Introduction

The frequency and significance of recombination in the evolution of bacteria is a subject of intense debate. Rhizobium population genetic studies show that there is a wide range of genetic structures in Rhizobium species, spanning from strictly clonal to essentially panmictic. We recently analyzed the genetic structure of a local population of R. etli bv. phaseoli (Silva et al. 1999). MLEE data revealed two genetic groups, linkage disequilibrium analysis showed equilibrium within each group, when only the different genotypes were taken into account, indicating a reticulated and epidemic genetic structure (Maynard Smith et al. 1993). The lateral transfer of the pSym in natural populations of Rhizobium is shown by the disparity of the groups formed by the chromosomal and the symbiotic loci. It is of interest to know if transfer of plasmids in the field involves only pSym or if the other (cryptic) plasmids are also exchanged. The present study employed a population genetics framework to elucidate the genetic structure of a local R. etli bv. phaseoli population in Puebla, Mexico, both at the chromosomal and plasmidic level over three consecutive years.

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