Figure 5.

Example of horizontal gene transfer to cyanobacteria: threonyl tRNA synthetase. This is a phylogenetic tree reconstructed from a data set in which the Anabaena sp. genome did not have a detectable homolog in its annotation. In this tree, sequences of three Prochlorococcus spp. group within Gamma-proteobacteria with high bootstrap support. The presence of an ancestrally transferred gene constitutes a shared derived character for the descendents (Andersson et al. 2005; Huang et al. 2005), suggesting that the three Prochlorococcus spp. form a monophyletic group. Note that despite coherency of the remaining cyanobacteria as a group, the relationships within cyanobacteria do not follow the plurality topology. tRNA synthetases are known to have complex evolutionary histories involving multiple HGT events (e.g., Wolf et al. 1999). Cyano-bacteria are shown in bold. To obtain bootstrap support values, a consensus tree was generated from 100 bootstrap sampled trees (see Methods for more details). Only bootstrap support values above 80% are shown. Additional examples of interphylum transfers are available in the Supplemental material.