Population Genetics of Plant Pathogens

Abstract

Abstract Population genetics consider the origin, maintenance and spatiotemporal distribution of genetic variation of species under the influences of mutation, gene flow, recombination, drift and selection. Mutation is the primary source of genetic variation and its rates vary significantly among genomes within or among pathogen species. Recombination increases genetic variation and plays a key role in the dissipation of linkage disequilibrium. Gene flow increases the genetic variation of local populations but decreases population differentiation. The principle determinant of gene flow is the dispersal mechanism of plant pathogens. Genetic drift is determined by effective population size. Pathogen populations with large effective sizes tend to have less genetic drift, therefore greater genetic variation and be more stable over time. Natural selection can increase or decrease genetic variation and population differentiation, depending on the type of selection. Directional selection can cause a rapid loss of major resistance agrochemical effectiveness. Key Concepts Genetic variation of plant pathogens results from their coevolutionary interaction with hosts and other ecological factors. Mutation by which new nucleotide sequences are generated is the primary source of genetic variation and pathogen evolution. Stochastic change in the frequencies of neutral alleles by genetic drift in the pathogen populations leads to nonadaptive genetic differentiation while selection for ecological traits results in adaptive genetic differentiation. Pathogen evolution is escalated in modern agroecosystems attributable to monoculture, agricultural intensification and globalisation. Plant diseases cause more damage to the hosts in the newly invaded territories than in their centre of origin as a result of the lack of host–pathogen coevolution. Selection is the primary force responsible for the loss of resistance gene and agrochemical effectiveness. Natural selection on plant pathogens can be manipulated by spatiotemporal deployment of host resistance and application of agrochemicals. Resistance gene rotation can delay pathogen evolution and maximise the life span of resistance genes. Effective field hygiene can reduce both the epidemic severity of plant diseases and the evolutionary rate of plant pathogens.