Bioinformatics for Genome Analysis

Nesbo, Boucher and Doolittle (2001) used phylogenetic trees of four taxa to assess whether euryarchaeal genes share a common history. They have suggested that of the 521 genes examined, each of the three possible tree topologies relating the four taxa was supported essentially equal numbers of times. They suggest that this might be the result of numerous horizontal gene transfer events, essentially randomizing the relationships between gene histories (as inferred in the 521 gene trees) and organismal relationships (which would be a single underlying tree). Motivated by the fact that the order in which sequences are added to a multiple sequence alignment influences the alignment, and ultimately inferred tree, they were interested in the extent to which the variations among inferred trees might be due to variations in the alignment order. This bears directly on their efforts to evaluate and improve upon methods of multiple sequence alignment. They set out to analyze the influence of alignment order on the tree inferred for 43 genes shared among these same 4 taxa. Because alignments produced by CLUSTALW are directed by a rooted guide tree (the denderogram), there are 15 possible alignment orders of 4 taxa. For each gene they tested all 15 alignment orders, and as a 16th option, allowed CLUSTALW to generate its own guide tree. If we supply all 15 possible rooted guide trees, they expected that at least one of them should be as good at CLUSTAL's own guide tree, but most of the time they differed (sometimes being better than CLUSTAL's default tree and sometimes being worse). The difference seems to be that the user-supplied tree is not given meaningful branch lengths, which effect the assumed probability of amino acid changes. They examined the practicality of modifying CLUSTALW to improve its treatment of user-supplied guide trees. This work became ever increasing bogged down in finding and repairing minor bugs in the CLUSTALW code. This effort was put on hold as we feel that our other proposed approaches will ultimately be better