Alternaria linariae is an economically important foliar pathogen that causes early blight disease in tomatoes. Understanding genetic diversity, population genetic structure, and evolutionary potential is crucial to contemplating effective disease management strategies. We leveraged genotyping-by-sequencing (GBS) technology to compare genome-wide variation in 124 isolates of Alternaria spp. (A. alternata, A. linariae, and A. solani) for comparative genome analysis and to test the hypotheses of genetic differentiation and linkage disequilibrium (LD) in A. linariae collected from tomatoes in western North Carolina. We performed a pangenome-aware variant calling and filtering with GBSapp and identified 53,238 variants conserved across the reference genomes of three Alternaria spp. The highest marker density was observed on chromosome 1 (7 Mb). Both discriminant analysis of principal components and Bayesian model-based STRUCTURE analysis of A. linariae isolates revealed three subpopulations with minimal admixture. The genetic differentiation coefficients (FST) within A. linariae subpopulations were similar and high (0.86), indicating that alleles in the subpopulations are fixed and the genetic structure is likely due to restricted recombination. Analysis of molecular variance indicated higher variation among populations (89%) than within the population (11%). We found long-range LD between pairs of loci in A. linariae, supporting the hypothesis of low recombination expected for a fungal pathogen with limited sexual reproduction. Our findings provide evidence of a high level of population genetic differentiation in A. linariae, which reinforces the importance of developing tomato varieties with broad-spectrum resistance to various isolates of A. linariae.
Keywords: asexual reproduction; linkage disequilibrium; population biology; population differentiation.