Some symbiotic associations are nasty, and some are nice – why is there so much variation in interspecific interactions?

Lots of experimental evolution studies show that the transmission of a parasite from mother to offspring (vertical) can select for reduced virulence, while horizontal transmission selects for increased virulence (e.g. Bull et al. 1991; Turner et al. 1998; Stewart et al. 2005). Under horizontal transmission, parasites continually shift between host lineages. There is little potential for coevolution. Under vertical transmission, a parasite and host lineage are paired for multiple generations. Tight coevolution is likely. Thus the potential for coevolution also varies with transmission mode. Does coevolution contribute to the evolution of reduced virulence?

We tested this hypothesis with experimental coevolution of the nematode Caenorhabditis elegans and its lethal bacterial parasite Serratia marcescens. Evolutionary transitions in virulence are relevant to this association. S. marcescens strains vary substantially in virulence to C. elegans. Serratia species are also cuticular or gut mutualists to rhabditid nematodes, including caenorhabditids. We manipulated the potential for coevolution while selecting for reduced antagonism between C. elegans and S. marcescens. We use the term “antagonism” here to emphasize the joint nature of the phenotypes of interest. Though virulence is not exclusively a parasite trait, it is commonly assumed to be.

Our results demonstrate that coevolution contributes to the evolution of reduced antagonism (i.e. reduced virulence). After 20 generations of selection, reduced antagonism evolved only when coevolution was possible. Independent selection on either partner failed to produce evolutionary change. Potentially coevolving lines were strongly locally adapted for reduced antagonism, further underlining the significance of coevolution (Gibson et al. 2015 Evolution).

Infection of C. elegans with S. marcescens. Microscopy and photography help from Erik Ragsdale.


  • Bull, J. J., I. J. Molineux, and W. R. Rice. 1991. Selection of benevolence in a host-parasite system. Evolution 45:875-882.
  • Stewart, A. D., J. M. Logsdon, and S. E. Kelley. 2005. An empirical study of the evolution of virulence under both horizontal and vertical transmission. Evolution 59:730-739.
  • Turner, P. E., V. S. Cooper, and R. E. Lenski. 1998. Tradeoff between horizontal and vertical modes of transmission in bacterial plasmids. Evolution 52:315-329.