Evolving Insights: Symbiosis Islands and Horizontal Gene Transfer

A common theme emerging from microbial genome studies is that the genes required for pathogenic or symbiotic interactions with eukaryotic hosts are often part of the accessory gene pool of the microbe, acquired by horizontal transfer (5). They may be clustered on plasmids or on the chromosome as genomic islands. Within a particular environment, such islands provide a selective advantage to organisms that carry them, leading to the term “fitness islands” (7), with the pathogenicity islands (PAIs) that endow many bacterial pathogens with their virulence constituting a particular subset (1). In many cases it appears that the transfer machinery of genomic islands has been lost through reductive evolution following their acquisition, anchoring the islands in the host chromosome (2). However, a particularly striking example of a genomic island that has retained its transmissibility is the Mesorhizobium loti strain R7A symbiosis island. The research that led to the characterization of this island represents an engaging and attractive “case history,” in which initial findings from a field experiment have logically progressed to the report by Sullivan et al. (10) in this issue of the sequence and annotation of the entire 502-kb island. This work impacts on diverse areas of microbiology, including the rhizobium-legume symbiosis, microbial ecology, and microbial evolution.