Evol Ecol Res 7: 1089-1107 (2005)     Full PDF if your library subscribes.

Plasmids survive despite their cost and male-specific phages due to heterogeneity of bacterial populations

Francisco Dionisio*

Instituto Gulbenkian de Ciência, Apartado 14, P-2781-901 Oeiras, Portugal

e-mail: dionisio@igc.gulbenkian.pt


Problem: Conjugative plasmids manipulate their bacterial hosts to express sex-pili – long, filamentous organelles whose genes are carried by the plasmids themselves. Sex-pili locate and attach to another bacterial cell for plasmid transfer. Bacteria differ in their ability to express sex-pili and hence to donate plasmids. Does this diversity in sex-pilus expression contribute to the survival of conjugative plasmids?

Method: Simulations of a chemostat containing bacterial cells, some harbouring a conjugative plasmid. Certain viruses, called male-specific phages (MSPs), enter the chemostat. These viruses infect only donor cells. I monitor whether plasmids continue to survive in each simulation.

Key assumption: A bacterium’s ability to donate is proportional to its susceptibility to MSPs.

Conclusions: Bacterial diversity in sex-pilus expression strongly increases the size of the parameter space for plasmid survival. A host with low expression of sex-pili – and hence a low plasmid transfer rate – represents a sink habitat for the conjugative plasmid. However, hosts with high expression of sex-pili also constitute sink habitats – although they have a high plasmid transfer rate, they are the most susceptible to MSPs. A system consisting of both types of bacteria (two sink habitats) constitutes a source habitat for the conjugative plasmid. At the same time, this diversity helps to maintain the MSP itself because extinction of the conjugative plasmids would also doom the MSP.

Keywords: maintenance of plasmids, male-specific-phages, polymorphism, sink habitat, source habitat.

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