Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2004
Publication Date: 12/1/2004
Citation: Barnes, C.W., Szabo, L.J., May, G., Groth, J.V. 2004. Inbreeding levels of two Ustilago maydis populations. Mycologia. 96:1236-1244. Interpretive Summary: Yield losses of corn due to infection by the corn smut fungus can be substantial, especially on some varieties of sweet corn. Genetic variation in the corn smut pathogen may allow the pathogen to infect different varieties of corn. The amount of genetic variation within a pathogen population is affected by the mating system of the organism. Like other fungi, the corn smut fungus is capable of both mating through outcrossing or inbreeding, leading to increased or decreased genetic variability, respectively. To determine the extent of the genetic variation, and the mating behavior of the fungus over a wide geographic range, we sampled infected corn in Le Seuer, Minnesota and Tarariras, Uruguay, representing geographic extremes of maize cultivation. The results of this study demonstrate that the corn smut fungus is largely outcrossing regardless of the location. High levels of genetic variation were found among the very different farming practices exhibited in the Minnesota and Uruguay cornfields. The results of this study also show that near the far ends of geographic distribution of the corn smut fungus in the Americas, the separate pathogen populations have become very distinct over a relatively short time frame. This information will be used by scientists working on the corn smut fungus and may be useful in developing better control strategies of this smut pathogen.
Technical Abstract: Little is known about the population breeding structure, ie. outcrossing vs. inbreeding, of the smut fungus Ustilago maydis DC (Corda). To determine the amount of inbreeding that occurs in local U. maydis populations, two cornfields were sampled, one in North America (NA) from Le Sueur, Minnesota and one in South America (SA) from Tarariras, Uruguay. These fields were chosen because of their geographic isolation and host management differences. Inbreeding coefficients (Fis) were calculated using Amplified Fragment Length Polymorphism (AFLP) loci. Mean Fis values estimated for both the NA (-0.08), and the SA (-0.02) populations are not statistically different from zero. The results of this study demonstrate that U. maydis populations in both cornfields are predominately from outcrossing and suggest that teliospores infrequently act as single infection units. The genetic differentiation between populations was high (Fst = 0.25).