THE ROLE OF SELECTION ON THE GENETIC-STRUCTURE OF PATHOGEN POPULATIONS - EVIDENCE FROM FIELD EXPERIMENTS WITH MYCOSPHAERELLA-GRAMINICOLA ONWHEAT

Coevolution refers to reciprocal genetic changes that occur in two or more ecologically interacting species. In agricultural ecosystems, we are especially concerned with the genetic response of pathogen populat ions to resistant cultivars produced by plant breeding programs. It wo uld be useful to be able to predict whether disease resistance is like ly to be durable or ephemeral before a cultivar is widely grown. Thoug h it may not be possible to predict durability in advance, knowledge o f the genetic structure of pathogen populations may prove useful for m aking predictions about the rate at which pathogens adapt to resistant varieties. Much has been learned about the genetic structure of popul ations of obligate fungal pathogens such as rusts and mildews, which h ave become paradigms for plant pathology. We have focused our effort o n the population genetics of the less known, non-specialized, necrotro phic pathogens, such as the Septorias of small grains. Our approach ha s been to use DNA fingerprinting and RFLP analysis to conduct field ex periments that elucidate how populations of fungal pathogens adapt in agroecosystems. Our results suggest that mating system may have a grea ter impact than natural selection on the genetic structure of populati ons of Mycosphaerella graminicola (anamorph Septoria tritici).