Host and parasite population structure in a natural plant-pathogen system

We investigated the genetic population structure in a metapopulation of the plant Silene latifolia (Caryophyllaceae) and its fungal pathogen Microbotr yum violaceum (Ustilaginales), a pollinator-borne disease. Population struc ture of the host plant was estimated using allozyme markers and that of the fungus by microsatellites. Both host and parasite showed significant diffe rentiation, but parasite populations were 12 times more strongly differenti ated than those of the hosts. We found significant isolation by distance fo r host populations but not for parasite populations. Higher population diff erentiation for the parasite may result from small effective population siz e, high selfing rates, or low migration rate. In this system, hosts are obl igate outcrossers and they migrate by seeds and pollen, whereas parasites c an self-fertilize and migrate only on pollinating insects. We discuss the e ffect of limited gene flow in this parasite on its coevolutionary interacti on with its host, and its potential for local adaptation on sympatric host populations.