An Ephemeral Sexual Population of Phytophthora infestans in the Northeastern United States and Canada

Phytophthora infestans, the causal agent of late blight disease, has been reported in North America since the mid-nineteenth century. In the United States the lack of or very limited sexual reproduction has resulted in largely clonal populations of P. infestans. In 2010 and 2011, but not in 2012 or 2013, 20 rare and diverse genotypes of P. infestans were detected in a region that centered around central New York State. The ratio of A1 to A2 mating types among these genotypes was close to the 50:50 ratio expected for sexual recombination. These genotypes were diverse at the glucose-6-phosphate isomerase locus, differed in their microsatellite profiles, showed different banding patterns in a restriction fragment length polymorphism assay using a moderately repetitive and highly polymorphic probe (RG57), were polymorphic for four different nuclear genes and differed in their sensitivity to the systemic fungicide mefenoxam. The null hypothesis of linkage equilibrium was not rejected, which suggests the population could be sexual. These new genotypes were monomorphic in their mitochondrial haplotype that was the same as US-22. Through parentage exclusion testing using microsatellite data and sequences of four nuclear genes, recent dominant lineages US-8, US-11, US-23, and US-24 were excluded as possible parents for these genotypes. Further analyses indicated that US-22 could not be eliminated as a possible parent for 14 of the 20 genotypes. We conclude that US-22 could be a parent of some, but not all, of the new genotypes found in 2010 and 2011. There were at least two other parents for this population and the genotypic characteristics of the other parents were identified

An Ephemeral Sexual Population of <i>Phytophthora infestans</i> in the Northeastern United States and Canada

<div><p><i>Phytophthora infestans</i>, the causal agent of late blight disease, has been reported in North America since the mid-nineteenth century. In the United States the lack of or very limited sexual reproduction has resulted in largely clonal populations of <i>P. infestans</i>. In 2010 and 2011, but not in 2012 or 2013, 20 rare and diverse genotypes of <i>P. infestans</i> were detected in a region that centered around central New York State. The ratio of A1 to A2 mating types among these genotypes was close to the 50∶50 ratio expected for sexual recombination. These genotypes were diverse at the <i>glucose-6-phosphate isomerase</i> locus, differed in their microsatellite profiles, showed different banding patterns in a restriction fragment length polymorphism assay using a moderately repetitive and highly polymorphic probe (RG57), were polymorphic for four different nuclear genes and differed in their sensitivity to the systemic fungicide mefenoxam. The null hypothesis of linkage equilibrium was not rejected, which suggests the population could be sexual. These new genotypes were monomorphic in their mitochondrial haplotype that was the same as US-22. Through parentage exclusion testing using microsatellite data and sequences of four nuclear genes, recent dominant lineages US-8, US-11, US-23, and US-24 were excluded as possible parents for these genotypes. Further analyses indicated that US-22 could not be eliminated as a possible parent for 14 of the 20 genotypes. We conclude that US-22 could be a parent of some, but not all, of the new genotypes found in 2010 and 2011. There were at least two other parents for this population and the genotypic characteristics of the other parents were identified.</p></div

Discriminant Analysis of Principal Components (DAPC) using 12 microsatellite loci.

<p>This scatterplot shows the first two principal components of the DAPC of <i>Phytophthora infestans</i> genotypes found in the United States. Groups are shown by different colors and inertia ellipses, while dots represent individual strains. Cluster 1 includes lineages US-6, US-7, US-11, US-12, and US-16; Cluster 2 includes isolates in lineage US-23; Cluster 3 includes lineages GDT-02, GDT-07, GDT-13, GDT-18, and GDT-20; Cluster 4 includes lineages GDT-03, GDT-04, GDT-08, GDT-08.1, GDT-14 and GDT-15; Cluster 5 includes lineages US-8, US-14, US-20, and US-24; Cluster 6 includes lineages GDT-05, GDT-06, GDT-09, GDT-10, GDT-11, GDT-12, GDT-16, GDT-17, and GDT-19; Cluster 7 includes lineages US-1; and Cluster 8 included lineages US-17, US-19, US-21 and GDT-01.</p

Spatial occurrence of the NYS-2010/11 population of <i>Phytophthora infestans</i> detected in western New York State.

<p>Genotypes that are underlined are those detected in 2010 all other genotypes were detected in 2011. Genotypes shown in bold are those that were found in several counties (GDT-01, GDT-04, and GDT-08). In New York State we detected only A1 individuals in six counties, and only A2 individuals in another three counties. However, both A1 and A2 individuals were reported from yet two other counties. Because of our limited sample size, we cannot conclude with certainty that both mating types were not present in counties where only a single mating type was detected.</p