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Genomic basis for host adaptation in Puccinia striiformis

Chongjing Xia: Washington State University

<div>Stripe rust fungi, <em>Puccinia striiformis </em>f. sp. <em>tritici</em> (<em>Pst</em>) and <em>P. striiformis </em>f. sp. <em>hordei </em>(<em>Psh</em>), are devastating pathogens of wheat and barley, respectively. However, the genomic basis of their host adaptation is not clear. To understand the evolutionary mechanisms of the different formae speciales, genomic DNA and cDNA of a <em>Pst</em> isolate and a <em>Psh</em> isolate were sequenced using next generation sequencing technologies. The assembled genomes with high continuity enabled us to determine their genome differences and understand the evolutionary history of <em>Pst</em> and <em>Psh</em>. The divergence of <em>Pst </em>and <em>Psh</em>, occurred 8.12 million years ago, has been driven by high nucleotide mutation rates. Extensive losses of gene families in both <em>Pst </em>and <em>Psh </em>have occurred separately after the divergence from their most recent common ancestor, in contrast to very few gene family gains, resulting in a large number of form-specific genes. These form-specific genes have unique genomic features compared to the conserved genes, including 1) significantly shorter in length; 2) significantly less expressed; 3) significantly close to transposable elements; and 4) redundant in pathways. Moreover, 116 and 119 genes were found to be exclusively expressed in <em>Pst </em>and <em>Psh</em>, respectively. Our data indicate that the different events of gene family losses, resulting in form-specific genes, have separate <em>Pst</em> from <em>Psh</em> and that the form-specific genes are responsible for their adaptation to different cereal crops.</div>