Genome-wide DNA Variants Identify Genetic Diversity and Population Structure of Leptosphaeria maculans in Western Canada

Abstract

Background: Leptosphaeria maculans is a serious concern for canola production in Canada. For effective management, knowledge of the pathogen’s genetic variability and population structure is a prerequisite. Despite some information on race dynamics of the western Canadian L. maculans population in recent years, genetic diversity based on a large number of genome-wide DNA variants has not been investigated.Results: From 1,590 L. maculans isolates collected from 23 field sites in three provinces: Manitoba, Saskatchewan and Alberta, Canada, in the years 2007-2008 and 2012-2014, 150 representative isolates were selected and whole-genome sequenced, and 31,870 polymorphic DNA variants (SNPs and InDels) were used to study L. maculans genetic diversity and population structure. Cluster analysis showed that the genetic diversity levels and isolate groupings varied with the number and genomic regions of the variants involved; isolates collected in 2012-2014 were more genetically diverse than those collected in 2007-2008 when genome-wide variants were considered. The genome wide association study (GWAS) detected variants in egn4_Lema_T86290 (AvrLm4-7), egn4_Lema_T86300 and egn4_Lema_T86310 associated with the year of collection, but no variants was found to be associated with the province or specific location from which the isolates were collected. Population structure analysis indicated the presence of three distinct sub-populations in western Canada. While isolates from Saskatchewan were mainly of one sub-population (sub-pop1), the Alberta isolates comprised two sub-populations (sub-pop1 and sub-pop2), and all the 3 subpopulations were found in Manitoba.Conclusion: The genetic diversity of the western Canadian L. maculans population varied among provinces. It was highly admixed in Manitoba, followed by that in Alberta. The Saskatchewan population had the lowest genetic diversity. Significant genome variation between 2007-2008 and 2012-2014 occurred in the genes egn4_Lema_T86290 (AvrLm4-7), egn4_Lema_T86300 and egn4_Lema_T86310), with AvrLm4-7 becoming much more common in the L. maculans population in the later period.