likelihood (?) inference from phylo. distribution

Arlin Stoltzfus arlin at is.dal.ca
Sun Mar 10 11:34:25 EST 1996


DEernisse at fullerton.edu (Doug Eernisse) wrote:
>> without evidence.  Apparently, the referee felt that the phylogenetic
>> distribution was irrelevant and was NOT unfavorable to an ancient
>> origin of C-to-U pan-editing followed by loss in genomes other than
>> those of angiosperm mitochondria.
>> 
>> I have seen a similar difference of opinion in other contexts.  My
>intuition suggests that 
>> the referee's view is wrong, but I have not seen
>> a rigorous and valid general argument against it.
>
>If this were the only character you are willing to bring into
>the discussion then, no, there is no strong argument against it.

I am assuming that a conventional phylogenetic tree (with plant
mitos being related to alpha-proteobacteria) is a given.

>If you are willing to bring the currently available evidence
>for plant mtDNA as a taxon in the tree of life then, yes, it
>would be unparsimonious to advocate that the tree of life
>is rooted with plant mtDNA. Such a claim would be necessary in order
>to claim the state "C-to-U pan-editing" as plesiomorphic rather than
>autapomorphic.

By what grounds would it be necessary to claim that the "tree of 
life is rooted with plant mtDNA" in order to argue that C-to-U
pan-editing is ancient?  Isn't this tantamount to assuming
that there is no homoplasy?  Let us restrict ourselves to the case 
where such an assumption cannot be made.  That is, in defense of the
ancient-editing hypothesis, one may suggest (and it has been
suggested in other contexts) that editing would be very difficult 
to gain after the RNA-world stage, but it is easy to lose and has been 
lost many times, such that non-editing is a homoplasy.  Surely 
there are characters in evolution that have been lost many times.  
For instance, if the common ancestor of cellular life was a thermophile, 
then thermophily has been lost many times.  

Arlin

Arlin Stoltzfus (arlin at is.dal.ca)
Department of Biochemistry
Dalhousie University
Halifax, NS  B3H 4H7 CANADA
ph. 902-494-3569; fax 902-494-3569





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