Abstract
Parasites ? defined broadly as disease causing organisms - are ubiquitous and exercise strong selection pressures on their host populations. The outcome is clearly visible, also in human society with high costs in terms of public health and in damage to agricultural crops. Over 60 years ago Haldane pointed out that infectious diseases have the capacity to bring about rapid evolutionary change and genetic polymorphism of affected populations. Host genetics can determine the incidence and severity of disease, and thus a deep understanding of the way disease shapes host population structure stands to enhance our efforts to mitigate damaging effects of disease and pests. Despite the strong selection by parasites direct demonstrations of parasite mediated selection under natural conditions are rare, even exceptional (in contrast to simplified laboratory or agricultural systems). The objective of our study is to demonstrate parasite driven selection in natural host populations. The basic question we ask is to what extend parasite driven selection really occurs in nature. Demonstration of parasite driven microevolution of the host is dependent on the strength of the selection. The current proposal is based upon a model host-parasite system in which the strength of selection is exceptionally high, offering good potential to observe selection under pressure of parasites. The model system consists of a freshwater diatom (Asterionella formosa) as a host and a chytrid fungus (Zygorhizidium planktonicum) as a parasite. Despite the known strength of selection in this system we have designed our study ? through a careful selection of fieldwork and closely related microcosm studies ? to include the most important confounding factors that could obscure patterns of host selection in nature. Potential constraints on parasite mediated selection include (i) environmental fluctuations possibly resulting in host genotype x environment interactions and (ii) gene flow from the lake sediment. Temperature in the lake under study changes rapidly during the spring diatom blooms, i.e. at the time of epidemic development of the fungus, and this in itself could steer clonal selection. A large ? viable ? population of host diatoms is present on the sediment of the lake we study. Under the conditions on the sediment the parasites are non-infective. Recurrent wind driven resuspension of non-infected hosts from the sediment could obscure a clear selection pattern in the planktonic population in the watercolumn. Overall the study should result in a deeper understanding of the ways disease shapes host genetic structure and of those confounding factors on parasite driven selection that operate in the complex natural environment of a lake.
Netherlands