Selection Pressures and the
Coevolution of Host-Pathogen Systems
Kurt J. Leonard
In classical biocontrol of weeds, the biocontrol agent is introduced at a few release points to spread naturally over the range of the target weed ( Templeton 1982). Biotrophic pathogens often are selected as classical biocontrol agents because they tend to be highly host specific and not damaging to crop or desirable native plant species. Host specificity, however, does not stop at the species level. During coevolution, plants accumulate genes for resistance to their pathogens, which in turn accumulate an array of virulence genes to overcome the effects of the resistance genes. This chapter explores in theoretical terms the evolutionary processes that may lead to accumulating resistance and virulence genes, and it examines how these processes may determine the long-term success of the use of biotrophic pathogens in classical biocontrol of weeds. Specifically, the chapter examines the theoretical possibilities that weed and pathogen populations may achieve equilibrium levels of resistance and virulence and defines the conditions that may determine the stability of such an equilibrium.
With diseases caused by biotrophic pathogens, the genetic variation typically follows a gene-for-gene pattern ( Day 1974; Thompson and Burdon 1992) (Fig. 9.1). Each gene for resistance in the host is matched by a gene for virulence that allows the pathogen to overcome that resistance. For example, Clarke et al. ( 1990) found twenty-four factors for powdery mildew resistance in groundsel, Senecio vulgaris, and a corresponding number of virulence factors in the powdery mildew fungus. Obviously, any attempt to control groundsel with powdery mildew would have to take this variation into account.
Typically, resistance genes are most common and most diverse in environments that favor disease development. This has been well documented for resistance to crown rust in wild oats ( Wahl 1970; Burdon 1987). As Burdon ( 1987) pointed out, this leads us to believe that the diversity is maintained in balanced rather than