Spatial Population Genetic Structure of Phytophthora sojae in Indiana, USA

The population genetic structure of Phytophthora sojae in the complex landscapes of Indiana, USA was assessed from 462 isolates collected from 136 sites of agricultural fields across the whole state. P. sojae specific primers were used to verify species identity. Isolates were genotyped using 12 microsatellite markers that were chosen from a larger set of 21 markers. Spatially explicit and non-explicit techniques were used to estimate the best K (describing the number of populations). Results indicate that there are about 17 populations of the pathogen in Indiana. AMOVA indicated significant yet different patterns of genetic variability both within and among populations at different spatial scales.  Spatial autocorrelation analyses showed that at smaller geographic distances, population dissimilarity reached a minimum of 0.1 at 500 m. Dissimilarity increased by distance to 0.74 at ≥ 2000 m. Average heterozygosity varied between 0.21 – 0.63. Some loci were in LD and departed from HWE, which could be attributed to limited sample size in some sites or comparing multiple loci pairs at different samples. We hypothesize that the presence of genetically structured populations indicates that P. sojae has different spatial range of dispersal. This dispersal is promoted mainly by environmental conditions (e.g. wind) and anthropogenic activities.  We will conduct a landscape genetics study that investigates the correlation between the pathogen’s gene flow between the populations and landscape connectivity between isolates. This will provide better understanding to the population dynamics of the P. sojae in the landscape.