Disease, Frequency-Dependent Selection, and Genetic Polymorphisms: Experiments With Stripe Rust and Wheat

Abstract

Abstract. Pathogens have the potential to maintain genetic polymorphisms by creating frequency-dependent selection on their host. This can occur when a rare host genotype is less likely to be attacked by a pathogen (frequency-dependent disease attack) and has higher fitness at low frequency (negative frequency-dependent selection). In this study, we used wheat genotypes that were susceptible to different races of the pathogen Puccinia striiformis to test whether disease created frequency-selection on its host and whether such selection could maintain polymorphisms for resistance genes in the wheat populations. Four different two-way mixtures of wheat genotypes were planted at different frequencies in both the presence and absence of disease. Disease created frequency-dependent selection on its host in some populations. Unknown factors other than disease also created frequency-dependent selection in this system because, in some instances, rare genotype advantage was observed in the absence of disease. Although the pathogen created frequency-dependent selection on its host, this selection was not sufficient to maintain genetic polymorphism in the host populations. In all cases where frequency-dependent selection occurred only in the diseased plots, one of the two genotypes was predicted to dominate in the population and the same genotype was predicted to dominate in both the presence and absence of disease. Only in cases where frequency-dependent selection was not caused by disease was there evidence that genetic polymorphisms would be maintained in the population. The frequencydependent selection described in this study is a consequence of epidemiological effects of disease and differs from the time-lagged frequency-dependent selection resulting from coevolution between hosts and parasites. The impact of this direct frequency-dependent selection on the maintenance of genetic polymorphisms in the host population is discussed.Key words. Disease, frequency-dependent disease attack, frequency-dependent selection, genetic polymorphisms, plant-pathogen interactions.Received March 19, 1999. Accepted October 18, 1999 Pathogens have been suggested as selective agents capable of modifying the genetic structure of plant populations. Pathogens may cause the maintenance of polymorphisms for resistance genes in plant populations (Haldane

THE COST OF BEING COMMON: EVIDENCE FROM NATURALDAPHNIAPOPULATIONS

“…Decreased fitness of the common host was also observed in the plant‐rust fungus system (Brunet and Mundt 2000), but in that study, the common clones were at a disadvantage in the absence of the disease as well. Thus, the negative frequency‐dependent selection was apparently caused by conditions other than parasites.…”

Complex genetic networks underlying the defensive system of rice ( Oryza sativa L.) to Xanthomonas oryzae pv. oryzae

“…Thus, PR is essentially ''weaker'' CR controlled by the complex genetic networks discussed above. The complex genetic networks are expected to maintain a high level of the allelic diversity at avirulence loci in pathogens by stabilizing selection (1, 2), which may maintain a high level of allelic diversity at R loci in the host by the frequency-dependent selection (18). Thus, a full understanding of the genetic networks underlying the plant defensive system requires a genome-wide approach and simultaneous study on a large number of loci involved.…”

Frequency‐dependent selection for rare genotypes promotes genetic diversity of a tropical palm

“…The observed rare genotype survival advantage is likely caused by either of two ecological mechanisms: frequency-dependent enemy attack or niche-partitioning. Pathogens and predators have been shown to respond in a frequency-dependent manner, where common genotypes of plants are preferentially attacked (Chaboudez & Burdon 1995;Brunet & Mundt 2000), though the majority of relevant studies are from agricultural systems or clonally reproducing plants. Despite the importance of predators and pathogens in regulating populations and communities in the tropics (Swamy & Terborgh 2010;Terborgh 2012;Bagchi et al 2014), the specificity of natural enemies to within-species genotypes in O. bataua and other tropical trees is currently not well understood, and represents a priority for future work.…”