Detecting phylogenetic signals in eukaryotic whole genome sequences.

Whole genome sequences are a rich source of molecular data, with a potential for the discovery of novel evolutionary information. Yet, many parts of these sequences are not known to be under evolutionary pressure and, thus, are not conserved. Furthermore, a good model for whole genome evolution does not exist. Consequently, it is not a priori clear if a meaningful phylogenetic signal exists and can be extracted from the sequences as a whole. Indeed, very few phylogenies were reconstructed based on these sequences. Prior to this work, only two reconstruction methods were applied to large eukaryotic genomes: the K(r) method (Haubold et al., 2009), which was applied to genomes of rather small diversity (Drosophila species), and the feature frequency profile method (Sims et al., 2009a), which was applied to genomes of moderate diversity (mammals). We investigate the whole genome-based phylogenetic reconstruction question with respect to a much wider taxonomic sample. We apply K(r), FFP, and an alternative alignment-free method, the average common subsequence (ACS) (Ulitsky et al., 2006), to 24 multicellular eukaryotes (vertebrates, invertebrates, and plants). We also apply ACS to the proteome sequences of these 24 taxa. We compare the resulting trees to a standard reference, the National Center for Biotechnology Information (NCBI) taxonomy tree. Trees produced by ACS(AA), based on proteomes, are in complete agreement with the NCBI tree. For the genome-based reconstruction, ACS(DNA) produces trees whose agreement with the NCBI tree is excellent to very good for divergence times up to 800 million years ago, medium at 1 billion years ago, and poor at 1.6 billion years ago. We conclude that whole genomes do carry a clear phylogenetic signal, yet this signal "saturates" with longer divergence times. Furthermore, from the few existing methods, ACS is best capable of detecting this signal.