Oligochaeta molecular phylogeny

Patrick MARTIN martin at kbinirsnb.be
Fri Jun 16 06:19:51 EST 2000

Dear Colleagues,

It is probably time for me to make my contribution to the discussion and to 
clear up some misunderstandings.

The first point is that I am a specialist of aquatic oligochaetes who, a few 
years ago, started a molecular analysis of my group and related taxa. 
Accordingly, I do not consider myself as a "molecular-oriented 
colleague", as my work on systematics was only based on morphological 
data so far. I just tried to use both kind of data, morphology and 
molecular, in order to solve clitellate relationships.  

The second point is that I have the strong feeling that most of the problem 
raised by K. Fitzhugh is due because the problem is considered within 
the framework of maximum parsimony (MP) analyses while I only used 
the maximum likelihood (ML) method. Briefly, the latter method enables to 
obtain the tree which statistically optimizes the probability of observing 
the data given a particular evolutive model. The important point here is 
that the temporal dimension is included in the model. Hence, rate has 
here a meaning since it is considered in a context of time.   

>Let's say one has a set of specimens, from which they "observe" that
>position 556 of each specimen has an A. Our perceptions tell us we "see"
>the same nucleotide among these individuals. Most systematists then go so
>far as to say, "this A at position 556 indicates homology."

This is exactly the point. Considered within a "most parsimonious" 
framework, this is indeed what most systematists will say. This is also 
why the MP method can be misleading when it is applied to molecular 
data: the method is unable to detect multiple changes on long branches 
(convergent, parallel, or reversed), resulting in the so-called "long branch 
attraction" phenomenon. In contrast, it is very likely that the maximum 
likelihood of obtaining the observed data, given a particular evolutive 
model, requires the following transitions, A-C-T-A, for one specimen, 
while for the other one, these successive transitions are not required.  

Very roughly, we measured in this study an amount of difference 
between the taxa considered (the number of substitutions per site 
separating the sequences, according to the evolutive model considered). 
We then noticed that some lineages, namely the leeches, leech-like 
worms and gutless worms, cluster along long branches in our 
phylogenetic inferences, by comparison with other oligochaetes. These 
differences in mutation rates were statistically significant, according to 
the evolutive model considered in ML analyses. This higher amount of 
difference between these groups and other oligochaetes could not be 
interpreted as the result of an ancestral condition since both molecular 
and morphological data suggest that these groups are located within 
oligochaetes. We then concluded that these lineages rapidly evolved.  

Hoping that this can be of some help in the discussion...

Patrick MARTIN
Dr. Patrick MARTIN
Institut royal des Sciences naturelles de Belgique
Biologie des Eaux douces
29, rue Vautier
B-1000 Bruxelles

Tel   : +32/2/627.43.17
Fax   : +32/2/627.41.13
Email : martin at kbinirsnb.be

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