
They looked at the sequences of five genes (ATPase, cytochrome oxidase I,
cytochrome oxidase II, beta-hemoglobin, and NADH) as well as 18S RNA.
The data is not unambiguous but it seems to indicate that molluscs, 
annelids and insects are all equidistant (in time) from chordates. Their
data also indicate that the protostomes are a monophyletic group.

>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

I don't think that this reflects the consensus among evolutionary biologists.
They would have molluscs more closely related to Drosophila than to mice.

>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.

It seems to me that the classification of molluscs as protostomes, a
monophyletic group, is not very controversial although I grant you that
it isn't as solid as I thought. Tunicates (urochordates) are almost
always classified as chordates as far as I know. This suggests that the
18S RNA tree is not accurate. (Hemichordates are not in the phylum
Chordata.) It is not surprising to find that the ribosomal RNA trees,
by themselves, can't resolve some of these branches since the nodes are
undoubtedly very close together.

>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 

Point taken. I will continue to assume that protostomes form a monophyletic
group but you are right to point out that this classification is subject
to revision.

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

This is correct but it doesn't seem to be relevant. If Aplysia isn't the
kind of intermediate that French Lewis was looking for then it isn't
important whether we know about snail neurobiology or not.


Larry Moran

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