EXACT AND APPROXIMATION ALGORITHMS FOR SORTING BY REVERSALS, WITH APPLICATION TO GENOME REARRANGEMENT

Motivated by the problem in computational biology of reconstructing the series of chromosome inversions by which one organism evolved from another, we consider the problem of computing the shortest series of reversals that transform one permutation to another. The permutations describe the order of genes on corresponding chromosomes, and a reversal takes an arbitrary substring of elements, and reverses their order. For this problem, we develop two algorithms: a greedy approximation algorithm, that finds a solution provably close to optimal in O(n(2)) time and O(n) space for n-element permutations, and a branch-and-bound exact algorithm, that finds an optimal solution in O(mL(n, n)) time and O(n(2)) space, where m is the size of the branch-and-bound search tree, and L(n, n) is the time tc, solve a linear program of n variables and n constraints. The greedy algorithm is the first to come within a constant factor of the optimum; it guarantees a solution that uses no more than twice the minimum number of reversals. The lower and upper bounds of the branch-and-bound algorithm are a novel application of maximum-weight matchings, shortest paths, and linear programming. In a series of experiments, we study the performance of an implementation on random permutations, and permutations generated by random reversals. For permutations differing by k random reversals, we find that the average upper bound on reversal distance estimates k to within one reversal for k < 1/2n and n less than or equal to 100. For the difficult case of random permutations, we find that the average difference between the upper and lower bounds is less than three reversals for n less than or equal to 50. Due to the tightness of these bounds, we can solve, to optimality, problems on 30 elements in a few minutes of computer time. This approaches the scale of mitochondrial genomes.