Clonal reproduction of Moniliophthora roreri and the emergence of unique lineages with distinct genomes during range expansion.

The basidiomycete Moniliophthora roreri causes frosty pod rot of cacao (Theobroma cacao) in the Western hemisphere. M. roreri is considered asexual and haploid throughout its hemibiotrophic lifecycle. To understand the processes driving genome modification, using long-read sequencing technology we sequenced and assembled five high quality M. roreri genomes out of a collection of ninety-nine isolates collected throughout the pathogen's range. We obtained chromosome-scale assemblies composed of eleven scaffolds. We used short-read technology to sequence the genomes of twenty-two similarly chosen isolates. Alignments among the five reference assemblies revealed inversions, translocations, and duplications between and within scaffolds. Isolates at the front of the pathogens' expanding range tend to share lineage-specific structural variants, as confirmed by short-read sequencing. We identified, for the first time, three new mating type A locus alleles (five in total) and one new potential mating type B locus allele (three in total). Currently only two mating type combinations, A1B1 and A2B2, are known to exist outside of Colombia. A systematic survey of the M. roreri transcriptome across twenty-two isolates identified an expanded candidate effector pool and provided evidence that effector candidate genes unique to the Moniliophthoras are preferentially expressed during the biotrophic phase of disease. Notably, M. roreri isolates in Costa Rica carry a chromosome segment duplication that has doubled the associated gene complement and includes secreted proteins and candidate effectors. Clonal reproduction of the haploid M. roreri genome has allowed lineages with unique genome structures and compositions to dominate as it expands its range, displaying a significant founder effect.