From comparative gene content and gene order to ancestral contigs, chromosomes and karyotypes

To reconstruct the ancestral genome of a set of phylogenetically related descendant species, we use the raccroche pipeline for organizing a large number of generalized gene adjacencies into contigs and then into chromosomes. Separate reconstructions are carried out for each ancestral node of the phylogenetic tree for focal taxa. The ancestral reconstructions are monoploids; they each contain at most one member of each gene family constructed from descendants, ordered along the chromosomes. We design and implement a new computational technique for solving the problem of estimating the ancestral monoploid number of chromosomes x . This involves a “ g -mer” analysis to resolve a bias due long contigs, and gap statistics to estimate x . We find that the monoploid number of all the rosid and asterid orders is x=9 . We show that this is not an artifact of our method by deriving x≈ 20 for the metazoan ancestor.