Genome Biology of Rust Fungi.

Rust fungi (order Pucciniales) form the largest group of plant pathogens. Early studies addressed infection processes and heritability of virulence but were limited by the complexity of these organisms. We present six lessons highlighting major discoveries about rust genome biology. First, during their main infectious stage, rusts carry two haploid genomes that can be highly heterozygous, and each genome is packaged in its own nucleus. Remarkably, this allows for the exchange of whole nuclei. Second, haploid genome size is extremely variable, ranging from ∼75 Mb to 1 GB. Third, genome expansions are driven by repetitive elements, but genomes are not compartmentalized. Fourth, although chromosome number and biotrophy-related gene content are conserved across the order, effectors are highly divergent in sequence. Fifth, resolution of the mating-type locus highlights its role in genetic exchange in populations. Sixth, individual haplotypes can now be tracked to reveal nuclear exchange events, delivering unprecedented insights into rust epidemiology.