Frank, S. A. 1997. Spatial processes in host-parasite genetics. Pages 325-352 in Metapopulation Biology: Ecology, Genetics, and Evolution, I. Hanski and M. Gilpin, eds. Academic Press, New York.
Host-parasite diversity can be described in two different ways. The first is simply the observed variability among the hosts and parasites in a particular population. For example, Burdon and Jarosz (1991) classified 67 wild flax plants into 10 distinct resistance genotypes when tested against six races of flax rust. One host genotype was completely resistant to all six pathogen races, whereas another genotype was susceptible to five of six races.
The second type of variability is the range of potential genotypes that can occur over space and time. For example, Parker (1985) used field transplant experiments to study the legume Amphicarpaea bracteata and its fungal pathogen Synchytrium decipiens. Fungal infection was heavy in each of three locations. However, a plant moved to a new location developed little or no infection, suggesting that the pathogen populations differ among sites. In a second experiment, host lines derived from different locations varied in their ability to resist a single pathogen isolate, indicating spatial differentiation among the host populations.
Parker's study suggests that the potential range of diversity over space and time is often greater than the variability observed in a single location. The potential diversity is limited by the biochemistry and morphology of host-parasite traits, whereas the observed diversity is controlled by the local dynamics of disease and the global processes of extinction and colonization in the metapopulation.
The first goal of this paper is to suggest that increasing potential diversity causes a qualitative shift in metapopulation dynamics. Local processes dominate when potential diversity is low. Colonization-extinction dynamics of alleles in the metapopulation become more important with an increase in the potential number of distinct genotypes. In the next section I present a simple model to illustrate the importance of potential diversity.
After briefly discussing the model, I review evidence that many host-parasite systems do in fact have high potential diversity. Examples include plant-pathogen genetics and bacterial defense systems against viral parasites and conspecific competitors. I also discuss the antagonistic interaction between cytoplasmic and nuclear genes in cytoplasmic male sterility.
Data from these studies suggest that spatial variation and colonization-extinction dynamics are important in the observed patterns of diversity. However, the data are difficult to interpret because of limited sampling over space and time. This difficulty leads to my second goal: the emphasis of space-time scaling when interpreting host-parasite diversity. Spatial scales that are small relative to migration distance have well-mixed populations dominated by local interactions. Local processes also dominate on temporal scales that are short relative to the expected times to extinction and recolonization of genotypes. By contrast, observations aggregated over long spatial and temporal scales may obscure colonizations, extinctions and rapid changes in genetic composition that occur on finer scales. Thus the patterns of observed variability are strongly influenced by the space-time scaling of colonizations and extinctions in the metapopulation.