SELECTION IN A HETEROGENEOUS POPULATION OF PUCCINIA-RECONDITA F-SP TRITICI

A heterogeneous population of Puccinia recondita f. sp. tritici developed from randomly mated pycnial infections on Thalictrum speciosissimum was selected for 12 generations on four wheat lines with different resistance genes to determine if unnecessary genes for virulence were deleterious to general levels of fitness in the wheat leaf rust fungus. Diversity of virulence phenotypes, as measured with the Shannon index, declined least in the population selected on the susceptible line Thatcher and most in the population selected on the resistant cultivar Roblin (Lr1, Lr10, Lr13, and Lr34). Phenotypes with virulence to six differential lines predominated in the populations selected on Thatcher, the Thatcher isogenic line containing Lr3ka (TcLr3ka) and TcLr11. The frequencies of virulence to Lr2a, Lr11, and Lr17 significantly increased in the population selected on Thatcher, and frequencies of virulence to Lr3ka and Lr30 significantly decreased. In the Roblin population, frequency of virulence to Lr2a significantly decreased, and frequency of virulence to Lr24 significantly increased. Frequency of virulence to Lr2c and Lr17 significantly increased in the population selected on TcLr3ka. Genes influencing fitness may be linked to the virulences that showed consistent and significant change in frequency over generations. Differences in the effective population size in the initial generation, caused by resistance genes in the host lines, may account for some of the differences in frequency of virulence phenotypes between the four selection populations.