Firas Swidan

Bio: Firas Swidan is an academic researcher from Technion – Israel Institute of Technology. The author has contributed to research in topics: Phylogenetic tree & Phylogenetics. The author has an hindex of 3, co-authored 6 publications receiving 104 citations.

Improved bounds on sorting with length-weighted reversals

Abstract: We study the problem of sorting integer sequences and permutations by length-weighted reversals. We consider a wide class of cost functions, namely f(l) = lα for all α ≥ 0, where l is the length of the reversed subsequence. We present tight or nearly tight upper and lower bounds on the worst-case cost of sorting by reversals. Then we develop algorithms to approximate the optimal cost to sort a given input. Furthermore, we give polynomial-time algorithms to determine the optimal reversal sequence for a restricted but interesting class of sequences and cost functions. Our results have direct application in computational biology to the field of comparative genomics.

Sorting by Length-Weighted Reversals: Dealing with Signs and Circularity

Abstract: We consider the problem of sorting linear and circular permutations and 0/1 sequences by reversals in a length-sensitive cost model. We extend the results on sorting by length-weighted reversals in two directions: we consider the signed case for linear sequences and also the signed and unsigned cases for circular sequences. We give lower and upper bounds as well as guaranteed approximation ratios for these three cases. The main result in this paper is an optimal polynomial-time algorithm for sorting circular 0/1 sequences when the cost function is additive.

On the repeat-annotated phylogenetic tree reconstruction problem

Abstract: A new problem in phylogenetic inference is presented, based on recent biological findings indicating a strong association between reversals (aka inversions) and repeats. These biological findings are formalized here in a new mathematical model, called repeat-annotated phylogenetic trees (RAPT). We show that, under RAPT, the evolutionary process - including both the tree-topology as well as internal node genome orders - is uniquely determined, a property that is of major significance both in theory and in practice. Furthermore, the repeats are employed to provide linear-time algorithms for reconstructing both the genomic orders and the phylogeny, which are NP-hard problems under the classical model of sorting by reversals (SBR).

On the repeat-annotated phylogenetic tree reconstruction problem.

Abstract: A new problem in phylogenetic inference is presented, based on recent biological findings indicating a strong association between reversals (i.e., inversions) and repeats. These biological findings are formalized here in a new mathematical model, called repeat-annotated phylogenetic trees (RAPT). We show that, under RAPT, the evolutionary process--including both the tree-topology as well as internal node genome orders--is uniquely determined, a property that is of major significance both in theory and in practice. Furthermore, the repeats are employed to provide linear-time algorithms for reconstructing both the genomic orders and the phylogeny, which are NP-hard problems under the classical model of sorting by reversals (SBR).

progressiveMauve: Multiple Genome Alignment with Gene Gain, Loss and Rearrangement

Abstract: Background Multiple genome alignment remains a challenging problem. Effects of recombination including rearrangement, segmental duplication, gain, and loss can create a mosaic pattern of homology even among closely related organisms.

Improved bounds on sorting with length-weighted reversals

Abstract: We study the problem of sorting integer sequences and permutations by length-weighted reversals. We consider a wide class of cost functions, namely f(l) = lα for all α ≥ 0, where l is the length of the reversed subsequence. We present tight or nearly tight upper and lower bounds on the worst-case cost of sorting by reversals. Then we develop algorithms to approximate the optimal cost to sort a given input. Furthermore, we give polynomial-time algorithms to determine the optimal reversal sequence for a restricted but interesting class of sequences and cost functions. Our results have direct application in computational biology to the field of comparative genomics.

An Integrative Method for Accurate Comparative Genome Mapping

Abstract: We present MAGIC, an integrative and accurate method for comparative genome mapping. Our method consists of two phases: preprocessing for identifying “maximal similar segments,” and mapping for clustering and classifying these segments. MAGIC's main novelty lies in its biologically intuitive clustering approach, which aims towards both calculating reorder-free segments and identifying orthologous segments. In the process, MAGIC efficiently handles ambiguities resulting from duplications that occurred before the speciation of the considered organisms from their most recent common ancestor. We demonstrate both MAGIC's robustness and scalability: the former is asserted with respect to its initial input and with respect to its parameters' values. The latter is asserted by applying MAGIC to distantly related organisms and to large genomes. We compare MAGIC to other comparative mapping methods and provide detailed analysis of the differences between them. Our improvements allow a comprehensive study of the diversity of genetic repertoires resulting from large-scale mutations, such as indels and duplications, including explicitly transposable and phagic elements. The strength of our method is demonstrated by detailed statistics computed for each type of these large-scale mutations. MAGIC enabled us to conduct a comprehensive analysis of the different forces shaping prokaryotic genomes from different clades, and to quantify the importance of novel gene content introduced by horizontal gene transfer relative to gene duplication in bacterial genome evolution. We use these results to investigate the breakpoint distribution in several prokaryotic genomes.

Sorting by weighted reversals, transpositions, and inverted transpositions

Abstract: During evolution, genomes are subject to genome rearrangements that alter the ordering and orientation of genes on the chromosomes. If a genome consists of a single chromosome (like mitochondrial, chloroplast or bacterial genomes), the biologically relevant genome rearrangements are (1) inversions—also called reversals—where a section of the genome is excised, reversed in orientation, and reinserted and (2) transpositions, where a section of the genome is excised and reinserted at a new position in the genome; if this also involves an inversion, one speaks of an inverted transposition. To reconstruct ancient events in the evolutionary history of organisms, one is interested in finding an optimal sequence of genome rearrangements that transforms a given genome into another genome. It is well known that this problem is equivalent to the problem of “sorting” a signed permutation into the identity permutation. The complexity of the problem is still unknown. The best polynomial-time approximation algorithm, recently devised by Hartman and Sharan, has a 1.5 performance ratio. However, it applies only to the case in which reversals and transpositions are weighted equally. Because in most organisms reversals occur more often than transpositions, it is desirable to have the possibility of weighting reversals and transpositions differently. In this paper, we provide a 1.5-approximation algorithm for sorting by weighted reversals, transpositions and inverted transpositions for biologically realistic weights.

Advances in phylogeny reconstruction from gene order and content data.

Abstract: Genomes can be viewed in terms of their gene content and the order in which the genes appear along each chromosome. Evolutionary events that affect the gene order or content are “rare genomic events” (rarer than events that affect the composition of the nucleotide sequences) and have been advocated by systematists for inferring deep evolutionary histories. This chapter surveys recent developments in the reconstruction of phylogenies from gene order and content, focusing on their performance under various stochastic models of evolution. Because such methods are quite restricted in the type of data they can analyze, we also present research aimed at handling the full range of whole-genome data.