PartTree: an algorithm to build an approximate tree from a large number of unaligned sequences

Motivation: To construct a multiple sequence alignment (MSA) of a large number (>similar to 10 000) of sequences, the calculation of a guide tree with a complexity of O(N-2) to O(N-3), where N is the number of sequences, is the most time-consuming process. Results: To overcome this limitation, we have developed an approximate algorithm, PartTree, to construct a guide tree with an average time complexity of O( N log N). The new MSA method with the PartTree algorithm can align similar to 60 000 sequences in several minutes on a standard desktop computer. The loss of accuracy in MSA caused by this approximation was estimated to be several percent in benchmark tests using Pfam. Availability: The present algorithm has been implemented in the MAFFT sequence alignment package (http://align.bmr.kyushu-u.ac. jp/mafft/software/). Contact: katoh@ bioreg.kyushu-u.ac.jp Supplementary information: Supplementary information is available at Bioinformatics online.