Gene Flow and Sexual Reproduction in the Wheat Glume Blotch Pathogen Phaeosphaeria nodorum (Anamorph Stagonospora nodorum)

Abstract

Restriction fragment length polymorphisms (RFLPs) were used to characterize the genetic structures of three field populations of Phaeosphaeria nodorum from Texas, Oregon, and Switzerland. Data from seven nuclear RFLP loci were used to estimate gene diversity and genetic distances and to make indirect measures of gene flow between populations. Three of the seven RFLP loci differed significantly in allele frequencies across populations. On average, 96% of the total gene diversity was found within populations. There was little evidence for population subdivision, suggesting that gene flow was not restricted among populations. Based on an average population differentiation of 0.04, we estimated that the exchange of 11 migrants among populations per generation would be needed to account for the present level of population subdivision. Genotype diversity based on DNA fingerprints was at a maximum for the Swiss population, whereas populations in Texas and Oregon had lower genotype diversities. Many multilocus haplotypes were found in each population. Ninety-five percent of RFLP allele pairs were in gametic equilibrium. The data were consistent with random mating within each population.

Genetic diversity and population structure of Ascochyta rabiei from the western Iranian Ilam and Kermanshah provinces using MAT and SSR markers

“…In the absence of gene flow, genetic drift causes different allele frequencies at neutral loci, leading to differentiation in isolate populations (Keller et al 1997). The low genetic differentiation in this study has several possible explanations.…”

“…2 Dendrogram of genetic relationships between seven populations of A. rabiei as reconstructed by UPGMA using Nei's genetic distance equilibrium, we can use GST to estimate Nm, the average number of migrants that would need to be exchanged among populations in each generation to account for their present lack of subdivision. Nm averaged 6.64 in all loci and populations, suggesting a level of gene flow that was 6 times greater than needed to prevent populations from diverging by genetic drift (Keller et al 1997). This result may help explain why resistance in new cultivars, periodically released by agricultural research centers and including material from the International Centre for Agricultural Research in the Dry Areas (INCARDA), has proven ineffective after 2-3 years (data not shown).…”

“…After a while, genetic drift and selection would limit the number of genotypes present in field populations, unless sexual reproduction generated new genotypes and wind dispersal distributed ascospores among populations every year (Shah et al 1995). Pycnidiospores are likely to play an important role in epidemiology over small distances (1-2 m), but ascospores probably play a more important role in the population biology and perhaps in the epidemiology of disease on a regional scale (tens to hundreds of kilometers; Keller et al 1997).…”

Detection of QTLs for Stagonospora glume blotch resistance in Swiss winter wheat

“…S. nodorum survives on the field debris and is able to infest all above-ground plant organs of wheat. The major primary inoculum are the winddispersed ascospores (sexual form) in late autumn and early spring (Mittelstadt and Fehrmann 1987;Keller et al 1997a;Bathgate and Loughman 2001). During the growing season, particularly under wet and warm weather conditions, the asexually generated and rain-splashdispersed pycnidiospores successively infect different leaf levels of individual plants (Scharen 1966;Keller et al 1997a).…”

Using dynamic diversity to achieve durable disease resistance in agricultural ecosystems

“…This higher pathogen density increases the likelihood of multi-infections in the same plant by different genotypes of the same pathogen (e.g. Linde et al 2002;Keller et al 1997;McDonald et al 1999). Multi-infections have long been thought to favor the development of higher virulence (defined here as the amount of damage done to the plant, but often referred to as aggressiveness or quantitative virulence in plant pathology) as a side effect of competition among strains for host resources (Van Baalen & Sabelis, 1995).…”