MEASURING IMMIGRATION AND SEXUAL REPRODUCTION IN-FIELD POPULATIONS OFMYCOSPHAERELLA-GRAMINICOLA

A field experiment was conducted to determine the relative contributio ns of immigration and sexual reproduction to the genetic structure of Mycosphaerella graminicola populations during the course of an epidemi c. The genetic structure of M. graminicola populations sampled from wh eat plots inoculated artificially with 10 isolates was compared with c ontrol plots infected naturally by airborne ascospores. Restriction fr agment length polymorphisms (RFLPs) were used to test the randomness o f associations among loci, and DNA fingerprints were used to identify clones. All isolates in the control plots had unique genotypes and RnP loci were at gametic equilibrium, findings consistent with random mat ing. The proportion of isolates in the inoculated plots with DNA finge rprints that differed from the 10 inoculated isolates increased from 3 % in the early to 39 and 34% in the mid- and late season, respectively . The degree of gametic disequilibrium was higher in the mid-season th an in the late-season population. By the end of the growing season, we estimate that 66% of the isolates in the inoculated plots were asexua l progeny of the 10 inoculated isolates, 10% were immigrants, and 24% were sexual recombinants. The proportion of infections caused by ascos pores increased over the growing season.