PHYLOGENETIC ESTIMATION OF PLASMID EXCHANGE IN BACTERIA

The existence of differential horizontal gene transfer may be assessed by comparing the phylogenetic trees derived from two different genes. We use this concept to estimate quantitatively the amount of plasmid exchange that has occurred in a bacterial population. By means of computer simulations we studied the effect of gene transfer on the topological distortion between two phylogenetic trees: one obtained from an euchromosomal gene and another from a plasmid-borne sequence, which may be subjected to horizontal transfer. The basic assumptions of our simulations were (a) that plasmid exchange had occurred recently (after the last population split); and (b) that either the amount of chromosomal horizontal exchange was negligible or that it was only a fraction of the amount of plasmid exchange in which case we will be estimating relative amounts of plasmid transfer. We found that the topological difference between two such trees is a function of the number of plasmid exchange events that have occurred. It can be explained by a logistic model that relates the average distortion index between two trees (d_T ) to the number of transfer events (x). The behavior remains the same under different conditions that were tested (symmetry of the topology, number of taxa in the tree, effect of reconstruction errors, mutation after plasmid transfer). We have also tried our method on empirical data from the literature and estimated the amount of gene transfer that may have occurred among Sym plasmids in agricultural field populations of Rhizobium leguminosarum biovar phaseoli. We found that between 15.77 to 29.98% of all genetic types in these populations have been either the source or the target of a plasmid transfer event. When the comparisons were made among trees derived exclusively from plasmid probes this value dropped to 2.00%. Phylogenetic trees derived from symbiotic and nonsymbiotic sequences were also used to infer the number of gene transfer events among 11 isolates from R. galegae. The estimated number of transfer events of symbiotic sequences was 10.515 (although we do not know out of how many genetic types). We concluded that intraspecific transfer of symbiotic sequences is widespread in these two species of the genus Rhizobium.