Estimating the relative contributions of mutation and recombination to clonal diversification: a comparison between Neisseria meningitidis and Streptococcus pneumoniae.

Both Neisseria meningitidis and Streptococcus pneumoniae are naturally transformable species and are known to be freely recombining in the wild. Large multilocus sequence typing (MLST) datasets have been generated for these species. Here we outline an approach which exploits these data sets in order to quantify the extent of recombination, thus enabling meaningful comparisons between the two species. Two parameters are estimated; the rate at which recombination changes alleles, compared to point mutation, and the rate at which recombination changes individual nucleotide sites, compared to point mutation. Estimates for the former parameter are 4:1 in the meningococcus (i.e. alleles are changed four-fold more frequently by recombination than by mutation), and 10:1 in the pneumococcus. However, estimates for the latter parameter are at least 80:1 in the meningococcus (i.e. an individual nucleotide site is at least 80-fold more likely to change by recombination than by mutation) and 50:1 in the pneumococcus. These data imply that recombination events, compared to mutational events, may be more common in the pneumococcus than in the meningococcus. However, because it is a more diverse species, each recombinational exchange in the meningococcus results in more nucleotide changes on average.