Impact of density and disease on frequency-dependent selection and genetic polymorphism: experiments with stripe rust and wheat

Authors: MUNDT, CHRISTOPHER - OREGON STATE UNIV; Brunet, Johanne; SACKETT, KATHRYN - OREGON STATE UNIV

Submitted to: Evolutionary Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2007
Publication Date: 6/17/2007
Citation: Mundt, C.C., Brunet, J., Sackett, K.E. 2007. Impact of density and disease on frequency-dependent selection and genetic polymorphism: experiments with stripe rust and wheat. Evolutionary Ecology. Available: 10.1007/s10682-007-9187-3.

Interpretive Summary: Disease has been implicated in affecting population and community diversity. This study examines whether and how disease affects genotype diversity in populations over a wide range of frequencies and densities. In a previous study, we found that stripe rust, a disease caused by a fungal pathogen, was less likely to attack a wheat cultivar in a population when rare. In addition, plants reproduced and survived better when rare thus disease created negative frequency-dependent selection on its host. Such negative frequency-dependent selection can help maintain different resistance genes (a polymorphism) in a population as a gene is more advantageous when rare. The role of disease in affecting species and genetic diversity depends on how widespread disease induced frequency-dependent effects are in plant populations. While our previous study examined the impact of disease at a single density, the current study examined its impact over a range of densities. We found that disease could create frequency-dependent selection on its host over a wide range of densities. However, as in our previous study, competitive interactions between the wheat cultivars interfered with the frequency-dependent effects of disease and prevented the maintenance of more than one resistance gene in the population at all densities. Thus in high selfers like wheat where genes for competitive abilities and disease resistance are tightly linked, the frequency-dependent disease effects are unlikely to maintain genetic polymorphisms in populations. However the presence of frequency-dependent disease effects over a range of densities suggests that disease can play an important role in maintaining genetic diversity in outcrossing species where crossing over breaks down associations between genes. Our data indicate that frequency-dependent disease effects are common in plant populations and thus are likely to play a significant role at least in plant populations with some outcrossing.

Technical Abstract: Frequency-dependent disease impacts may contribute to the maintenance of genetic diversity and sexual reproduction in plant populations. In earlier work with experimental wheat (Triticum aestivum) populations at a single density, we found that stripe rust (caused by Puccinia striiformis) created frequency-dependent selection on its host but competitive interactions reduced the potential for maintenance of genetic polymorphisms in this highly self-pollinated species. Here we report on results with wheat stripe rust in which we altered both the frequency and density of host genotypes in factorial combinations of two-way mixtures where each host genotype was attacked by its own specialized race of rust. We expected that lower planting densities would decrease the level of competition, thereby allowing frequency-dependent disease impacts to play a greater role. We found no clear evidence to suggest that plant density had an important influence on any frequency-dependent disease effects or on the potential for polymorphism to be maintained when density was varied eight-fold. Disease could create frequency-dependent selection on its host at all densities but, as in our previous study, the dominance of competition effects usually prevented the maintenance of genetic polymorphisms.