Visualizing and assessing phylogenetic congruence of core gene sets:: A case study of the γ-proteobacteria

Here, we address a much-debated topic: is there or is there not an organismal tree of gamma-proteobacteria that can be unambiguously inferred from a core of shared genes? We apply several recently developed analytical methods to this problem, for the first time. Our heat map analyses of P values and of bootstrap bipartitions show the presence of conflicting phylogenetic signals among these core genes. Our synthesis reconstruction suggests that at least 10% of these genes have been laterally transferred during the divergence of the gamma-proteobacteria, and that for most of the rest, there is too little phylogenetic signal to permit firm conclusions about the mode of inheritance. Although there is clearly a central tendency in this data set (it is far from random), lateral gene transfers cannot be ruled out. Instead of an organismal tree, we propose that these core genes could be used to define a more subtle and partially reticulated pattern of relationships.