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MBE Advance Access published online on November 30, 2005
Molecular Biology and Evolution, doi:10.1093/molbev/msj069
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© The Author 2005. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org
Accepted November 21, 2005

Research Article

Maximum Likelihood Jukes-Cantor Triplets: Analytic Solutions

Benny Chor 1, Michael D. Hendy 2 *, and Sagi Snir 3

1 School of Computer Science, Tel-Aviv University, Tel-Aviv 39040 Israel
2 Allan Wilson Centre for Molecular Ecology and Evolution, Massey University, Palmerston North, New Zealand
3 Mathematics dept. University of California, Berkeley, CA 94720, USA

* To whom correspondence should be addressed.
Michael D. Hendy, E-mail: m.hendy{at}massey.ac.nz


  Abstract

Maximum Likelihood is a popular method for inferring a phylogenetic tree of the evolutionary relationship of a set of taxa, from observed homologous aligned genetic sequences of the taxa. Generally the computation of the maximum likelihood tree is based on numerical methods, which in a few cases, are known to converge to a local maximum on a tree, which is sub-optimal. The extent of this problem is unknown, one approach is to attempt to derive algebraic equations for the likelihood equation, and find the maximum points analytically. This approach has so far only been successful in the very simplest cases, of three or four taxa under the Neyman model of evolution of two-state characters.

In this paper we extend this approach, for the first time, to four-state characters, the Jukes-Cantor model under a molecular clock, on a tree T on three taxa, a rooted triple. We employ spectral methods (Hadamard conjugation) to express the likelihood function parameterised by the path-length spectrum. Taking partial derivatives we derive a set of polynomial equations whose simultaneous solution contains all critical points of the likelihood function. Using tools of algebraic geometry (the resultant of two polynomials) in the computer algebra packages (Maple), we are able to find all turning points analytically. We then employ this method on real sequence data and obtain realistic results on the primate-rodents divergence time.

Keywords: Maximum likelihood; phylogenetic trees; Jukes-Cantor; Hadamard conjugation; analytical solutions; symbolic algebra.
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