Piron, M.-C. | bioRxiv Science

bioRxiv2020

Identification of the first oomycete mating-type locus sequence in the grapevine downy mildew pathogen, Plasmopara viticola

Mating types are self-incompatibility systems that promote outcrossing in plants, fungi and oomycetes. Mating-type genes have been widely studied in plants and fungi, but have yet to be identified in oomycetes, eukaryotic organisms closely related to brown algae that cause many destructive animal and plant diseases. We identified the mating-type locus of Plasmopara viticola, the oomycete responsible for grapevine downy mildew, one of the most damaging grapevine diseases worldwide. Using a genome-wide association approach, we identified a 570 kb repeat-rich non-recombining region controlling mating types, with two highly divergent alleles. We showed that one mating type was homozygous, whereas the other was heterozygous at this locus. The mating-type locus encompassed 40 genes, including one encoding a putative hormone receptor. Our findings have fundamental implications for our understanding of the evolution of mating types, as they reveal a unique determinism involving an asymmetry of heterozygosity, as in sex chromosomes and unlike other mating-type systems. This identification of the mating-type locus in such an economically important crop pathogen also has applied implications, as outcrossing facilitates rapid evolution and resistance to harsh environmental conditions.

evolutionary biology↗

bioRxiv2018

A high-quality grapevine downy mildew genome assembly reveals rapidly evolving and lineage-specific putative host adaptation genes

Downy mildews are obligate biotrophic oomycete pathogens that cause devastating plant diseases on economically important crops. Plasmopara viticola is the causal agent of grapevine downy mildew, a major disease in vineyards worldwide. We sequenced the genome of Pl. viticola with PacBio long reads and obtained a new 92.94 Mb assembly with high continuity (359 scaffolds for a N50 of 706.5 kb) due to a better resolution of repeat regions. This assembly presented a high level of gene completeness, recovering 1,592 genes encoding secreted proteins involved in plant-pathogen interactions. Pl. viticola had a two-speed genome architecture, with secreted protein-encoding genes preferentially located in gene-sparse, repeat-rich regions and evolving rapidly, as indicated by pairwise dN/dS values. We also used short reads to assemble the genome of Plasmopara muralis, a closely related species infecting grape ivy (Parthenocissus tricuspidata). The lineage-specific proteins identified by comparative genomics analysis included a large proportion of RxLR cytoplasmic effectors and, more generally, genes with high dN/dS values. We identified 270 candidate genes under positive selection, including several genes encoding transporters and components of the RNA machinery potentially involved in host specialization. Finally, the Pl. viticola genome assembly generated here will allow the development of robust population genomics approaches for investigating the mechanisms involved in adaptation to biotic and abiotic selective pressures in this species.\n\nDATA AVAILABILITYRaw reads and genome assemblies have been deposited in GenBank (BioProjects PRJNA329579 for Pl. viticola and PRJNA448661 for Pl. muralis). Genome assemblies, gene annotations and analysis files (e.g. orthology relationships, full tables for GO enrichment analyses, pairwise dN/dS values and branch-site tests) have been deposited in Dataverse (Pl. viticola assembly and annotation: doi.org/10.15454/4NYHD6, Pl. muralis assembly and annotation: doi.org/10.15454/Q1QJYK, analysis files: doi.org/10.15454/8NZ8X9). Links to the data and information about the grapevine downy mildew genome project can be found at http://grapevine-downy-mildew-genome.com/.

genomics↗