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Analysis of the genetic structure of soil populations provides abundant evidence indicating that Rhizobium spp. exchange DNA. For instance, among native field populations, the same symbiotic plasmid can be found in otherwise unrelated strains, and vice versa, chromosomally related strains may harbour different symbiotic plasmids (Vlassak and Vanderleyden, 1997 and references therein). Other examples show that transfer is not limited to plasmid DNA, but it can also involve chromosomal genes. Sullivan et al. (1995) showed transfer of a chromosomal “symbiotic island” from a R. loti inoculant into non-symbiotic soil bacteria. Other authors have proposed transfer of taxonomically important chromosomal genes to explain discordant results obtained with different methodologies (Eardly et al., 1995). Chromosomal DNA transfer may not be unfrequent, and in fact it is starting to be recognized as an important mechanism for antibiotic resistance spreading among clinic bacterial isolates (Francois et al., 1997; Scott et al., 1997). However, experimental quantification of these events under natural conditions has proven to be extremely difficult. Even when DNA exchange in soil has been evidenced, many times it is unknow which are the selective pressures favoring gene transfer. For example, the transferable “symbiotic island” identified by Sullivan et al. (1995) also provides vitamin prototrophy to the recipients. Thus, answers to the questions of where, when, how and why gene transfer occurs are required in order to understand the ecology and evolution of bacterial populations. |
